C-LOGIC 250 Instruction Manual page 30

4.2 Action principles
4.2.1 Concepts of illuminance scales
One lux (lumen) indicates the illuminance got on a surface of one square me-
ter, all points of which are one meter from a uniform source of one candela.
One foot-candle (Fc) indicates the illuminance got on a surface of one square
foot, all points of which are one foot from a uniform source of one candela.
4.2.2 Unit conversion for illuminance scales
• 1 Fc = 10,764 lux
• 1 lux = 0,09290 Fc
4.2.3 Conversion formula for illuminance and light intensity
• E = I / r 2
When
- E: illumination value (unit: Lux)
- I: light intensity of the light source (unit: cd)
- r: distance from the luminous surface of light source to the optical detector
(unit: m).
During the measurement, the minimum distance between the luminous surface
of light source and the opto-sensor should be more than 15 times greater than
maximum size of the luminous surface (or opto-sensor).
4.3 Typical practice cases
• In the following practice cases, the user stands under a light source. Re-
move the protection cover of the C-LOGIC 250 multifunctional light meter
sensor and place it at right angle to the light source, as shown in figure 4-1.:
C-LOGIC 250 • Digital light meter
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