Polychromatic Light

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

What is Polychromatic Light?

Light is called polychromatic when it has multiple optical frequencies, i.e., if it is not monochromatic. In some cases, polychromatic light has a mixture of some number of discrete wavelength components, while in other cases its optical spectrum is continuous.

Light may still be considered as quasi-monochromatic if its optical bandwidth is so small that the behavior of interest (for example, the light propagation properties) is not significantly different from that of monochromatic light. For example, diffraction patterns will not be significantly modified if the bandwidth is only a small fraction of the mean optical frequency.

When modeling the propagation of laser beams with methods of numerical beam propagation, it is sometimes essential not to assume that the light is monochromatic — even if its optical bandwidth is rather small. This is because different transverse modes generally have somewhat different optical frequencies, and those frequency differences prevent interference effects which would artificially come up in monochromatic simulations [1].

In many cases of technical interest, light is substantially polychromatic, i.e., its optical bandwidth is not small compared with the mean frequency. The most important example is sunlight, covering a huge spectral range, and the part of it which is relevant for imaging with human eyes is still rather broad: it ranges from about 400 nm to 700 nm in terms of wavelength or 430 THz to 750 THz in terms of optical frequency. Therefore, chromatic aberrations may be substantial if an optical system (e.g. a photographic objective or a microscope) is not designed to work well with polychromatic light.

Typical technical sources of broadband (strongly polychromatic) light are incandescent lamps (including halogen lamps) and superluminescent sources.

It is generally not possible to directly convert polychromatic light into monochromatic light. However, one may e.g. use the original polychromatic light for optically pumping a laser which emits monochromatic light.

Various types of calculations in optics are based on the assumption of monochromatic light. Sometimes one can use the results simply by calculating them for some suitable set of optical wavelengths and calculating average values from those, for example. In other cases, that is difficult, or the calculations inherently need to be done for polychromatic light.

Light pulses are inherently polychromatic, even if the instantaneous frequency is constant throughout the pulse. In the case of ultrashort pulses, the minimum possible optical bandwidth can be many terahertz.

The strongly polychromatic nature of sunlight is also relevant for solar power generation since the conversion efficiency of photovoltaic cells is substantially wavelength-dependent and can be optimized only for a limited spectral range.

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