Page 75 - Sensors and Control Systems in Manufacturing

The Role of Sensors in the 21st Century
1.15.7 The Difference Between Lumen and Watt 37
Lumen is a unit of the photometric system and Watt belongs to the
radiometry system.
Both characterize a power of light flow. However, lumen is power
“related” to human eye sensitivity. Therefore, lights with the same
power in watts, but that are different colors have different luminous
fluxes, because the human eye has different sensitivity at different
wavelengths. At a wavelength of 555 nm (maximum eye sensitivity),
1 watt equals 683 lumens (lm).
High-power sources of infrared radiation produce no lumen out-
put, so the human eye can’t see it. However, in order to calculate the
total power absorbed by a surface to estimate temperature increase,
for example, lumen flux must be transferred to watts. This can be
done by using a spectral luminous efficiency curve, which can be
found in many photometry handbooks.

1.15.8 Calculating Beam Angles
If the distance from a fixture to the screen is known (which is a much
larger value than fixture length), as well as the image diameter
(Fig. 1.22), then:

D
α= 2arctan
2L

D - image diameter

Screen

L - distance

α

Fixture

FIGURE 1.22 Calculation of beam angle.