APPLICATION OF A “BLINKING” FILTER FOR DISTINGUISHING COLORATIONS SIMILAR IN COLOR

It is known that in photographs taken through a dark-red filter, greenery comes out very light. The reason for this is that chlorophyll almost does not absorb light with $\lambda > 700\ \mathrm{m}\mu$. A typical reflection curve of plant objects is shown in Fig. 1 (curve $B$), borrowed from the work of Shurcliff and Stierce cited below[^1]. The curve gives a characteristic reflection maximum at $550\ \mathrm{m}\mu$ and a sharp rise from $690\ \mathrm{m}\mu$ to $750\ \mathrm{m}\mu$ (the reflection coefficient here increases almost 10-fold).

Most artificial green paints, however, give only a very weak increase of the reflection coefficient at the red end of the spectrum (see Fig. 1, curve $A$), as a result of which complete imitation of natural greenery is difficult. It is precisely this “weak point” of masking paints that is used for their deciphering[^2]. The use of photography through dark-red filters for detecting objects camouflaged as greenery is well known, as is also (as reported by Shurcliff and Stierce) the use of two-color filters (such as, for example, Wratten No. 97) for visual observations.

Fig. 1. Reflection curves for “poorly imitated greenery” $A$ and natural greenery $B$. Transmission curves of red filters $C$ and $D$, through which natural greenery appears equally light.

Fig. 2. Reflection curves of standard brown $A$ and of the tested brown coloration $B$. Transmission curves of yellow filters $C$ and $D$, through which the standard brown coloration $A$ appears of the same shade and lightness.

Taking into account certain shortcomings of the two-color-filter method, the authors mentioned proposed another method of visual deciphering, consisting in the following: in front of the eye (protected from extraneous light), two red filters, selected so that (see Fig. 1, curves $C$ and $D$) natural greenery appears equally light through these filters, are rapidly alternated with one another. An artificial green coloration that does not give a rise of reflection in the region $\lambda > 700\ \mathrm{m}\mu$ appears, when viewed through one filter, much darker than when observed through the other. With rapid alternation of these filters before the eye, “poorly imitated” green objects will seem to blink and will therefore be easily noticed.

The construction of the apparatus itself is not complicated—a disk with filters is rotated by a small motor; the apparatus is quite portable and light.

Of interest are Shurcliff and Stearns’s considerations on the applicability of the variable-filter method to other cases requiring rapid selection of specimens close in shade and differing in the course of the spectral reflectance or transmittance curves. For example, two brown colors (Fig. 2), identical under visual examination, will be readily distinguishable with the aid of variable filters having transmittance curves \(C\) and \(D\). In this case specimen \(B\) will blink, since through filter \(D\) it will appear much hotter than through filter \(C\).

The authors’ brief remark on the applicability of their method for the rapid detection of differences in the spectral properties of light sources and detectors is also significant. The latter is especially interesting. It is quite possible that, on the basis of the blinking-filter method, simple instruments can be designed for the rapid and accurate selection of persons with anomalies of color sensation.

A. Ilyina.

CITED LITERATURE

1. W. A. Shurcliff and E. J. Stearns, J.O.S.A. 36, 473 (1946).
2. Optics in Military Affairs, 3rd ed., 1945, p. 235.