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Page 1: Conversion filters for scientific photography

Conversion filters for scientific photography

D. Hoeschen Physikalisch-Technische Bundesanstalt, Bundesallee 100, 3300 Braunschweig, Federal Republic of Ger­many. Received 11 August 1979. 0003-6935/80/020198-03$00.50/0. © 1980 Optical Society of America.

For the calibration of photographic equipment such as light meters and cameras with automatic exposure control, light sources with a relative spectral distribution equal to that of blackbody radiation at 4700 K are needed. For sensitom-etry of color films, light sources with distribution temperatures of 3200 K or 3400 K are sometimes used.1 To obtain light sources with the desired distribution temperatures, light sources of known spectral radiance are taken together with conversion filters. Davis-Gibson liquid filters, recommended by many standards, are widely used.2 With these filters a good approximation is obtained in the spectral region between 400 and 780 nm to any desired spectral radiance distribution with distribution temperatures up to 10,000 K when the initial light source has a distribution temperature of ~2850 K.

For most purposes of scientific photography, a spectral region of 400-780 nm is sufficient because the spectral sensi­tivity of many light meters is essentially zero outside that region. But silicon detectors are more sensitive in the IR region, so care must also be taken for the spectral region up to 1000 nm at least if the calibration of light meters with un-filtered silicon detectors is concerned.

Because it is sometimes tedious to make and use the liquid filters and because the spectral transmittance of these filters is far too high in the spectral region above 800 nm, a set of blue-glass filters was developed. In Fig. 1 the spectral transmittances of these filters together with that of a Davis-Gibson liquid filter are shown.

The filters consist of one or more single glasses of appro­priate thickness, cemented together with EPO-TEK 305 (Epoxy Technology, Watertown, Mass.). The conversion factors (mired values) are so chosen that by combining dif-

198 APPLIED OPTICS / Vol. 19, No. 2 / 15 January 1980

Page 2: Conversion filters for scientific photography

Table I. Thicknesses of Single Glasses

Fig. 1. Spectral transmittance of six glass filters and one Davis-Gibson liquid filter.

Fig. 2. Relative spectral distributions of two Planckian radiators (curves 1 and 2) and of three combinations of Planckian radiators (2856 K) with glass and liquid filters (curves 3-5).

15 January 1980 / Vol . 19, No. 2 / APPLIED OPTICS 199

Page 3: Conversion filters for scientific photography

ferent filters, every mired value up to about 400 mired can be reached in steps of 10 mired. Table I shows the composition of the filters.

The thicknesses of the single filters were computed by minimizing the sum of the quadratic deviations between the relative spectral distributions of a desired high-temperature Planckian radiator and a given low temperature Planckian radiator (2586 K) together with an appropriate conversion filter. This was done first for the spectral region from 400 to 780 nm without a heat-absorbing KG4 filter because it was felt that this region is of principal interest. Afterwards, the same procedure was carried out with a KG4 filter in the region from 790 to 1050 nm.

In Fig. 2 the relative spectral distributions of two Planckian radiators (3224 and 4759 K) together with the distributions of a Planckian radiator (2856 K) with different filters are il­lustrated. All curves are so normalized that the area below

the curves is the same in the region between 400 and 780 nm.

The results show that the fitting of the glass filter curves is quite good. These glass filters can undoubtedly compete with Davis-Gibson filters in the visible region of the spectrum. They have the advantage of being far superior to Davis-Gibson filters in the IR. They are also thin, stable, and easier to handle than liquid filters.

References 1. G. Vieth, Sensitometric Testing Methods (Focal Press, London,

1974). 2. R. Davis and K. S. Gibson, "Filters for the Reproduction of Sun­

light and Daylight and the Determination of Color Temperature," U.S. National Bureau of Standards Misc. Publ. No. 114 (U.S. GPO, Washington, D.C., 1931).

200 APPLIED OPTICS / Vol. 19, No. 2 / 15 January 1980