Obtaining Ground Truth For Multispectral Photography

Abstract

The ratio of the intensity of the radiation directionally reflected from a background or reference target in a selected portion of a spectral region extending from substantially 260 nanometers to substantially 3,000 nanometers or emitted by the target in a selected portion of a spectral region extending from substantially 3,000 nanometers to substantially 20,000 nanometers to the intensity of incident solar radiation at a point near the target is determined at intervals of 5 nanometers. Similar ratios are determined for a specimen of an anomalous target visually similar to the background target. Filters are selected for multispectral photography having maximum transparency in the bands where the ratios are different. Multispectral photographs employing such filters are optimally adapted to distinguish between the two types of targets.

Description

BACKGROUND OF THE INVENTION

This invention relates to multispectral photography and, more particularly, to methods and apparatus facilitating the establishment of ground truth by which multispectral photography can be improved as a remote-sensing tool.

The burgeoning population of the earth, the increasing pollution of the environment and the urgent quest for higher standards of living for all have placed great emphasis on the need for more prudent ecological management and more efficient means for ascertaining the earth's resources.

Both desiderata require a vast information input and the ability to survey large territories quickly, economically and accurately. The data of interest are quite varied and include, for example, information regarding the spread of conditions such as wheat rust and Dutch elm disease and, indeed, any information regarding changes, however minor, in vegetation; information regarding changes in the purity or salinity of water or in the moisture content of soils; and evidence of mineral deposits.

Information can, of course, be gathered by investigators on the ground, who can drill the earth, perform chemical analyses and ultimately extract a wide range of information with a high degree of reliability; but ground surveys, while important in many cases for final verification, are very costly. Much of the earth is virtually inaccessible on the ground except at great expense, because of the absence of roads, extremes of temperature, and the paucity of local supplies of food, water and power. Moreover, even though ground surveys can be conducted for a price, they are extremely slow.

There is accordingly much interest in remote-sensing tools adapted for mounting in an airplane or satellite, whereby large territories can be surveyed quickly, economically and accurately.

Various remote-sensing tools are available, but the usual ones all have well-known limitations. Magnetometers, radar, laser scanning and conventional black-and-white or color photography offer some possibility of ascertaining from a distance the nature of the terrain over which an aircraft or satellite passes, but these tools are ill suited to detect certain phenomena related to subtle differences in the reflectance characteristics of surface targets.

Multispectral photography is the one remote-sensing tool that facilitates detection of such phenomena. The general principles of multispectral photography, sometimes referred to as spectral-zonal photography, are disclosed in my copending application Ser. No. 519,854, filed Jan. 11, 1966, for "Spectral-Zonal Color Reconnaissance System". A number of images, typically four, are simultaneously formed on a black-and-white panchromatic film through a plurality of lenses associated with filters respectively having maximum transparency in different regions of the electromagnetic spectrum. The film is developed, and the several images are projected in a special viewer having a separate lens system for each image. The projection lenses of the viewer project the several images in accurately superimposed relation to form a single composite image on a viewing screen. Colored filters are respectively associated with the projection lenses, but the filters are, in general, not the same as the filters employed in taking the pictures. The composite image is therefore in false color, and, by adjusting the chromaticity, brightness and saturation associated with each projected image much information regarding the target can be extracted.

The inventor has found, however, that, if multispectral photography is to work with maximum effectiveness, the selection of the filters used in taking the pictures cannot be random or arbitrary. Rather, the filters are preferably selected with regard for the nature of the target--or, more precisely, with regard for the differences between the reflectance characteristics of an anomalous target of interest and a background target that may resemble the anomalous target so closely as to be indistinguishable from it by conventional remote-sensing means. In the absence of good ground truth, the filters selected for multispectral photography will in general not be optimally adapted to extract the desired information.

SUMMARY OF THE INVENTION

An object of the invention is to remedy the deficiencies of the prior art noted above. In particular, an object of the invention is to provide a method and apparatus facilitating the obtaining of ground truth improving the accuracy and utility of multispectral photography.

The foregoing and other objects are attained in accordance with the invention by determining, at each of a plurality of closely-spaced-apart frequencies of any suitable region of the electromagnetic spectrum, the ratio of the intensity of electromagnetic radiation reflected from or emitted by a background or control target, at the wavelength associated with such frequency and at a given time, to the intensity of solar radiation, at the same wavelength and at the same time, incident at a point near the target. In this way, the "spectral signature" of the background target is determined. The process is repeated for a representative, known specimen of an anomalous target of interest, and the spectral signatures of the anomalous and background targets are compared to identify a plurality of discriminating spectral bands in which the spectral signatures are different. Filters are then selected respectively having maximum transparency in the discriminating spectral bands. The filters are employed to take multispectral photographs, whereby it is possible to distinguish with a reliability heretofore unattainable between overflown targets some of which are similar to the background target and others of which are similar to the anomalous target.

BRIEF DESCRIPTION OF THE DRAWING

An understanding of additional aspects of the invention can be gained from a consideration of the following detailed description of a representative embodiment thereof, in conjunction with the appended figures of the drawing, wherein:

FIG. 1 is a diagrammatic view of spectroradiometric apparatus constructed in accordance with the invention;

FIG. 2 is a schematic view in greater detail of a portion of the apparatus of FIG. 1;

FIG. 3 is a schematic wiring diagram of apparatus in accordance with the invention; and

FIG. 4 is a graph showing spectral signatures of background and anomalous targets.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows apparatus 10 constructed in accordance with the invention. The apparatus 10 includes a mobile platform such as a light truck 12, a periscope tower 14 mounted on the truck 12 and responsive to radiation reflected from or emitted by a target T, and spectroradiometric apparatus 16, responsive to incident solar radiation, mounted beside the truck 12 and connected thereto by two incident input cables 18, which may be coaxial and about 20 feet in length. The periscope tower 14 is mounted with its axis parallel to the gravity vector and supports an upper periscope mirror 20 and a calibration plate 22. Motive means such as a line or rope 24 is provided connected to a linkage 26 pivoted about a shaft 28 by means of which the calibration plate 22 can be adjusted relatively to the upper mirror 20. The calibration plate can thus be made selectively to occupy fully, or be entirely out of, the field of the periscope. A motor-generator 30 supplies power through a cord 32 to the instrumentation carried in the truck 12 (which is shown in FIG. 2 and described in detail below).

In general outline, the apparatus of FIG. 1 functions as follows:

The truck 12 is driven to a position adjacent to, and on the sunny side of, a target T of interest, which may be a background target or an anomalous target. If one intends to use the ground truth obtained in order to prospect for a particular mineral, the background target may be a tree known to be growing in soil that does not contain the mineral in an appreciable quantity, and the anomalous target may be a tree of the same species known to be growing in soil that contains a deposit of the mineral in sufficient quantity to be commercially attractive and also to stress the tree and alter its spectral signature.

The periscope tower 14, which may be pivoted to a horizontal position for transport, is erected so that its axis is parallel to the gravity vector and rotated so that the upper periscope mirror addresses the target T.

The upper periscope mirror 20 looks down on the target from an elevated position. The elevated position is within the volume of a right circular cone having an apex at the target and being generated by a right triangle of which the leg about which the triangle is rotated extends between the target and the sun and the hypotenuse forms an angle of 45.degree. with respect to that leg and intersects the leg at the target. The upper periscope mirror 20 thus looks in a direction divergent by not more than 45.degree. from the direction in which the solar rays travel in illuminating the target.

Simultaneously, the incident spectroradiometer 16 looks up at the incident solar radiation. Apparatus shown in FIGS. 2 and 3 is provided for measuring the intensity of the electromagnetic radiation reflected from or emitted by the target T and the intensity of the electromagnetic radiation incident at a point near the target. On the basis of these measurements, one determines the ratio of the former intensity to the latter. Such ratio is determined at discrete wavelengths throughout a desired region of the electromagnetic spectrum which may extend from substantially 260 nanometers to substantially 20,000 nanometers and preferably at least from substantially 350 nanometers to substantially 1,300 nanometers. Each ratio in each scan is separately calculated, and three ratios for each selected wavelength are averaged. The wavelengths are selected at close intervals, preferably substantially 5 nanometers. Four complete scans of the target are made in this fashion, plus a similar scan of the calibration plate 22 before the first scan of the target T and again after the fourth scan of the target T. The averaged ratios determined in the four scans are averaged at each wavelength to yield the final ratio characteristic of the target at each wavelength. The calculations can be performed by pencil and paper but are preferably performed by computer. The calibration scans facilitate calibration of the apparatus in a way described in detail below.

The measurements are recorded in any form, for example in machine-readable form on punched paper tape or magnetic tape, and the ratios calculated therefrom define the spectral signature of the target. A typical spectral signature for a certain species of tree (a background tree that is unstressed) is shown in solid outline in FIG. 4. The percentage of incident radiation reflected (shown on the ordinate) as a function of the wavelength (shown on the abscissa) varies from a low value (around 5 percent) towards the violet end of 500 nanometers to a small peak (around 15 percent) between 500 and 600 nanometers then decreases to a low value (around 5 percent) at about 650 nanometers. From there it increases rapidly to nearly 80 percent between 700 and 800 nanometers then declines gradually with increasing wavelength in the infrared region of the spectrum.

The process described above is repeated for another target, which may be an anomalous target, and a spectral signature for the anomalous target is determined. A hypothetical spectral signature for the same species of tree (an anomalous tree that is stressed by growing in soil containing a high concentration of a mineral of interest) is shown in broken outline in FIG. 4. The spectral signatures of the two targets may be sufficiently similar that observation by the naked eye or ordinary color or black-and-white photography would not reveal the difference.

FIG. 4 clearly reveals the difference, however, in that there are four spectral bands A, B, C and D where the reflectances of the two trees differ. Specifically, in bands A and C the reflectance of the anomalous tree exceeds that of the background tree, while in bands B and D the reflectance of the background tree exceeds that of the anomalous tree.

On the basis of this preliminary information or ground truth, four filters are constructed respectively having maximum transparency in bands A, B, C and D. These four filters are used for multispectral photography in the manner disclosed in my copending application identified above.

Thus, in accordance with the present invention, the spectral signatures of two targets known to differ in some respect of interest to an investigator are determined with great precision and compared; discriminating spectral bands are identified in which the spectral signatures are different; filters are selected respectively having maximum transparency in the discriminating spectral bands; and the filters are then employed to take multispectral photographs, whereby it is possible to distinguish readily between targets similar to the background target and targets similar to the anomalous target, even though the two types of targets may be visually indistinguishable by ordinary means.

It may be expected that the spectral signatures of two targets that are visually nearly identical will generally exhibit "crossovers", as indicated in FIG. 4. That is, there will be certain spectral bands in which the reflectance of the background target exceeds that of the anomalous target and other spectral bands in which the reflectance of the anomalous target exceeds that of the background target. In accordance with the invention, one preferably selects discriminating bands that include crossovers, since such selection definitely facilitates target discrimination. Beyond that, it is desirable that the absolute magnitudes of the differences in reflectances, regardless of their sign, be as large as possible.

The implementation of the general principles of the invention disclosed above can be effected by many means. The embodiment that is preferred as the best mode known to the inventor for practicing the invention is the one illustrated in the figures, and now described in detail.

FIG. 2 shows schematically the instrumentation mounted in the truck 12. The bottom of the periscope tower 14 is provided with handles 34 for manual rotation of the periscope. An upper mirror "up-down" switch is provided for controlling the upper mirror in elevation by means of a motor (not shown).

The lower periscope assembly includes two optical paths 36 and 38, a lower mirror 40 and a "VIS-I.R." switch 42 for adjusting the position of the lower mirror 40 about a shaft 44 so that the reflecting plane of the lower mirror 40 selectively assumes angles of 45.degree. clockwise or counterclockwise (as seen in FIG. 2) with respect to the vertical axis of the periscope. Depending on its position, the lower mirror 40 directs light reflected from the upper mirror 20 along the optical path 36 or 38.

The optical path 36 contains instrumentation adapted to process information relating to electromagnetic radiation in the region of substantially 350 nanometers to substantially 750 nanometers, and the optical path 38 contains instrumentation adapted to process information relating to electromagnetic radiation in the region of substantially 700 nanometers to substantially 1,300 nanometers. The two optical paths are otherwise similar.

The reason for providing a visible path and an infrared path is simply that instrumentation is unavailable for making precise measurements throughout the portion of the electromagnetic spectrum of principal interest, namely the portion extending from 350 nanometers or less to 1,300 nanometers or more. It is also within the scope of the invention to employ a single optical path and this would indeed be preferred if the requisite instrumentation were available to cover with great precision the entire range. With the instrumentation now available, the double optical path is the preferred embodiment.

Considering the optical path 36 first, it includes a filter holder 46 for insertion of a filter that passes electromagnetic radiation within the range of substantially 350 nanometers to substantially 750 nanometers and a telescope 48 for visible information including an eyepiece 50 to enable to direct viewing of the target in the visible region of the electromagnetic spectrum. The telescope 48 passes the visible information to a monochrometer 52 that is associated with a motor drive 54. The monochrometer 52 may be in the form of a diffraction grating, and the drive 54, by moving the grating stepwise, permits very precise control of the wavelength (and hence the frequency, which is related to wavelength by the formula c = .lambda.f, where c is the velocity of the electromagnetic radiation, .lambda. is its wavelength, and f is its frequency) that is passed to a visible reflectance detector head 56.

The visible reflectance detector head 56 is provided with a power supply 58 and may take the form of a photomultiplier that produces an analog output signal proportionate to the intensity of the electromagnetic radiation incident upon it.

As FIG. 3 shows, the visible reflectance detector head 56 supplies the analog output signal representative of such intensity over a line 59 to an analog-to-digital converter such as a self-ranging picoammeter 60. The picoammeter 60, which may be a conventional kind such as the Model 930A manufactured by EG&G, generates a digital signal as a function of the analog signal and supplies the digital signal through a lead 61 to a data coupler 62. The data coupler 62 is also conventional and may be, for example, Model 29B-2 manufactured by Coldspring Instrument Corp.

As FIG. 3 also shows, a "visible" wavelength transducer 64, responsive to the position of the grating 52, supplies an analog signal over a line 66 to the data coupler 62. The data coupler 62 thus is supplied with two signals, one respresentative of the intensity of radiation reflected from the target and the other representative of the wavelength of the radiation of which the intensity is being monitored at that particular time. The data coupler 62 includes an analog-to-digital converter (which need not be self-ranging in view of the small range involved) to convert the signal received through the lead 66 from analog to digital form so that the signals as processed by the data coupler 62 are both in digital form.

Simultaneously, the data coupler 62 receives signals indicative of the intensity of radiation incident near the target at exactly the same wavelength. This information comes from a "visible-incident" detector head 68 shown in FIGS. 1 and 3 and forming part of the incident spectroradiometer 16. Means including a servosystem (not shown) is provided for changing the wavelength to which the visible-incident detector head 68 responds exactly in step with the change in the wavelength to which the visible-reflectance detector head 56 responds.

The infrared optical path 38 (FIG. 2) includes instrumentation generally analogous to that previously described in connection with the optical path 36 except that the instrumentation is sensitive in the infrared region of the spectrum. The wavelengths to which the "visible" instrumentation and the "infrared" instrumentation respond overlap, the "visible" instrumentation being responsive throughout the range from substantially 350 nanometers or less to substantially 750 nanometers and the "infrared" instrumentation being responsive throughout the range from substantially 700 nanometers to substantially 1,300 nanometers or more.

The optical path 38 includes an infrared filter holder 70 adapted to hold a filter passing wavelengths within the range of about 700 nanometers to about 1,300 nanometers, an infrared telescope 72 having an eyepiece 74, an infrared monochrometer 76 having a wavelength drive 78, and an infrared reflectance detector head 80. A cooling controller unit 82 having a power supply 84 is also provided.

FIG. 3 shows the infrared reflectance detector head 80 supplies an analog output signal representative of the intensity of the reflected infrared radiation at the monitored wavelength over a line 86 to the self-ranging picoammeter 60. The picoammeter 60 generates a digital signal as a function of the analog signal and supplies the digital signal through the lead 61 to the data coupler 62. The lead 61 is available for such service because it is not necessary to reserve it simultaneously for use in transmitting signals representative of the information in the visible region of the spectrum; on the contrary, information relating to the visible and information relating to the infrared are always processed sequentially.

As FIG. 3 also shows, a transducer 88, responsive to the position of the grating 76, supplies an analog signal over a line 90 to the data coupler 62. The data coupler 62 is thus supplied with two signals, one representative of the intensity of radiation reflected from the target and the other representative of the wavelength of the radiation of which the intensity is being monitored at that particular time. As noted above, the data coupler 62 includes an analog-to-digital converter (which need not be self-ranging in view of the small range involved) to convert the signal received through the lead 90 from analog to digital form so that the signals as processed by the data coupler 62 are both in digital form.

Simultaneously, the data coupler 62 receives signals indicative of the intensity of radiation incident near the target at exactly the same wavelength. This information comes from a "visible-incident" detector head 92 shown in FIGS. 1 and 3 and forming part of the incident spectroradiometer 16. The spectroradiometer 16 is conventional and may be, for example, a Model 580/585 manufactured by EG&G. Means including a servosystem (not shown) is provided for changing the wavelength to which the infrared-incident detector head 92 responds exactly in step with the change in the wavelength to which the infrared-reflectance detector head 80 responds.

The signals from the visible-incident detector head 68 and from the infrared-incident detector head 92 are supplied to the data coupler 62 by way of an incident self-ranging picoammeter 94 through leads 96 and 98, respectively. The incident picoammeter 94 converts the signals from analog to digital form and supplies them to the data coupler 62 through a lead 100. The picoammeter 94 is also conventional and may be, for example, a Model 445 made by Keithley Instruments.

The data coupler 62 activates the visible-reflectance detector head 56 or the infrared-reflectance detector head 80 at the appropriate time by signals supplied through leads 104 and 106, respectively. Whenever one of the reflectance heads is activated to supply information to the data coupler 62, the other is deactivated by signals from the data coupler. In a similar manner, the visible-incident detector head 68 and the infrared-incident head 92 are activated and deactivated at the appropriate times.

The data coupler 62 causes recording of all of the information supplied to it on a tape punch 108 through signals supplied over a line 110. The tape punch is conventional and may be, for example, a high-speed tape punch BRPE manufactured by Teletype Corporation. The punched paper tape can be inspected visually and the ratios indicative of the spectral signature calculated by pencil and paper. Preferably, however, the tape is punched in machine-readable form and is supplied as-is to a general purpose digital computer, which is programmed to make the necessary calculations.

In order to obtain accurate readings, a number of calibrations must be made.

The purpose of the calibration plate 22 (FIG. 1) is to compensate for any differences in the instrumentation for measuring the incident radiation and that for measuring the reflected radiation. More particularly, with the calibration plate 22, which is white and as nearly perfectly reflecting as possible, positioned so that it completely fills the field of view of the periscope, the instrumentation responsive to the reflected radiation should produce signals that correspond exactly or in a predetermined way to the signals produced by the incident spectroradiometer 16. Such correspondence should exist throughout the scanned range. To assure the desired fit, readings are taken at intervals of 5 nanometers in the region of 350 nanometers to 1,300 nanometers (or whatever the region of interest is). This constitutes the calibration scan. A series of scans on the target is then made, preferably four in number, and each extending over the same spectral range. Another complete scan of the calibration plate is then made, and any relative drift between the instrumentation responsive to the radiation received by the periscope and the instrumentation responsive to the incident radiation is noted. Compensation for any relative drift that may occur between the two calibration scans (which may occur, for example, because of changes in the response of the photomultipliers as a function of temperature) is made by linear interpolation. While this can be done with pencil and paper, it is preferred that it be done by computer, which is a straightforward matter if all data, including data derived from the calibration scans, are recorded in machine-readable form.

The gratings are calibrated by selecting a mercury, helium or other lamp with known, well-defined, well-spaced spectral lines in the infrared, visible and ultraviolet. The line lamp is permitted to illuminate the grating 52 or 76 (depending on the position of the mirror 40) in connection with which the calibration is to be performed, other light being excluded. A port 112 is provided in the lower periscope assembly (FIG. 2) for inserting the spectral calibration lamp. The grating being calibrated is stepped manually or by means of a motor so that each visible spectral line (if the grating 52 is being calibrated) or each infrared spectral line (if the grating 76 is being calibrated) successively illuminates the photomultiplier 56 or 80. The position of the grating associated with each peak is noted and recorded. The recording can be effected automatically an punched paper tape under the control of the data coupler 62. This accurately correlates grating position with wavelength.

The photomultipliers are calibrated by illumination with a standard lamp at a standard distance. Conventional photomultipliers are available that have such lamps incorporated in them. With the photomultiplier so illuminated, it is adjusted to give a standard output, e.g., 1.000 .times. 10.sup..sup.-6 amperes. The lamp is turned off, and, after decay of the output reading, the decayed or dark reading is entered on the tape (again automatically under the control of the data coupler 62). The dark reading and standard reading permit interpolation (or extrapolation, if that should be necessary) so that any reading by the photomultiplier can be accurately associated with a given light intensity. The photomultipliers are calibrated in this way at the beginning and end of each scan involving the photomultiplier in question, and the readings are corrected by linear interpolation. Again, the interpolation is preferably effected automatically.

The data coupler always waits until all readings are stable before activating the tape punch 108 to make the recordings. After three recordings at a given wavelength, it causes stepping to the next wavelength.

Tests of apparatus constructed in accordance with the invention have demonstrated that the thousands of readings involved in determining the spectral signature of a given target can all be made and recorded in little more than 10 minutes. And, of course, since the recordings are in machine-readable form, the calculations to be made from the recording can be done at computer speeds.

Thus there is provided in accordance with the invention novel and highly-effective methods and apparatus facilitating the obtaining of good ground truth for multispectral photography. Such ground truth can be obtained extremely rapidly because of the automation of the process. By means of the ground truth, filters can be selected, represented schematically by bands A, B, C and D in FIG. 4, that greatly improve the value of a multispectral camera as a remote-sensing tool.

Many modifications of the representative embodiment of the invention disclosed herein will readily occur to those skilled in the art. In particular, anyone skilled in the art will be able upon reading the present disclosure to assemble many different combinations of instruments that will perform the method of the present invention. Also, it is not necessary that the periscope view the target within the 45.degree. cone disclosed above on cloudy days, though best results are obtained by viewing from a position within such cone on sunny days. Further, provision can be made for recording the time of each reading in order to facilitate calibration. Accordingly, the invention is to be construed as including all of the embodiments thereof within the scope of the appended claims.

Claims

I claim:

1. A method of establishing ground truth for multispectral photography comprising the steps of measuring the intensity of electromagnetic radiation reflected from or emitted by a target of interest at a given time at the wavelength associated with a first frequency; measuring the intensity of solar radiation at the same time and at the same wavelength incident at a point near said target; computing the ratio of one of said intensity measurements to the other; and making corresponding measurements and computing the ratios thereof at the wavelength associated with a second frequency closely spaced apart from said first frequency; these steps being repeated at a sufficient number of closely-spaced-apart frequencies of a region of the electromagnetic spectrum to determine the spectral signature of the target.

2. A method according to claim 1 wherein said region of the electromagnetic spectrum extends from substantially 260 nanometers to substantially 20,000 nanometers and the intervals between the wavelengths respectively associated with said plurality of frequencies within said region of the electromagnetic spectrum are substantially 5 nanometers.

3. A method according to claim 1 wherein said region of the electromagnetic spectrum extends from substantially 350 nanometers to substantially 1,300 nanometers and the intervals between the wavelengths respectively associated with said plurality of frequencies within said region of the electromagnetic spectrum are substantially 5 nanometers.

4. A method of multispectral photography in which a plurality of separate lens systems form a corresponding plurality of related black-and-white photographic images of a common scene, comprising the steps of determining the spectral signature of a control target, determining the spectral signature of an anomalous target, comparing said spectral signatures to identify a plurality of discriminating spectral bands at which said spectral signatures are different, mounting in combination with each of said lens systems a different filter, said filters respectively having maximum transparency in said discriminating spectral bands, and forming said related black-and-white images respectively through said lens-filter combinations, whereby, when said images are developed and viewed additively, it is possible to distinguish visually between targets similar to said control target and targets similar to said anomalous target.

5. A method according to claim 4 wherein there is a crossover of said spectral signatures between at least two of said discriminating spectral bands.

6. A method according to claim 4 wherein the absolute differences in said spectral signatures are greatest in said discriminating spectral bands.

7. A method of multispectral photography in which a plurality of separate lens systems form a corresponding plurality of related black-and-white photographic images of a common scene, comprising the steps of

determining, at each of a plurality of closely-spaced-apart frequencies of a region of the electromagnetic spectrum, the ratio of the intensity of electromagnetic radiation reflected from or emitted by a background target at the wavelength associated with such frequency and at a given time to the intensity of solar radiation at the same wavelength and at the same time incident at a point near said background target, whereby the spectral signature of said background target is determined,

determining, at each of a plurality of closely-spaced-apart frequencies of the same region of the electromagnetic spectrum, the ratio of the intensity of electromagnetic radiation reflected from or emitted by an anomalous target at the wavelength associated with such frequency and at a given time to the intensity of solar radiation at the same wavelength and at the same time incident at a point near said anomalous target, whereby the spectral signature of said anomalous target is determined,

comparing said spectral signatures to identify a plurality of discriminating spectral bands in which the associated reflectance/incident ratios of the control and anomalous targets are different,

mounting in combination with each of said lens systems a different filter, said filters respectively having maximum transparency in said discriminating spectral bands, and

forming said related black-and-white images respectively through said lens-filter combinations, whereby, when said images are developed and viewed additively, it is possible to distinguish visually between targets similar to said control target and targets similar to said anomalous target.

8. Apparatus for establishing ground truth for multispectral photography comprising downward-looking means responsive to electromagnetic radiation of a selected wavelength reflected from or emitted by a target of interest for producing a signal representative of the intensity of said reflected or emitted electromagnetic radiation at said wavelength, upward-looking means mounted near said target and responsive to electromagnetic radiation of the same wavelength for producing a signal representative of the intensity of electromagnetic radiation simultaneously incident at said wavelength, data-coupler and recording means responsive to said signals for recording indicia representative of the intensity of said reflected or emitted electromagnetic radiation and the intensity of said incident electromagnetic radiation, wherefrom the ratio of the former intensity to the latter is calculable, and control means for varying the wavelength to which said downward looking means and said upward-looking means are simultaneously responsive, wherefrom a series of ratios is calculable indicative of the spectral signature of said target.

9. Apparatus according to claim 8 comprising standard reflectance means and motive means for permitting said downward-looking means to respond selectively to electromagnetic radiation reflected from or emitted by said target and to electromagnetic radiation reflected from or emitted by said standard reflectance means, whereby indicia respectively representative of the intensity of electromagnetic radiation reflected from or emitted by said standard reflectance means at various wavelengths and indicia respectively representative of the intensity of electromagnetic radiation simultaneously incident at such wavelengths are recorded from time to time to facilitate calibration of said apparatus.

10. Apparatus according to claim 8 comprising a lamp of standard brightness forcalibrating the measurement of said intensity by said downward-looking means.

11. Apparatus according to claim 8 comprising a spectral-line lamp for calibrating said control means for accurate selection of said wavelength.

12. Apparatus according to claim 8 wherein said downward-looking means and said upward-looking means both comprise means responsive to electromagnetic radiation in a first region of the spectrum and means responsive to electromagnetic radiation in a second region of the spectrum slightly overlapping said first region, whereby the obtaining of accurate measurements is facilitated over a wide spectral band.