Light Measurement Handbook phần 2 potx

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Light Measurement Handbook © 1998 by Alex Ryer, International Light Inc.

Visible Light

The lumen (lm) is the photometric equivalent of the watt, weighted to

match the eye response of the “standard observer”. Yellowish-green light

receives the greatest weight because it stimulates the eye more than blue or

red light of equal radiometric power:

1 watt at 555 nm = 683.0 lumens

To put this into perspective: the human eye can detect a flux of about 10

photons per second at a wavelength of 555 nm; this corresponds to a radiant

power of 3.58 x 10-18 W (or J s-1). Similarly, the eye can detect a minimum

flux of 214 and 126 photons per second at 450 and 650 nm, respectively.

Use of a photopic correction filter is important when measuring the

perceived brightness of a source to a human. The filter weights incoming

light in proportion to the effect it would produce in the human eye. Regardless

of the color or spectral distribution of the source, the photopic detector can

deliver accurate illuminance and luminance measurements in a single reading.

Scotopic vision refers to the eye’s dark-adapted sensitivity (night vision).