Page 73 - Sensors and Control Systems in Manufacturing

The Role of Sensors in the 21st Century
unit area. We won’t delve any further into this subject because it is so 35
seldom used. Many different standard units of measurement exist for
luminance, some of which are listed next.
• Candela per square inch (cd/in²)
• Foot-lambert (luminance of a surface emitting one lumen per
square foot)
• Lambert (similar, but per square cm).

1 cd/in² = 452 foot-lambert
1 lambert = 929 foot-lambert = 2.054 cd/in²

Our eye sees brightness, not illumination. Every visible object has
brightness. Usually, brightness is proportional to the object’s illumi-
nation, so a well-illuminated object seems brighter. For a properly
diffusing reflecting surface:

foot-lamberts = foot-candles × surface refl ectance

1.15.4 Foot-Candle to Lux Conversion
One foot-candle is the illumination produced by one lumen uniformly
distributed over one square foot of surface, and lux is the illumina-
tion over one square meter of surface (Fig. 1.20). Therefore, one lux =
0.0929 foot-candles. Or, approximately:

≈1 Fc = 10 lux

Illumination at the center = E 2

Illumination at the center = E 1

Distance = D 2

Distance = D 1

Point Source

FIGURE 1.20 Illumination inverse square law representation.