Spectrometer apparatus

A spectrometer apparatus uses a spectrum resolving sensor containing an opto-electronic monolithic array of photosensitive elements, each preferably sub-millimeter in size and pitch, and a continuous variable optical filter, that is permanently aligned with the array. Polychromatic light passing through the variable filter is spectrally resolved in accordance with the local filter transmission function, and is incident upon the photosensitive elements in the array. The electrical output of each element in the array is thence a function of the local transmission function of the variable filter and the output of all the elements provides the spectral contents of the polychromatic light. High spectral resolving power is obtained by subtraction of the output signal of adjacent elements in the array.

FIELD OF THE INVENTION 
This invention relates to opto-electronic spectral measurement devices and, 
more particularly, relates to a spectrometer apparatus for imaging and for 
electromagnetic radiation analysis application. 
BACKGROUND 
Spectrometer apparatus are known and used for measuring and analyzing the 
spectral, or "color", contents of electromagnetic energy in the frequency 
range or spectrum of optical wavelengths, defined herein as being from 
ultra violet, visible, through infra red, that portion of the 
electromagnetic radiation which produces photo-electric effects, 
collectively referred to herein as "light." Those kinds of opto-electronic 
apparatus are used for both imaging application, as by inspecting the 
spectral reflectance characteristics of a two-dimensional object, and for 
non-imaging applications, as by analyzing the spectral emissions of 
thermal radiation by a material subjected to intense heat. The apparatus 
are also referred to by many different names depending upon the 
application to which they are put in part and upon the attached 
accessories, such as spectrograph, spectroscope, spectroreflectometer, 
spectrophotometer, spectrofluorometer, spectrobolometer and the like, 
including those others identified hereinafter. For present purposes such 
apparatus are collectively referred to as "spectrometer apparatus". Such 
spectrometer apparatus are considered in some detail next, beginning with 
non-imaging techniques. 
Spectrometric measurements of optical radiation are performed basically in 
two ways; using a dispersing (refracting or a diffracting) element, and by 
using a filter-based device. Typical resolving powers from 1,000 to 
50,000, and 5,000 to 500,000 are obtained using a prism or diffraction 
grating, respectively. Resolving power of a hundred to several thousands 
are obtained with a filter. A third method, using an interferometer, with 
resolving powers from 10,000 to 5,000,000 is reserved for a very high 
spectral resolving power applications, and is of no interest as background 
to the present invention. 
In the dispersion based approach to spectral measurement, a radiation 
dispersion device is used to separate the incident polychromatic light 
into its spectral contents. The spectrally separated light is then 
projected onto a photodetector to measure the relative intensity in each 
spectral range. The dispersion device may be a prism or diffraction 
grating; in either case the spectrally separated light moves in a 
diverging or spreading beam. After traveling inside an enclosure over a 
sufficient distance the spectral bands are adequately separated, 
spatially. The bands of light are then directed, with reflective optics, 
at the detection device. A single photodetector, such as a solid state 
detector or a photomultiplier tube, depending on the spectral region and 
the intensity, may be used to measure the intensity of one narrow spectral 
band of the incident light. The dispersion system is then rotated slightly 
by a mechanical device to a new position in order to direct other narrow 
bands of the light onto the photodetector and the next intensity 
measurement is taken. In this sequential manner it is possible to scan and 
measure the complete spectrum of the light bands that the spectrometer is 
capable of dispersing. 
Such an instrument, often referred to as a monochromator, is commercially 
available, but is large in weight and size. It requires a physically large 
enclosure for the diverging light to achieve adequate spatial separation 
of the light spectra. In addition the instrument is delicate, 
inappropriate for use in harsh environments, and easily falls out of 
alignment. That instrument, moreover, cannot provide simultaneous 
measurement of all the spectral contents in the received light. 
To overcome the latter limitation, a polychromater is commonly used. In 
this apparatus the dispersed light may be directed onto a linear array of 
photodetectors. The array is positioned such that each individual 
photodetector in the array measures a different band of the dispersed 
light being received. For similar reasons to those mentioned earlier in 
regards to the monochromater, such a polychromater is still large in 
weight and size, and is too delicate for a harsh field environment. 
Furthermore, none of these instruments has a selectable spectral 
resolution or band-width. In order to change the instrument's resolution, 
the dispersion element has to be changed. 
For those less skilled in the art, details of the construction of 
non-imaging spectrometers, spectrographs, spectroradiometers and 
spectroscopes are presented by Philip N. Slater, in "Remote Sensing; 
Optics And Optical Systems", published by Addison-Wesley Publishing 
Company, 1980, Chapter 7; and by H. S. Chen in "Space Remote Sensing 
Systems", published by Academic Press, 1985, Chapter 5, to which the 
interested reader may make reference. 
A purpose of the present invention, which employs a photodetector array, is 
to overcome the difficulties associated with the size, weight, and 
limitation on spectral accuracy in detection due to mechanical motion, the 
band-width selection, and the delicate structure of the previously 
described dispersion-based instruments. 
The second approach for measuring the spectral contents of the light 
utilizes optical filters and photodetectors. A single band-pass filter may 
be placed over a detector to measure a single spectral band of the 
incident light. In the "poor man's" spectrometer, a multiplicity of 
band-pass filters, each of which is used in conjunction with one of a 
multiplicity of detectors, is used to form a multi-channel instrument. 
Such a device may be used to simultaneously measure several spectral bands 
of light, such as in the devices presented in U.S. Pat. Nos. 3,973,118 and 
3,737,239. Two or three-channel versions of this concept are commonly used 
in the so-called 2-or 3-color pyrometers for non-contact temperature 
measurements. 
Of the preceding two patents, the method used by LaMontagne in U.S. Pat. 
No. 3,973,118 measures simultaneous and discrete spectral bands by the use 
of several detector and spectral band-pass filter combinations, packaged 
in a single housing. Although in theory, such approach may be expanded to 
a very large number of detectors, the discrete photodetectors in 
LaMontagne's device cannot be positioned in the housing with the spatial 
precision required for imaging applications or for very accurate 
spectroscopic measurements, those requiring resolution on the order of but 
a few Angstroms in wavelength. Further, since no known set of discrete 
optical filters of the type presented in LaMontagne are available, in 
which each narrow band-pass filter is slightly spectrally shifted as 
compared to the adjacent filter, the technique is limited for use in low 
resolution spectral measurements and to non-contiguous bands only. 
In imaging applications, the spatial and the spectral resolution require 
the use of very small individual detectors, which are positioned very 
close to each other in great accuracy. Therefore, for similar reasons it 
would not appear possible to make an acceptable imaging array with 
LaMontagne's method; a large scale integration, LSI, technology must be 
applied for adequate imaging performance. Yet another severe limitation of 
this method is that the band-pass of each detector is determined by and 
cannot be better than the band-pass of the filter. Moreover, on a more 
practical note, since each photodetector in the LaMontagne device is 
connected to an associated external connector pin, a spectrometer device 
with a very large number of pins, for example, in the thousands or 
millions, as would be required for high resolution, requires an electrical 
connector that is completely impractical. Further, since all 
photodetectors are connected to a common lead, the capacitance of the 
system with many detectors would be too large to be practical. 
The present invention also employs an optical filter. However, as becomes 
apparent, the limitations characteristic of LaMontangne's device are 
eliminated in the present invention. 
Other variations on the described filter-based technique are common. A 
filter-wheel, on which several filters are mounted, is used in conjunction 
with a single photodetector or several photodetectors. The wheel is 
mechanically rotated to position one filter at a time above the 
photodetector to provide non-simultaneous, sequential, spectral 
measurements. Examples of prior knowledge of this form of spectrometer 
apparatus are presented in U.S. Pat. Nos. 4,477,190; 4,291,985; 4,082,464; 
3,963,351; and 3,877,812. In yet another variation, the discrete filters 
in the disk are replaced with a continuous circular variable filter, 
"CVF", which is placed over a detector. Continuous or circular variable 
filters are addressed in the literature: "Circular variable Filters", Yen, 
Optical Spectra Magazine, 1982; "Have You Considered Using Variable Band 
Pass Filters", Laser Focus World, September 1989; Circular Variable 
Filter, Illsley et. al., U.S. Pat. No. 3,442,572. The CVF may be rotated 
to allow the measurement of a continuous, but not simultaneous, spectrum 
of light. Alternatively the CVF may be placed over several detectors to 
provide simultaneous spectra in a limited number of bands, as presented, 
for example, in U.S. Pat. Nos. 4,657,398; 3,929,398; 3,811,781; and 
3,794,425. Finally, a limited number of spectral bands may be obtained by 
a combination of beam splitters and dichroic filters. This technique is 
also limited, for practical reasons, to a few bands only. The present 
invention as is shown hereafter has the spectral measurement capability of 
hundreds of contiguous bands of light. 
Imaging type spectrometers are next considered as additional background. An 
imaging device which uses a charge coupled device, CCD array, is described 
by Goetz and Landauer in U.S. Pat. No. 4,134,683. In the Goetz and 
Landauer invention, four such CCD arrays are used, each in conjunction 
with a corresponding band-pass filter, to give four overlapping spectral 
images of the same scene in a method similar to that used in some 
broadcast television cameras. The difficulty with that approach to 
spectral imaging, stems from the need to precisely align all four detector 
arrays with the scenery being viewed. Even if the alignment is correctly 
accomplished, however, only a four color resolution is obtained. An 
extension of this technique to additional colors appears impractical 
because of the alignment difficulties of multiple arrays and the 
impracticality of simultaneously handling the great amount of data output 
of dozens or more such arrays to obtain a better spectral resolution. An 
aspect to the present invention includes the adaption of CCD devices 
within a versatile high resolution imaging spectrometer, one that is not 
limited to four color resolution. 
Inventions presented in U.S. Pat. No. 4,764,670 and U.S. Pat. No. 4,081,277 
each teach how to make solid state imaging devices by depositing a filter 
array over a multi-sensor device. Both techniques discuss the use of the 
three primary color filters, red, green and blue. Although presently 
marketed color video technology utilizes such an approach, it is also 
limited in the spectral resolution, primarily due to the broadband filters 
used. It is deficient in the spatial resolution, since it takes at least 
three photosensitive elements to cover a single point, each at one of the 
primary colors. Details of imaging systems are given by P. N. Slater and 
H. S. Chen in the reference cited previously. Embodiments of the present 
invention provide an essentially "infinite" spectral resolution at no loss 
of spatial resolution. 
Space borne applications for spectrometer apparatus in both imaging and 
non-imaging application have been extensive. For example, a 12-channel 
prism, and a 9-channel grating spectrometers were constructed for 
space-borne sensing of terrestrial resources. A 13-band multispectral 
scanner, flown on the Skylab, measured spectral bands from 20 to 100 nm 
wide in the range between 410 nm and 2350 nm. 
The LANDSAT-D satellite used two scanning-type instruments, the Thematic 
Mapper, .TM., and the Multi-Spectral Scanner, MSS. The latter sensor used 
four channels: a 500 to 600 nm, green; 600 to 700 nm, yellow; 700 to 800 
nm, red and near infrared; and 800 to 1100 nm, near IR. The Coastal Zone 
Color Scanner, CZCS, flown on the Nimbus satellite used a grating 
spectrometer and five visible-near IR channels with a spectral band of 
about 20 nm in the visible, centered at 443 nm, blue; 520 nm, green; 550 
nm, yellow; and 670 nm, red; a 100 nm band in the near IR centered at 750 
nm, and a 2 micrometer band in the far IR centered at 11.5 micrometer. A 
dichroic beam splitter was used to separate the far IR radiation band from 
the visible radiation band. In a scanning system, a moving mirror is used 
to scan different parts of the scene across the array of detectors in 
order to get multispectral images, each detector operates at a different 
wave-band. 
A 14-channel radiometer, using detector/filter combinations was used on the 
Earth Radiation Budget, ERB, sensor flown on the Nimbus 7 satellite. The 
Solar Backscatter Ultraviolet, SBUV, used a moving grating spectrometer to 
monitor 12 selected narrow wavelength bands, and a filter photometer to 
measure a fixed band. The Total Ozone Mapping Spectrometer, TOMS, measured 
six discrete wavelength with 1 nm band. Both instruments have also flown 
on the Nimbus satellite. 
The French SPOT satellite employs two High Resolution Visible, HRV, imaging 
sensors. The multispectral sensor uses CCD arrays with filter based 
spectral bands centered at 550 nm, green; 650 nm, red; and 840 nm, near 
IR, all with about 80 nm band-pass. The panchromatic CCD has a band-pass 
from 500 to 900 nm. NASA's multispectral Linear Array, MLA, uses four 
fixed band CCD channels with band-pass from 460 to 470 nm, 560 to 580 nm, 
660 to 680 nm, 870 to 890 nm, and two near IR CCDs with fixed bands from 
1230 to 1250 nm, and 1540 to 1560 nm. 
The Airborne Visible Infrared Imaging Spectrometer, AVIRIS, placed in 
service by NASA in 1987, is one of the most advanced imaging spectrometers 
uses 244 bands with about 9.6 nm bandwidth. Finally, the new generation 
imaging spectrometers scheduled to be constructed and flown by NASA 
on-board the Space Station in the late 1990s are the High Resolution 
Imaging Spectrometer, HIRIS, and the Moderate-Resolution Imaging 
Spectrometer, MODIS. Both instruments use area arrays to obtain spectrally 
resolved images of a one-dimensional scene. The spectral resolution, 
though, is achieved with a diffraction grating with a 10 nm band-width. 
Further particulars of various space-borne instruments are described in a 
Report to the Congress titled Space-Based Remote Sensing of the Earth, 
prepared by NOAA and NASA in 1987; in the Nimbus 7 Users' Guide, published 
by NASA August 1978; in Remote Sensing of the Environment, by J. Lintz and 
D. S. Simonett, published by Addison-Wesley, 1976; in the Earth Observing 
System--Instrument Panel Report, volumes IIb, MODIS, and IIc, HIRIS, 
published by NASA in 1987, and in the two references cited earlier. 
The foregoing imaging devices are limited to a few spectral bands, often 
provide non-simultaneous spectra, and, depending on the application, 
require a scanning system, which is less reliable and less accurate than a 
system with non-moving parts. The spectral resolution of these devices is 
low and is fixed by the hardware design and cannot be changed in 
operation, thus limiting versatility. As it is shown herein the present 
invention is believed to alleviate the aforementioned deficiencies, 
through elimination of all moving parts, and increase the operational 
capabilities of the measuring methods discussed above through software 
control of the spectral resolution. 
An object of the present invention, therefore, is to provide an improved 
spectrometer apparatus for use in any of the applications described in the 
foregoing section that is small, lightweight, rugged and permanently 
aligned, inexpensive, solid state device of versatile application, 
suitable for manufacture at least in part using LSI technology so that the 
size and weight can be at least one thousand times smaller than such 
conventional spectrometer apparatus. 
An additional object is to provide spectrometer apparatus having enhanced 
spectral and spatial resolution than previously existing filter types 
without use of dispersion devices and which avoids the need for alignment 
adjustments in use. 
A further object of the invention is to provide a simplified method for 
measuring simultaneously the contiguous spectra of a polychromatic optical 
radiation by use of a solid-state array and filter combination without the 
use of moving parts with the arrays being line type or area type in 
alternative embodiments. Such spectral measurements may be provided in a 
narrow-band pass mode, a wide-band pass, a long-wave pass, or a short-wave 
pass modes and for various portions of the optical spectrum; the 
ultra-violet, the visible, or the infra-red. 
A still further object of the invention is to provide spectrometer 
apparatus that has a selectable spectral resolution from very fine, 
several Angstroms, to coarse, several nanometers, by means of an 
associated signal processing technique. 
And, it is an additional object of the invention to provide like benefits 
in spectrometer apparatus, regardless of whether used for non-imaging 
applications or in imaging applications. 
SUMMARY OF THE INVENTION 
Spectrally resolved measurements of polychromatic light are achieved with 
an array of photosensitive elements, such as photodetectors or "pixels", 
and a continuous linear variable filter, LVF, mounted in a single housing. 
The variable filter may be formed in alternative embodiments by optical 
coatings deposited directly onto the photodetector array. Fine resolution 
is achieved with any of an edge-type, such as a long-wave pass, a 
short-wave pass, or a band-pass LVF in the described combination. In 
accordance with the foregoing objects, an improved spectrometer apparatus 
according to the invention contains a continuous linear variable 
wavelength filter fixed in overlying relationship with a photodetector 
array with the photosensitive sites in the array, referred to as pixels, 
being formed on a single monolithic substrate, as a one piece assembly. 
The filter extends along the length of the array, parallel to the array's 
axis, so that each pixel is located behind a different portion of the 
filter, whereby each pixel is exposed to a different spectral band of the 
polychromatic light incident upon the filter corresponding to the spatial 
position of that pixel along the length of the filter. Each pixel, 
therefore, responds to a different spectral content of an incident beam of 
polychromatic light. 
With the monolithic substrate the photosensitive sites, elements, or 
pixels, as variously termed, may be formed in a small size and be packed 
close together with accurate spacing therebetween. Suitably, a housing 
serves to support at least the filter and the array in a unitary sensor 
assembly. Electronic means interrogate or, as variously termed, scan or 
poll, the individual pixels in the array and thereafter process and/or 
store the information thereby obtained for contemporaneous or future 
readout or display. In a specific embodiment, some of the electronic means 
referred to is formed upon the same substrate as the photosensitive array, 
allowing for a spectrometer on a chip, so to speak, that may be connected 
to an external display. In addition to an external display, such as a 
video monitor or a conventional cathode ray tube device, in other 
embodiments, the display may be of liquid crystal type, LCD, and may be 
carried by the same housing as the sensor portion. 
Suitably the electronic means includes conventional processing means for 
receiving outputs, often referred to as "video" signal, representing 
individual pixels in the array as by scanning or polling and the like. In 
still additional embodiments the processing means may include self 
scanning circuitry associated with the array to provide appropriate input 
to circuits or buffers in other parts of the processing means. The 
electronic means may include a microprocessor for further data processing, 
for controlling the array scanning rate and frequency and for general 
interfacing with other apparatus. 
In accordance with a more specific aspect of the invention, the arrays used 
in the invention are either of the parallel output type or of the various 
charge storage or transfer, photon integration, type, which are 
customarily referred to as Charge Coupled Device, CCD, Charge Injection 
Device, CID, Charge Coupled PhotoDiode Array, CCPD, Self-Scanned Photo 
Diode Array, SSPD, or several other like types. These devices include a 
monolithic or hybrid integrated circuit which contain the electronics for 
sequentially scanning and reading the signal of each pixel in the array, 
and are manufactured utilizing LSI technology. 
In a still additional aspect to the invention, the linear variable filter 
within the spectrometer is an edge type of the short-wave pass type or of 
the long-wave pass type or, alternatively, is of the band-pass type; and 
the electronic means includes means, such as an algorithm means, to 
subtract the output of one pixel in the array from that of a preceding 
pixel to derive information as to spectral intensity in the band defined 
by the two pixels physical displacement along the filter. As shown 
hereinafter, with a typical pixel dimensions of several micrometers, as an 
example, a resolution of a few Angstrom is obtained by such subtraction. 
The necessity of a band-pass type filter of such a fine resolution, even 
if possible of construction, is avoided. 
The spectrometer apparatus thus utilizes an array of photodetectors in 
conjunction with a linear variable optical filter as a sensor, and is 
capable of resolving and/or measuring simultaneously a contiguous spectrum 
of polychromatic light incident upon the sensor. The improved spectrometer 
apparatus can be utilized for non-imaging as well as imaging applications 
and in scanning or non-scanning systems. 
With the invention substantial simplification of the instrumentation 
previously required for spectral measurements of polychromatic light is 
obtained since the dispersion element (refracting or diffracting) is not 
needed and the long path for spatial separation is avoided. Not only is 
the weight and size of a spectrometer reduced by the miniaturization 
provided by this invention, but so is its cost. Size and weight are often 
very important considerations, in particular with portable or space-borne 
instruments. Cost is reduced because all the mechanical elements of a 
spectrometer are eliminated and the number of components in the assembly 
of the present invention is minimal. The invention, therefore, opens the 
door for applications in industry and science which were not practical in 
the past due to the equipment limitations. 
The disclosed invention permits construction of a ruggedized spectrometer 
instrument which is fabricated with permanent alignment between the 
variable filter and the photosensitive elements and can operate under a 
harsh field environment. With the invention a continuous spectra over a 
wide band of light may be measured, by using arrays containing anywhere 
from a few, to several thousands or millions of pixels. The spectral 
resolution may also be adjusted by the proper selection of the array and 
the filter's spectral spread. This is believed to be a major improvement 
over the use of discrete filters which can measure only a few discrete 
bands of radiation. 
With proper care, such as by cooling the array, the apparatus operates at a 
very low signal-to-noise ratios, allowing the detection of very low 
intensity radiation, a decided advantage. Further, the signal integration 
time may be electronically controlled for either very short durations, 
milliseconds, when the incident light is high, or very long durations, on 
the order of minutes or even hours when the light is faint. Another 
advantage of the invention is the very wide dynamic range made possible by 
an array-based spectroscopic instrument. The spectrometer is also immune 
to "light shock", i.e., it rapidly recovers from over-exposure. 
In still additional embodiments of the invention operation at extremely low 
light levels is possible. For such applications, the linear variable 
filter component is mounted over a conventional image intensifier, such as 
a second or third generation microchannel plate type intensifier, which in 
turn is mounted over the photo sensor array. The image intensifier, as is 
conventional, contains a photocathode and a phosphor screen on its input 
and output sides, respectively, amplifies light incident on the input, 
some 10,000 to 20,000 times, and provides the amplified light as a 
brighter image appearing on the output screen, the latter of which 
supplies the light output to the photodetectors. 
High spectral resolution, not possible with the conventional three primary 
colors, red, green, and blue, or similar schemes, is achieved by the 
invention. The signal processing technique, implemented in the associated 
electronic circuits, allows this high resolution, as little as several 
Angstroms in wavelength, to be obtained by use of either a long-wave pass 
or a short-wave pass variable filters. It does not require a band-pass 
filter. 
The high degree of geometrical accuracy of the array improves the spectral 
and spatial accuracy compared to that of mechanical scanning devices. 
Further, the fine spatial separation of the pixels, a few micrometers, 
provides the fine spectral resolution obtained by this technique. The 
spectrum resolving sensor, may even be fabricated on a curved surface to 
meet the needs of a large focal plane array, FPA. 
The spectral resolution and the band-pass may be selected by the user by 
means of the associated signal processing and data handling elements, a 
further advantage. Signal processing can also be used for calibration, 
pixel spectral response correction, pixel uniformity corrections, 
background signal suppression, dark current suppression, time delay 
integration, and various other signal enhancement techniques. Random 
access capability is available; it allows one to sample any pixel of 
interest while eliminating the need to process a large amount of unwanted 
data, permitting enhanced versatility. 
Although the measurement of the spectrum of electromagnetic radiation has 
been practiced for years, none of the practiced techniques suggest or 
imply the use of the detector's geometry in conjunction with a variable 
filter to effect the band-width of the measured readiation. In view of the 
complexity of the prior art spectrometric systems, the simplicity of the 
invention is believed to provide new, unexpected and superior results. The 
invention permits the elimination of the prism or the diffraction grating 
in lieu of the single solid state device reducing the size and weight of 
prior art spectrometer apparatus by more than a 1,000 times. Further, the 
use of a long-wave or short-wave pass variable filter in conjunction with 
the detector array and the described processing procedure, to obtain a 
fine band-pass resolution, provides a result not heretofore taught or 
suggested. 
The foregoing and additional objects and advantages of the invention 
together with the structure characteristic thereof, which was summarized 
in the foregoing passages, becomes more apparent to those skilled in the 
art upon reading the detailed description of a preferred embodiment, which 
follows in this specification, taken together with the illustration 
thereof presented in the accompanying drawings.

DETAILED DESCRIPTION 
Referring to FIGS. 1a, 1b and 1c, a spectrum resolving sensor assembly 10 
is illustrated in side section view, top plan view and right side view, 
respectively. The assembly includes a monolithic area array, 12, such as, 
for example, an NEC PD 35400 device, containing 728.times.493 
photosensitive sites, pixels, 12.3 micrometers.times.13.5 micrometers each 
in dimension, which is housed in a typical electronic package, 14; and a 
linear variable filter, "LVF", 16, mounted overlying and covering the 
photosensors. The linear variable filter is illustrated partially cut-away 
to reveal the underlying pixels 18. The LVF is positioned such that the 
spectral transmission varies along an axis indicated by the arrow. The 
spectral transmission characteristic does not change in the lateral 
direction and depends only on the axial position. In this embodiment, the 
package contains ledges 13, on either side of the channel in which the 
photosensor array is positioned, to which the linear variable filter is 
cemented in a position slightly elevated, spaced from the photosensors. In 
another embodiment, described elsewhere herein, the filter is coated 
directly over the sensor 12. 
When spectrum resolving sensor 10 is illuminated with a polychromatic 
light, each lateral row of pixels, 18, is exposed to a different spectral 
band of this light due to the continuously variable transmission 
characteristic of the filter. The flow of electronic commands into, and 
data from the sensor is accomplished via the contact pins, 15, in the 
conventional manner as is later discussed in greater detail. 
To complete the spectrometer apparatus, the sensor assembly is installed on 
a printed circuit board containing the appropriate electrical connector 
sockets to contact and seat the pins 15, which is conventional. The 
printed circuit board may also support the appropriate electronics later 
herein described, which is also of conventional circuitry. 
An alternative form of sensor contains a line of photosensitive sites to 
form a line array, in contrast to the two dimensional area array earlier 
described. This alternative form is presented in FIG. 2A in top view in 
which for convenience like sensor elements are denominated by the same 
numbers as in the prior figures and in which corresponding elements are 
given the same number as the counterpart in the prior figures, but primed. 
As illustrated, the large number of photosensitive elements or sites, 17, 
are presented not to scale, form a straight line or axis. Those 
photosensitive sites are covered by linear variable optical filter 16, the 
latter of which is partially cut away in the figure. By way of example the 
array may be of the type marketed by the EG&G Company in the EG&G SB 
series devices. The side and section view of this alternative are the same 
in all essential respects as that of FIGS. 1A and 1C and thus do not 
require separate illustration. 
As shown to an enlarged scale in FIG. 2B the spacing between the one edge 
of the pixel, or photosensitive element, to the corresponding position of 
the next, X.sub.c--c, is about 25 micrometers the width of the element, 
x.sub.p, is 15 micrometers, the heighth, a, may be from 25 to 2500 
micrometers. These dimensions are typical of line CCD arrays. While the 
number of pixels in the array may be as small as 64, 128, or 256, it is 
preferably greater than 512 or 1024. In an area array, as was graphically 
depicted in FIG. 1, the element may be square or rectangular, with a few 
micrometers spacing or no spacing at all, such as in the "Kodak Megaplus" 
camera, marketed by The Kodak Company, which contains a 1340.times.1037 
square pixels, 6.8 .mu.m on a side, with zero spacing between. 
The mode of operation of the form of spectrometer apparatus containing the 
sensor of FIG. 2A is the same as that described for FIG. 1, the 
electronics being less complex as there are fewer pixels to poll. The 
sensor of FIG. 2A is also mounted on a printed circuit board containing 
the appropriate electrical connections for pins 15' and that printed 
circuit board may also contain the conventional electronic elements to 
poll the pixels and process the spectral information derived from each 
pixel. 
A linear variable filter 16 is shown to an enlarged scale pictorially in 
FIG. 3. The filter is made of a base or substrate, 22, of relatively 
rectangular shape and of a material which is selected on the basis of the 
desired range of wavelengths in which the variable filter operates. 
Typical materials for ultraviolet, visible, and near infra-red apparatus 
are fused silica, vicor, or germanium. The substrate is covered with 
variable thickness coatings, 24, formed into a wedge shape as illustrated, 
of a material whose composition also depends on the desired spectral range 
of operation. Metal-dielectric are typical coating materials. Although the 
preferred embodiment uses a separate element, the coating material may be 
deposited directly onto the surface of the array, resulting in a 
monolithic assembly as discussed elsewhere herein. 
Continuously variable wavelength filters in a circular pattern have been 
available for many years. The method of manufacturing such circular 
filters is described in U.S. Pat. No. 3,442,572. The same technology has 
been commonly used to produce a linear variable filter, LVF. Details on 
the construction and coating processes may be found in "Optical Thin 
Films; Users Handbook", by J. D. Rancourt, published by McGraw Hill in 
1987. 
In recent years CCD devices have been replacing the traditional vidicon 
tube in video cameras. Recent advances have produced such devices with 
enhanced sensitivity in the ultra-violet, 200 to 400 nm, using Si, and in 
the infra-red range using PtSi, HgCdTe or other materials. Enhanced UV 
sensitivity can also be obtained by coating a Si array with various 
phosphor-based materials, or using a thinned backlighted Si array. All 
such devices are commercially available, are described in the 
manufacturers' catalogs, and are well known to those skilled in the art. 
For instance, the complete theory of operations, circuit diagrams, and 
application notes can be found in the "Image Sensing Products" catalog by 
EG&G Reticon, and in the "CCD Sensors, Systems & Developmental Technology" 
catalog by Fairchild Weston. Most CCD light sensitive devices produce a 
video-compatible electrical output signal. 
A line-array based sensor assembly 10 is shown, symbolically, installed in 
a hand held camera case, 25 in the partially exploded view of FIG. 4 to 
which reference is made. A circuit board 26, only partially illustrated, 
contains the other electronic elements, partially illustrated, and 
batteries, not illustrated, and is housed in the case. Through the circuit 
board the pins 15 connect the sensor array in electrical circuit with the 
other electrical components in the circuit board to permit polling or 
"read-out" of the photodetector array. The image transfer optics, or a 
lens system, 27, and a shutter, 28, are attached to the camera in a manner 
as is done with conventional cameras. The lens system used with the array 
may be of the anamorphic type, such as a cylindrical lens, in order to 
transfer a point source of light presented at the front of the lens into a 
line image to the lens' rear, as described hereafter. A start switch 29 
closes the appropriate electrical circuit and allows the operator to take 
the measurement. Cable 23 connects the unit to a video monitor for 
display, not illustrated, externally located. 
In the embodiment of FIG. 4 exposure control is accomplished in one of 
several available methods. In a continuous scan mode, the exposure time is 
equal to the integration time of the array, and no shutter is required at 
all, although an iris with an automatic gain control may be used--a common 
technique used in all home video cameras. For a single exposure, a 
conventional mechanical shutter of the kind within still single lens 
reflex type cameras is used. An electronic shutter may be used instead of 
the mechanical one; one such electronic shutter operates by electrically 
polarizing a Liquid Crystal plate for blocking the passage of light. Other 
commercially available devices, for instance Varo Electronic Devices of 
Garland, Texas, and ITT Electro-Optical Products of Roanke, Va., use an 
electronic gated shutter, a technology well known to those skilled in the 
art. 
The complete spectroradiometric system is illustrated in block diagram form 
in FIG. 5. The fore-optics, 30, such as the lens 27 in the preceding 
figure, "conditions" the incident radiation, focuses the light onto 
spectrum resolving sensor 10'. The sensor, 10' controlled by the 
electronics, 32, picks up the signal and passes it for further analysis 
and display, 34. Mass storage media, 46, may be used to save the data for 
future use. 
The sensor readout electronics includes "on-board electronics" and a sensor 
interface. The details of the on-board electronics depends on the specific 
sensor selected. Several examples of sensors and interfaces are discussed. 
First, a parallel output photodiode array is used in which each pixel is 
connected to a pin in the electronic package. For instance, the EG&G 
Photon, PDA series contain 20, 32, 35 or 38 pixels, 4 mm, by 0.94 mm with 
1 mm center-to-center spacing, in a package. Each pixel, in this case, 
must be connected to a standard readout circuit, which is common and well 
known to those skilled in the art, for a parallel readout. In a second 
example, the PDA series array can be connected to a multiplexer, such as 
the EG&G Reticon, M series, parallel-in-serial-out device which allows 
reading the pixels sequentially. The foregoing is a well known technique. 
EG&G's published data sheets for that device describe how it may be used 
in a hybrid or non-hybrid application. In a third example, a Si CCD array 
is utilized as a monolithic package with the readout electronics. In this 
case integral parallel and serial registers are used to shift the 
electrical charge from the array, through an on-board preamplifier to a 
serial video output signal. The input to the array is typically a clock 
timer. In a fourth example, a PtSi, InSb, or HgCdTe infrared area array 
detector is packaged with a CCD multiplexer, such as the EG&G Reticon 
RA0128M, in a hybrid package. The readout is taken as before with a 
monolithic Si CCD. The foregoing is also a well known technique. 
Further, the CCD data readout may be of various architectures. For 
instance, a full frame may be transferred from the pixels to a masked set 
of pixels from which the data is then read out in the conventional manner. 
An inter-line transfer architecture allows each line of pixels to be 
transferred to a masked, adjacent line of pixels for subsequent readout. 
Random access allows reading of only selected zones of pixels out of the 
complete array. These and other architectures may be used for image 
control. 
Arrays of the types discussed herein are commonly manufactured by several 
companies, RCA, EG&G, Mitsubishi, Hughes, Kodak, Tektronix, Ford Aerospace 
and others. These devices are used in common instruments such as home 
video camera, fax machine, optical character reader, bill exchanger, 
thermal imaging cameras, and others. The electronic circuitry and readout 
techniques which is used in this invention and only symbolically 
illustrated, therefore, is well known to those skilled in the art, and 
need not be elaborated upon in detail herein. Any conventional electronic 
circuit and processing apparatus may be used to interrogate the sensor 
array and process the information thus obtained according to the 
directions herein given. 
As is evident from the various examples discussed above, a principal 
advantage of using the disclosed invention as compared with the prior art, 
is in eliminating the need for a large and delicate dispersion system, 
which requires often re-alignment. In the foregoing paragraphs the 
components and the assembly of the invention were discussed. In what 
follows the performance and operations of the invention are further 
discussed. 
When the linear CCD array is combined with a edge type or a band-pass LVF, 
each pixel is covered by a portion of the LVF with a different spectral 
transmission. The spectral transmission curves for a typical long-wave 
pass edge type LVF, and a band-pass LVF are shown in FIGS. 6 and 7. Such 
characteristics of filters are well known to those skilled in the art. The 
spectral map of several pixels, showing their location in the array and 
the corresponding spectral range with the overlying filter in place, are 
shown in FIGS. 8 and 9. For clarity, only every second pixel is shown in 
the figures. The band-pass filter has a 5 nm band width, and the pixels in 
these figures are about 15 micrometers wide with a 25 micrometer 
center-to-center spacing, similar to those shown in FIG. 2. A 
two-dimensional representation of the pixels' spectral response with a 
band-pass LVF is shown in FIG. 10. A typical aperture response of the CCD 
array of FIG. 2 is shown along the x-axis in the figure. The corresponding 
spectral response, .DELTA..lambda..sub.p, of two typical pixels, pixels 
number 2 and 4, is shown along the y-axis, .lambda.. The band-pass, 
.DELTA..lambda..sub.f, of the LVF relative to the pixels position is also 
plotted. 
Several examples of a spectrometer apparatus utilizing the spectrum 
resolving sensor in both imaging and non-imaging devices are discussed in 
the following paragraphs and are illustrated in the additional drawing 
figures. The optical settings are discussed first, and the electronics, 
common to all the examples, were discussed previously. The data processing 
techniques are discussed in greater detail following these examples. 
Thus, a non-imaging spectrometer embodiment is presented in FIGS. 11a and 
11b in top and side views, respectively. A line array sensor, 40, of the 
structure earlier described is used to measure the spectrum of a point 
source, 44, of light radiation. A anamorphic optical system, 42, is used 
to transfer the image of the source onto the line array. The advantage of 
anamorphic optics is that it has a different focal length, or 
magnification level in perpendicular planes to the optical axis, thus acts 
to focus the radiation in one plane but not in the transverse plane. The 
radiation source could be a polychromatic light, which has passed through 
a sample which absorbed certain spectral lines or bands, such as in 
transmission spectrometer, or may be the radiation emitted from high 
temperature substance such as in an Induction Coupled Plasma - Atomic 
Emission Spectroscopy, ICP-AES, instrument. 
In another system pictorially illustrated in FIG. 12 a collimated light 
beam originating from source 50 is incident upon line array sensor, 46. 
The collimation in this case is achieved by means of a parabolic 
reflector, 48, with a source, 50, at the focal point of the reflector. 
This arrangement would work well with a reflectometer to measure the 
spectral characteristic of light reflected from a sample, 49, to be 
studied. A spatial filter system, 45, is used to remove all reflected 
beams not normal to the sensor's surface. 
An imaging spectrometer is shown in still another embodiment in the partial 
symbolic illustration of FIGS. 13a and 13b, which are considered together, 
with the latter figure illustrating a section of the former figure, M, to 
an enlarged scale. In this combination an air-borne area array 10' 
sensor-based imaging spectrometer is used for ground survey. The direction 
in which the filter's spectral transmission changes is indicated in the 
figure. A strip of a scene, 51, observed by the imaging optics, 54, is 
imaged onto the area array 10'. An element, or ground pixel, "A" of the 
scene strip, 51, located to the left end of the scene strip is imaged 
across the sensor array, as at A', to provide a spectral map of that one 
ground element. Other elements of strip 51 are simultaneously imaged, as 
shown with the ground pixel "X" to the right end of ground strip 51 and 
its image X' on array 10'. In this way a single strip of the scene is 
spectrally imaged. As the air-borne platform moves, a new strip of the 
ground scene, 56, is imaged. The spatial resolution for the system is 
determined by the pixel size and the imaging optics 54, or the optical 
telescope, in use. The system does not use a scanner and therefore is 
commonly referred to as a "pushbroom" type system. In the foregoing 
embodiment, each ground pixel, A, was imaged across a row of pixels in the 
array. To improve the signal-to-noise ratio of the spectrometer system, 
each ground pixel, A, instead may be imaged across two, or more, rows of 
pixels, 58 and 59, as shown in the expanded partial view of FIG. 14. 
A functional block diagram for one embodiment of the imaging spectrometer 
in a pushbroom space-borne system is shown in FIG. 15. Four major 
subsystems, indicated by the dotted lines, are included: the fore-optics, 
80, the focal plane array, 86, the "frame grabber", 92, and the 
electronics and data processing, 100. Data storage and telemetry is 
provided, in this case, by the host platform, via interface 108 or the 
satellite. This is considered in greater detail. 
The ground area to be imaged is viewed by the optical system, 82, which may 
be a telescope. A mechanical shutter or electronic gating, 84, is used as 
described earlier in discussing FIG. 4. The spectrum resolving sensor, 88, 
is a monolithic area array of the design earlier described in detail, 
which contains the readout shift registers and a preamplifier, 90, 
on-chip. A microprocessor, 104, controls the image acquisition timing and 
data processing. The parallel and serial clock, 102, inputs to the sensor 
are provided via a sensor interface, 94, which also transfers back the 
video signal from the sensor. The image data are digitized, 96, and stored 
in an image buffer, 98. 
A "frame grabber" is a collective name for the electronics for interfacing 
and capturing a full set of output signals of a line or area array. These 
devices usually interface with a computer or may come in a stand-alone 
configuration, but may use the computer for further data processing and 
display. Frame grabbers for microcomputer applications are commercially 
available from many manufacturers, e.g., Data Translation of Marlboro, 
Mass., DataCube, Epix, and others, and are commonly in use for scientific 
imaging, LANDSAT image analysis, medical imaging and many other 
applications. Because of their wide use details of their operation are 
well known and are not discussed further. 
The raw signal is then processed to: a) subtract the dark current signal 
which is obtained from a previously collected calibration data and stored 
in RAM, b) correct for pixel non-uniformities, and c) correct for pixel 
spectral sensitivity. The correction vector to be applied to the image is 
stored in a look up table, LUT, in RAM, 110. Finally, the desired spectral 
resolution is selected and the analysis is performed by the subtraction of 
sequential pixel signal as explained in more detail below. A different 
spectral resolution may be selected at various portion of the spectrum 
simply by controlling the data processing. This technique will become 
clear from reading the quantitative performance analysis which follows in 
this specification. Further, if the scene of interest includes a moving 
target, background frame subtraction is used to accentuate the moving 
target image in the various spectral bands. All these operations are 
efficiently performed by a dedicated digital signal processor, DSP, 106. 
The image data vector, or matrix, X.sub.i, is operated upon by the dark 
current and pixel non-uniformity correction vectors, D, and the spectral 
sensitivity vector, R.sub..lambda., as follows: X.sub.f =}X.sub.i 
+D}.multidot.R.sub..lambda. to yield the corrected data. These data are 
then reduced to obtain the desired spectral resolution. The spectral 
intensity, I.sub.i,.lambda. of pixel i is obtained by, I.sub.i,.lambda. 
=[X.sub.f ].sub.i -[X.sub.f ].sub.i-1. 
The procedure is shown in FIG. 15b. The data vector obtained from the 
spectrum resolving sensor 88 is marked as line A(i). A copy of the data 
vector is made in memory, and then it is shifted by one memory register as 
marked by line A(i+1). Now the two vectors may be subtracted, pixel for 
pixel, in their corresponding registers. The resulting vector, marked 
A(.sub.i)-A(.sub.i+i), is the spectrum of the signal with the highest 
resolution possible for the specific sensor configuration (explained in 
the quantitative analysis which follows). 
If a lower spectral resolution is acceptable, the copy vector A(i) may be 
shifted by more than one register. Vector A(i+4) is a copy of A(.sub.i) 
shifted by 4 registers. When the A(i)-A(i+4) operation is performed a 
vector with a lower spectral resolution is obtained as can be seen in the 
figure (the resolution of the fine structure is lost). This operation 
requires the calculation of only one-fourth of the points than the former. 
Thus, the trade off is of speed versus resolution (as the sampling 
frequency is reduced, though, aliasing error may increase). The resolution 
may be selected by software control and may be different at different 
portion of the spectrum. 
Any of the above operations may be performed in conjunction with other 
common data processing routines. The data processing may be performed 
on-board the spacecraft, or at the receiving ground station if the raw 
data are transmitted to the ground first. Further, as it may be clear to 
those experienced in the art, this embodiment may be used for imaging as 
well as non-imaging applications, and for laboratory apparatus as well as 
for space-borne systems. 
In the discussion of FIG. 13 a mobile spectrometer was used with a 
stationary scene. If no relative motion exists between the scene and the 
sensor then a scanner is used to sequentially project different portions 
of the scene onto the array. Each portion is projected after the previous 
frame has been read. Scanner systems are in common use in robotic vision 
systems, and in supermarket checkout systems, and are well known to those 
skilled in the art. 
The invention may be improved for situations involving low light levels. A 
sensor array operating in conjunction with an image intensifier is shown 
in FIG. 16. The LVF, 60, is placed on top of a common image intensifier, 
of the type produced by several manufacturers, e.g., ITT and Litton 
Industries. The LVF allows photons of only selected frequency, or 
wavelength, to pass through onto the intensifier. The microchannel plate, 
MCP, 62, in this embodiment, includes the front photocathode and the rear 
phosphor surface, produces an electron multiplication effect, producing 
light output at the rear phosphor coated output window. The backplate, 64, 
which could be a quartz window or a coherent fiber optic bundle, transmits 
the enhanced optical image to the array, 66. Because of the initial 
filtering of the radiation by the LVF, the image at every point on the 
array corresponds to the spectrum of the incoming radiation. 
In another embodiment, the spectrum resolving sensor may be modified to 
incorporate a "thinned" CCD array. A thinned Si array is used for 
enhancement of the ultra violet, UV, response of the array. Typically the 
front surface of a CCD array is covered to a large extent by thin metal 
deposits which serve as the electrical gates. Although the longer 
wavelength of the spectrum go through, the UV radiation is blocked by 
these gates resulting in a reduced sensitivity of the array. By etching 
the backside of the Si to a thickness of less than 1 mm, the UV radiation 
may penetrate through the thin wall from that direction and effect charge 
accumulation. This arrangement is shown in FIG. 17. The back of the array 
10' is etched, 70, and is exposed to the UV radiation rather than the 
front, 74, on which the gates are deposited. The LVF, 76, in this 
embodiment, is placed over the etched area of the array. 
An alternative construction for the sensor array illustrated in FIGS. 1a-1c 
in side section view is shown in FIG. 18. For convenience the elements are 
identified by the same numbers as were used to identify like elements of 
the prior construction and are primed. This includes the package 14' 
containing connector pins 15' and the photodetector array 12', which is 
either a line array or an area array as earlier described in connection 
with the preceding embodiment, attached by an appropriate adhesive to the 
bottom of the u-shaped channel in package 14'. However, the linear 
variable filter 19 is coated directly over the top of the array, which 
serves as a substrate. There is no spacing or gap between the filter and 
the array. The coating covers all interstitial spaces between pixels in 
the array as well. Inasmuch as the quartz glass in the filter of the 
preceding embodiment serves as a physical support for the optical 
coatings, the elimination of that glass does not materially detract from 
the optical filtering achieved. However, in this instance the entire 
photodetector array must be included in the optical filter coating 
process. 
A large focal plane is often required to increase the spatial resolution of 
an image, as partially illustrated in FIG. 19. Due to geometrical optics, 
the focal zone of the optical system, 76, is slightly curved. If a plane 
array is used in such a case, some of the image is out of focus. The 
sensor in this embodiment, 78, is slightly curved to improve the spectrum 
resolving focusing and resolution of the image. 
The quantitative performance of the sensor is analyzed as follows: The 
n.sub.th and the (n+.sub.1).sub.th pixels of a sensor assembly constructed 
with a long-wave pass LVF, receive radiation at a wavelength above 
##EQU1## 
Here, .lambda..sub.1 and .lambda..sub.2 are the lower and upper wavelength 
ranges of the LVF and N is the total number of pixels covered along the 
axial direction by the LVF between these wavelengths. 
By subtracting the signals of these two pixels, the radiation intensity 
above .lambda..sub.n but below .lambda..sub.n+1 is obtained. This becomes 
then the spectral band-pass, or the resolution of the spectrum resolving 
sensor. 
##EQU2## 
By subtracting the signal of consecutive pixels, the complete spectrum with 
the above band-pass is obtained. The systems resolving power is defined 
as, 
##EQU3## 
Further, in cases the spectrum does not change abruptly it is possible to 
improve on the resolution by factoring out the "dead space" between the 
pixels. In that case the spectral band pass and resolving power of a pixel 
becomes, 
##EQU4## 
Where, X.sub.c--c is the center-to-center distance of the pixels in the 
array, and X.sub.p is the width of a single pixel as shown in FIG. 2b. 
If a wider band-pass is desired then the-subtraction may be done over 
several pixels rather than consecutive ones. Therefore, the band pass that 
may be measured is: 
##EQU5## 
Where i=1, 2, 3 . . . is an integer. The control of the band-pass is done 
in the signal processing computer only; no changes are required in the 
sensor's hardware. Hence, band-pass may be changed at any time during a 
scan process. 
For a 1024 pixel array, with a 25 micrometer center-to-center and 15 
micrometer wide pixels, operating with an edge-type LVF over the visible 
spectrum from 400 to 700 nm, the spectral band-pass is 0.176 nm (1.76 
Angstrom) or selectively 0.293i nm (2.93i Angstrom) where i=1, 2, 3 . . . 
the resolving power of the system is at best 1707. 
The spectral resolution of the invention using a band-pass LVF is shown as 
an example of performance analysis for other sensor designs. The resolving 
power of a common optical filter at a wavelength .lambda. and band-pass 
.DELTA..lambda..sub.f is customarily defined as 
##EQU6## 
The resolving power of a linear variable filter with a lower and upper 
wavelength transmission of .lambda..sub.1 and .lambda..sub.2 will be 
defined in a slightly different way as: 
##EQU7## 
In this form the resolution of a LVF is the same at all wavelengths. From 
FIG. 10, the band-pass of a single pixel in a CCD array which is covered 
with the above LVF is 
##EQU8## 
where, as before, X.sub.c--c is the center-to-center distance of the 
pixels in the array, X.sub.p is the width of a single pixel, and N is the 
number of pixels in the array. The first term on the right hand side 
reflects the finite length of the pixel which spreads the band-pass. An 
ideal, square-wave, response of the pixels in the array and ideal, 
square-wave, filter band-pass were assumed. 
The system's resolving power may be defined as 
##EQU9## 
The performance of each pixel, or of the invention, may be compared with 
that of the LVF alone as follows: 
##EQU10## 
where the resolving power of a single pixel is defined, as before, in 
terms of its relative size in the array, as 
##EQU11## 
The overall system performance for a 1024 pixel array, with a 25 micron 
center-to-center and 15 micron wide pixels, operating with a LVF over the 
complete visible spectrum from 400 to 700 nm at a 5 nm band-pass is 
R.sub.p =1707, R.sub.f =60, and R.sub.sys /R.sub.f =0.966, which means 
that the system resolving power is degraded by about 3.4% as compared to 
the resolving power of the filter alone. As in the previous embodiment, a 
much higher resolution may be obtained by subtracting the signals of 
adjacent pixels. 
As evident from the foregoing discussion, in non-imaging applications, the 
invention may be used for the measurements of spectral transmission 
through samples such as in medical, pathological, biological, geological, 
or chemical laboratory work; for molecular absorption and emission 
spectra; for spectral reflectance measurements in similar fields; for 
pollution and emission control by measuring the transmission or the 
absorption of radiation through a stack or exhaust plume; for remote 
sensing of air pollution, of ozone in the atmosphere, using a variety of 
ground, air-borne, or space-borne instruments; for astronomical spectral 
analyses of stellar radiation; for pyrometry by measuring the thermal 
radiation emitted by heated bodies at several wavelengths; underwater 
spectrometry; and other usages in which spectrometers, 
spectro-photometers, or spectrographs are currently utilized. 
In imaging applications the invention may be used for color copying 
machines; for color printing; for color facsimile machines; color 
picture-phone; color page scanning; robotic vision; aerial mapping; 
air-borne and space-borne resources monitoring; reconnaissance and 
surveillance; sorting of parts, mail, currency, food; non-contact 
inspection; missile guidance; star tracking; and other applications 
requiring color resolution using either line or area image devices. 
I believe that the foregoing description of the preferred embodiments of my 
invention is sufficient in detail to enable one skilled in the art to make 
and use the invention. However, it is expressly understood that the 
details of the elements which I have presented for the foregoing purpose 
is not intended to limit the scope of my invention, in as much as 
equivalents to those elements and other modifications thereof, all of 
which come within the scope of my invention, will become apparent to those 
skilled in the art upon reading this specification. Thus my invention is 
to be broadly construed within the full scope of the appended claims.