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Chapter 5:
General Light Beam Measurements
Light has several intrinsic properties that can be measured: phase, intensity, color, coherence, and polarization. Each can be measured at a point or as a function of position. Light also exhibits directionality (pointing). All properties can evolve temporally and thus be measured as a function of time. In the last chapter we focused our attention on phase or wavefront properties. In this chapter we discuss measurements on the remaining parameters. No matter what property of light is considered, a detector of some sort is needed to sense that property and act as an interface between the field and the experimenter. The human eye is such a detector. The electric field amplitude is not directly sensed because (a) the frequency of the field is much too rapid to follow; and (b) the amplitude goes negative as well as positive (and intensity is always positive). Consequently, what detectors sense is proportional to the square of the electric field averaged over many cycles. This is called power and the units are watts. (For example, typical HeNe lasers emit 1–5 milliwatts.) If we measure power over a known finite area and form the ratio (power∕area), we obtain a quantity called irradiance with units watts∕cm2. Some detectors respond to energy instead of power. This is simply (power x— time interval). The units of energy are joules (or ergs). Photographic film is such a detector. When you adjust the exposure time on your camera, you are regulating energy accumulation on the frame of film.
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