Intensity of Light

On measuring the intensity of light, we are merely measuring the most intense wavelength of the spectrum obtained, so in principal, it is the same thing as measuring the spectra of light.  The difference is that instead of getting a readout of all the wavelengths, you get only the major ones, the most intense ones.  Therefore the explanation of this optical principle is the same as the one for the spectra of light, with the addition of the diagram at the very bottom.

The principle of obtaining the spectra of light is basically refraction.  Simple, single wavelength refraction consists of the incident beam, the reflected beam and a single refracted beam.  


When measuring the intensity of light, the integral under the intensity x wavelength function and the spectral sensitivity of the sensor itself  is taken, and this area corresponds to the overall intensity.  For instance, in the diagram below, the incoming light is blue and the spectral sensitivity is in red, and the intensity is a function of the area which intersects both curves.
Intensity of the spectrum.  This diagram shows which wavelengths are usually most intense to the human eye, with the most intense being at around 550 nm (the color we call yellow-green)

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Data from Fourth Edition of Fundamentals of Physics