Page 72 - Sensors and Control Systems in Manufacturing

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Cha p te r
O n e

4000
3000
Intensity (counts) 2000

1000

0
400 500 600 700
Wavelength (nm)

FIGURE 1.19 Spectrographic analysis of a 10-mm non-phosphor LED, when
driven at 350.7 mA. (See also color insert.)

angular zone; amount of absorbed light, etc. However, if you need to
calculate a characteristic that is not related to the human eye—for
example, the temperature increase due to absorbed light, you cannot
use luminous flux. Instead, you must use the correct unit of power—
the watt.

1.15.2 Illumination
Illumination is the density of luminous flux on a surface This param-
eter shows how “bright” the surface point appears to the human
eye. The appropriate units of measure are foot-candle and lux. One
foot-candle is the illumination produced by one lumen uniformly
distributed over one square foot of a surface, or conversely this is the
illumination at the point of a surface which is one foot from, and per-
pendicular to, a uniform point source of one candela. So, foot-candles
incident on a surface = lumens/area (sq. feet). Lux is used in the
International System. Both have a similar objective, but meters are
used for lux and feet are used for candelas.
Therefore, one lux = 0.0929 foot-candles.

Or,

≈ 1 fc = 10 lux

1.15.3 Luminance (Brightness)
Luminance (or brightness) is the luminous intensity of a surface in a
given direction per unit of projected area of the surface. It can be
expressed in two ways: in candelas per unit area, or in lumens per