Patent GB-1285301-A (GB)
Title: Solar stimulated fluorescent radiation detection method and apparatus
IPC: G01J3/44
Filed: 1970-03-20
Abstract: 1285301 Prospecting PERKIN ELMER CORP 20 March 1970 [1 April 1969] 13644/70 Heading E1F [Also in Divisions G2 and H4] An image comprising solely solar stimulated fluorescent radiation from objects or materials in a scene, e.g. viewed from an aircraft prospecting for such fluorescent materials, is produced by forming one image solely of light from the scene (illuminated by sunlight) over a wavelength band inside a preselected Fraunhofer absorption band and another image consisting solely of light from the scene near the same Fraunhofer absorption band in the continuum, normalizing the two images and then substracting the "continuum" image from the "Fraunhofer" image and displaying the result. In Fig. 1, light from the scene is projected via a tunable filter 14 on to an image tube 15 such as a vidicon. The filter is first tuned to have a transmission peak centred on a preselected Fraunhofer absorption band and a transmission passband about the same width or smaller than the bandwidth of the preselected Fraunhofer absorption band, and is then tuned so that its transmission passband is in the continuum a few Angstroms away from the preselected Fraunhofer absorption band. The resulting two consecutive electrical images are then normalized and combined into a single image by subtraction and the resulting pattern, which corresponds solely to the solar stimulated fluorescent radiation in the scene, displayed on a CRT display 17. The brightness of the image is proportional to the relative concentrations of the fluorescent materials or objects. Normalization may be achieved in a variety of ways such as by calibration with known targets pr by selecting portions of the scene known to contain no fluorescence and therefore required to cancel in subtraction, i.e. instead of being effected in the computer 16 it may be effected in the image tube 15. In a modification the two images may be produced simultaneously by using a beam splitter, two nontunable filters instead of filter 14 and two image tubes. . In Fig. 2, light from the scene is projected via a beam splitter 24 and nontunable filters 25 and 26 on to photographic films 27 and 28 disposed behind shutters 29 and 31. Filter 25 is centred on a pre-selected Fraunhofer absorption band and has a transmission passband about the same width or smaller than the bandwidth of the preselected Fraunhofer asborption band. Filter 26 is centred in the continuum a few Angstroms away from the preselected Fraunhofer absorption band. The resulting pictures formed on films 27 and 28 are normalized such as by adjusting the exposure time or developing time. The fluorescent (subtracted) image is produced by converting one of the pictures into a positive and other into a negative, and then directing a beam of light therethrough when superimposed on to a screen (Fig. 3 not shown). The brightness of the image is proportional to the relative concentration of the fluorescent materials or objects. Normalization may be achieved in a variety of ways such as by calibration with known targets or by selecting portions of the scene known to contain no fluorescent and therefore required to cancel in subtraction. In a modification the beam splitter is eliminated and, instead of nontunable filters 25 and 26 a tunable filter is employed, the "Fraunhofer" and "continuum" image being recorded consecutively on a moving length of film. If the apparatus is used to sense Rhodamine B, which fluoresces at the Fraunhofer absorption band at 5890A, filter 14 is designed to peak at 5890A and have a passband width of 0.7, which is equal to the 5890A Fraunhofer absorption bandwidth.