WO2012098297A1

WO2012098297A1 - High speed chemical imaging based on fabry-perot interferometer

Info

Publication number: WO2012098297A1
Application number: PCT/FI2012/050046
Authority: WO
Inventors: Heikki Saari; Jouko Malinen
Original assignee: Teknologian Tutkimuskeskus Vtt
Priority date: 2011-01-19
Filing date: 2012-01-19
Publication date: 2012-07-26

Abstract

The present publication discloses a method and device for high speed chemical imaging system based on Fabry-Perot interferometer, the method comprising illuminating a sample by broadband light(1)through a High Pass Filter (22) and Low Pass Filter (21), using a Collimator lens (23) for gathering and collimating the scattered light from the target(2), using Piezo-actuated Fabry-Perot Interferometer (20) for filtering the incoming light,with a Filter wheel (24) to enable using high orders,using an Imager lens (24) to deliver the image on a 2D IR detector (25),and finally predicting chemical maps from measured spectral images.

Description

High speed chemical imaging system based on Fabry-Perot interferometer

The present invention relates to a method, and an apparatus intended to apply the method.

The prior art is described for example in article "Comparative performance studies between tunable filter and pushbroom chemical imaging systems", Jouko Malinen, Heikki Saari, Gabor Kemeny, Zhenqi Shi and Carl Anderson, Proceedings of SPIE Vol 7680 (2010).

1. Background

Problem description. Chemical Imaging instruments based on tuneable filters for NIR spectral reflectance or absorbance measurements are available based on LCTF filters and diffractions gratings (e.g. Malvern and SisuChema/Specim accordingly) but the available instruments are expensive and their measurement times are too long for many applications. This invention uses tunable Piezo actuated Fabry-Perot interferometer to achieve fast measurement time and cost effective instrument construction.

The chemical imaging concept is described in the following in connection with figures 24 and 25. The heart of the instrument is the Piezo actuated Fabry-Perot Interferometer 20 which can provide fast tuning of the wavelength band (switching time less than 2 ms). The measurement of the spectral data cube consisting of 125 spectral bands can be performed in less than 1 s with the invented Piezo FPI spectral imager to be compared with the corresponding recording time of 100 s of the state of the art LCTF instruments.

Furthermore, users have needs to validate performance of NIR Chemical Imaging systems while they are considering potential instrumentation for certain use, and while they are assessing day-to-day performance of their equipment. Essential part of any validation procedure will be validation of the wavelengths scale and wavelength resolution. Methods available today for wavelength validation of NIR imaging instruments are typically based on either rare earth oxides or polystyrene material. None of these methods and materials has completely solved user needs, because of the following limitations:

• optical characteristics of the target materials are spatially non-homogeneous, which makes it difficult to see spatial calibration problems in the validated instruments

• absorption peaks in these materials are relatively broad, in which case

mathematical corrections on the results will be needed to calculate instrument resolution. The invention is intended to eliminate at least some defects of the state of the art described above and for this purpose create an entirely new type of method and apparatus for reading spectral and furthermore chemical properties of a material.

The invention is based at least on the following definitions:

1 A chemical imaging spectrometer for measuring characteristic information from sample material moving or stationary at a distance, the spectrometer comprising: a) a broadband light source 1, such as a halogen incandescent lamp. A light beam of the light source is collimated with lens in order to lead the light beam through a High Pass Filter 22 and Low Pass Filter 21 which define the pass band of the instrument measurement spectral range. The lens after the High and Low pass filters focuses the light to 2D Chemical Image target 2.

c) The Imager Collimator lens 23 gathers the scattered light from the target and collimates this light and directs it to the Piezo-actuated Fabry-Perot Interferometer 20, d) The Piezo-actuated Fabry-Perot Interferometer 20 filters the incoming light by transmitting only light at one or several narrow spectral bands pending on the air gap between the mirrors of the Fabry-Perot Interferometer.

e) Filter wheel 24 contains several band pass filters that are matched to the the specific Fabry-Perot Interferometer orders. This enables to use high orders of the Fabry-Perot

Interferometer whose spectral widths can chosen pending on the application, this is because the spectral width of a Fabry-Perot Interferometer pass band as a function of the FPI order number is a descreasing function. f) Imager lens 24 is used to form the image of 2D Chemical Image target 2 on a 2D IR detector 25.

g) The 2D IR detector 25 is typically a InAsGa or Mecrury-Cadmium-Telluride (MCT) type of detector in the wavelength range relevant to chemical imaging

2. Method or procedure for high speed spectral and chemical imaging spectrometers, the method comprising the steps of:

a) irradiating a small area on moving sample material at a distance from a broad band light source;

b) receiving scattered light

c) splitting the broadband radiation into wavelength components with a Fabry-Perot tunable filter;

d) converting optical radiation into series of images recorded at different pass bands of the Fabry-Perot tunable filter with a two dimensional detector matrix.

e) processing electrical signals from the pixels into spectral information; and f) maintaining the spectral information, comprising of spectral values and wavelengths, stable by cooling and stabilizing the operating temperature of the 2D detector array and the Fabry-Perot tunable filter, while the spectrometer is operating in variable environmental conditions.

g) predicting chemical maps from measured spectral images by applying chemometric prediction models.

3. Device for testing and validating wavelength performance of imaging spectrometers, the device comprising:

at least one laser diode positioned to illuminate a spectrally white diffuse reflecting target,

means for regulating the forward current and the operating temperature of each laser diode to predetermined set values,

electrical memory device to store predetermined the predetermined set values.

More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 3 and 6. For its part, the apparatus according to the invention is characterized by what is stated in the characterizing portion of Claims 1 and 14..

Considerable advantages are gained with the aid of the invention.

Commercial significance. This technology enables high-performance, low-cost chemical imaging in pharmaceutical applications, where materials have suitable optical properties in terms of reflection, absorption and extinction coefficients within the measurable range of the instrument.

VCSEL based method can display information on peak wavelength and band width for each pixel across the image, which can clearly illustrate any instrumental errors in wavelength calibration and wavelength resolution. Thanks to narrow line width of the VCSEL source, the observed line width is a direct measure of imager line width and spectral resolution. An array of multiple VCSEL sources may be used, which provides information about nonlinearity of the wavelength scale. The proposed method includes calibrating the VCSEL wavelengths using traceable methods, storing the calibrated values of laser current and operating temperature in an electronic memory device and regulating the wavelengths using electronic means during actual use of the method.

In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

Figure 1 shows several aspects of the present invention as figures of phases of the invention.

Figure 2 shows a further aspect of the present invention, especially a chemical imaging setup.

Figure 3 shows a further aspect of the present invention as a graphical presentation. Figure 4 shows a further aspect of the present invention, especially a chemical imaging setup for lactose.

Figure 5 shows a further aspect of the present invention, especially a chemical imaging setup for lactose and ibuprofenum.

Figure 6 shows a further aspect of the present invention, especially SNR tests graphically.

Figure 7 shows a further aspect of the present invention, especially noise characteristics with different integration times.

Figure 8 shows a further aspect of the present invention, especially spectral bandwidth and accuracy of the passband over image area in connection with laser tests.

Figure 9 shows a further aspect of the present invention, especially temperature stability of the FPI as a graph.

Figure 10 shows a further aspect of the present invention, especially as a photograph of the setup and a summary of major results of NOSPI WP4.

Figure 11 shows a further aspect of the present invention, especially mechanical design of the 2^nd iteration laboratory setup for the NOSPI imaging setup

Figure 12 shows a further aspect of the present invention, especially as photographs implementation of the 3^rd laboratory setup with large aperture R-PP-FPI and 13 b) with improved illumination. Photos have been taken before the blocking of the visible wavelengths from the illumination.

Figure 13 shows a further aspect of the present invention, especially optical design layout and Modulus of the OTF as a function of spatial frequency.

Figure 14 shows a further aspect of the present invention, especially a) chemical image from one detector row of the lactose tablet in the wavelength window of 1680-2170nm b) spectrum from one pixel of the lactose tablet

c) reference spectrum of the lactose from the literature [RD 9.]

Figure 15 shows a further aspect of the present invention, especially a) chemical image of lactose (upper tablet) and ibuprofenum (lower tablet) tablets at 1680nm wavelength b) chemical image of one column from the two tablets at wavelength window of 1680- 2170nm. The region between the tablets has higher signal level.

Figure 16 shows a further aspect of the present invention, especially graphically . SNR over image area at 1500nm with 12ms integration time. Measured with high gain. Figure 17 shows a further aspect of the present invention, especially graphically Detector noise map at 1500nm with 12ms integration time and dark object. Measured with high gain..

Figure 18 shows a further aspect of the present invention, especially a) FWHM at 1512nm and b) peak wavelength and the tilt in the peak wavelength over the image plane of the chemical imaging setup. Figure 19 shows a further aspect of the present invention, especially graphically temperature stability of the Fabry Perot filter at 1900nm in the temperature window of 19 to 31°C.

Figure 20 shows a further aspect of the present invention, especially schematically a second measurement setup in accordance with the invention.

Figure 21 shows a further aspect of the present invention, especially as a photograph a second measurement setup in accordance with the invention.

Figure 22 shows one aspect of the present invention as a schematical side view.

Figure 23 shows a further aspect of the present invention as a photograph.

Figure 24 shows a further aspect of the present invention as a graphical presentation.

Figure 25 shows a further aspect of the present invention graphically.

In figure 1 a summary of Linear detector array module development for use in high speed Fabry-Perot imager is presented:

Aims :

• Develop generic tunable MCT line or 2D imaging modules for fast, online imaging spectroscopy

^• MCT arrays for NIR (1 - 3 μιη) and IR (3 - 6 μιη) wavelengths

^• Spatial line imaging resolution from 32 to 128 pixels

^• Fast electrically scanned wavelengths or multiple simultaneous wavelengths thanks to large aperture FPI filtering from WP4

(VTT Patent application FI20065348)

• Development of optimized testing platform consisting of the MCT line imaging array, FPI module, optical and electrical functions, hardware and software; with integration density upgraded during project process

Potential exploitation:

• Line and 2D imaging spectroscopy for PAT applications at NIR and IR wavelengths (Chemical Imaging...)

Figure 6 presents summary of the first developed high speed chemical imager prototype: Parameter NOSPI CHEMIM Remarks

Spectrometer detector 3 -stage Peltier cooled 2D

MCT array

Image size 384x288 pixel array 2 ms frame time

Spectral dispersion technology Piezo tunable Fabry-Perot

filter

Illumination Halogen light source with

long pass filters from four

directions plus FP order

selection filters

Spectral range 1200-2200nm Short end limited by order selection filters.

Long end limited at the moment by the tunable bandgap in Fabry Perot

Spectral resolution FWHM 20nm @ 1512nm

FWHM 25nm @ 1877nm

Typical scan time >400ms / lambda The software has not been

Detector frame time ~ optimized yet.

2ms 30ms/lambda is a goal for the scan time.

Dimensions/System size 50 cm (H) x 40 cm (W) x The system is realized as

50 cm (D) plus the PC a laboratory prototype. No and the power supplies effort has been done to build up an integrated system.

;ure 6 are presented SNR tests, where

SNR 99%s pectralon-dark • New AIM electronics

Improved illumination

• High gain

1.2 ms x 10 repeats

=> one averaged frame

• On SNR measurements, 20 frames/lambda

• Lamp power 48 V

(nominal 12V&20W/lamp, L519-G gold reflector halogen).

• Signal power maximized for 1500nm

· Theoretical maximum with 3 OOke^" well and 10 repeats

SNRmax=V300000*VlO=1732

Figure 10 presents a photograph of the Fabry-Perot imager prototype, with key specifications:

· VTT has proven that spectral imaging setup based on

• TE-Cooled MCT detector and

• Large aperture Fabry Perot Interferometer works and it has been utilized as a Chemical Imaging demonstrator

• Specifications at the moment are

· 25nm spectral resolution

• ~ 13 lp/mm spatial resolution

• Measurement time / lambda ~ 400ms

• SNR average over image area = 1554 V/V @ 1500nm with 12ms integration time

· By small redesign the system can be upgraded to

• Different wavelength area

• Totally different Field of View

• 30ms/lambda measurement time The proposed wavelength validation technique in accordance with invention is illustrated in Figures 22 and 23.

Typical instrumentation for implementing laser based validation with spectral imaging instruments is illustrated on figure 22. Either one or multiple laser diodes 30 are assembled to illuminate a target sample 2 at an angle of approximately 45 degrees relative to the surface. The sample used for the wavelength calibration experiment should be diffusely reflecting in order to avoid problems due to specular (mirror) reflections. Suitable target samples are available with certain amount of reflectance, such as Spectralon TM material. Alternatively this target sample may be patterned with gray and white (or with low and high spectral reflectance) lines or other patterns. In this case multiple validation experiments may be carried out with the same instrumentation. For example spatial and spectral resolution may be studied simultaneously. Traditionally spatial resolution is studied with patterned targets (e.g USAF 1951) using white light illumination, but similar information at a single wavelength may be obtained using similar experiment and monochromatic or narrowband laser illumination. Spatial resolution is usually defined as resolved linepairs/mm and is frequently studied separately for different wavelengths and for different parts of the image (center vs edges and corners).

Initially the validation instrumentation and system (figure 1) must be calibrated by monitoring the laser wavelength by suitable wavelength measuring device. This traceable measurement setup 31 should be stored in controlled environment and should be tracking to known reliable wavelength calibration, with controlled uncertainty (NIST etc). During initial calibration of the laser based validation system, the exit wavelength of the laser is observed and compared to a chosen target value. Next the laser wavelength is adjusted by changing either the laser current or the operating temperature until the chosen wavelength is achieved. This kind of wavelength tuning is illustrated in figure 3, where current tuning is used in the left hand picture and temperature tuning in the right hand picture. Temperature tuning is feasible option with many laser diodes and in this case apparatus for changing the temperature of the laser chip is integrated inside the laser diode module 30. Furthermore a temperature sensor is also integrated inside the laser diode module 30. The apparatus typically used for temperature regulation is thermoelectric cooler, which are commercially available as miniature device suitable for cooling or heating electrical and electro-optical components. Additional means needed for changing the temperature or the laser current is the current and temperature regulation device 33. This device includes the necessary electrical circuits supplying both the activation current to the laser diode 30 (so called forward current) and activation current to the thermoelectric cooling device inside the laser diode module 30. The needed electrical circuits included in device 33 are temperature measurement and controller circuits, typically using a Pi-controller, which compares the set temperature and the measured true temperature and produces electrical output in the form of regulated current fed to the thermoelectric cooler device. During operation the circuit produces suitable output until the error between the set and true values is reduced to essentially zero. Furthermore, the regulation device produces stabilized current output for driving the laser diode, the current of which can be electrically adjusted to the desired value. Initial calibration of the validation system of figure 1 is finished by storing the correct temperature and current values or parameters for obtaining those values in the calibration memory device 32. Preferably these temperature and current values or parameters are originally in analog or continuous form, but they can be converted to digital form in order to store them in digital type of calibration memory 32. It is clear that the discussed electronics features may be implements using either analog or digital electronics, for example the Pi-controller may be implemented with either analog or digital circuits. Most preferred approach is combing certain amount of both circuit techniques in the validation system. If multiple laser diodes are included in the validation system, then analogous supporting features 31, 32, 33 will be needed for every laser, and the discussed initial calibration must be arranged for all of these lasers.

It is true that single supporting features 31, 32, 33 may be designed so that the work with all the lasers simultaneously. Also the initial calibration then can be implemented as a single procedure, calibrating each of the lasers in sequence. After the initial calibration is finished the validation system can be used in normal operation. The validation system may be used as separate equipment connected with each spectral imaging instrument 34 in turn, when validation is needed for that instrument. Alternatively the validation system may be assembled to be integral part of every produced spectral imaging instrument 34. In the latter option, the imaging instrument may include software which carries out automatic performance monitoring activating the validation system at predetermined time intervals or when faulty operation is suspected. The key idea for using the validation system is based on the fact that true laser wavelength is known and exactly the same, when it illuminates all spatial points on the sample 2. When the sample is imaged using a spectral imaging instrument to be validated 34, then internal errors and technical limitations will lead to a situation that the observed wavelength as seen by the instrument 34 may be slightly erroneous and furthermore this error may be different for each pixel in the image. Furthermore, even though the light produced by the laser diode 30 is close to monochromatic, still the spectral imaging instrument will see certain spectral bandwidth (FWHM, full width at half maximum) which is a measure of the spectral resolution of the spectral imaging instrument. This spectral bandwidth or spectral resolution is often also different for each image pixel, due to optical limitations of the imaging system 34. These possibilities for using the validation system are illustrated in figure 25. Additional software will be needed to implement normal operation, but this software is not shown in figure 22.

During normal operation the following procedure will be activated:

1) Suitable target sample 2 is mounted in the sample space

2) Calibrated current and temperature values are read from the calibration memory 32 and sent to the regulation device 33

3) Stabilized current values are created at the regulation device 33 and delivered to the laser diode 30

4) The system waits until the calibrated current and temperature is reached at the laser diode 30, after which the laser beam illuminating the sample has correct wavelength and other characteristics

5) A set of spectral images are taken with the spectral imaging instrument to be validated, forming a validation datacube (parameters of the datacube will depend on the specifications of the spectral imaging instrument, typically the datacube has WxLxH pixels, where W= wavelength and LxH are image length and height in pixels)

6) The validation datacube is mathematically processed to produce pictures which illustrate key performance figures of the validated spectral imaging instrument, as illustrated in figure 4

7) The results produced are automatically analyzed comparing observed wavelength and the observed line width against the correct wavelength and target instrumental line width, accordingly. These both comparisons can be made for all pixels over the recorded image (representing the datacube).

8) The spectral imaging instrument 34 will pass the validation if the wavelength and line width values, over the image pixels, are correct within predetermined error limits stored in the validation software.

Correct wavelength performance of any imaging instrument means that the wavelength axis must be stable and the wavelength readings must be "true" i.e tracking to existing standards. Furthermore correct wavelength performance should also mean that instrumental line width is correct and remains stable over time and over image pixels. Wavelength performance is very important for producing meaningful results with spectral imaging instruments. Faulty wavelength performance can lead to f.ex. erroneous chemical maps, if the spectral imaging instrument is used for chemical imaging applications. The proposed validation device and method can be used to check performance of spectral imaging or chemical imaging instruments, which are used for important measurements such as quality control in pharmaceutical production. The proposed procedure can be upgraded to cover validation of spatial resolution over the image, if the validation sample includes suitable line or other patterns and if the normal procedure is extended to cover this need in the similar way.

Alternatively, according to the proposed technique, a white diffuse reflectance target 2, such as Spectralon-99 will be illuminated by a single mode diode laser source 30, as illustrated in Figures 22 and 23. In a preferred version of the procedure, a Vertical Cavity Surface Emitting Laser (VCSEL) will be used as the light source.

The system in accordance with figure 22 can be used for calibration of a spectrometer in accordance with figure 20. In this system shown in figure 20 the information from the sample material moving at a distance is measured by a broad band light source 1 for delivering irradiance to a small area on moving sample material 2 at a distance. The spectrometer further comprises light collimating lens 4 which collimates the light from the light source 1. The high pass filter 22 and the low pass filter 21 define the

measurement spectral range of the spectrometer and reduce the heating of the sample. The focusing lens 5 focuses the light to the sample. The imager collimator lens 23 collimates the light reflected and scattered by the target and directs it to the Piezo actuated Fabry-Perot Interferometer 20, which transmits spectral bands determined by the air gap between the mirrors of the FPI module. The filter wheel 24 contains the FPI order sorting filters. The light from FPI goes through the selected order sorting filter 24 which guarantees that only the spectral band of the selected FPI order is transmitted to the 2D detector and measured. The image focusing lens 6 forms a 2D image of the target on the 2D IR detector 25 at one selected spectral band. The spectral image is formed by recording the images of the target at all selected wavelength bands.

VCSEL diodes are advantageous for the purpose, because they are single mode lasers and therefore the line width is narrow, typically a fraction of a nanometre. Therefore, for a typical NIR imaging instrument, the observed instrument line width is almost entirely due to the spectral resolution of the validated instrument. Furthermore, the wavelength of VCSEL sources can be tuned by regulating the operating temperature and /or the operating current. This capability can be used for initial calibration of the VCSEL to obtain emission wavelength traceable to an accepted global reference (NIST). These initially selected values of operating current and temperature can be stored electronically and used subsequently to regulate the VCSEL device, essentially maintaining traceable wavelength characteristics over its operating life. Typical range of current and temperature tuning for 1512 nm VCSEL is approximately 4nm, as illustrated in Figure 24. Even one VCSEL device can be used for single wavelength validation. More comprehensive validation may be implemented using an array of VCSEL diodes.

VCSEL technology was originally developed for telecommunication needs, aiming for mass produced, low cost laser diodes at telecommunications wavelengths. Recently a wide range of VCSEL wavelengths have become available for spectroscopy

needs, too. One of the suppliers, German Vertilas GmbH offers these diodes for wavelengths ranging from 1340 to 2050 nm. Supplementing similar devices from other manufacturers, there are VCSEL devices available to cover quite well the complete region used for Near Infrared Spectroscopy, i.e. 700 -2500 nm. The proposed technique was recently demonstrated by VTT Technical Research Centre of Finland and Duquesne University Centre for Pharmaceutical Technology (DCPT), as illustrated in Figure 25. The top two images compare single point wavelength validation for MatrixNIR instrument using the VCSEL procedure (left) and a typical diffuse wavelength target (right). The VCSEL method clearly shows approx 0.5 nm gradient in the observed laser wavelength, which is expected to be due to instrumental wavelength error sources. In practise this means that even if the LCTF filter is tuned to 1510nm, different pixels can see slightly different peak wavelength values. The traditional diffuse white wavelength target cannot clearly see these wavelength errors, because the image is dominated by target non-homogeneity of the order of 2 nm, centred around the 1261 nm peak in the target material.

The bottom two images compare performance of the same procedures for defining spectral resolution across the recorded image. The spectral resolution was defined as Full Width at Half Maximum (FWHM) calculated from uncorrected raw measurement data. The mean image FWHM values obtained were 8.6 nm @1512 nm and 22.2 nm @1261 nm, for the VCSEL and diffuse target methods accordingly. The former is expected to be very close to true spectral resolution of the validated instrument, because it is broadened by the laser line width of only 0.5 nm. The latter gives larger FWHM, because the result is actually a convolution of the true instrument band width and the line width of the 1261 nm peak of the target material. Therefore the VCSEL procedure appears to be a practical method to validate wavelength resolution of NIR Chemical Imaging systems, where the results may be presented as FWHM values across recorded images.

The emission peaks from VCSEL diodes are typically very symmetrical as illustrated in figure 3. This is contrary to absorbing wavelength targets, where peaks are often non- symmetrical at least to some extent. Thanks to symmetrical performance the observed wavelength will not depend on the spectral resolution of the validated instrument. This characteristic should be useful for validating different types of instruments, which may have very different wavelength resolution characteristics.

1.1 System concept

The system concept of the invention is based on illumination source, large aperture Fabry Perot filter module with order selection filters, imaging optics, 2D detector module, and supporting electronics and mechanics. The setup has been iterated three times, where the first setup used CEDIP's Titanium InSb infrared camera and SP FPI made of expensive Kovar mechanics. The second setup used AIM's 2D MCT infrared camera, less expensive large aperture R-PP-FPI, and the illumination unit based on focusing optics and a 45W halogen lamp (Fig. 14).

The final NOSPI setup (Fig 12. a)) uses four halogen lamps (model L519-G, 20W/12V) as an illumination source (Fig 12. b)), where an option of blocking visible spectrum has been established (Schott RG850 long pass filters), in order to cut down the out of the band optical power that the sample experiences.

1.2 Optical and mechanical design

The optical performance has been documented on RD 4 WP 4 slides. As a summary the measured spot size (LSF FWHM) of 36μιη gives a spatial frequency of 13.9 cycles per mm, which gives modulus of the OTF value greater than 0.5 also at the corners of the detector, Fig 13., i.e. the measured values are very close to design.

VTT's setup is done on chemical imaging purposes with short focal distance, however by small optical iterations the setup can be tuned to applications where focus on infinity is needed with different fied of views [RD 8].

Mechanical design has been done in several steps. The goal has been to built up a laboratory prototype, and therefore the mechanics looks a bit like built from lego blocks and the size is what it is. 1.3 Electrical Design

The electronics of the chemical imaging setup consists of detector and its supporting electronics, Fabry Perot module and its supporting electronics, peltier element controller and of laboratory power supplies. The detector related electronics are AIM's product, where as the other electrical parts are VTT made. The VTT electronics consist of Faby Perot controlling electronics and of thermoelectric cooler controller electronics. The Fabry Perot controlling electronics is a product of WP2 and the thermoelectric cooler electronics has been developed earlier at VTT.

2. Characterization test results of the built chemical imaging prototype

The measurements have mostly been reported on the RD 4. There are measurements to be done before the March, 2009 steering group meeting. These will be reported on [RD 5]. Figure 14. presents the spectral image and the measured spectrum of the lactose tablet in the wavelength window of 1680-2170nm. Spectral image of one detector row is presented in Fig 14. a), spectrum of the one pixel is presented in Fig 14. b), and reference spectrum of the lactose is presented in Fig 14. c).

Figure 15. a) presents the spectral image of two tablets, where upper tablet is the lactose tablet and the lower is a ibuprofenum tablet, measured at 1680nm wavelength. Figure 15. b) presents the spectral image from one column of the two tablets measured at wavelength window of 1680-2170 nm.

The performance results will be presented in [RD 5] more thoroughly and only summarized results will be presented here,

o SNR:

^■ Spectralon diffuse 99% target, with integration time of 12ms

( 1.2ms * 10=>1 frame) SNR average over image area

• 953 @ 1300nm

· 1554 @ 1500nm

• 1006 @ 1700nm

• 843 @ 1900nm

• 627 @ 2100nm

• 243 @ 2200nm o Theoretical maximum SNR with 12ms integration method and with High Gain mode of the detector is roughly Λ/300000 X /ΪΟ = 1732

Figure 16 presents the SNR over image area at 1500nm with 12ms integration time o Total Measurement time

^■ At the moment the measurement time is roughly 400ms/lambda, i.e. roughly 60s/1501ambdas

^■ Goal value for 150 lambdas measurement, with integration time of 20ms over the wavelength area of 1-2.5μιη is 9.5s, and if combination of two Fabry Perots could be used, this time could be shortened to roughly 5 s

o Noise

^■ Detector NEP 0.6*10-13W @ High Gain and 3*10-14W @ Low Gain

^■ Detector noise map over image area is presented on Figure 21. The noise level is in counts, where the dynamic range goes from 0 to

16384, i.e. detector has a 14bit ADC.

0 - o p ti ea I p e r f o r m a n c e o f tli e d e te c to r

^ D . G ct o r^* 0 ***· JL 1.(3 J o^*i c. S

• Detector Well: High Gain mode 0.3Me^" and Low Gain mode l.lMe^"

• esp R [LSB/photon] = 0.02 @ High Gain and 0.006 @ Low Gain

• NEP 0,8*10 ³ @ High Gain and 3*10 ⁴W@ Low Gain

• 0.3% of the pixels are not functional, and all fault pixels are single pixels

o Linearity of the system against reflectance standards has been documented on [RD 4]

o Straylight levels to be documented on [RD 5]

o Resolution

^■ Spatial resolution has been documented on [RD 4]

^■ Spectral resolution has been measured with a setup where a laser source was illuminating a reflectance standard and the setup was taking spectral images over the laser wavelength. This gives spectral resolution's FWHM value, Fig 18 a) and information of possible tilt in the Fabry Perot filter at laser wavelength over the image plane, as well information of the peak wavelength, Fig 18 b).

o Temperature stability of the Fabry Perot filter is presented on Figure 19. It was tested in a setup where water cooling block was thermally attached to Fabry Perot module and a BioRad FTIR spectrometer was used to measure the temperature drift. A 0.2nm/°C @ 1900nm drift was seen. o Stress test of the Fabry Perot filter

^■ FP was stressed by applying back and forth movements between minimum and maximum gaps. The measurement was done in a test bench, where FP was implemented in the optical path of BioRad FTIR spectrometer.

^■ More than 500000 movements during 14h continuous scanning were done twice and less than 0.5nm drift could be seen, which could be due to temperature effects or FTIR accuracy, i.e. no drift could be seen on cycling.

In accordance with one aspect of the invention in accordance with figures 20 and 21 the target 2 is illuminated with a broadband light source 1. The low 21 and high pass filters 22 are used to limit the target 2 illumination spectrum to the required spectral band thus avoiding the heating of the sample. The target reflects the light source radiation modifying the spectrum of the reflected light. The light reflected by the target is collected by the collimator lens 23 which collimates the light beam. The collimated light beam goes to the Piezo actuated Fabry-Perot Interferometer (FPI) 20 which transmits spectral bands determined by the air gap between the mirrors of the FPI module. The filter wheel 24 contains the FPI order sorting filters, that are matched to the specific Fabry-Perot Interferometer orders. This enables to use high orders of the Fabry- Perot Interferometer whose spectral widths can be chosen depending on the application gap width, because the spectral width of a Fabry-Perot Interferometer pass band is a function of the FPI order number is a descreasing function, the light from FPI goes through the selected order sorting filter 24 which guarantees that only the spectral band of the selected FPI order is transmitted to the 2D detector and measured. The wavelength calibration of the 2D-spectral image with the aid of laser source is the second novel idea. An array of Vertical Cavity Surface Emitting Laser (VCSEL) diodes 30 (Fig. l) may be used for checking and validating wavelength performance of imaging spectrometers and NIR Chemical Imaging Systems. Existing methods for wavelength validation are based on target samples made of rare earth oxides or polystyrene particles. These are not very good for use with imaging systems, because of spatially non- homogeneous characteristics.

The measurement illumination and imaging optics are also novel. The design of optics was performed by Kari Kataja.

3. Executive summary, table of applications, technologies, cost, potential

manufacturer in the NOSPI consortium

Spectral imaging can be utilized in several application areas; e.g. pharmaceutical, chemical, agricultural and different monitoring applications.

VTT's setup is done on chemical imaging purposes with short focal distance, however by small optical iterations the setup can be tuned to applications where focus on infinity is needed with different fied of views [RD 8] for different spectral imaging applications.

The production of NOSPI Chemical Imaging prototype like spectral imager can be organized for example by a network of:

• The embedded electronics (FPI, Peltier and illumination controllers + powers) and the integration of the whole system by Exens Development

• The mechanics by Mansner

• The detector modules and their supporting electronics by AIM Infrarot-Module GmbH

• The commercial manufacturing of large aperture FPI modules is an open

question at the moment and VTT has project proposal in TEKES for arranging the commercial production of FPI modules and FPI module based devices. At the moment VTT can produce the FPIs, but larger scale production needs a recognized and evaluated value chain, which is the main purpose of the TEKES proposal. There are also potential end users in the NOSPI consortium to which more detailed specification and development of for example application specific software of the spectral imager would be allocated. The possible end users are GlaxoSmithKline, Inficon, Metso, Mettler Toledo, and PVTT, in alphabetical order.

4. Summary and Conclusions

NOSPI chemical imaging prototype demonstrates potentially cost effective, high performance spectral imaging technique, that can be utilized in the broad field of spectral imaging applications. NOSPI project has successfully demonstrated chemical imaging of tablets. The system has shown adequate spectral and spatial resolutions, and it is potentially fast technology. There are no moving parts needed in the system and the only parts which lifetimes are limited are the standard halogen bulbs used in the chemical imaging setup, where illumination source is needed. In the applications where discrete wavelengths are known and that can be covered by e.g. SLED sources the service needed for the system can be cut down.

4.1 Abbreviations and terms

Term or abbreviation Explanation

A/D Analogue to Digital

ADC Analogue to Digital Converter

ADU Analogue to Digital conversion Unit

AFE Analogue Front End

DAC Digital to Analogue Converter

FPGA Field Programmable Gate Array

I/O Input/Output

I/F Interface

NIR Near Infra Red (wavelength range)

PC Personal Computer

TBC To Be Confirmed TBD To Be Defined

TBW To Be Written

WBS Work Breakdown Structure

WPD Work Package Description

5. Design description of the NOSPI WP4 Chemical Imaging prototype

5.1 User and Technical requirements

Original NOSPI Chemical Imaging Spectrometer specification (presented at kick-off meeting).

Parameter NOSPI CHEMIM Remarks

Spectrometer detector 3 -stage Peltier cooled

MCT linear array

Image size 256 pixel linear array with parallel readout

Spectral dispersion technology Piezo tunable Fabry- Pat. application

Perot filter operating at FI20065348

multiple orders

simultaneously

Illumination Multi-LED light source Readout of each type of

8-20 LED different types LED separately together cover the whole

spectral range

Spectral range 900-2300nm (2500nm) 2300 to 2500 nm

covered with halogen lamp

Spectral resolution 3...10nm @ 1600nm Resolution depends on mirror coatings and beam opening angle

Typical scan time 10ms for 160 3.2 s for 320 x 256 pixel wavelength bands and @ 160 WL bands 256 pixels Dimensions/System size 20 cm (H) x 15 cm (W) x

15 cm (D)

Final NOSPI Chemical Imaging Spectrometer (will be presented at final meeting)

Parameter NOSPI CHEMIM Remarks

Spectrometer detector 3 -stage Peltier cooled

2D MCT array

Image size 384x288 pixel array 2 ms frame time

Spectral dispersion technology Piezo tunable Fabry- Perot filter

Illumination Halogen light source

with long pass filters

from four directions plus

FP order selection filters

Spectral range 1200-2200nm Short end limited by order selection filters. Long end limited at the moment by the tunable bandgap in Fabry Perot

Spectral resolution FWHM 20nm @

1512nm

FWHM 25nm @

1877nm

Typical scan time >400ms / lambda The software has not

Detector frame time ~ been optimized yet. 2ms 30ms/lambda is a goal for the scan time.

Dimensions/System size 50 cm (H) x 40 cm (W) The system is realized as x 50 cm (D) plus the PC a laboratory prototype. and the power supplies No effort has been done to build up an integrated system. An array of Vertical Cavity Surface Emitting Laser (VCSEL) diodes may be used for checking and validating wavelength performance of imaging spectrometers and NIR Chemical Imaging systems. Existing methods for wavelength validation are based on target samples made of rare earth oxides or polystyrene particles. These are not very good for use with imaging systems, because of spatially non-homogeneous

characteristics. VCSEL based method can display information on peak wavelength and band width for each pixel across the image, which can clearly illustrate any instrumental errors in wavelength calibration and wavelength resolution. Thanks to narrow linewidth of the VCSEL source, the observed linewidth is a direct measure of imager linewidth and spectral resolution. An array of multiple VCSEL sources may be used, which provides information about nonlinearity of the wavelength scale. The proposed method includes calibrating the VCSEL wavelengths using traceable methods, storing the calibrated values of laser current and operating temperature in an electronic memory device and regulating the wavelengths using electronic means during actual use of the method.

NIR Chemical Imaging is being increasingly applied to important issues in

pharmaceutical development and other applications. Due to regulated nature of pharmaceutical industry there is continuous need to qualify Chemical Imaging instruments for certain use and to characterize day-to-day performance. Significant part of this characterization and qualification relates to the wavelength characateristics, such as checking calibration and resolution of the wavelength axis. Currently available wavelength calibration targets are not optimal for use with imaging systems due to spatial variation in key characteristics. These problems can be solved with the proposed VCSEL based device and measurement procedure.

Claims

Claims:

1. A spectrometer for measuring spectral information from sample material moving or stationary at a distance, characterized in that the spectrometer comprises:

a) a broadband light source (1), such as a halogen incandescent lamp, a light beam of the light source is collimated with a lens (4) in order to lead the light beam through a High Pass Filter (22) and Low Pass Filter (21) which define the pass band of the instrument measurement spectral range,

b) a lens (5) after the High and Low pass filters focuses the light to 2D Chemical Image target (2),

c) an Imager Collimator lens (23) gathers the scattered light from the target and collimates this light and directs it to a Piezo-actuated Fabry-Perot Interferometer (20), d) The Piezo-actuated Fabry-Perot Interferometer (20) filters the incoming light by transmitting only light at one or several narrow spectral bands depending on the air gap between the mirrors of the Fabry-Perot Interferometer,

e) a Filter wheel (24) contains several band pass filters that are matched to the specific Fabry-Perot Interferometer orders, this enables to use high orders of the Fabry-Perot Interferometer whose spectral widths can be chosen depending on the application, f) Imager lens (6) is used to form the image of 2D Chemical Image target (2) on a 2D IR detector (25).

2. A spectrometer according to Claim 1 where the 2D IR detector (25) is an InAsGa or Mecrury-Cadmium-Telluride (MCT) type of detector in the wavelength range relevant to chemical imaging

3. Method or procedure for high speed spectral and chemical imaging spectrometers, characterized in that the method comprises the steps of:

b) receiving scattered light

e) processing electrical signals from the pixels into spectral information; and

4. Method or procedure according to Claim 3 characterized in that the spectral information, comprising of spectral values and wavelengths, is maintained stable by cooling and stabilizing the operating temperature of the 2D detector array and the Fabry- Perot tunable filter, while the spectrometer is operating in variable environmental conditions.

5. Method or procedure according to Claim 3, characterized in that the spectrometer is predicting chemical maps from measured spectral images by applying chemometric prediction models.

6. Testing and validating wavelength performance method for imaging spectrometers, characterized in that the method comprises:

illuminating by at least one laser diode (30) a spectrally white diffuse reflecting target (2),

regulating the forward current (33) and the operating temperature of each laser diode (30) to predetermined set values, and

storing predetermined values in electrical memory (32).

7. Method according to Claim 6, characterized in that vertical cavity surface emitting laser diodes (VCSEL) are used as diodes (30).

8. Method according to Claim 6 or 7, characterized in that validation instrumentation and system (figure 1) are be calibrated by monitoring the laser (30) wavelength by suitable wavelength measuring device.

9. Method according to any previous Claim, characterized in that a traceable measurement setup (31) is stored in controlled environment and is tracked to known reliable wavelength calibration, with controlled uncertainty (NIST etc).

10. Method according to any previous Claim, characterized in that during initial calibration of the laser based validation system, the exit wavelength of the laser is observed and compared to a chosen target value.

11. Method according to any previous Claim, characterized in that the laser wavelength is adjusted by changing either the laser current or the operating temperature until the chosen wavelength is achieved.

12. Method according to any previous Claim, characterized in that temperature tuning is performed for changing the temperature of the laser chip is integrated inside the laser diode module 30.

13. Method according to any previous Claim, characterized in that after the initial calibration is finished the validation system is used in normal operation.

14. Device for testing and validating wavelength performance of imaging spectrometers, characterized in that the device comprises:

at least one laser diode (30) positioned to illuminate a spectrally white diffuse reflecting target (2),

means (33) for regulating the forward current and the operating temperature of each laser diode (30) to predetermined set values and

electrical memory (32) device.

15. Apparatus according to Claim 14, characterized in that vertical cavity surface emitting laser diodes VCSEL) are used as diodes (30).

g) The 2D IR detector (25) is typically a InAsGa or Mecrury-Cadmium-Telluride (MCT) type of detector in the wavelength range relevant to chemical imaging.

15. Method or procedure for high speed spectral and chemical imaging spectrometers, characterized in that the method comprises the steps of: a) using a broadband light source 1, such as a halogen incandescent lamp, a light beam of the light source is collimated with lens in order to lead the light beam through a High Pass Filter (22) and Low Pass Filter (21) which define the pass band of the instrument measurement spectral range, the lens after the High and Low pass filters focuses the light to 2D Chemical Image target (2),

c) using a Imager Collimator lens (23) for gathering the scattered light from the target and collimates this light and directs it to a Piezo-actuated Fabry-Perot Interferometer (20),

d) using The Piezo-actuated Fabry-Perot Interferometer (20) for filtering the incoming light by transmitting only light at one or several narrow spectral bands pending on the air gap between the mirrors of the Fabry-Perot Interferometer,

e) using a Filter wheel (24) contains several band pass filters that are matched to the the specific Fabry-Perot Interferometer orders, this enables to use high orders of the Fabry- Perot Interferometer whose spectral widths can chosen pending on the application, f) using an Imager lens (24) to form the image of 2D Chemical Image target (2) on a 2D IR detector (25),

g) using as The 2D IR detector (25) typically a InAsGa or Mecrury-Cadmium-Telluride (MCT) type of detector in the wavelength range relevant to chemical imaging.