1006 LETTERS TO THE EDITOR Vol. 53 Infrared Fluorescence of Green Leaves JEAN BERCHTOLD Bell & Howell Research Center, Pasadena, California (Received 20 December 1962) I T is well known that green plants are reproduced as nearly white when photographed on infrared-sensitive emulsion. The explanation commonly given in textbooks for this effect is that chlorophyll is transparent to infrared and that the internal leaf structure is highly reflective to same radiation. Although this is correct, it does not cover the whole story. It does not seem to be commonly known that leaves of plants emit fluorescent radiation in the near infrared when illuminated by visible light. This fluorescence can easily be demonstrated by illuminating leaves with light filtered from all infrared radiation and imaging them by means of an image-converter tube made sensitive only to infrared. The light source used for these experiments was a 250-W projection lamp whose light was projected on the tested objects through infrared absorbing filters Schott BG 18, 1 mm (millimeter
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1006 L E T T E R S T O T H E E D I T O R Vol. 53
Infrared Fluorescence of Green Leaves JEAN BERCHTOLD
Bell & Howell Research Center, Pasadena, California (Received 20 December 1962)
IT is well known that green plants are reproduced as nearly white when photographed on infrared-sensitive emulsion. The
explanation commonly given in textbooks for this effect is that chlorophyll is transparent to infrared and that the internal leaf structure is highly reflective to same radiation. Although this is correct, it does not cover the whole story. It does not seem to be commonly known that leaves of plants emit fluorescent radiation in the near infrared when illuminated by visible light. This fluorescence can easily be demonstrated by illuminating leaves with light filtered from all infrared radiation and imaging them by means of an image-converter tube made sensitive only to infrared.
The light source used for these experiments was a 250-W projection lamp whose light was projected on the tested objects through infrared absorbing filters Schott BG 18, 1 mm (millimeter
August 1963 L E T T E R S T O T H E E D I T O R 1007
FIG. 1. Curve 1: relative energy from a 3000°K tungsten lamp filtered with Schott filters BG 18, 1 mm and KG 2, 2 mm. Curve 2: relative sensitivity of S-1 type photocathode filtered by Wratten filter 88 A.
thickness) and KG 2, 2 mm. Curve 1 shows the spectral distribution of radiant flux calculated from the filter-manufacturer's data and the spectral emittance of a tungsten lamp at 3000°K.
The infrared receiver used was an RCA 6032 image-converter tube with a Wratten filter 88 A placed in front of the imaging lens in order to remove its sensitivity in the visible. The two curves show evidence of a high degree of rejection between the two spectral ranges.
When looking at the light through the image-converter tube with the filters interposed, the filament of the tungsten lamp is just barely visible. Nonfluorescent objects, illuminated with this light source, are rendered black when viewed in the infrared
range, whereas green plants exhibit a distinct glow; in fact, they appear appreciably brighter than the light source itself. This gives experimental evidence of fluorescence.
Foliage from several species of plants and dried leaves also shows essentially the same phenomenon. Light of blue, green, or yellow color does excite fluorescence, whereas red light is without effect.
Experiments indicate that infrared fluorescence of green leaves is likely to be at least partly responsible for the near-white rendering of plants by infrared photography.1
1 Since this information was submitted, a picture was published by Kodak in an advertisement (Science, April 1963, p. 81) showing essentially the same phenomenon, but recorded on infrared film.
