TECHNICAL FIELD

This invention relates generally to a spectrometer built using LEDs of different core frequency as light sources, with said LEDs coded to allow extraction of the individual LED expression in a target material, and to also allow for the implicit mitigation of ambient light.

BACKGROUND ART

A Spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, and is typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light, normally expressed as some fraction of a meter, but is sometimes expressed as some unit directly proportional to the photon energy, such as wave number or electron volts, which has a reciprocal relationship to wavelength. Spectrometer is a term that is applied to instruments that operate over a very wide range of wavelengths, from gamma rays and X-rays into the far infrared. If the region of interest is restricted to near the visible spectrum, the study is called spectrophotometry.

In general, any particular instrument will operate over a small portion of this total range because of the different techniques used to measure different portions of the spectrum.

A Light Emitting Diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device, across the p-n junction forming the diode itself. The wavelength of the light emitted, and therefore its colour, depends on the band gap energy of the materials forming the p-n junction. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near- ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colours.

An LED is usually a small area (less than 1 mm ² ) light source, often with optics added directly on top of the chip to shape its radiation pattern and assist in reflection [1][2]. The colour of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Since LED's have quite specific light emitting behaviour, and this behaviour is relatively stable, one could take a group of LEDs of different wavelengths and combine them to provide a composite light source. By allowing this light to fall onto a sample and then collecting the returned light and analysing it via a spectrometer, a simple, robust self- lighted spectrometer system may be formed. However, the quality of the data is closely linked to the quality of the spectrometer, and aside from cost there is little advantage over a broad-spectrum light source (e.g. halogen bulb) coupled to the spectrometer itself. Accordingly, there is no advantage gained by using LEDs with the spectrometer systems presently known by those skilled in the art. LEDs have another very useful capability, in that they switch from on to off (and vice versa) extremely quickly relative to conventional incandescent or other light sources. Furthermore, their settling time (the time taken to reach a steady state) is also very short. Both these times are typically of the order nanoseconds or less, compared with hours to reach equilibrium for a typical halogen bulb. Taking advantage of this capability, one could switch individual LEDs on and off very quickly, while collecting the light in a simple non wavelength specific diode or similar detector, thereby obtaining a sequential spectrograph of the light impact on some sample of, interest. However, the time taken to measure such sequentially, although fast, is not negligible, and furthermore there is no compensation for ambient light changes over this time, or over longer time periods.

On the other hand, suppose one were to code the LED array switching following, for example, a Hadamard scheme. This scheme has a series of steps, where for every step about half the LEDs are on, and the scheme then flips all the LEDs from on to off and vice versa. The precise detail of which LED is on or off for the various parts of the cycle is critical, but well understood. One could then collect the light in a simple non wavelength specific detector for each step (on/off) of the cycle, then post-process this detector information to derive a spectral response for every LED of the sequence. Using an appropriate Hadamard coding scheme provides almost complete compensation for ambient light changes through the collection time, or for any longer time period (as when comparing scans taken some time apart).

Since the mathematics is purely integer, there is no rounding error as with complex real- number matrix analysis, and furthermore since there are no floating-point calculations in the processing, the outcome will be significantly quicker. This becomes particularly important when one considers that part of the mathematical process involves inverting an MxM matrix (M being the number of LEDs), which would normally be an intensive computing task. However, choosing the Hadamard transform method (or one of several other such methods related to this) means the inverse matrix is very simply related to the original matrix, without mathematical recourse (usually the simple transpose, with a divisor factor).

A further advantage to this approach [3] is that the amalgamation of several LED sources constitutes a noise reduction amounting to as much as M/2 for low intensity light detection. In the case where one might consider replacing a conventional spectrometer, this might constitute 40 or more LEDs, giving a noise reduction factor of 20 or better over an individual LED measurement.

In order to provide essential output stability as LEDs and/or other electronic components age, it is essential that the light output from the LED sources be itself measured simultaneously with the light returned from the sample of interest. As discussed in detail below, this may be achieved by a second detector of similar or identical design to the primary sample detector, and a method of bringing a representative portion of the LED light upon this detector. One simple way to achieve this is by a shielded fibre optic ring located forward of the LEDs which is shielded to avoid collection of light coming from the sample. Contrasting the sample derived light against the reference light for every Hadamard step provides an essential measure of the device performance and a method of compensating for any changes in performance over time. The reference collection need not be identical to the sample directed LED light, it merely needs to be optically constant, and exclude any sample effects. In this way, a relative change in reference intensity may be interpreted as a correction for the collected sample intensity. Such a device, as detailed in the appended claims, would have many advantages over conventional incandescent spectrometers. LEDs are extremely robust, and have very low energy drain, allowing an LED based spectrometer of this design to be very rugged, and able to operate on quite modest power requirements. This device would have no moving parts at all, yet be insensitive to ambient light without needing to directly measure that light. The modest power dissipation would allow for a completely sealed unit, making it ideal for measurements in explosive environments and/or where local heating could be an issue. A sealed unit would also be suitable for measurements in damp or even very wet circumstances, and would tend to be more robust with respect to industrial cleaning practises.

The device would be self-lighted, inherently insensitive to ambient light, moisture, vibration and would also be shock proof. The low power requirement means simple power reticulation and lessened power supply requirements. This would make such a unit ideally suited to industrial measurements taken close to the sample such as (but not limited to) paddock measures, processing environments, any measurements needed with poor vibration control, and the like.

In comparison to suitable quality spectrometer light-separating systems, LEDs are quite cheap. Thus the unit is likely to be very cost effective, able to be manufactured for a reasonable premium over even cheap conventional spectrometers, without sacrificing noise control, and would display considerably enhanced robustness. Such a device would have myriad of uses, from single hand-held units to measure targeted products through to research tools and industrial control devices, and to systems that could be conveniently mounted in a bracket or holder for continuous or on-line applications. Furthermore, the system could be designed to operate via a battery operated system where the battery could be replaced or recharged as required, either by a docking station or communication cable or the like, although it should be appreciated that these are listed by way of example and are not intended to be limiting in any way. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided an LED based spectrometer comprising: a plurality of LED light sources that are capable of exhibiting different wavelengths and bandwidths, arranged in any convenient manner about a detector module wherein the sample and reference optical paths are distinct from each other.

The LED sources may be switched in any convenient manner and in preferred embodiments the LEDs are switched in a Hadamard complement coded sequence.

It should be appreciated that in such preferred embodiments the LED individual signal may be decoded using the inverse Hadamard matrix.

It should also be appreciated by one skilled in the art that the reference optical path should largely exclude light returned from the target or sample.

It should also further be appreciated that the reference collection uses at least one fibre optic glass cable, and in preferred embodiments, the fibre optic glass cable is bare. In some embodiments, the sample and reference detectors may be identical in design.

In preferred embodiments, however, the sample and reference detectors are substantially identical in design.

It should also be appreciated that spectral responses may be recorded, along with relevant controls and other details, into non-volatile memory within the spectrometer, such as a Secure Digital card and associated protocol or the like, but not limited to such.

The specific arrangement of the LEDs relative to the detector may be altered for different desired applications, including but not limited to the light transmitting through the sample or target before detection (transmission), or reflecting from the sample surface, or in other embodiments, the light being constrained so that surface reflected light may not reach the detector, but light entering the sample is able to do so (interaction or transflection mode). Clearly these different requirements would need specific, different arrangements of the LEDs with respect to the detectors, as would be appreciated by those skilled in the art.

In preferred embodiments, communication between the spectrometer and other devices may be accomplished using the FieldTrans protocol, the USB protocol, or the TCP/IP protocol (or both) although this should not be seen to be limiting in any way as other communication protocols may also be used.

According to another aspect of the present invention there is provided an LED based spectrometer as detailed above which uses LED's of different core wavelengths and/or band widths specific to a desired application. In order to address the challenges mentioned above, it should be appreciated that the present invention is an embodiment of a plurality of LED sources at different central wavelengths arranged in order to illuminate a target evenly. The LED's are switched in groups following a Hadamard complement coding scheme. Light is reflected from (or transmitted through) a target material and collected for each code instance separately. The collecting detector may be any convenient type, chosen so its response range includes the light emission wavelengths of the LED's. There may be several detectors to cover the LED wavelength range if desired.

Simultaneous with the target returned light, a reference light detector, or detectors, collects light directly from the LED's, without any collected reference light having interacted with the sample. This may be accomplished in a number of ways that arrange the optics of the system to allow a small amount of LED scatter light to impinge on a collector element (or plurality thereof) and be directed toward the reference detector(s). Critically, however, this reference light path must exclude light that returns from the sample. The reference detectors themselves, in preferred embodiments, would be of the same type as those of the sample detectors. It should be clear, however, to those skilled in the art, that reference detectors of dissimilar design may also be suitable, with suitable conversion factors.

The intensity of light returned from the sample is recorded for each of the code steps in the Hadamard positive and complement or negative patterns. Depending on the detailed electronics of the particular system, there may be advantages to using the first code line of the Hadamard to set the signal gain prior to digitising this signal, thereby maximising the capability of the digitising system, however this is not a core requirement. The normal first code pattern of the Hadamard complement positive matrix is to have all components turned on. Since all other positive and negative codes have precisely half the components switched on, there can be dynamic range issues if the detector system settings are appropπate for the particular energy level when all subsequent codes are about half that intensity. Hence in one embodiment, there is provided a Modified Hadamard Complement scheme where this first positive code is omitted and substituted with the SUM of the second positive and second negative code intensity readings. In such a case, the first negative reading must be doubled. The advantage of such a Modified Hadamard Complement scheme is to provide almost a doubling of the effective dynamic range of the instrument in comparison to the usual Hadamard Complement scheme.

After decoding the signal following the Hadamard Complement inverse or the Modified Hadamard Complement inverse, or following another coding scheme as may have been employed, the resultant spectrum will comprise a single value for each LED in the array. These spectra can be treated in the normal manner for analysis, as is the case with any other spectrum.

The signals from all the detector diodes may be treated separately, or combined (added) in any convenient manner, as best fits the electronic and/or processing advantages and limitations of the specific design employed.

According to another aspect of the present invention, there is provided a method of use of an LED based spectrometer as described above characterised by the steps of orienting the LED based spectrometer toward a sample of interest, activating the LED based spectrometer, and collecting the results.

According to yet another aspect of the present invention, there is provided a device capable of being conveniently hand held that includes an LED based spectrometer as described above.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which: Figure 1 is a cross sectional view of one embodiment of the LED spectrometer of the present invention.

Figure 2 is a diagram of the LED spectrometer in front view, showing the arrangement of LED' s around the central detector optics.

Figure 3 is a diagram of the LED Spectrometer in cross section, showing the principle of aiming the central beam direction of the various LEDs.

Figure 4 is a diagram of the principle of using an optical element to focus an individual LED output.

Figure 5 is a diagram of the LED Spectrometer in front view showing the spatial distribution of different LED types.

Figure 6 is a diagram of the LED Spectrometer in front view showing the location of an example reference fibre relative to the LEDs. Figure 7 is a functional block-diagram of the circuit suitable to control an LED spectrometer of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Figure 1 shows a cross sectional view of one embodiment of an LED spectrometer generally indicated by arrow 1, which includes a reference fibre (with shield not shown) (2), LEDs (each of which includes individual optics) (3) and LED controller electronics (4). Also shown are the detector electronics (5), the sample or target light detector (6), the reference or LED light detector (7), the detector optics (8) and detector optics mounting device (9), which are optional for some applications, and a window to allow light in and out (10). In preferred embodiments, the LED's are arranged in annuli or concentric rings around a central detector core. It should be understood that other physical arrangements may be preferred for various reasons, such as grouping the LED assemblies or specific locations of LED's for transmission rather than reflectance measures.

Figure 2 shows the LED spectrometer with the LEDs arranged in rings as generally indicated by arrow 11. The LEDs (3) are located around the central sample light detector optics (7) and mounting device (8).

In preferred embodiments, the LED's (3) are individually focused using the lenses moulded into the LED bodies themselves. It should be understood that many other form- factors, and mounting systems including direct wire bonding or surface mounted ceramic (SMC), with or without secondary lenses and either imaging or non-imaging optics are also possible, with appropriate modification to other elements in order to accommodate those modifications.

In preferred embodiments, the LED (3) light is partially focused by each individual LED (12) with an incorporated lens (13), and the axes of these beams (14) are focused to a point appropriate to the intended application (15), approximately 200mm from the plane of the LEDs, as shown in Figure 3. An alternate focusing mechanism could be to use a planar- convex lens (16), or other optical element(s), mounted close to the LED units (12), with focal length appropriate for the desired application, as shown in Figure 4.

It should be understood that different focal lengths maybe desirable for different applications, and this may be achieved by a number of methods including, but not limited to, one or several of: altering the individual LED beam direction directly; using a shaped lens or lenses mounted in the beam path; tilting or altering the LED mount(s).

In the preferred embodiment shown in Figure 5, three LED's (17) are implanted for each desired wavelength. These are arranged in any convenient manner, with the restriction that each of the three LED's are 120 degrees apart, in order to ensure even sample illumination. It should be understood that different arrangements of the LED's may be designed for specific cases, such as for varying transmission, and/or for particular target attributes.

In preferred embodiments, the reference collector is a glass fibre (18), of suitable diameter, looped around the LED clusters (17) and masked from light that may have returned from the sample or target, as shown in Figure 6. Both ends of this fibre are drawn down and directed onto the detection region of the reference detector (7) shown in Figure 1. It should be understood that other methods for collecting this reference light may be more appropriate for different designs.

Figure 1 shows a final glass window (10) sealing the system. In preferred embodiments this window may be constructed from any convenient material. Consideration of the window's optical properties is, of course, important to ensure robust spectrometer performance.

Preferred embodiments of the present invention employ a Hadamard sequence switching of the LED sources. While any convenient switching control may be suitable, the Hadamard complement sequence offers signal to noise advantages, as discussed in detail elsewhere [3]. In general, the inventors consider a Hadamard sequence to be a mathematical coding expressed in matrix form where each row of the matrix represents a given overall state of the LED array (a mixture of on and off switched LEDs) for a given capture value of the detector(s). Each column of the matrix represents the various ON (+1) and OFF (-1) states for a given individual LED, across the various LED array states. For a simple case of 4 LEDs, the matrix is:

1 1 1 1

H (4) = 1 -1 1 -1

1 1 -1 -1

1 -1 -1 ^" 1 Collecting the +1 and -1 cases, this can be re-expressed as:

and these two matrices can be treated as independent LED switching cases, where a T represents LED ON, and a '0' LED off. This is called the Hadamard Complement approach. The detector measures the response to the LED array case represented by a given row, and finds the Hadamard coded total response following the appropriate row difference above.

For simplicity, this is represented in general as:

tf(m) = Where

• H(m) is the Hadamard matrix for m-LEDs

• P _j = they-th positive complement row (e.g. Pi = [1 1 1 1] for H(4))

• N _j = the_/-th negative complement row (e.g. N; = [0 0 0 0] for H(4J)

Generally, one might expect that ambient light (or other noise source) is not completely perfectly excluded from the spectrometer. In the Hadamard Complement approach, this can be expressed as an error term in the detector response for a given LED array case (positive-and negative):

Α = Pi + ξp,i where the 'ξ' term represents this noise for the specific case.

The LED array may be switched arbitrarily quickly. In particular, if the negative complement detector response is measured immediately following the corresponding positive complement case, and assuming the rate of change of the actual noise value (for example from ambient light leakage) is relatively slow relative to the LED switch time, then ξ _{P ι} « ξ _{N i} = ξ _t , and so for the i-th row case of the Hadamard this results in:

HiTn) ₁ = [P ₁ ] - [7T ₁ ) = [P ₁ + ξ _t ) - [N ₁ + ft] = [P ₁ ] - [N ₁ ] That is, the noise effect has been cancelled out. In reality, even a very fast switching LED will retain a small change in background noise, but it is clear that this effect will assist greatly in reducing the noise within the final Hadamard signal. This effect is discussed in more detail elsewhere [3].

Figure 7 shows a block diagram of the circuit used to implement the LED switch coding. The diagnostics interface (19), the temperature sensor(s) (20), the real-time clock (21) and the main power supply (22) are all connected to the microcontroller(s) (23). This microcontroller(s) (23) sends a signal to the LED controllers) (24), which in turn sends a signal to the LED Bank(s) (25). The LED Bank(s) (25) then produce a source of light (26) which is reflected off the sample of interest (28). The light is then reflected/returned back (29) from the sample (28) to the detector(s) and related amplifiers (30). The detector(s) (30) then send a signal to the analogue to digital converter(s) (31) and the signal is then fed back into the microcontrollers) (23) Simultaneously to producing a source of light directed at the sample (26), the LED Bank also produces light (27) directed toward the reference fibre (18). This reference light (27) is taken onto the detectors and related amplifiers (30). The reference detector(s) then send a signal to the analogue to digital converter(s) (31) and the reference signal is also fed back into the microcontroller(s) (23). The reference and sample signals may be kept separate or combined within the microprocessor(s) (23), and may incorporate corrections due to the various other inputs (temperature sensor(s) (20), Real-time clock (21), Main power supply (22), or other performance measures as may be available) as may be convenient for the specific outcome. The microprocessors) (23) then send the signal(s) out to various outputs such as a nonvolatile memory (32), a USB interface (33), an Ethernet interface (34) and/or various control inputs and indicators (35). Electronic control of each LED switches these on or off according to the current Hadamard code pattern desired.

At each code pattern, sufficient time is allowed for the LED's to reach a settled light intensity, then the readings are recorded from the sample detector or detectors and the reference detector or detectors. These readings are combined in the desired coding sequence. In preferred embodiments, the Hadamard complement system is used in the manner defined above.

In preferred embodiments, the combined response is decoded to individual LED responses using the inverse Hadamard matrix. One reason the Hadamard matrix is an ideal candidate is that the inverse Hadamard matrix is just the transpose of the original matrix, with a scaling factor. One may apply the scaling factor whenever convenient, and in particular, one may leave this until a final step. The detector signal is digitally converted from the analogue detector output, and as such is represented by integer values. The Hadamard coding also uses integer values, as shown above. By deferring the scaling until a final step, the matrix deconvolution remains an integer arithmetic step, and hence accrues no floating- point rounding. This is especially important for large matrices, with many hundreds of multiplications required to calculate the inverse. By retaining integer mathematics, a further signal to noise advantage is obtained.

Decoding the Hadamard detector signal produces a spectrum where each decoded response corresponds to the expected reading that would have been obtained had a single LED been switched on.

In preferred embodiments, spectral responses are recorded along with all other relevant control instructions onto an internal memory system such as a Secure-Digital card. It should be understood that other storage systems may be suitable for different applications, including directly sending these spectra to an external storage or analysis system. In preferred embodiments, communication to the device is accomplished by USB V 1.0 or better cables and protocols. It should be understood that other comparable capability communications systems may also be employed, such as but not limited to TCP/IP, RS232, and the like.

The device as described is self-contained in a single module. This may include an internal power source, or the power source may be external to this module. It should be understood that in some applications it may be convenient to split the system into two or more module.

Such a latter device may include an integral trigger system as per conventional bar-code readers and the like. The choice of power supply may vary depending on particular application and should be sufficient to operate for a convenient length of time. Said power supply may include, but not be limited to, an attached power source such as an electric battery.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.

References Cited

1. Moreno I. and Sun C, Modelling the radiation pattern of LEDs. Optics Express, Feb 2008, 16, 3, ppl808-l 819

2. Moreno L, LED Intensity Distribution. International Optical Design, Technical Digest. ISBN 1-55752-8144, June 2006. 3. Streeter L., Burling-Claridge G.R., Cree M.J., and Kύnnemeyer R., Optical full Hadamard matrix multiplexing and noise effects, Applied Optics 48, 11, pp2078-2085, 2009.