An Interference-Filter Monochromator System for the Irradiation of Biological Material

The determination of action spectra for biological photoreactions requires that large areas be irradiated uniformly at high inteiisities and with narrow band widths for the resolution of spectral details. When the material is changing irreversibly with time or when long irradiation periods are required, it may be necessary to irradiate samples at ten or more different points in the spectrum simultaneously. This excludes use of the conventional single-exit-slit monochromator and requires a spectrograph with a series of wide slits arranged in the focal plane of the spectrum. The necessity of both high radiant power and narrow band width are difficult to meet with the prism or grating spectrograph. These instruments are capable of producing band widths of a small fraction of a millimicron but the intensity of the beam falls rapidly as the waveband is decreased. Large prism (8) and grating (7) spectrograph systems using 5 to 15 kw carbon-arc sources have been described for the simultaneous irradiation of relatively large objects with monochromatic energy at many wavelengths. The band width of such systems is usually from 5 to 20 ??? when the radiant power is sufficiently high. The properties and application of these systems have been discussed elsewhere (14). The principal practical limitations of the spectrograph system are: 1) the large space required, 2) the unequal irradiance distribution over the spectrum, and 3) the cross-scattering of energy between wavelength stations. Because of the magnification required of the optical system, it is necessary to project the beam over relatively large distances of 5 to 15 meters. The relative intensity distribution between the various wavelength stations is arbitrarily determined by the spectral energy distribution of the source and transmission of the spectrograph. Therefore, it is not possible to obtain an equal irradiance or equal quantum-intensity spectrum. While this is not. always essential for action spectrum studies, it is certainly desirable for convenience of analysis and. interpretation of results. It is especially desirable for biological systems for which reciprocity does not hold. The interference-filter monochromator presents several important advantages over the prism and grating instruments. A system of 10 or 20 wavelength stations can be placed in a relatively small constantcondition room. The spectral bands can be completely isolated in small cabinets and the intensity of each wavelength station adjusted independently by electrical control of the source or by neutral density filters so as to produce an equal irradiance or equal quantum-intensity spectrum. The system is simple optically, does not require high quality optical components, and is much easier to adjust than grating and prism instruments. The interference-filter monochromator system to be described has been developed for the determination of action spectra in plants and for kinetical studies where it is desired to irradiate the material with high intensities of monochromatic energy of high spect al purity.