Radiometry and Photometry What you Get and What you

What the eye or photographic film or instruments such as photomultiplier tubes get is incident radiant energy over a time interval (radiant power). But film and instruments cannot be said to see anything. Only the eye can see, and seeing is mostly the processing of radiant energy by the organs of seeing - retina, optic nerve, and, most important, the brain - not the simple formation of images on the retina. It is sometimes said that the eye is just like a camera. It would be more accurate to say that the eye is not just like a camera except in the most trivial sense, namely, they both contain arrangements of lenses to form images. Yet even the most complicated camera, a mere chunk of metal and glass, cannot begin to perform the feats routinely done with ease by the human and animal eye-brain combination. Our brains create a visual world beginning with light as the raw material.

Wavelength is not a synonym for color nor is radiant power a synonym for brightness. But we begin with these strictly physical properties of light (radiometry), then proceed to the sensations it produces in the human observer (photometry).

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