Filters for the Isolation of Narrow Regions in the Visible and Near-Visible Spectrum.

Investigations of photochemical reactions in higher plants and other biological systems frequently require the irradiation of large areas with welldefined regions in the visible and near-visible spectrum. Large glass filters or dyed gelatin filters of the Wratten type are costly. Inexpensive tlheatrical gelatin filters, dyed cellophane, or other dyed plastics are available, but these are limited in variety of spectral characteristics and few of tlhemii have sharp absorption bands. The laboratory preparation of large dyed gelatin films is very inexpensive. These films are particularly useful for studies in plant photochemistry because the transmission bands can be adjusted to meet the special requirements of each problem. On the basis of laboratory experience and of various reports in the literature (6, 7, 10, 11, 13), the following informiiation is presented on the technique of preparation of dyed gelatin films and the composition and transmission data for a few typical filters. In addition, data are presented on the transmission of water and solutions of copper sulphate and ferrous ammonium sulphate, which are useful as primary filters for the removal of the infrared and certain other regions of the visible and nearultraviolet spectrum. With water or aqueous solutions as primary filters and dyed gelatin films as secondary filters, it frequently is possible to isolate single lines of the mercury arc spectrum and fairly narrow regions from incandescent lamp sources. Since the gelatin films can be prepared conveniently in sizes up to at least one square meter, relativ ely high irradiances can be produced over areas of a square meter or more, making it possible to grow muature plants with spectral regions that are well defined and sufficiently narrow for many physiological studies. The dyed gelatin films to be described were prepared by the general method of casting hot solutions of dye and gelatin onto plate glass. After the layer has gelled, it is allowed to dry. If the glass has been treated previously with silicone, the film can be stripped from the casting plate and mounted between panes of window glass.