Anything related to *photometry*, the field of physics that study the theory and the techniques used to measure light as it is perceived by the human eye. In engineering photometry is applied especially to all problems of lighting design.

Anything related to photometry, the field of physics that study the theory and the techniques used to measure light as it is perceived by the human eye.

In engineering photometry is applied especially to all problems of lighting design.

Note that photometry is different from radiometry, which studies the theory and techniques of measurement of any electromagnetic radiation. When radiometry is applied to visible light it doesn't take the human perception in any account and thus it is seldom used in lighting design.

In other words, for instance, when you want to know how much "light" is needed to illuminate a room it is not important to know how many watts of EM power a bulb emits, but how many lumens of luminous flux instead.

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