The present invention refers to an optical device for providing ultraviolet (UV) imaging capability according to independent claim 1 , a combination of a device comprising a casing and an optical sensor, as well as an optical device according to claim 10 and a method for processing light by a device comprising an optical sensor according to claim 13.

Prior Art

In the prior art, devices that achieve UV imaging or processing capabilities are commonly known. In this regard, for example scintillator sensors are known that are able to receive light in the UV spectrum and to transform it into light of a longer wavelength and thus, photons of a lower energy.

From US 9 173 570 B2 and US 8913 118 B2, methods and systems for capturing, processing and representing to a user multispectral images, including ultraviolet light and its interactions with ultraviolet light-interactive compounds are known. Ultraviolet-light related information can be provided to a user to allow the user to have awareness of UV characteristics and the user's risk to UV exposure.

Further, EP 3449240 describes a device for assessing sunscreen coverage on a person that includes a casing and a lens assembly extending from about a front facing surface of the casing and allowing transmissivity to light energy in a wavelength range of about 300 to about 400 nm. A filter is in optical communication with the lens assembly and having a high optical density above about 390 nm and a low optical density below about 390 nm. A sensor is in optical communication with the filter, the sensor having a signal/noise ratio that is greater than about 36 db. A controller is configured for receiving input from a user to control the device. A display screen may be in communication with a controller for displaying an image associated with the filtered light.

From US 2014/092238 A1 , a device and system are known that provide UV sensing capability. This is achieved by providing a transparent enclosure housing a camera that can be attached to a mobile electronic device so that light passing through the transparent enclosure and reflected by the transparent enclosure is captured by the camera. The camera is connected to a processing unit receiving signals from the camera and produces UV measurement data indicative of UV exposure level, wherein the UV measurement data is extracted by analysing the UV spectral component of light captured by the camera. This requires, in addition to the mobile electronic device, a plurality of electronic components, thereby making the respective device for providing the UV capability quite expensive.

A similar system is also known from KR 101774748 B1, with the same disadvantages.

Also, US 2002/0096728 A1 provides an UV light photodetector comprising silicon photodiodes using which light that was received at a scintillator layer and transformed into light of a larger wavelength, compared to the previous UV wavelength, is detected.

From “Characterisation of a smartphone camera as a response to ultraviolent A radiation” by Igoe, D., Parisi, A., & Carter, B. (2013) in Photochemistry and Photobiology, 89 (1), 215 to 218, a corresponding system is described where an integrated optical sensor (in this case thecamera of the mobile device), directly detect the UV light after optical filtering using neutral density and bandpass filters. However, the detection efficiency of the camera’s image sensor in ultraviolet spectral range is intrinsically low and the optics employed are not optimised for high optical transmission in this range. In general, this limits the usability of standard image sensors, designed for the direct detection of visible light for imaging ultraviolet radiation.

All the above examples show that a plurality of components is necessary, in addition to the optical components already known to be present in current mobile systems like smartphones, for detecting and processing ultraviolet light.

This makes the respective devices comparably expensive, which limits widespread access to ultraviolet imaging to the general public, particularly using mobile devices such as smartphones. Providing this capability to the general public would however be useful for a multitude of applications including, but not limited to: insight into personal sun protection by imaging sunscreen coverage, assessment of skin lesions, as well as plant, surface and environmental monitoring and inspection.

Furthermore, the above devices are active, which means that they require a power supply as well as dedicated software running on the mobile platform they are used in conjunction with, which reduces their universal compliance with the broad range of mobile platforms available.

Finally, these devices are only sensitive to ultraviolet light, which limits their application and the information that can be extracted from the images produced by them. Instead, a device that enables imaging beyond the ultraviolet spectral range would have many more potential applications given that it provides more information. Problem

In view of the known prior art, the technical problem addressed by the present invention is to provide an optical device that provides UV imaging capability at low cost with high efficiency. Furthermore, the design of this device should i) provide universal compatibility with current mobile platforms and ii) enable simultaneously imaging and distinguishing light information in spectral ranges beyond the ultraviolet spectral range.

Solution

This problem is solved by the optical device according to independent claim 1 and the combination of an optical device with a device comprising an optical sensor and a casing according to claim 10 as well as the method for processing light by a device comprising an optical sensor according to claim 13. Preferred embodiments of the invention are provided in the dependent claims.

The optical device for providing UV-imaging capability according to the invention is adapted to be attached to an optical sensor, like a camera, wherein the optical device comprises a receiving section at which light can be received and a propagation component for propagating at least a portion of light received at the receiving section to a transmission section for transmitting light to an optical sensor, wherein the propagation component comprises, in propagation direction of light received at the receiving section, an optical filter for blocking at least a portion of visible light and transmitting UV light, a first optics for focusing the UV light, a screen comprising a fluorescent material for transforming the incident UV light into light of a longer wavelength and a second optics for relaying light having a longer wavelength from the screen to the transmission section. In some embodiments, a protective window may be placed before the optical filter.

In the context of the present invention, the transformation of the incident UV light into light of a longer wavelength refers to light having a longer wavelength that can in fact be detected by the optical sensor. For most commonly employed optical sensors, this will be visible light. Thus, for example, the incident ultraviolet light received at the screen will be transformed by the fluorescent material in the screen to light of, for example, a wavelength that corresponds to green, red or blue light.

In case the optical sensor is capable of also detecting other wavelengths outside of the visible spectrum and to display them in a manner meaningful to a user (for example, infrared or near- infrared cameras), a transformation of the incident ultraviolet light to such wavelength is also encompassed by the invention. In particular, devices that simultaneously detect and image ultraviolet, visible and near-infrared light are advantageous for applications in dermatology, such as sun protection monitoring, solar skin damage and the identification and classification of skin lesions and conditions as well as the detection of the presence and density of topical substances such as skin creams and similar. In addition, such imaging systems may be employed to monitoring of plant health and spills of oil and other organic compounds, industrial surface inspection to detect organic residues, cracks and coatings as well as art conservation and defence applications such as camouflage detection, and others.

Therefore, although we will usually refer to the transformation of the UV light into light within a specific portion of the visible spectral range, such as green or red light in the present invention, other wavelengths and colours of the transformed light are also encompassed by the invention, if not explicitly stated otherwise.

The propagation component is preferably designed such that the image information, in the form of light from a scene incident on the receiving section is maintained until the light is transmitted at the transmission section. This is to be understood such that an optical sensor that receives light from the transmission section is able to detect an image based on the light received from the transmission section that corresponds, in view of its image information, particularly its spatial intensity distribution but not necessarily the colour information, to the image information received at the receiving section of the optical device.

With this optical device, it is thus possible to not only detect whether there is ultraviolet light present in a scene from which light is received at the receiving section, but it is possible to present an image to a user making use of an optical sensor to which the optical device can be attached where image information is obtained. Additionally, the information obtained in the ultraviolet part of the electromagnetic spectrum is made visible to a user, thereby providing UV imaging capability to the optical sensor when the device is used in combination with an optical sensor.

Since the optical device only comprises passive components and does not require any power supply or processing capability, it is compatible with a large range of mobile devices equipped with an optical sensor and can be produced cost-effectively, thereby making it available to the general public. Furthermore, this can reduce the mass of the device compared to an active one, such that it may easily be employed in airborne imaging systems, such as drones, or other applications where mass is critical. Additionally, the properties of components of the optical device can be chosen to enable the detection of a specific portion of the ultraviolet spectral range, as well as portions of the electromagnetic spectrum beyond ultraviolet radiation, such as portions of the visible and near-infrared spectral ranges. In one embodiment, at least one of the optical elements before the screen in propagation direction comprises at least one lens, or a different type of refractive, diffractive or reflective optical element that achieves a similar focusing effect.

Using lenses as optical elements at this position allows focusing incident light on the screen and subsequent relaying of fluorescence from the screen, thereby making it possible to not only efficiently process the light at the screen, but to also provide an output at the transmission section that can be reasonably processed by an optical sensor or the platform into which it is embedded to create an image meaningful to the user.

Further, the fluorescent material of the screen is adapted to transform the incident UV light into light of a wavelength that is detectable by the optical sensor, for example visible light within a certain spectral range.

This makes the optical device adapted to be used together with commonly known image sensors in cameras and specifically in cameras of smartphones that are now widely used.

The fluorescent material of the screen can further comprise at least one of fluorescent molecules, fluorescent ions, fluorescent chemical compounds, crystal defects, quantum dots; and/or the fluorescent material may be embedded into a solid matrix with an optical transmission efficiency of more than 0.5 or more than 0.8, for at least one wavelength in the visible light spectrum, particularly for wavelengths at which the fluorescent material emits light. In embodiments where an optical sensor with different channels is used, for example an optical sensor with red, green and blue colour channels, the fluorescent material may be chosen such that the spectrum of light emitted by it overlaps significantly with one of these channels.

By using those materials, efficient and cost-effective transformation of the incident UV light on the screen to light of a longer wavelength can be achieved. By provision of matrix materials that achieve a high transmission efficiency for both the incident UV light and the emitted fluorescence to achieve efficient excitation and coupling of fluorescence out of the screen, the image information based on the incident ultraviolet light can be provided to the optical sensor in a manner that allows for creating an image comprising this information in a recognisable way to the user.

In a further embodiment the optical device comprises a connecting means for connecting the optical device to an optical sensor.

With this, a secure connection between the optical device and the optical sensor can be realized, thereby making a proper arrangement of the optical device relative to the optical sensor possible, thus ensuring that the image information transmitted from the transmission section is properly received at the optical sensor.

More specifically, the connecting means may comprise at least one of: a clamping mechanism, a magnetic holding mechanism, a threaded connection, a holding means for engaging with a connector at an optical sensor.

These means can be used together with commonly known devices that comprise optical sensors such as digital cameras and smartphones.

In one embodiment the filter is further adapted to transmit light having a wavelength of more than 600 nm or more than 700 nm and wherein the screen is transparent to light having a wavelength of more than 600 nm or more than 700 nm. More generally, the filter may be adapted to absorb or reflect light in a portion of the visible wavelength range corresponding to the spectral range in which the fluorophores in the screen emit light and transmit light outside of this visible wavelength range, where the screen is also transparent to this light transmitted by the filter.

With this, near-infrared information that can also be detected by some commonly used optical sensors, can be transmitted through the system and detected by the optical sensor.

In one embodiment, the optical device comprises a second optical filter located after the screen and before or after the second optics, which partially or fully blocks light having a wavelength of more than 600nm or more than 700nm. This allows modifying the relative intensity between the transformed ultraviolet signal and signals in additional wavelength ranges transmitted through the device, for instance NIR signals.

It can be provided that a polarizer having a polarization axis perpendicular to the optical axis of the optical device is provided between the first optics and the screen.

Furthermore, it can be provided that a first polarizer having a polarization axis perpendicular to the optical axis of the optical device is provided between the first optics and the screen, and a second polarizer having a polarization axis perpendicular to the optical axis of the optical device is provided between the screen and the second optics, wherein, optionally, a rotating mounting is provided that is adapted to adjust the relative orientation of the polarization axes of the first and second polarizer

Here, the optical axis is a line passing along the length of the optical device that passes through the centre of optical elements in the device and is parallel to their axis of symmetry. In addition, a second linear polarizer also having a polarization axis perpendicular to the optical axis may also be provided between the screen and the second optics, wherein the polarization axes of the first and the second polarizers, may be fixed or adjustable. In this sense, the polarization axes of both polarizers lie in the same plane perpendicular to the optical axis. Within this plane, the polarization axes may be set at different angles to each other by rotation of one or both polarizers about the optical axis. This embodiment encompasses perpendicular arrangements of the polarization axes but also other arbitrary arrangements of the polarization axes. The first polarizer is chosen to display high transmission and high extinction ratio at least in the ultraviolet spectral range (100-400 nm), while the second polarizer is chosen to display high transmission and extinction ratio at least in the least in the visible spectral range (400- 700 nm).

With the optional rotating mounting, at least one of the polarizers can be rotated about the optical axis. This allows tuning of the optical device’s visible light transmission from a minimum value when the polarization axes are perpendicular, to a maximum value when the polarization axes are parallel. In this way, visible light is transmitted through the optical device to a user- controllable degree.

Using this combination of polarizers with their polarization axes set to be perpendicular in their common plane perpendicular to the optical axis, visible light incident on the receiving section of the optical device can be efficiently blocked and prevented from transmission to the transmission section. In contrast, a significant fraction of the incident ultraviolet light transformed to visible light emitted from the screen is transmitted through the second polarizer due to the random polarization of the emitted light due to the generally random orientation and high density of fluorophores embedded in the screen. Overall, this increases the signal-to- background ratio of the transformed ultraviolet light information, thereby also improving the transformed ultraviolet image quality at the transmission section.

In some embodiments where it is desirable for a portion of light in the visible spectral range (for instance, wavelengths longer than 400 nm) to be transmitted to the transmission section of the optical device, the angle between the polarization axes of the two polarizers may also be set to be oblique or parallel, i.e. any angle that is not 90° or 270°, provided that the polarizers display a high polarization efficiency in those wavelength ranges.

In some embodiments, one or both polarizers display optical transmission above 0.5 for wavelengths longer than 600 nm or wavelengths longer than 700 nm but low polarization efficiency in both of these ranges, such that e.g. NIR light is transmitted through the system regardless of the relative angle of the polarization axes to each other. According to the invention, a combination of a device, the device comprising a casing and an optical sensor, and an optical device is provided wherein the optical device is attached to the casing of the device such that light transmitted from the transmission section can impinge on the optical sensor.

This endows a device with an image sensor, such as a smartphone, with the imaging properties of the optical device. In one embodiment, the device is one of: a smartphone, a mobile, a laptop, a camera, a tablet. Furthermore, any other device that comprise or are associated with some form of optical sensor may be combined with the optical device. For example, PCs, industrial PCs or any other entities that are combined with or associated an optical sensor may be equipped with the optical device according to the invention in an appropriate manner. In principle, the invention can also be combined with a device that captures images in an analogue manner by using films of chemical components that react to light.

The casing may comprise a connector and the optical devices comprises a holding means adapted to engage with the connector to removably connect the optical device with the device.

This realization of the connector and the holding means makes it possible to easily attach and detach the optical device as need may be.

The method for processing light according to the invention by a device comprising an optical sensor, wherein an optical device according to any of the previous embodiments is attached to the device, method comprising receiving, at the optical sensor, light incident from the transmission section of the optical device, wherein the optical sensor captures an image having colour values in at least two colour channels and wherein the converted UV light having a longer wavelength is received in at least one of the channels, and light transmitted through the optical system is detected in a different channel.

In some embodiments where the optical device transmits both ultraviolet light that is transformed to a portion of visible light as well as a portion of light in the near-infrared wavelength range to an optical sensor with at least two colour channels, ultraviolet light transformed by the screen is detected on one colour channel, while light in the near-infrared wavelength range is preferably detected in a second, distinct channel.

In some embodiments where the optical device transmits both ultraviolet light that is transformed to a portion of visible light, a portion of light in the visible wavelength range (e.g. blue light) as well as a portion of light in the near-infrared wavelength range, the optical sensor may have at least three colour channels, such that light transformed by the screen is detected on one colour channel, while light in the blue spectral range is detected in a second channel and near-infrared wavelength is detected in a third channel, given a sensor with suitable sensitivity to near-infrared radiation.

In one embodiment, the method further comprises separately processing the information received in each of the colour channels to obtain modified information for each colour channel and wherein providing the image of the actual scene comprises providing the image using the modified information of at least two of the colour channels. This allows to, for example, visually amplify characteristics of the image presented to a user. By this, for example, portions of the image indicating high UV-exposure of the skin of the user can be shown more prominent than other portions, like the visible or near-infrared images showing the actual skin of the user.

Furthermore, image information from different channels may be combined to enhance the signal from a single colour channel, for example by weighted addition or subtraction of pixel values in each channel, or the identification of areas of interest and features in one channel combined with the evaluation of image information in those areas of interest in one or more other channels. After this enhancement, image information may be combined, overlaid and presented to the user.

It can be provided that the image is processed further to provide information on UV intensity to a user. This can be done for example by imaging a UV source directly, or indirectly by imaging a target of known UV reflection or absorption illuminated by a UV source

Such information can, for example, focus on the ultraviolet information or the combination of ultraviolet, visible or near-infrared image information.

Further, the information may comprise information indicative of presence and/or absence of UV absorbing, scattering or reflecting materials, like sunscreen.

Brief Description of the Drawings

Figure 1 shows one embodiment of an optical device according to the invention in connection with a device comprising an optical sensor.

Figures 2A-2D show optical properties of components of the optical device over the ultraviolet, visible and near-infrared spectral ranges for different use cases. Figures 3A 3C show different embodiments of connecting means for connecting the optical device to an optical sensor or another device comprising an optical sensor.

Figure 4 shows a block diagram illustrating the process of multichannel image processing and intensity calibration.

Figure 5 shows the optical device employed for direct or indirect light intensity measurement.

Detailed Description

Figure 1 shows an optical device 100 according to one embodiment of the invention only in a very schematic depiction. In Figure 1, a device 130 is also depicted that comprises an optical sensor 131 (like a camera) onto which light that is finally transmitted from a transmission region 140 of the optical device 100 can impinge when the optical device is positioned appropriately.

It is noted that the depiction in Figure 1 does not show the size of the optical device 100 in relation to device 130 in its actual dimensions. Instead, the optical device 100 is schematically depicted in a significantly enlarged way. This means that the optical system will usually be much smaller, preferably on the order of the size of the optical sensor of the device 130 or at least within a range of dimensions comparable to the size of the device 130. For example, suitable dimensions of the optical device 100 range from 1x1x1 cm to 2x2x2cm or to 4x4x4cm and any combination of the values in between or above those values.

The depiction in Figure 1 is thus only schematic.

The device 130 may be any device that comprises an optical sensor 131. Preferably, the device 130 is either a smartphone or a camera commonly used by a plurality of users. The optical sensor, thus, may usually consist of an image forming system, comprising for instance one or more lenses, in conjunction with an image sensor, for instance a CCD or CMOS-type image sensor. This image sensor is at least capable of receiving and detecting light in the visible spectrum (defined here as 400-700 nm) and processing this light further because the image sensor comprises pixels that are sensitive to at least three different wavelength ranges in the visible spectrum corresponding to colour channels, e.g. red, green and blue (RGB) channels. In addition, the pixels of the image sensors may be partially sensitive to near- infrared (NIR) radiation in the range of typically 700-1000 nm. The invention however, is not limited to such optical sensors - “analogue” sensors, making use of chemically processed films of material, can also be used.

In Figure 1, the optical device may be considered to comprise a casing in which the other components of the optical device are provided. This casing can be made of an opaque material such as plastic and also serves to surround, include and hold the components of the optical device in place.

The optical device depicted in Figure 1 comprises one propagation component 150 along which light received from a receiving section 120 can travel. This light is shown with arrows, where the arrows also indicate the propagation direction of the light through the optical device 100.

The receiving section 120 may be an opening in the casing of the optical device, allowing for transmission of light into the casing and onto the components of the optical device. The device may also include a protective window 155, which is transparent to ultraviolet (wavelength 100- 400 nm), visible and near-infrared light.

The propagation component 150 comprises, according to the invention, at least two optics 152 and 154. In propagation direction before the first optics 152, a filter 151 can be provided. This filter comprises one or more layers of a material that blocks at least a portion of the incident light where this portion belongs to the visible light by absorption, reflection or other optical effects such as interference. In one embodiment, all of the visible light of the incident light beam is filtered out by the filter 151.

The filter, however, is at least transparent to ultraviolet light or has an absorption coefficient and a thickness that does at least not result in a complete extinction of the incident ultraviolet light so that ultraviolet light passes the filter 151 at least partially.

In propagation direction, it then impinges on the first optics 152. These first optics might be provided in the form of a lens in order to focus the ultraviolet light onto the component 153 that will be explained below in further detail. It can also be provided in the form of a mirror or other suitable means as long as the first optics 152 achieves focusing at least the UV light on the component 153. In some applications, optics 152 is chosen to minimize the optical field curvature on component 153, for example by comprising at least one meniscus lens.

Instead of lenses, other optical arrangements that achieve an optical effect corresponding to that of a lens may be used. For example, a pinhole or a combination of a pinhole and a lens may be used instead of a lens. The invention is not limited in this regard. Therefore, whenever the term “lens” is used throughout this description, any optical arrangement, including mirrors, or other refractive, diffractive and reflective optical elements that achieve an optical focusing effect comparable to that of a lens should be considered encompassed under the term “lens”.

The component 153 is, according to the invention, a screen that comprises at least some fluorescent material that is adapted to transform the incident ultraviolet light from the optics 152 into light of a longer wavelength. Thereby, the ultraviolet light can preferably be transformed from the ultraviolet range of the electromagnetic spectrum into the visible range of the electromagnetic spectrum. This is achieved by using a screen containing fluorescent material that emits longer-wavelength electromagnetic radiation upon absorption of ultraviolet radiation.

In preferred embodiments, the filter 151 has a sufficiently high optical density - referred to as “blocking” in the following - for light in the visible portion of the electromagnetic spectrum that corresponds to the longer wavelength of the transformed ultraviolet light, as illustrated in Figure 2A. In Fig. 2A, the filter, with a transmission spectrum indicated by the dashed line as T, blocks all wavelengths except the UV spectrum up to a wavelength of approximately 400nm. The screen material has an absorption spectrum denoted as A and thus absorbs this UV light and subsequently emits light of a longer wavelength, within a spectral range denoted as F. In such embodiments, it can be preferred that the filter only blocks this specific portion of the electromagnetic spectrum that corresponds to the longer wavelength of the transformed ultraviolet light but transmits the remainder of the visible electromagnetic radiation/visible light and beyond, for example in the near-infrared spectral range, as illustrated in Figure 2B. Here, the filter T blocks only the light in the visible spectral range as shown in Fig. 2B which essentially corresponds to the emission wavelengths F of the screen.

The structure of the fluorescent material and of the screen is preferably such that the image information that was originally received at least with respect to the ultraviolet portion of the originally received image information is maintained also after the light having a longer wavelength leaves the screen 153. In a preferred embodiment, the screen 153 is a material of thickness below 5mm, more preferred below 3mm, more preferably below 1 mm.

For example, the screen can be made up of a material comprising at least one of: fluorescent ions, fluorescent molecules, fluorescent chemical compounds, crystal defects or quantum dots that exhibit the respective property of being able to transform the incident ultraviolet light into light having a longer wavelength, such as red, green or blue light. Such materials are, in principle, already widely known. In a preferred embodiment, the fluorescent material chosen should display a high absorption in the ultraviolet spectral region transmitted by the optical filter.

In some embodiments, the screen may also contain a mixture of two or more fluorescent materials, where some materials display high absorption in a certain band of the ultraviolet spectrum, for example UVB (280-315 nm) and corresponding fluorescence in a certain visible wavelength range, while other materials display high absorption in the UVA spectral region (315-400 nm) and corresponding fluorescence in a distinct visible wavelength range to the first material. This embodiment is illustrated in Figure 2D for a screen comprising two different fluorescent materials, with UV absorption bands denoted here as A1 and A2, respectively. Light in the spectral range of these absorption bands is transmitted by the filter, while the filter blocks visible light that basically corresponds to the light having a longer wavelength (F1 and F2) that are emitted by the two different materials in the screen. As shown in Fig. 2D, other portions of the electromagnetic spectrum may likewise not be blocked by the filter.

In some embodiments the screen may comprise a plurality of screens, each consisting of a fluorescent material embedded in a host matrix such that the fluorescent materials in different screens display high absorption in a certain band of the ultraviolet spectrum, for example UVB (280-315 nm) and corresponding fluorescence in a certain visible wavelength range, for example 400-500 nm, while the materials in other screens display high absorption in a different spectral region, for example UVA (315-400 nm) and corresponding fluorescence in a distinct visible wavelength range, for example 500-600 nm, to the material in the first screen. This embodiment is also illustrated in Figure 2D. To ensure that the image information is not lost when the incident light is transformed by the fluorescent material within the screen, e.g. due to absorption, the fluorescent material can be embedded into a transparent matrix, such as a polymer, a glass or a crystal. This matrix should be made of a material that is at least transparent in the wavelength range of the light having a longer wavelength, that is emitted by the fluorescent material. This ensures that no unintended loss of image information occurs during the transformation of the light. The material of the matrix holding the screen may be a solid material. However, in some embodiments, the material of the matrix may be a liquid material or a gel or some other viscous material. Using a solid material for the matrix gives the advantage that the screen is well fixed.

In this regard, it is also preferred if the matrix (independent of whether it is solid or liquid or a gel) has a high transmission efficiency within at least the wavelength range corresponding to the ultraviolet spectral range transmitted by the optical filter as well as the spectral range corresponding to the fluorescence of the fluorescent material. Preferably, the transmission efficiency of the matrix is at least 0.1 , preferably larger than 0.3, more preferably even larger than 0.5 and, most preferred, larger than 0.8. Thereby, it is not only ensured that most of the image information is maintained, but also that processing of the transformed light is easier as the signal strength is comparably large. In order to ensure that the respective image information is properly relayed to the optical sensor 131 of the device 130, a second optics 154 can be arranged in propagation direction after the screen 153 and can ensure, for example, relaying of the light transmitted from the screen. In some implementations, this can be advantageously used to reduce the focal length of the optical sensor (like a camera) to which the optical device is attached because it enables the reduction of the dimensions of the optical device. In other implementations, optics 154 can be chosen such that it forms an image of the screen at infinity in the direction of propagation. In all cases, optics 154 is arranged such that light propagating along the propagation component 150 is then directed to transmission section 140.

In some embodiments, the optical properties of optics 152 and 154, in particular their focal lengths, are chosen such that the image captured by the optical sensor 130 with the optical system in place represents a magnified or reduced image of the scene compared to the intrinsic field of view of the optical sensor in the absence of the optical device.

In some embodiments, at least one of the spaces 156 and 157 between the optics 152, 154 and screen 153 may be filled with a solid or liquid material or combination of materials, the material or the materials having a refractive index higher than air, for example an oil or a polymer. This improves focusing light onto screen 154 and collection of emitted light from it, leading to an overall increase of transformed signal levels at the transmission section 140.

In some embodiments, the optical density of filter 151 may not be sufficiently high to completely block unwanted light, particularly visible light in the wavelength range of the screen 153 emission. Such light could result in an increase of background signal in this wavelength range, resulting in an overall reduction of the relative signal strength of the converted ultraviolet light emerging from screen 153.

In order to prevent this, two polarizers may be provided where one of them is provided in propagation direction of the light before the screen and the other is provided in propagation direction of the light after the screen. Different positions for the first polarizer are shown with items 161, 162 and 165. The first polarizer, however, may be placed anywhere before the screen. The second polarizer may be provided at positions 163 or 164 or at any other positions after the screen but within the optical device. Preferably, both polarizers have an extinction ratio of at least 10:1 , preferably more than 100:1 and most preferably more than 1000:1 in the wavelength range corresponding to the visible light emitted by the fluorescent material in screen 153. In some embodiments, the polarizer at position 165 or 161 is chosen to additionally display an extinction ratio of at least 10:1 , preferably more above 100:1 and most preferably more above 1000:1 for light in the ultraviolet wavelength range transmitted by the optical filter 151. Preferably, the polarizer at positions 165 or 161 is transparent to ultraviolet, visible and near-infrared light, while the polarizer at positions 162 or 163 is at least transparent to visible and near-infrared light. The polarizers can also be provided to polarize visible and near-infrared light.

Preferably, the first polarizer has a polarization axis that is perpendicular to the propagation direction of the light and may be denoted as P1. P1 may be provided in arbitrary direction as long as it is perpendicular to the propagation direction of the light.

The second polarizer then has preferably a polarization axis that is also perpendicular to the propagation direction of the light (i.e. positioned in the same plane as the polarization axis of the first polarizer) and may be denoted with P2. Most efficiently, the polarization axes P1 and P2 are also, although being provided in the same plane, perpendicular to each other. Thereby, light that passes the first polarizer and then impinges on the second polarizer will be strongly attenuated due to the perpendicular arrangement of the polarization axes. However, any relative arrangement of the first polarization axis and the second polarization axis relative to each other, can be thought of. Therefore, according to some embodiments, the angle between the first and the second polarization axis is larger than 0° but is different from 90° and from 180° and from 270°. Any other relative arrangement of the polarization axis, like 30°, 45°, 60° can be thought of. In some embodiments, the angle between the polarization axes P1 and P2 may also be freely chosen by mounting either one or both polarizers within the optical device such that they may be rotated about the optical axis perpendicular to the plane in which both P1 and P2 lie. By this, the actual impact of the polarizers can be selectively adjusted by, for example, a user of the device.

By providing the polarization axes perpendicular to each other, it is made sure that only visible light that is emitted by the fluorescent material in screen 153 is actually relayed to the optical sensor 131.

Thereby, it is made sure that only light that was generated by the screen 153 leaves the first propagation component. In some embodiments, it may be preferred to provide the first and the second polarization axes at an angle relative to each other that is different from 0°, 90°, 270° and 180°. Thereby, the visible light of the scene is dimmed but not completely blocked, thus still enhancing the signal strength of the light converted by the screen compared to background light.

In some embodiments, a rotating mounting system for at least one of the polarizers can be used such that this angle, and thus the degree to which visible light is rejected, can be arbitrarily chosen by the user. The rotating mounting can be provided for one polarizer only or it can be provided that a rotating mounting is provided for each of the polarizers, allowing for independently rotating the polarizers. For example, if two rotating mountings are provided, the accuracy with which one of them can be moved to adjust the angle of the polarizer can be different from the accuracy with which the other polarizer can be rotated.

In some embodiments, only the first polarizer at positions 161 , 162 and 165 can be provided such that the optical device images polarized light, for example to reduce specular reflections from surfaces.

However, these polarizers do not need to be provided but they can be advantageous in order to prevent light that has not been converted to fluorescence or is otherwise undesirable in some implementations from travelling from the receiving section to the transmission section, thereby improving the quality with which the image information is forwarded to the optical sensor 131.

It is noted that it can also be provided that the optical elements employed in the device, specifically the optical filter 151 as well as the screen 153 could be transparent to near-infrared light in the wavelength range of 700-1000 nm (referred to as NIR light), thereby enabling NIR light to be transmitted through the device and subsequently impinge on and imaged by the optical sensor. Commonly used CCD or CMOS image sensors are usually sensitive to NIR light to a certain degree, even when covered by a colour filter such as a Bayer filter. In some embodiments, the fluorescent material in the screen 153 is chosen such that the spectral range of its fluorescence does not fall into the NIR range, thus enabling simultaneous NIR and ultraviolet imaging by the optical sensor in different colour channels, as shown in Figure 2B.

Additionally, the optical filter 151 may be chosen to also transmit a small portion of visible light (VIS, for example in the range of 400-500 nm), which does not overlap with the spectral range of the fluorescence of the fluorescent material used in the screen. Provided that all optical elements employed in the device are transparent to the VIS spectral range, this signal can thus be transmitted through the optical device and subsequently imaged on the optical sensor, given that commonly used CCD and CMOS image sensors are sensitive to this wavelength range. In some embodiments, optical filter 151 is chosen such that it displays an optical transmission in the ultraviolet wavelength range (denoted by T in Fig. 2C) of preferably 0.4 or above, an optical transmission of preferably 0.2 or above within a portion of the visible wavelength range (for example within the range 400-450 nm) and an optical transmission of preferably 0.1 or above of in the NIR wavelength range (e.g. 700-1000 nm) as well as high optical density (optical transmission preferably below 0.01) in a portion of the visible wavelength range (e.g. 450-700 nm). In addition, the fluorescent material in screen 153 may be chosen to fluorescence mainly in the wavelength range F (e.g. 450-700 nm). This embodiment enables simultaneous imaging of light in the ultraviolet, visible and NIR wavelength ranges in different colour channels of the optical sensor, as shown in Figure 2C.

This information might not always be used for providing image information to a user in the form of, for example, an image. However, the additional information contained in these channels can be mixed, added, subtracted or otherwise combined to enhance the image quality of one or more of the channels, or to detect the presence of materials in the image with a specific combination of optical properties in one or more of the different colour channels, thus allowing its presence to be detected.

It can also be that the UV light is transformed by means of the screen into such near-infrared light. Most preferable, the optical sensor is sensitive to wavelengths of 400 nm or more, or 600 nm or more in addition to the visible light. Most preferable, the screen can then transform the UV light into light having a corresponding wavelength of more than 400 nm or more than 600 nm.

In embodiments that comprise an optical filter 151 with transmission in the ultraviolet wavelength range as well as in additional wavelength ranges (e.g. transmission bands in the NIR range), the optical device may include a second filter after the screen at positions 163 or 164 that partially or completely blocks this additional wavelength range. This allows modifying the relative intensity between the transformed ultraviolet signal and signals in additional wavelength ranges transmitted through the device, for instance NIR signals.

In order to ensure that the optical sensor is correctly arranged with respect to the optical device and vice versa, Figure 3 shows some embodiments of connecting means that are adapted to provide a connection between the optical device on the one side, and the optical sensor and/or the device comprising the optical sensor on the other side. Such connecting means may form a single unit that directly connected to the device or may also comprise two parts, one connected to the device and the other to the optical sensor, to facilitate attaching and detaching the device. In Figure 3A, the device 201 comprising the optical sensor (like a camera) is schematically depicted. This device 201 may for example be a smartphone but it may also be any other device that comprises an optical sensor. For example, it can also be a personal camera that is only adapted to take images and does not have furtherfunctionalities. Though a smartphone is depicted in Fig. 3, this is thus not limiting the invention.

In the embodiment shown in Figure 3A, the optical device 100 comprises a clamp 202 that can be engaged with the casing of the device 201. This clamp can for example, be adjustable in order to provide a force-tight connection between the optical device on the one side and the device 201 on the other side. This adjustability can also be used to detachably connect the optical device with the device in a manner that allows for application of the optical device to a plurality of devices 201. For example, considering the clamp 202 as a specific case of a holding means, its geometrical shape (for example the portion encasing a portion of the device 201) can be provided in a modifiable or adjustable manner, thereby allowing matching it to the casing of a plurality of different smartphones. Thereby, the optical device is combinable with a plurality of known smartphones. The same holds for cameras as well as camera objectives, for which a corresponding clamping, holding or screwing mechanism can also be provided.

Figure 3B shows an alternative embodiment that could still be combined with the embodiment of Figure 3A. In this case, the device 201 with the optical sensor 250 comprises one or more connectors 211. In the present situation in Figure 3B, those connectors are basically provided in the form of slots or openings 212. Those openings or slots 212 are provided such that they can receive corresponding holding means 213 in the form of pins 213 which are provided at the optical device 100. Thereby, a detachable connection between the device 201 and the optical device 100 can be realized.

Other realizations which may also be considered combinable with each of the embodiments in figures 3A and 3B can be thought of according to Figure 3C. In this case, the device 201 comprises a receiving means (as one realization of a connector) 221 in which a magnet 222 is arranged. This can be provided for example, in proximity of the optical sensor 250.

The optical device 100 then comprises one or more corresponding magnets 223 that can be positioned relative to one or more magnets 222 and as engaging the receiving means 221 such that once this magnet force connection is established, the optical device is properly positioned relative to the optical sensor 250.

Other means can also be thought of in order to establish a connection between the optical device and the optical sensor. Preferably, those means realize a detachable connection between the optical device and the device comprising the optical sensor or a detachable connection with the optical sensor itself.

Thereby, an optical device is provided that does not need any source of energy or any processing capability but passively provides UV imaging capability to the optical sensor 131 of a device 130.

Once the light is received at the optical sensor, it can be processed further. Typical image sensors such as CCD or CMOS-type image sensors used in the optical sensor will usually comprise one or more colour channels and each image sensor pixel receives a portion of the image information of light leaving the transmission section of the optical device and impinging on the optical sensor. This image can comprise image information in the spectral range of the transformed UV, as well as transmitted visible and NIR light.

Figure 4 shows exemplary steps for processing captured multispectral light from the optical device detected by the optical sensor, which the optical device is in communication with. These steps can be implemented in software running on a computer processor in communication with the optical sensor.

First, an imaging mode 400 may be selected, which preferably depends on the multispectral signature to be detected and defines the parameters 401 used in the subsequent image processing. For example, a given imaging mode might be used to detect a specific combination of absorption and reflection over various spectral bands. The imaging mode may be user-selected or can be fixed for a specific use case. Once selected, the optical sensor captures an image 402 relayed to it by the optical device. This image may then be split into its channels in step 403, each channel preferably corresponding to a certain wavelength range and consisting of a matrix of pixel values. In typical three-channel image sensors, the channels correspond to red, green and blue colour ranges. Next, the image information in each channel is processed preferably individually in step 404, for instance by adjusting contrast, brightness and gamma values and performing operations such as histogram equalisation and other operations. The processing of the image information in the channels may also be done together for all the channels or a subset of the channels. The parameters of this channel optimisation step 404 may depend on the initially selected imaging mode.

Now, image information from different channels can be combined in step 405 to, for example, enhance certain spectral features of interest. In particular, this step may include but is not limited to operations such as image pixel value addition, subtraction, multiplication and weighted combinations thereof, as well as logic operators acting on multiple channels. Furthermore, this type of image information processing may be used to compensate crosstalk due to undesired spectral overlap between the optical response of components in the optical device and to remove background signals. The outcome of the channel mixing step may be one or more images. In some embodiments, this information may be further processed in step 406 to detect certain features of interest, which may also be defined by the selected imaging mode, using, for example, automated image processing methods including but not limited to neural networks, deep learning and artificial intelligence.

Finally, the thus processed images, or overlays of the same, as well as features of interest may be displayed to a user on a screen in step 407, and/or stored in step 408, and/or processed further in some way in step 409. The described series of steps after image mode selection may also be performed continuously. This means that upon displaying an image in step 407, it is returned to step 402 where a new image is captured. This is indicated by the arrow 410. In this regard, the steps 402 to 407 may also be performed in parallel for consecutive images. For example, while a first image taken is processed throughout steps 403 to 407, it may be provided that a subsequent image is already taken in step 402 and is then processed further just after the first image has be processed by the immediately following step. This can be advantageous for instance for video image processing and real-time display of multispectral features.

In embodiments of the optical device comprising one or more polarizers, the polarization axis angle of each polarizer may also be either taken as an input parameter for a given imaging mode or may be set to a given value, for instance in embodiments comprising one or more rotatable polarisers. This adjustment of the optical device’s transmission properties can support the detection of some types of multispectral signatures, including but not limited to suppression of specular reflection as well as visible light transmission.

In addition to the steps illustrated in Figure 4, image processing may also include an additional intensity measurement step that enables a measurement of light intensity from a scene in different spectral bands transmitted through the optical device to optical sensor. This intensity measurement requires knowledge of the spectral response of the combination of the optical device and the optical sensor, defined by the optical device’s transmission and the optical sensor’s exposure time, gain and spectral sensitivity. In particular, the response of the combination of the optical device and the optical sensor can be calibrated for each optical sensor channel using a broadband light source of known and controllable intensity over the spectral ranges detected by each channel. Once the response of the combination is calibrated in this way, unknown light intensities in multiple spectral bands can be measured with it. Figure 5 illustrates the direct measurement (Fig. 5A) of light source 500 intensity by the combination of the optical device 501 and the optical sensor 502 as well as an indirect measurement (Fig. 5B) of light source 503 intensity by capturing an image of a target with known diffuse reflection properties, such as a label or marking 506, for example on a bottle or container 507. For example, under solar illumination, this enables in situ measurement of the ultraviolet index (UVI) at the user’s location.

At the same time, this calibration can entail readout of the chemical composition of topical creams and/or lotions, such as sunscreen for example, if the label or marking contains encoded product information, for example as UPC, QR or similar codes. The optical properties of the product, such as its absorption and reflection in the spectral ranges that are detected by the optical device, may then be accessed directly via this code, or inferred using its chemical composition. In turn, these optical properties can then be used to set the channel optimisation and mixing parameters of the imaging method to improve the optical assessment of skin coverage by the topical product, together with additional information such as skin type, for example.

This can preferably be done in an automated fashion, i.e. upon reading the respective code on the bottle of sunscreen, the computing device may automatically set the respective imaging mode as explained above with relation to Fig. 4. For example, an “app” can be provided on the computing device (or any other suitable computer-readable instructions) that cause the device, upon reading the respective code, to select an imaging mode. This selection can be based on a table or other data format storing both, the code or a plurality of codes that may be found on a bottle of sunscreen and a corresponding imaging mode. If a code is recognized, a matching of this code to codes in the table can be attempted. If a match is found, the corresponding imaging mode can be selected.