The absorption and transmission characteristics of Kodak Wratten filters are published in the Kodak Photographic Filters Handbook (Publication No. B-3), which includes tabulations of spectral data at various wavelengths. A typical visible light absorption spectrum for one of these filters, the Wratten number 34a, is illustrated in Figure 1. In this figure, absorption (diffuse density) is plotted as a function of wavelength from 300 to 700 nanometers.

The 34a filter has a strong absorption band at 370 nanometers, in the ultraviolet region of the spectrum, and another centered at 560 nanometers in the green-yellow transition region of the visible spectrum. This filter passes or transmits wavelengths whose diffuse density is less than 0.1, so the wavelength ranges transmitted by the 34a filter are 410 to 470 nanometers (blue light) and 660 to 700 nanometers (red light).

The transmission bands of filters are conveniently represented as a bar graph, such as the one illustrated in Figure 2. The darker areas represent regions of transmission by a filter that exceed 10 percent, and the lighter areas represent wavelengths blocked by the filter. This type of graph provides a simple index for comparing a set of filters to quickly determine the suitable filter for any particular application.

Color Characteristics of Filters for Black & White Photography Filter or Filter Combination Light Transmittance Over 10 Percent  47B (deep blue) 400-470    47B + 2E (pale yellow)   420-470    47B + 3   440-470    47 (blue)   410-500    47 + 3   440-500    47 + 4   460-500    47 + 8 (yellow)   440-500    45   450-510    47A (light blue) 380-520      47A + 4   460-520    47A + 8 (yellow)   480-520    44 (blue green)   440-540    44 + 8 (yellow)   480-540    44 + 12 (deep yellow)   500-540    64   440-560    61   500-560    58 (green)   500-580    58 + 15 (deep yellow)   520-580    57   480-580    57 + 12   500-580    57 + 15   520-580    92   630+  29 (deep red)   610 +  25 (red)   590 +  22 (deep orange)   560 +  15 (deep yellow)   520 +  12 (deep yellow)   510 +  8 (yellow)   480 +  Wavelength (Nanometers) 400 500 600 < Ultraviolet         Infrared >

Table 1

A compilation of bar charts for the common Kodak Wratten color filters are listed in Table 1. Use this table as a reference in choosing filters to enhance contrast in stained specimens using black & white photomicrography.

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