Light Sources and Related Accessories

Abstract

Often, there is little consideration of the type of light source required for gas exchange measurements, and the determining factor is generally availability rather than suitability. Obviously, the particular light source will depend on whether the lamps are to be used for growing algae or for direct measurements of photosynthesis or fluorescence. The former requires a number of low-cost lamps, with a low heat output, which can provide a relatively uniform Irradiance over a large area, whereas in most gas-exchange studies narrow beam or point sources are required. Failure to consider the merits of different light sources prior to experimental investigations can result in considerable methodologic problems due, for instance, to low outputs, excessive overheating, or poor spectral resolution. There are several factors to consider before choosing a particular light source. The incident irradiance will depend on the lamp power output, the distance between the lamp and the illuminated sample, and the type of optical system used. Filters or monochromators will drastically reduce the original light output. 12-V, 100-W tungsten-halogen lamps are adequate for most photosynthetic and absorption measurements (maximum irradiance ≈ 2,000 μmol·m^-2·s^-1, λ = 400–700 nm) using simple optical arrangements. More complicated systems using monochromators may require considerably higher output lamps. The source should have a continuous emission spectrum and be efficient in the photosynthetically active wavebands (λ = 350–720 nm). It should also have a long lifetime with a constant output that exhibits little variation with lamp age. Ideally, the lamp should have a minimal heat output or the housing should have a built-in cooling system.