Referring to this question How can I measure the amplitude of a light wave? I'm curious about what is a amplitude of a light wave. Especially for light from a thermic source.
Answer
You cannot immobilize light, but you can absorb it, convert it to heat. A first step in measuring light intensity (watts/square meter) is to make a black body to absorb the light. Then, by conservation of energy, the heating of that black body tells you how much light is being absorbed, in watts. The illuminated area of the black body is the "square meters" part.
There are many other kinds of light sensors (electronic, acoustic, chemical) suitable for special cases, but they require different calibration curves for different light sources. Color, beam divergence, polarization, and other variables other than light intensity will affect those sensors.
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