FIELD OF THE INVENTION

The invention relates to a working light for issuing light with adjustable spectrum. The invention further relates to a lighting system comprising such a working light, and to usage of both the working light and lighting system.

BACKGROUND OF THE INVENTION

Light is a fundamental part of life and affects us in a variety of ways - visually, psychologically and biologically. The most obvious effect of light on humans is to enable vision, 83% of information we receive from the world, comes through our eyes. The human eye is a very specialized functional system that can function from very dark to very bright levels of light; and from near-by to far-away vision. The ability to perform a near-by visual task is strongly related to the effort it takes to distinguish the details from the background, i.e. a good contrast. This is the result of the complex system of light reflected from the task, subsequently captured by the eye and finally sent to our brain. This effort can be minimized by creating circumstances for optimum vision by ophthalmo logical correction in the first place, supported by the right indoor light conditions. By taking into account the visual and biological effects of light, a lighting solution has the power to support near-by visual tasks, of which dimming/boosting is one of the tools for such lighting. Reading typically is a near-by visual task which requires good contrast and can induce high eye-strain upon prolonged reading or inadequate lighting conditions.

A working light for issuing light with an adjustable spectrum is known from US20120176767A1. The known light source comprises a plurality of light emitting devices (LEDs). The combined output of the various LEDs renders the light source to have a white emission spectrum with an intensity, and hue or chromaticity that offers viewing comfort to persons. The emission spectrum issued by the known working light is dimmable in brightness and adjustable in color to enhance the visual acuity and to improve the comfort of lighting to the eyes of a human. Though by the known working light the comfort and the visual acuity is improved compared to no light, it yet has the disadvantage that both the aimed comfort to the eyes of a human and the aimed improvement in visual acuity is still relatively poor. SUMMARY OF THE INVENTION

It is an object of the invention to provide a working light comprising a light source of the type as described in the opening paragraph in which at least one of the abovementioned disadvantages is counteracted. Thereto the working light of the type as described in the opening paragraph comprises a light source being configured to generate source light of a white light emission spectrum having a CCT in a range of 2500-20000K and a control unit being configured to control a lighting element for tuning of the source light with respect to a ratio between a first emission peak in a wavelength range of 440-5 OOnm and a second emission peak in a wavelength range of 400-430nm, the lighting element being at least one of a tunable light filter, switchable lighting element, dimmable lighting element, and the light source is tunable in light intensity and that upon boosting the light intensity and/or lowering the CCT the ratio between the first and second emission peak increases..

When aging, the human eye lens yellows, which has two effects: blue light is absorbed more and blue light is scattered more. For older people this introduces problems like:

Less blue light reaches the retina. Because less blue light reaches the eye, the melanopsin is less activated, potentially having bad effects on the circadian rhythm.

Blue light is scattered more, causing lower contrast and hence reading capabilities are reduced.

To solve these problems, the working light comprises the feature that the light source is tunable in light intensity and that upon boosting the light intensity and/or lowering the CCT the ratio between the first and second emission peak increases. A typical lighting element to be controlled is at least one of a dimmable lighting element, a switchable lighting element, and a tunable light filter. Preferably, the lighting element is a dimmable blue light lighting element of the light source and/or a switchable blue light emitting lighting element of the light source, or a blue light filter. The lighting element then preferably comprises a first lighting element issuing light having a first maximum emission peak in a wavelength range of 440-500nm during operation, and a second lighting element issuing light having a second maximum emission peak in a wavelength range of 400-43 Onm during operation. The control unit to control the light emitting elements can, for example, be a switch, a power knob, a pulse width modulation (PWM)-unit, amplitude modulation (AM) unit, current control unit. Ways to control the filter can be via a variable voltage source, transverse shift of a filter having variable thickness or dope concentration transverse to the propagation direction of light as issued by the light source that passes through said filter.

It appeared that in dependency of light intensity and/or color temperature, in particular in the blue range of the visible spectrum, i.e. in the wavelength range of 400- 500nm, both the visual acuity of human eye and appreciation of the provided light are sensitive for the spectral distribution in said range. For example, when a lower intensity light setting is used for reading, for example when one is reading in bed and the reader does not want to disturb his partner who is sleeping, the comfort of reading decreases. From user tests it is concluded that male readers in this situation at relatively low lighting levels, for example in a range of 100 to 200 lux, prefer the LED light spectrum with the second blue peak in the range 400-430 nm next to the first blue peak in the range of 440-5 OOnm. For female readers this preference is smaller, and many prefer also the white light without the second blue peak in the range 400-43 Onm. Therefore an embodiment of the working light is characterized in that the second lighting element is independently switchable between an off-state and an on- state. Furthermore, more appreciation of the spectrum was obtained with a spectrum in which the first peak is in the range of 450-490nm, most preferably close to 450nm, i.e. in a range of 450-460nm.

One aspect of viewing comfort involves discernment of colors and fine details in work scenes. Human eyes tend to do this best with higher levels of illumination, for example at 800-1200 lux. However, human vision involves not only the eyes, but also the brain. A person doing a visual task can easily benefit from a magnitude of illumination other than that which optimizes acuity of the eyes alone. One example is of a person whose brain is adapted to a lower illumination level, and who experiences discomfort from experiencing a jolting blast of a higher illumination level even if that person's eyes work better at a higher illumination level.

Some persons maximize their viewing comfort in most to all work situations with higher illumination levels that favor greater visual acuity. Other persons can lose productivity by having illumination level suddenly changing from one that such persons are adapted to, to a greater one that such person's visual systems do not quickly adapt to due to discomfort arising from effort required in the brain to adapt to a change in illumination level.

Apart from dimming or boosting being one of the tools for a lighting solution to support near-by visual tasks, another tool is a tunable spectral composition of the emission spectrum to release eye-strain. Tunable spectral composition means that the ratios between wavelengths in different parts of the spectrum are mutually changeable. Light has been reported to influence near-by vision by creating the contrast between the detail and background, which in turn strongly depend on both the intensity and the spectral composition of the light source. As a result, people experience an improved clarity of details and may even see smaller details. Next to the physical illumination of a task, light has been reported to influence vision physiologically by playing a role in the pupil contraction, accommodative response, enhancement of the visual acuity and vision blurring.

Task illuminance affects the visual acuity and contrast while a cooler tone of light enables people to read faster. Additionally, qualitative research demonstrates that personalized light tones and intensities leads to optimal visual comfort. It is demonstrated that people experience difference between warm and cool tones on the clarity of details.

Since most consumers associate visual comfort with reading comfort, a desklight for optimal reading comfort by introducing the personalization of not only the brightness but also the color of light was tested. It appeared that 90-95% % of the consumers (n=20) who tested the desklight with the feature of personalization of the color of light and the brightness confirm that their choice delivers sharper vision, optimal eye comfort, seeing smaller details and improved contrast. However, the choice they made in intensity and color temperature ranged from low to high intensities and from low to high color temperatures underpinning the fact that each eye is unique and hence that innovative luminaires need personalization.

The expression "working light" is to be understood as a lighting device which has for its main purpose to illuminate an area or space for people to work, recover, rest and/or read, for example, a luminaire for illumination of a room or space in an office, hospital, nursing home, psychiatric center, restaurant, library, study center, at home or of an outside space like parking lot, terrace, or billboard.

The expression "white light" refers to the chromaticity of a particular light source or the "color point" of the light source. For a white light source, the chromaticity may be referred to as the "white point" of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator heated to a given temperature. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the color or hue of the white light source. White light typically has a CCT of between about 2500 K and 20000 K. The term white light for general lighting especially is generally in the range of about 2700 K and 6500 K, and for backlighting purposes especially in the range of about 7000 K and 20000 K, and especially within about 15 SDCM (standard deviation of color matching) from the BBL (black body locus), especially within about 10 SDCM from the BBL, even more especially within about 5 SDCM from the BBL. White light with a CCT of about 4000 has a yellowish color. White light with a CCT of about 8000K or higher is more bluish in color, and may be referred to as "cool white" or "crisp white". "Warm white" may be used to describe white light with a CCT of between about 2500K and 4000K, which is more reddish in color.

The expression "emission peak" means a local maximum in the emission wavelength which is at least twice the intensity in the number of photons of the emission of near/adjacent emission wavelengths.

In an embodiment of the working light the light source is tunable in light intensity (dimmable). The expression "dimmable" in this respect means that the intensity or brightness of the light is controllable in a continuous manner or in at least three steps, i.e. it can be gradually boosted or dimmed and eventually turned off/on. In particular LEDs are suitable for tuning at least one of the intensity or spectral distribution of the emission spectrum as these are easily dimmable and in view of the usually large number of LEDs for generating the spectrum, the fraction of active operating LEDs is easily changeable. Thereto an embodiment of the working light is characterized in that the first lighting element comprises a first LED and in that the second lighting element comprises a second LED. In most cases the working light also comprises a, preferably tunable/dimmable, green light emitting LED and a, preferably tunable/dimmable, orange-red or red light emitting LED as a third respectively as a fourth lighting element, for example for obtaining white light with a CCT of 10000K or lower.

An embodiment of the working light is characterized in that the light source is tunable in light intensity and that upon dimming, the ratio between the first and second emission peak decreases. Experiments were done using different, i.e. the light levels being 150 lux, 500 lux and 1000 lux. The group of n=23 respondents was about equally divided over males and females. The outcome of the test was that males increasingly preferred the emission spectrum in which the second peak was increased as the light level was decreased; at the lowest light level 90% of males preferred the emission spectrum with the second peak being higher than the first peak. For females there was the same, but yet somewhat smaller, effect.

An embodiment of the working light is characterized in that the CCT of the white emission spectrum is not affected by, or in other words, is not causal related to the tuning of the ratio between the first and second emission peak. To attain this effect, the sensor of the working light measures the spectral composition of the initial spectrum and calculates from that the CCT. Subsequently the spectrum of the follow-up light spectrum is adapted in emission intensity in the longer wavelength ranges, i.e. the green to red-part of the spectrum, to compensate for and/or reverse the effect on and/or the shift the CCT caused by the difference in the second emission peak between the initial and follow-up spectrum. It is appreciated by users if one wants to switch from general background lighting to task lighting, such as reading, an said switching involves a different ratio between the first and second emission peak, without the CCT of the emission spectrum to change. In particular this is appreciated if two people are in the same room that, when said switch made on behalf of the first person, said switch is not noticed by the second person because the CCT remains constant.

Embodiments of the lighting device issue light having a CCT in the range of 2500K to 6000K. At these relatively low CCTs the contribution of the blue radiation in the spectral output is relatively low, and hence for usually applied indoor lighting levels the risk on retinal damage for older people with an eye disease is acceptably low. Normal indoor lighting levels are generally in a range of 600 to 1000 lux.

The invention further relates to a lighting system comprising a working light according to the invention, a user carried device, and a sensor configured to sense sensor data during operation, said sensor data comprising a location of the user carried device, and (ambient) spectral lighting conditions, the sensor is further configured to provide the control unit with a sensor signal based on the sensor data which sensor signal is processed by the control unit to tune both the ratio between the first and second emission peak and their absolute emission intensity during operation.

The carrier device can be uploaded with general data which normally renders the lighting system to provide good visual acuity and comfort to an average person and which are adapted to the (ambient) spectral lighting conditions. Yet, an embodiment of the lighting system is characterized in that the user carried device is uploaded with personal user data, for example gender, age, race and personal eye-characteristics, like for example wearing glasses or contact-lenses. Both said personal data and sensor data are processed by the control unit to adjust both the emission spectrum and intensity to the personal user during operation.

Working conditions and visual comfort are then personalized and hence can be optimized for a specific person.

Preferably, an embodiment of the lighting system is characterized in that exposure to the lighting conditions relates to the wavelength range of 400-5 OOnm, which is the most relevant wavelength range for providing and control of visual comfort and visual acuity.

The invention still further relates to the use of the working light according to the invention and to the use of the lighting system according to the invention for providing favorable working conditions and controlling experienced appreciation of light. Appreciation or discomfort of light is most often related to experienced contrast. Our eyes adapt to the most prevalent brightness in our field of view, and when the brightness is fairly uniform we are generally comfortable, whether the levels are high or low. Our eyes can adapt to a wide range in brightness, from a moonlit night to a bright sunny day. The discomfort occurs when our eyes try to adapt to two levels at once, like trying to read a bulletin board with bright windows on either side. Discomfort also occurs when the contrast is sudden, like exiting from a darkened theater into a sunlit street. Our tolerance for high contrast also varies depending on the circumstances, for example age. In general, people are more tolerant of extremes if the light source is natural, if they have the option to make adjustments to avoid discomfort, and if there is little pressure to perform difficult visual tasks. Occupants in a library may choose a chair in bright sunlight, even though reading is more difficult. But in working environments, the goal is to avoid visual discomfort and provide acceptable levels or contrast. Total uniformity is not the goal. A totally uniform, shadowless luminous environment also lacks interest and visual stimulation, like a fully overcast sky. A small amount of contrast creates soft shadows that are desirable to make people and objects look three-dimensional and natural. For example, a maximum 3: 1 or 1 :3 ratio is recommended between the luminance of the task and the immediate surround, and a maximum of 1 :40 between the task and a bright window. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated by means of the exemplary, non- limiting schematic drawings, in which:

Fig. 1 shows a general view of a standing working light according to the invention;

Fig. 2A-B show an example of a first respectively a second emission spectrum as issued by the working light according to the invention;

Fig. 3 shows reflected light spectra from white paper for the emission spectra of Fig. 2A and 2B; Fig. 4 shows the preference of respondents for the spectra of Fig. 2A and 2B as function of light level.

DETAILED DESCRIPTION

Figure 1 shows a working light 1 , in the figure a desklight, comprising a light source 3 inside a housing 5 with reflector 7, the housing being connected via a flexible joint pole 9 to a base 11. The base contains a control unit 13, an intensity adjustment knob 15 and a first control knob 17. The working light is connectable to mains via an electric cable 19. The light source comprises a plurality of LEDs 21 comprising at least a first 23 and a second lighting element 25. The embodiment shown in the figure further comprises as a third lighting element at least one green light emitting LED 22 and as a fourth lighting element at least one orange-red light emitting LED 24. Both the first and second lighting element can be a single LED or a plurality of LEDs. The working light by its light source issues a beam 31 of a, preferably white, spectrum which is tuned source light in intensity and/or in spectral composition (in particular of the ratio between the first and second emission peaks) controlled via control knob 17. The intensity of light issued by the at least first lighting elements can be controlled by knob 17 independently from the second lighting elements and vice versa. The intensity of both the first and the second lighting elements can be adjusted by dimming or boosting or by turning on/off a fraction of the respective plurality of LEDs. The intensity of the beam 31 as issued from the working light is adjustable by knob 15, through a light exit window 33 of the reflector to the exterior. Additionally or alternatively, the reflector accommodates a tunable filter 27 for tuning of the spectral composition of the beam 31 issued by the working light, which is also shown in figure 1 and which is tunable by a second control knob 29. To accommodate for the fact that each eye is unique and experiences sharp vision and eye comfort under different circumstances, a light that is tunable in spectrum and in intensity is thus provided. Hence, a working light, for example as shown in figure 1 , is provided that is dimmable and enables different spectral emission resulting in eye comfort and/or visual acuity for each individual.

Fig. 2A-B shows an example of a first 41 respectively a second emission spectrum 43 as issued by the working light according to the invention. The spectrum of figure 2A has two peaks in the blue part of the spectrum, i.e. a first maximum 45 at about 455nm and a second maximum 47 at about 410nm. The spectrum of figure 2B has only one peak in the blue part of the spectrum, i.e. a first maximum 45 at about 455nm. Both spectra have a correlated color temperature (CCT) of about 3000K. To attain the same CCT, the omission of the second maximum in the non-CW spectrum compared to the CW-spectrum is accounted for via slight modifications of the spectrum in the longer wavelength ranges, for example in that the amount of radiation in the green part 46 of the spectrum is somewhat diminished and in that the peak 49 in the orange-red part of the spectrum is somewhat shifted towards the orange wavelength range of the spectrum. Though the CCT of both spectra is the same, the spectra each have specific properties and effects which become apparent, for example, in the experienced visual acuity by respondents. The spectrum of figure 2A is generally referred to as a crisp white spectrum (CW), the spectrum of figure 2B is generally referred to as a normal white spectrum (non-CW). The CW spectrum is somewhat less in efficacy than the non-CW spectrum, but the CW-spectrum in general being perceived by most of male users and part of female users as being more comfortable than the non-CW spectrum.

Fig. 3 shows a reflected light spectrum 51 from white paper for the emission of a CW-spectrum (Fig. 2A) and a reflected light spectrum 53 for the emission of a non-CW spectrum (Fig.2B). The white paper comprises whitening agents. The effect of the extra blue LED peak at 410 nm is that the fluorescent whitening agents which are present in normal" white paper are addressed .These agents render the reading surface to appear more bluish white which is interpreted as an increase in brightness. This is particularly appreciated by male readers at lower light levels, for example 150 lux on a reading surface.

Fig. 4 shows the preference of respondents for the spectra of Fig. 2A respectively 2B as function of the light level for a CW-spectrum (shown in Fig. 2A) respectively for a non-CW-spectrum (shown in Fig. 2B). The appreciation for a LED reading light with two different light spectra was compared in a user test with adults in the age between 45 and 55 years old. The test has been done at different light levels being 150 lux, 500 lux and 1000 lux. The group of n=23 respondents was about equally divided over males and females. The outcome of the test was that males increasingly preferred the CW-spectrum as the light level was decreased; at the lowest light level 90% of males preferred the CW- spectrum. For females the trend was the same as for males, but the effect on preference between the two spectra was smaller.