Monochromatic Light

There are five ways to produce narrow spectrum or monochromatic light.

1. Absorption filters are made of glass, gelatin, or liquids in which colored agents are dissolved or suspended. Gelatin filters are perhaps the most versatile type of filter and are made by mixing organic dyes in gelatin. They have the advantage of being relatively inexpensive and can be made very large, but are not stable over time (they can change their transmission spectrum with use and age) and tend to have fairly broad transmission spectra (Fig. 4E). Also be aware that short-wavelength (blue) filters often have a second long-wavelength (red) transmission that is frequently not reported by the manufacturer.

2. Cutoff filters essentially transmit or block light above or below defined cutoff wavelengths. These can be particularly useful in the generation of narrow-band UV light sources when combined with fluorescent lights that have strong emission peaks in the UV (e.g., ref. 25).

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Wavelength (nm)

Fig. 3. Black-body or cavity radiation at a range of temperatures from 1000 K to 5000 K. The wavelength of maximum emission moves to shorter wavelengths as temperature increases.

Fig. 4. Emission spectra for a variety of commonly encountered white-light (A-D) and monochromatic (E-H) light sources. (A) Tungsten-halogen bulb. (B) Fluorescent tube. (C) Xenon-arc light. (D) Sunlight (cloudy day in London). (E) Gel filter (Kodak safelight). (F), 460-nm interference filter. (G) Diffraction grating monochromator set to 550 nm. (H) "Red" light-emitting diode. All measurements made using an USB2000 fiberoptic spectrometer (Ocean Optics).

Fig. 4. Emission spectra for a variety of commonly encountered white-light (A-D) and monochromatic (E-H) light sources. (A) Tungsten-halogen bulb. (B) Fluorescent tube. (C) Xenon-arc light. (D) Sunlight (cloudy day in London). (E) Gel filter (Kodak safelight). (F), 460-nm interference filter. (G) Diffraction grating monochromator set to 550 nm. (H) "Red" light-emitting diode. All measurements made using an USB2000 fiberoptic spectrometer (Ocean Optics).

3. Interference filters produce high-quality monochromatic light using an arrangement of highly reflective surfaces that ensure that only a narrow band of wavelengths are transmitted (see Note 4). These filters are made by successively evaporating dielectric and silvered films on glass. Interference filters are defined on the basis of their wavelength of maximum transmission (^max) and on the basis of their half-maximal bandwidth (A^)0.5 (Fig. 4F). An important point to note is that the angle of incidence of the light falling on these filters greatly modifies their transmission spectra; it is critical that the incident light strike the filter at 90° to the surface (thus the source beam needs to be collimated). Also note that the mirrored surface should face the light source. Although they are able to produce higher-quality monochromatic light than gelatin filters, interference filters are expensive, and because of their small size, cannot be used to bathe large areas in monochromatic light. Interference filters, coupled with powerful tungsten-halogen (or similar) bulbs and high-quality optic fibers for light delivery, have formed the basis of most monochromatic light sources for circadian experiments (6).

4. Monochromaters or diffraction gratings are designed to disperse incident light into its spectral components—from which any desired narrow band of wavelengths can be isolated (Fig. 4G). Although very versatile, they are expensive, produce relatively little monochromatic light, and, like interference filters, cannot be used to irradiate large areas.

5. Light-emitting diodes (LEDs) emit monochromatic light of high purity, although their half-maximal bandwidth is usually broader than interference filters. The early LEDs had a relatively low irradiance confined to a fairly narrow portion of the spectrum, usually in the yellow, red, or infrared (Fig. 4H). However, the latest generation of LEDs can produce large amounts of monochromatic light at wavelengths that span the whole spectral range. Thus, for some applications, LEDs may provide a suitable substitute for interference filters.

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