Development of a Miniature Snapshot Multispectral Imager 

by Neelam Gupta, Philip R. Ashe, and Songsheng Tan 

ARL-TR-5320 September 2010 

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Army Research Laboratory 

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ARL-TR-5320 September 2010 

Development of a Miniature Snapshot Multispectral Imager 

Neelam Gupta 

Sensors and Electron Devices Directorate, ARL 

Philip R. Ashe* and Songsheng Tan 

Infotonics Technology Center, 5450 Campus Drive 

Canandaigua, NY 14424 

*

Presently at SpectralSight, Inc. 

2 Mae Meadow, Rochester, NY 14624 

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Development of a Miniature Snapshot Multispectral Imager 

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Neelam Guptaa, Philip R. Asheb*, and Songsheng Tanb

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U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD, USA 20783-1197 

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Infotonics Technology Center, 5450 Campus Drive, Canandaigua, NY, USA 14424 

*

Presently at SpectralSight, Inc., 2 Mae Meadow, Rochester, NY, USA 14624 

14. ABSTRACT 

We present a miniature snapshot multispectral imager that operates in the short wavelength infrared (SWIR) region and has a 

number of applications. The system is low weight and portable with a miniature platform, and requires low power. The imager 

uses a 4×4 Fabry-Perot filter array operating from 1487 to 1769 nm with a spectral bandpass ~10 nm. The design of the filters 

is based on using a microelectromechanical system (MEMS) shadow mask technique to fabricate a Fabry-Perot etalon with 

multilayer dielectric mirrors. The filter array subsystem is installed in a commercial handheld indium gallium arsenide 

(InGaAs) camera, replacing the imaging lens with a custom designed 4×4 microlens array with telecentric imaging 

performance in each of the 16 sub imaging channels. We imaged several indoor and outdoor scenes. The microlens optics and 

filter design is quite flexible and can be tailored for any wavelength region from ultraviolet (UV) to longwave infrared 

(LWIR) and the spectral bandpass can also be customized to meet requirements. In this report, we discuss the design and 

characterization of the filter array, microlens optical package, and imager and present the imaging results. A main motivation 

behind development of such a compact imager is to be able to detect chemicals used in improvised explosive devices (IEDs). 

15. SUBJECT TERMS

Fabry-Perot filter, multispectral, SWIR, microlens optics, IED detection 

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iii 

Contents 

List of Figures iv

List of Tables v

1. Introduction 1

2. Snapshot Multispectral Imager Concept 2

3. Design of Micro Optic Assembly 4

3.1 Optical Performance ........................................................................................................5

3.1.1 Optical Parameters ..............................................................................................5

3.1.2 Tolerance Analysis ..............................................................................................6

3.1.3 1st-order Radiometric Analysis ...........................................................................6

3.2 Microlens Assembly Prototypes ......................................................................................6

3.3 Characterization of Microlens Assembly ........................................................................7

4. Fabry-Perot Filter Array 7

4.1 Modeling of F-P filters ....................................................................................................8

4.2 MEMS F-P Filter Array Fabrication Process ................................................................10

4.3 Experimental Characterization of F-P Filter Array .......................................................13

5. Snapshot Multispectral Imager 14

6. Results and Analysis 15

7. Summary and Conclusions 21

8. References 22

List of Symbols, Abbreviations, and Acronyms 24

Distribution List 25 
iv 

List of Figures 

Figure 1. An illustration of snapshot multispectral imager using a commercial camera. A 4×4 

filter array is installed in front of the FPA and in front of the filter array a micro optic 

assembly with telecentric microlenses, baffles, and light stop array are placed. .......................3

Figure 2. Single imaging channel of multiple channel imager composed of (as viewed from 

left to right)—front lens, rear lens, narrow-band F-P filter, and FPA. Not shown in the 

figure is the optical stop in front of lens 1. ................................................................................4

Figure 3. Design of micro optic assembly. Off-the-shelf plano-convex lenses were selected 

and reshaped to fit the assembly. Plano-convex lenses focus a collimated beam to the 

back focus and collimate light from a point source. Two different plano-convex lenses 

were chosen one with a focal length of 15 mm and a second with a focal length of 10.2 

mm with anti-reflection coating for the operation in the desired spectral region. The F# of 

the micro optic for each channel is 10.2. ...................................................................................5

Figure 4. Simulation showing in-band and out-of-band images using the optical assembly 

and F-P filter array. The out-of-band image leakage is very small when compared to the 

light rays captured at the detector edge. ....................................................................................5

Figure 5. Different views of microlens assembly. ..........................................................................7

Figure 6. Monochromatic images captured from 16 channels of the microlens assembly 

installed in front of the SWIR camera. ......................................................................................7

Figure 7. Values of refractive indices of SiO2 and TiO2 shown are from TFCalc software 

and SiO2(T) and TiO2(T) are from laboratory measurement of thin films of SiO2 and 

TiO2. ...........................................................................................................................................8

Figure 8. Results of modeling showing FWHM for dielectric mirror operating at 1650 nm 

with 8 layers ( ), 9 layers ( ), 10 layers ( ), and 11 layers ( ) using 

TFCalc software. ........................................................................................................................9

Figure 9. Schematic drawing of the side view of F-P filter array showing four filters 

operating at 1, 2, 3, and 4 wavelengths. Each F-P filter has the same 11 alternating 

TiO2 (n = 2.3) and SiO2 (n = 1.46) layer upper and lower mirrors. The thickness of SiO2

cavity is different for each wavelength roughly equal to half of the center wavelength of 

the filter. .....................................................................................................................................9

Figure 10. Calculated transmittance of F-P etalon with 11-layer TiO2/SiO2 mirrors using 

TFCalc......................................................................................................................................10

Figure 11. Shown are the four shadow masking steps used to grow the 16 different cavity 

layers for 16 F-P filters in the array. The mask (a) on the top left is used to grow SiO2

layer on filters in column 2 and 4, then mask (b) on top right is used to grow SiO2 layer 

on filters in columns 3 and 4, next mask (c) on bottom left is to used to grow SiO2 layer 

on filters in rows 2 and 4 and the last mask (d) on bottom right is used to grow SiO2 layer 

on filters in rows 3 and 4..........................................................................................................11

Figure 12. Four shadow masks used to grow the cavities of 16 F-P filters. .................................11

Figure 13. Filter array with numbered filters on a die. .................................................................12
v 

Figure 14. SEM photos of fabricated filter as well as detailed layer structure. .............................12

Figure 15. Optical setup used to characterize filter array and imager. ........................................13

Figure 16. Measured transmission spectra of interference filters used to calibrate the optical 

test setup...................................................................................................................................14

Figure 17. Measured spectral transmission of each of the 16 spectral channels in the filter 

array. ........................................................................................................................................14

Figure 18. Snapshot multispectral imager with 16 spectral channels as shown in figure 17. ......14

Figure 19. Sixteen spectral images of the object on the left obtained with the snapshot 

imager. .....................................................................................................................................16

Figure 20. Sixteen spectral images of the plant on the left obtained with the snapshot imager. ..16

Figure 21. Cropped view of a fiber optic bundle holder. ..............................................................16

Figure 22. Fiber at filter # 1, 1487 nm. .........................................................................................17

Figure 23. Fiber at filter # 2, 1504 nm. .........................................................................................17

Figure 24. Fiber at filter # 3, 1522 nm. .........................................................................................17

Figure 25. Fiber at filter # 4, 1540 nm. .........................................................................................17

Figure 26. Fiber at filter # 5, 1562 nm. .........................................................................................18

Figure 27. Fiber at filter # 6, 1582 nm. .........................................................................................18

Figure 28. Fiber at filter # 7, 1601 nm. .........................................................................................18

Figure 29. Fiber at filter # 8, 1623 nm. .........................................................................................18

Figure 30. Fiber at filter # 9, 1641 nm. .........................................................................................19

Figure 31. Fiber at filter # 10, 1662 nm. .......................................................................................19

Figure 32. Fiber at filter # 11, 1681 nm. .......................................................................................19

Figure 33. Fiber at filter # 12, 1699 nm. .......................................................................................19

Figure 34. Fiber at filter # 13, 1717 nm. .......................................................................................20

Figure 35. Fiber at filter # 14, 1736 nm. .......................................................................................20

Figure 36. Fiber at filter # 15, 1753 nm. .......................................................................................20

Figure 37. Fiber at filter # 16, 1779 nm. .......................................................................................20

List of Tables 

Table 1. Specifications of F-P filters. ...........................................................................................15

 
vi 

INTENTIONALLY LEFT BLANK. 
1 

1. Introduction 

Development of compact multispectral imaging sensors with no moving parts is required for 

many defense, medical, environmental monitoring, and variety of other applications. A 

multispectral imager uses an optical device that can separate the colors to obtain the spectral 

content in the scene. Such an optical device could be a grating or prism, a filter wheel, a 

diffractive optic lens, a Fabry-Perot (F-P) etalon, or a tunable filter. All of these optical devices 

are used with a focal plane array (FPA) and suitable optics for a given spectral region. All such 

imagers collect images in a time sequential manner. Recently, some multispectral imagers have 

been developed that collect images of the scene at all the spectral bands simultaneously (1–5). 

We describe one such imager that has been developed using microelectromechanical system 

(MEMS) technology to fabricate a filter bank covering a spectral region with each filter 

operating at a specific peak wavelength with a design spectral width. Such a filter array is then 

combined with a microlens assembly based on telecentric design to image the same scene at each 

spectral band without much overlap with neighboring bands using a honeycomb baffle design at 

a subsection of the FPA. Recently, a snapshot miniature multispectral imaging system operating 

in the shortwave infrared (SWIR) region has been developed by the U.S. Army Research 

Laboratory (ARL) working with the Infotonics Technology Center (ITC), Canandaigua, NY. 

One of the main motivations behind development of such a compact imager is to be able to 

detect chemicals used in improvised explosive devices (IEDs). 

The two main components used in the design of such a snapshot multispectral imager are the 

multichannel F-P filter array and the microlens assembly. The filter array uses 16 F-P etalons, 

each tuned to a different peak wavelength. The filter bank consists of a 4×4 two-dimensional 

(2-D) array of fixed wavelength filters each with a narrow bandpass. The filter design was 

arrived at after a lot of research effort had been devoted to finding the proper materials, mirror 

design, and thin-film deposition techniques. Instead of using two high reflectivity metal mirrors 

with a cavity in between to fabricate each of the 16 F-P filters, a multilayer dielectric stack 

mirror design was chosen to obtain optimal filter performance. Each of the mirrors on either side 

of the cavity in the F-P etalon was fabricated using 11 alternating layers of two different 

dielectric materials: titanium dioxide (TiO2) and silicon dioxide (SiO2) with different refractive 

index values. In the mirror design, the layer thickness for each material was chosen equal to one 

quarter of the selected center wavelength of the filter to obtain the desired spectral full width at 

half maximum (FWHM) with high filter finesse. A SIO2 cavity between the two dielectric 

mirrors was chosen with a corresponding thickness for each of the peak wavelengths. Each of the 

16 F-P filters had the same top and bottom multilayer dielectric mirror but the cavity thickness 

for each filter was different, equal to around one half of the corresponding center wavelength of 

the filter. The F-P filters were fabricated by deposition of evaporated thin material films. 

Different cavity layer thicknesses for the 16 spectral filters were achieved by using only four 
2 

shadow masks for deposition of the evaporated SiO2 films. The other main component that is 

required to obtain simultaneous multispectral images of a scene is a microlens assembly that can 

image the scene into 16 different spectral channels, coming through each filter on a subsection of 

the FPA of the SWIR camera independently, i.e., the out-of band leakage from the surrounding 

filters has to be made as small as possible. This was achieved by using two arrays of 4×4 

microlenses with field stops and baffles. Commercial-off-the-shelf lenses were used with some 

machining to match the size of the individual filter. The f-number of microlenses, baffle 

thickness, and field stop distances were chosen after careful radiometric considerations to 

optimize the light transmission to the FPA. The filter bank operates from 1487 to 1769 nm with a 

spectral bandpass ~10 nm. The design of the filter array can be customized for any spectral 

region from the ultraviolet (UV) to the longwave infrared (LWIR). Also, filter and custom optics 

sizes can be matched to optimize performance using a variety of cameras with different size 

focal planes. The fabricated filter was installed in front of a commercial camera with an indium 

gallium arsenide (InGaAs) FPA and the camera imaging lens was replaced by the microlens 

assembly. The microlens assembly was carefully tested with the commercial camera for its 4×4 

independent image formation capability. A number of filter arrays were fabricated and carefully 

characterized for their spectral characteristics. The snapshot multispectral imager with 16 

independent channels was carefully calibrated to evaluate its imaging performance. A number of 

indoor and outdoor scenes were imaged and the images were analyzed. In this report, we discuss 

the snapshot imager concept (section 2); the micro optic assembly design and testing (section 3); 

the filter design, fabrication and characterization (section 4); the snapshot imager and its imaging 

performance (section 5), and finally the results and conclusions (sections 6 and 7, respectively). 

2. Snapshot Multispectral Imager Concept 

The snapshot imager is designed using a commercial camera without its imaging lens, a 2-D 

filter bank that contains a number of filters each transmitting at a different peak wavelength in 

front of the FPA, and a microlens assembly in front of the filter array to ensure a telecentric light 

path through the F-P filters and to image the scene on the corresponding subsections of FPA. In 

figure 1, we show a computer aided design of such an imaging system. A cut-off filter is shown 

in front of the camera to control the spectral spread of the incident light such that only the first 

order images from the F-P filters will be formed on the FPA. The micro optic package was 

designed to image the light transmitting through each filter independently on a subsection of the 

FPA without any leakage from the surrounding filters. This requires using a light stop array and 

baffles with the microlens array to ensure telecentric light paths to meet the F-P etalon 

requirements. The main design consideration is to form images on the FPA using only close to 

normal incident light. 
3 

Figure 1. An illustration of snapshot multispectral imager using a commercial 

camera. A 4×4 filter array is installed in front of the FPA and in front of 

the filter array a micro optic assembly with telecentric microlenses, 

baffles, and light stop array are placed. 

To find the required IR camera, the following parameters were considered: range of operation, 

camera resolution, pixel size, FPA size, temperature setting required for operation, ease of 

operation, availability, and cost. The camera that best fit these requirements was a Goodrich 

Sensors Unlimited model SU3201.7RT camera with 320×240 pixels with a 12.8×9.6 mm size 

InGaAs FPA with a 40-m pixel pitch and a spectral range of 0.9–1.7 m at room temperature 

operation (6). The size of the individual filters in the array and the custom optical package with 

microlenses was designed to work with this camera. The other camera that was considered was a 

Goodrich model SU640KTSX, which has an InGaAs FPA with 640×512 pixels and a 25-m 

pixel pitch. In addition, Goodrich is developing a 1920×1080 pixel IR camera in the near future. 

As stated earlier, the filter and microlens package can be redesigned to fit other FPAs and lens 

working distances. 
4 

3. Design of Micro Optic Assembly 

The optical design goal was to create a lens system that would output a matrix of identical 

images of the captured scene simultaneously and project these parallel images through the image 

stop, microlenses, light baffle, another set of micro lenses, and filter array and onto the sensor as 

described in patent application U.S. 11/995362 (7). The design goals were to maximize the 

camera output performance, maintain the light ray angular specification of an F-P, and meet the 

operating requirements of the imager. 

The optical design* illustrated in figure 2 has been customized to match the focal plane of an 

InGaAs camera model SU3201.7RT with a 320×240 FPA format with a 40-m pixel pitch. The 

design is essentially a compact array of 16 (4×4) sub-imaging optical systems. Each sub-imager 

consists of an optical stop and two lenses with the front lens spaced from the rear so that the 

imaging is telecentric on the image side, as shown in figure 2. 

Figure 2. Single imaging channel of multiple channel imager composed of (as viewed from left to right)—front 

lens, rear lens, narrow-band F-P filter, and FPA. Not shown in the figure is the optical stop in front of 

lens 1. 

The telecentricity in each channel is achieved by placing the front lens one focal length (rear lens 

focal length, F2) away from the rear lens. Telecentricity accomplishes the objective of 

minimizing the angles through the narrowband F-P filter. A free parameter is the focal length F1 

assigned to the front lens. This parameter determines the field coverage and the F# of each 

channel, as shown in figure 3. The distance, h, to the FPA, as shown in figure 3, is 2 mm. The xand y-pitch to meet the desired cell size constrains much of the design. In the present design, the 

top and bottom lenses were mechanically modified to fit the optical assembly. In future, when 

significant volumes are desired, monolithic lens arrays may be preferable. One system tradeoff is 

with the system F# (Fno_det) and the roll-off in the field due to vignetting of the ray bundle. 

Filling the front lens aperture diameter (2y1) produces the lowest F#, but roll-off begins 

immediately. It can be shown that the sensitivity of InGaAs FPA allows F#’s between 10–15 

with good signal-to-noise ratio from sunlit scenes. This, in turn, allows much of the sub-image 

 *Optical design and simulations by Chris Brophy, OES, Rochester, NY. 

Lens 1 

Lens 2 

F-P Filter 

FPA
5 

area to be unvignetted. An array of 1-mm-diameter aperture stops is placed in front of the front 

lenses. The out-of-band image leakage is very small when compared to the light rays captured at 

the sub-section of FPA for each channel. Edge roll-off and sub-image size with a light baffle 

wall thickness of 0.3 mm gives an image size of 48×70 pixels. Simulation results for this are 

shown in figure 4. 

Figure 3. Design of micro optic assembly. Off-the-shelf plano-convex lenses were selected and reshaped to fit the 

assembly. Plano-convex lenses focus a collimated beam to the back focus and collimate light from a point 

source. Two different plano-convex lenses were chosen one with a focal length of 15 mm and a second 

with a focal length of 10.2 mm with anti-reflection coating for the operation in the desired spectral region. 

The F# of the micro optic for each channel is 10.2. 

Figure 4. Simulation showing in-band and out-of-band images using the optical assembly and F-P filter 

array. The out-of-band image leakage is very small when compared to the light rays captured 

at the detector edge. 

3.1 Optical Performance 

3.1.1 Optical Parameters 

• Working F# (channel): 10.2 

• Strehl Ratio (worst-case field, (1.4 < λ < 1.8 m)): 97% 
6 

Full Field of View: 10.2° (along 2.4 mm length ) × 13.6° (along 3.2 mm length ) 

3.1.2 Tolerance Analysis 

• Worst-case sensitivity to relative decentering of the two lenses (field tilt): Strehl (±0.1 mm 

decenter) = > 91%; (±0.052 mm decenter) = > 95% 

For either case, there is a negligible reduction in image quality due to the large (40-m) pixel 

pitch. In fact, a sensor with pixels half this size would still experience negligible image 

degradation. 

3.1.3 1st-order Radiometric Analysis

Irradiance, Ir, the incident radiant power per unit area on the FPA is given by the following 

expression (8): 

 (1) ܶ,ܮ ൌ௥ ܫ

where L is the source radiance,  is the solid angle of collection, and T is the overall optical 

transmission of the imaging system, given by 

ܶൌܶ௢௣ܶி௉, (2) 

where ܶ௢௣is the transmission of the optics including the cutoff filter before the microlenses and 

ܶி௉is the transmission of the each FP etalon. For the InGaAs FPA, sensor noise equivalent 

irradiance, NEI = 0.13 nW/cm2 (near 1.55 m, with quantum efficiency, QE = 80%) (6). We will 

denote NEI by In. Since the solid angle, , for optics for each channel is given by /(4F#2), we 

can obtain an expression for L by using the fact that minimum irradiance on FPA that gives rise 

to NEI: 

்௅ ൌ௥ ܫ ؠ ܫܧܰ

ସி#మ. (3) 

Using equations 2 and 3, we can write 

ܮ ൌ ସி#మூ೙

்೚೛்ಷು

. (4) 

Based on the imager design, the values of these parameters are In = 0.13 nW/cm2

; F# = 10.2; ܶி௉

= 0.8, and ܶி௉ = 0.5. Substituting these values in equation 4, we obtain L = 43 nW/cm2

-ster. 

Since the noise-equivalent radiance, NEL, is defined as the dimmest scene radiance that gives 

rise to the camera NEI, from the above calculations, NEL ~ 43 nW cm–2-ster–1. This is a 

reasonable value for a scene illuminated by the sun. The incident radiance around 1.55 m on the 

camera is over 2000 times greater than NEL, which means that there should be adequate S/N for 

the camera with the chosen design parameters. 

3.2 Microlens Assembly Prototypes 

Figure 5 shows the microlens assembly viewed from top and bottom. 
7 

Figure 5. Different views of microlens assembly. 

3.3 Characterization of Microlens Assembly 

The microlens assembly was installed in front of the IR camera and some images were acquired 

to understand the image formation and see the effect of the baffles, as shown in figure 6. An 

object was illuminated with a monochromatic light using a 1560-nm filter in front of an 

incandescent light source and images were captured with the camera. 

Figure 6. Monochromatic images captured from 16 channels of the microlens 

assembly installed in front of the SWIR camera. 

4. Fabry-Perot Filter Array 

A F-P filter consists of two highly reflective mirrors separated by a spacing or cavity. For fixed 

wavelength operation, the cavity thickness is equal to one half of the wavelength of light that it 

transmits. The operation of the F-P filter is based on interference of light. F-P filter arrays have 

been developed for various applications (9–12). In many applications, instead of using two 

traditional mirrors with high reflectivity coatings, two multilayer dielectric stack mirrors made 

with alternating high and low refractive index transparent materials are used. The performances 

of the F-P filter array used in multispectral imagers should be characterized for the following 

four major parameters: bandwidth, FWHM, peak transmittance, and the transmittance round the 

peak. Modeling several choices of materials for the dielectric mirrors used in the F-P filters 

operating in the 1.5 to 1.8 m region indicated that the best choice for the dielectric mirrors is 
8 

alternating layers of zinc sulfide (ZnS) and SiO2, however since ITC had no prior experience 

with ZnS, the next best choice of TiO2 and SiO2 was selected for fabrication of dielectric mirrors 

(13–15). 

4.1 Modeling of F-P filters 

The analysis results carried out using a commercial thin-film modeling software program TFCalc 

by Software Spectra Inc. (16) showed that by choosing a larger number of layers for each mirror, 

the FWHM of the filter can be narrowed. Many simulations were carried out to understand the 

effect of change in refractive index of the material until the desired results were obtained. The 

filter design was optimized by using measured values of refractive indices. In figure 7, values of 

refractive indices for TiO2 and SiO2 obtained from the TFCalc software and also from laboratory 

measurement are shown as a function of wavelength. Thin layers of TiO2 and SiO2 on glass 

substrate were grown using a Leybold thin-film deposition machine and the refractive indices 

were measured by a Woollam ellipsometer. These measured values were used in the thin-film 

coating software to obtain the dielectric stack with desired FWHM. Figure 8 shows the change in 

FWHM of the filter by using different number of layers in the design of the dielectric mirror. An 

11-layer dielectric stack was selected to fabricate each mirror structure, as shown in figure 9. The 

thickness of the SiO2 and TiO2 layers in the mirror was equal to one quarter wavelength at 

1650 nm corresponding to the center of the spectral region, while the values of the cavity 

thickness were approximately equal to half of the center wavelength corresponding to each of the 

16 filters. 

Figure 7. Values of refractive indices of SiO2 and TiO2 shown are from TFCalc software 

and SiO2(T) and TiO2(T) are from laboratory measurement of thin films of SiO2 and TiO2. 
9 

Figure 8. Results of modeling showing FWHM for dielectric mirror operating 

at 1650 nm with 8 layers ( ), 9 layers ( ), 10 layers ( ), 

and 11 layers ( ) using TFCalc software. 

Figure 9. Schematic drawing of the side view of F-P filter array showing four filters operating at 

1, 2, 3, and 4 wavelengths. Each F-P filter has the same 11 alternating TiO2 (n = 2.3) 

and SiO2 (n = 1.46) layer upper and lower mirrors. The thickness of SiO2 cavity is 

different for each wavelength roughly equal to half of the center wavelength of the filter. 

Based on the TFCalc simulations of the 11-layer dielectric mirror, it was shown that the each 

mirror has a reflectance, R, of around 98% from 1500 to 1800 nm and the finesse, F, of the F-P 

filter defined by the following well known expression: 

ܨ ൌ గ√ோ

ሺଵିோሻ , (5) 

which is high equal to 155.5. The calculated transmission for each F-P filter from 1500 to 

1800 nm is around 90% as shown in figure 10. In reality, actual process of fabrication and 

measurement uncertainties may yield a lower value. 
10 

Figure 10. Calculated transmittance of F-P etalon with 11-layer TiO2/SiO2 mirrors using TFCalc. 

4.2 MEMS F-P Filter Array Fabrication Process 

A large number of 4×4 F-P filter arrays were fabricated by using only four masks on a common 

glass plate. The filter array was fabricated using a Leybold e-beam evaporator system. The 

TiO2/SiO2 layer thicknesses for both the mirror structures for each of the 16 filters in the array 

were the same, although the cavity thickness was different for each filter in the array. One way to 

do this is to use 16 separate cavity layer depositions. This requires a total of 18 process steps, 

which is too many. Thus, the main challenge was to minimize the number of steps involved in 

depositing 16 different thicknesses of SiO2 cavity layers for the 16 filters. The approach used 

here uses far fewer layer deposition steps to achieve this. This was achieved by using a set of 

four shadow masks, as shown in figure 11, which enabled us to use only six process steps. The 

first process step used during the film deposition was to deposit the 11-layer TiO2/SiO2 bottom 

mirror, as shown in figure 9, and a base SiO2 layer was deposited on all the filters controlled by 

the thickness controller. The middle four steps were used to adjust the cavity thicknesses of the 

16 filters, which were controlled by the quartz thickness controller. During these four steps first 

the shadow masks (a) was used to deposit layer of SiO2 on filters 2, 4, 6, 8, 10, 12, 14, and 16, 

then second mask (b) was used to deposit SiO2 layer on filters 3, 4, 7, 8, 11, 12, 15, and 16, after 

that the mask (c) was used to deposit SiO2 layer on filters 5–8, 13–16 and at the end the mask (d) 

was used to deposit SiO2 layer on filters 9–16. The last step was to deposit the second mirror 

structure, which was also controlled by the optical thickness controller. The full size of the filter 

array is 12.8×9.6 mm with individual filter size of 3.2×2.4 mm. 

TiO2/SiO2 FP filter array with 11 layer mirror

0

10

20

30

40

50

60

70

80

90

100

1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900

Wavelength (nm)

Transmittance (%)
11 

Figure 11. Shown are the four shadow masking steps used to grow the 16 different cavity 

layers for 16 F-P filters in the array. The mask (a) on the top left is used to 

grow SiO2 layer on filters in column 2 and 4, then mask (b) on top right is 

used to grow SiO2 layer on filters in columns 3 and 4, next mask (c) on 

bottom left is to used to grow SiO2 layer on filters in rows 2 and 4 and the last 

mask (d) on bottom right is used to grow SiO2 layer on filters in rows 3 and 4. 

The actual shadow masks used to grow the 16 different thicknesses of SiO2 cavity are shown in 

figure 12. The numbering system for the filter array on a die is illustrated in figure 13. Scanning 

electron microscope (SEM) photos of fabricated filter as well as detailed layer structure on a 

0.75-mm-thick fused quartz plate is shown in figure 14. 

Figure 12. Four shadow masks used to grow the cavities of 16 F-P filters. 
12 

Figure 13. Filter array with numbered filters on a die. 

Figure 14. SEM photos of fabricated filter as well as detailed layer structure. 

11 Layer Mirror TiO2

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

Cavities‐

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

SiO2

TiO2

Al Array

IDs

SiO2

TiO2

Glass

23 layer Dielectric Stack
13 

4.3 Experimental Characterization of F-P Filter Array 

A fully computer controlled optical setup was used to characterize the spectral properties of the 

fabricated F-P filter array. It consists of a tunable light source from 320 to 2200 nm with a 

wavelength accuracy of ±0.6 nm, a fiber bundle, and a power meter with two different detectors. 

The tunable light source system was a model AST-XE-175-BFEX from Spectral Products with a 

175-W xenon light source and a 1/8-meter monochromator. The nominal maximum spectral 

resolution of the output light is about 1 nm. A Newport dual channel power meter model 2832 – 

C with both silicon (Si) (model 818-ST) and germanium (Ge) (model 818-IR) detectors was 

used. The size of the Si detector is 10×10 mm2 and the diameter of the Ge detector is 3 mm. 

Since the filter array operates in the 1.5- to 1.8-m region, the Ge detector was used. The 

operation of the optical setup, including data collection with a graphical output, was 

computerized. A computer was used to communicate with the monochromator and the power 

meter to perform the spectral scan. This setup was used to detect the optical interference peaks, 

to calibrate the bandwidth resolution and measure the light intensity distribution across the F-P 

filters. 

Figure 15 is a schematic of the test system. The light output from monochromator is coupled into 

a fiber bundle with an entrance slit size of 0.2×4.2 mm and an output circle with a diameter of 

1 mm. The detector is mounted on a linear stage in order to move the detector “in” and “out.” 

When the detector is at “in” position, the optical spectrum can be scanned. The active area of 

detectors is large enough to collect the entire light beam. When the detector is at “out” position, 

image of the sample (labeled TF) can be observed on the monitor through the lens and chargecoupled device (CCD) camera for carrying out optical alignment. 

Power 

meter 

TF 

Light in

Fiber bundle

CCD 

Camera 

Computer 

Xenon 

light 

source 

Monochromator 

Lens set

Monitor 

2832

C

Detector 

Figure 15. Optical setup used to characterize filter array and imager. 

The filter test system was calibrated using commercial inference filters of known bandpass 

values with center wavelengths: 1460, 1555, 1670, 1723, 1810, and 1900 nm, as shown in figure 

16. The spectral transmission of each of the fabricated F-P filter in the filter array was measured 

with the above optical setup, as shown in figure 17. 
14 

Figure 16. Measured transmission spectra of interference filters used to calibrate 

the optical test setup. 

Figure 17. Measured spectral transmission of each of the 16 spectral channels in the filter array. 

5. Snapshot Multispectral Imager 

The camera lens of one InGaAs camera model SU3201.7RT with a 320×240 FPA was removed. 

The fabricated F-P filter array was installed in front of the FPA and then the microlens assembly 

was installed in front of the filter array, as shown in figure 1, to obtain the snapshot imager with 

16 spectral channels. A long-wave cutoff filter was also installed to block the shorter wavelength 

light. A photograph of the snapshot multispectral imager is shown in figure 18. The camera was 

then used to image a number of indoor and outdoor scenes. 

Figure 18. Snapshot multispectral imager with 16 

spectral channels as shown in figure 17. 
15 

6. Results and Analysis 

Table 1 lists the specifications of the filter array. Both the target specifications and 

corresponding measurement results are listed for comparison. It is quite clear from this table that 

the measurement results are quite close to the target specifications. The measured bandpass of 

each of the 16 filters was quite narrow (12–18 nm), as shown in figure 14 and listed in table 1. 

The measured transmittance of each of the filter in the array, as shown in figure 17, was far 

below the predicted value of 90% shown in figure 10 even though it met the target specification. 

One of the main reasons for this reduced value is the uneven thickness of the thin films grown 

for the multilayer dielectric mirror and another reason is the lower resolution of the measurement 

system shown in figure 15 due to the short length (1/8 m) of the monochromator used, which can 

be accounted for by analyzing the results for calibrated interference filters shown in figure 16. 

The vendor specified value for 1460 nm filter is 70% while the measured value shown in figure 

16 is around 50%. When this is taken into account, the effective transmission should be at least 

40% more than the values shown in figure 17. Another reason for getting lower value of 

transmittance in the measurement is due to not applying antireflection coating on the filter array. 

These problems will be fully addressed in near future to improve optical transmission of filters. 

Table 1. Specifications of F-P filters. 

Parameter Target 

Specification 

Measurement Results 

Spectral region (nm) 1500–1800 1487–1769 

Width of spectral 

region(nm) 

300 282 

Peak to peak separation 

(nm) for 16 channels 

20 16–22 

FWHM for each channel 

(nm) 

10–15 12–18 

FP1 -18, FP2 - 17, FP3 - 17, FP4 - 18, FP5 - 13, FP6-13, 

FP7 - 13, FP8 - 13, FP9 - 12, FP10 - 12, FP11 - 13, 

FP12 - 14, FP13 - 13, FP14 - 14, FP15 - 15, FP16 - 16 

Filter peak wavelength for 

each channel (nm) 

 FP1-1487, FP2 - 1504, FP3 - 1522, FP4 - 1540, FP5 - 1562, 

FP6 - 1582, FP7 - 1601, FP8 - 1623, FP9 - 1641, 

FP10 - 1662, FP11 - 1681, FP12 - 1699, FP13 - 1717, 

FP14 - 1736, FP15 - 1753, FP16 - 1769 

 Peak transmittance (%) >40 50–90 (corrected) 

The imager shown in figure 18 was used to image a number of indoor and outdoor scenes. Some 

examples of images obtained from this imager are shown in figures 19 and 20. It is clear from 

these images that each spectral channel operated independently without noticeable out-of-band 

leakage and the dark pixels between the sub images are due to the baffle. The sub images seem 

to be slightly tilted due to misaligned placement of the filter array on the FPA and is quite easy 

to fix. 
16 

Figure 19. Sixteen spectral images of the object on the left obtained with the snapshot imager. 

Figure 20. Sixteen spectral images of the plant on the left obtained with the snapshot imager. 

A fiber optic bundle was used as a variable wavelength target to demonstrate the spectral 

performance of the multispectral camera system. Figure 21 shows the image of the fiber bundle 

holder. The fiber bundle was illuminated from 1400 to 1900 nm in 5-nm increments. Screen 

captures of the video that show the light cycling from filter #1 position to filter #16 were created 

and pasted in figures 22–37. Each image has 16 images of the fiber-optic bundle and holder and 

only the proper wavelength corresponding to the filter passband lights up. 

Figure 21. Cropped view of a fiber optic bundle holder. 

Cropped view of the fiber bundle

fiber bundle
17 

Figure 22. Fiber at filter # 1, 1487 nm.

Figure 23. Fiber at filter # 2, 1504 nm.

Figure 24. Fiber at filter # 3, 1522 nm.

Figure 25. Fiber at filter # 4, 1540 nm. 
18 

Figure 26. Fiber at filter # 5, 1562 nm.

Figure 27. Fiber at filter # 6, 1582 nm.

Figure 28. Fiber at filter # 7, 1601 nm.

Figure 29. Fiber at filter # 8, 1623 nm. 
19 

Figure 30. Fiber at filter # 9, 1641 nm.

Figure 31. Fiber at filter # 10, 1662 nm.

Figure 32. Fiber at filter # 11, 1681 nm.

Figure 33. Fiber at filter # 12, 1699 nm.
20 

Figure 34. Fiber at filter # 13, 1717 nm. 

Figure 35. Fiber at filter # 14, 1736 nm.

Figure 36. Fiber at filter # 15, 1753 nm.

Figure 37. Fiber at filter # 16, 1779 nm. 
21 

This experiment clearly validated that the multispectral imager is working over the entire range 

of wavelength of operation from 1487 to 1779 nm. 

7. Summary and Conclusions 

A miniature multispectral snapshot imager with 16 spectral channels operating in the SWIR 

region (1.5–1.8 m) was developed using a fabricated F-P etalon based filter array, custom 

designed optical frontend, and a commercial SWIR camera with an InGaAs FPA. The filter array 

was fabricated using MEMS shadow mask technology using only 4 masks to grow 16 different 

cavity thicknesses for the filter array. The filter array met the target specifications and in future 

even greater performance can be obtained by making some minor changes in the fabrication 

process. A 16-channel optical frontend assembly that replaced the camera lens was fabricated to 

obtain independent spectral image from each channel. The present frontend optics uses machined 

commercial lenses with honeycomb baffles and light stops to ensure telecentric light propagation 

and minimize out of band spillage on each of the 16 spectral channels. In future, monolithic 

microlens arrays will be used in the optical assembly, which will reduce the cost further and the 

optics will become more ruggedized. The filter and frontend optics designs are quite flexible and 

can be modified to meet a variety of operational requirements and mission needs. Many 

different scenes were imaged and high quality spectral images were obtained. The snapshot 

imager will be used to carry out chemical analysis in the near future. 

The snapshot multispectral imager design described here is extremely versatile and adaptable to 

meet the requirements of different optical bands, FPAs, camera hardware, and bandpasses. The 

MEMS-based fabrication process is quite easy to implement and cost effective and can be 

tailored to design a number of custom snapshot imagers for a variety of applications. Automating 

the image analysis would greatly enhance the performance of the multispectral imager. 

 
22 

8. References 

1. Hinnrichs, M; Morris, G. M. Image multispectral sensing, U.S. patent 5479258, 1995. 

2. Volin, C. E.; Garcia, J. P.; Dereniak, E. L.; Descour, M. R.; Hamilton, T.; McMillan, R. 

Midwave-Infrared Snapshot Imaging Spectrometer. Appl. Opt. 2001, 40, 4501–4506. 

3. Mathews, S. A. Design and Fabrication of a Low-cost, Multispectral Imaging System. Appl. 

Opt. 2008, 47, F71–F76. 

4. Gao, L.; Kester, R. T.; Tkaczyk, T. S. Compact Image Slicing Spectrometer (ISS) for 

Hyperspectral Fluorescence Microscopy. Opt. Express 2009, 17, 12293–12308. 

5. Kong, L.; Sprigle, S.; Yi, D.; Wang, C.; Wang, F.; Liub, F.; Wang, J.; Zhao, F. Detecting 

Early Stage Pressure Ulcer on Dark Skin Using Multi Spectral Imager. Proc. SPIE, vol. 

7494, 74940Q, 2009. 

6. Goodrich/Sensors Inc. Web site. http://www.sensorsinc.com/cameras.html (accessed July 

2010). 

7. Mir J.; Boysel, M. R. High Speed, Optically-Multiplexed, Hyperspectral Imagers and 

Methods Thereof, U.S. Patent Application 11/995362, 2008. 

8. Smith, W. J. Modern Optical Engineering, Chapter 8; McGraw-Hill Professional, 2000. 

9. Jayaraman; et al. WDM Multiplexer – Demultiplexer Using Fabry – Perot Filter Array. U.S. 

Patent 5835517 (1998). 

10. Minas, G.; Ribeiro, J. C.; Martins, J. S.; Wolffenbuttel, R. F.; Correia, J. H. An Array of 

Fabry – Perot Optical – Channels for Biological Fluids Analysis. Sensors and Actuators A: 

Physical 2004, 115, 362–367. 

11. Correia, J. H.; Bartek, M.; Wolffenbuttel, R. F. High-selectivity Single-chip Spectrometer for 

Operation at Visible Wavelengths. Electron Devices Meeting, 1998 – IEDM '98 : Technical 

Digest, International, 1988, 467–470. 

12. Guy, D.R.P.; Taylor, L. L.; Bass, S. J.; Apsley, N. High Quality InP/InGaAs Fabry-Perrot 

Etalons Grown by AP MOCVD. Semicon. Sci. Technol. 1987, 2, 466–467. 

13. Szipocs, R.; Ferencz, K. Chirped Multilayer Coatings for Broadband Dispersion Control in 

Femtosecond Lasers. Opt. Letts. 1994, 19, 201–203. 

14. Netterfield, R. P.; Schaeffer, R. C.; Sainty, W. G. Coating Fabry-Perot Interferometer Plates 

with Broadband Multilayer Dielectric Mirrors. App. Opt. 1980, 19, 3010–3017. 
23 

15. Chinnayelka, S.; Bessler, R. S.; McShane, M. J. Modeling and Fabrication of Mirrors used 

in Microspectrometers for Infrared Analysis of Biofluids. Proc. 2nd Joint EMBS/BMES 

Conference, 2002, 1658–1659. 

16. Software Spectra Inc. Web site http://www.sspectra.com/ (accessed March 2010). 

 
24 

List of Symbols, Abbreviations, and Acronyms 

ARL U.S. Army Research Laboratory 

F-P Fabry-Perot 

FPA focal plane array 

FWHM full width at half maximum 

IEDs improvised explosive devices 

ITC Infotonics Technology Center 

LWIR long-wave infrared 

MEMS microelectromechanical system 

SEM scanning electron microscope 

SiO2 silicon dioxide 

SWIR shortwave infrared 

UV ultraviolet 

 
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