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Many measurements in radiometry start with the use of a source. It illuminates an optical system or a detector. It may be part of a measurement of reflectance or transmittance. Sources are part of radiometry.
A comprehesive list of sources and their properties is not possible in the scope of this text. This chapter is intended to provide an overview and perspective rather than handbook data. Two good references for more detail, and for more references may be consulted.
Sources may be categorized in a number of ways, but surely they separate into laser and non-laser sources. Laser sources need to be described in terms of their wavelength of operation, power, and whether or not they are continuous. If they are not continuous, then their repetition rate and pulse characteristics are important. For both types, beam spread and lifetime are also important. The other sources consist of cavity radiators, arc lamps, incandescent (mostly tungsten) lamps, and special types.
6.1 Laser Sources
There are many types of lasers: gas, diode, dye, excimer, and more. They also operate either as fixed-wavelength sources or can be tunable. The latter differentiation is made here.
6.1.1 Fixed-Wavelength Lasers
The type that comes immediately to mind is the ubiquitous helium-neon (He-Ne) laser, which is a cheap, stable workhorse for almost every laboratory. It can be operated at several wavelengths, but the most common is the red line at 0.6328 μm. In this mode, it has an output power from 1 to 10 milliwatts. This laser can also be operated at 1.15 and 3.5 μm but with reduced performance in terms of both stability and power output.
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