An Evaluation System for the TSL2550 Ambient Light Sensor The Count Value for each channel is obtained by the following equation: Count Value = Chord Value + (Step Number x Step Value) Eq. 1 Count Value = (INT (16.5 x (2
Chord Number – 1)) + (Step Number x 2
Chord Number ) Eq. 2 Illuminance (i.e. ambient light brightness) is derived using the values read from Channel 0 and Channel 1 in the units of Lux to approximate the human eye response. The formula follows: Light Level (lux) = Ch0 Counts x 0.46 x e
-3.13(Ch1 Counts/Ch0 Counts) Eq. 3 Figure 4. Modify TSL2550 Equation The coefficients of the equation can be modified from the ‘Settings/Lux Formula’ pull down menu item. See Figure 4.

C

OLOR

T

Light Source Lux Ch0 Counts Ch1 Counts Ch1/Ch0
EMPERATURE As shown on Figure 3, a relative indication of the color temperature of the light source can be made for the purpose of light source classification. Color temperature can be estimated by taking the ratio of infrared light to the combination of visible and infrared light (i.e. Ch1/Ch0). Fluorescent lighting, for example, represents almost all visible light. Incandescent lighting, on the other hand, represents a combination of visible and infrared light (i.e. about half-visible and half-infrared). Fluorescent 300 831 68 0.082 Daylight (shade) 100 895 343 0.38 Incandescent 50 959 671 0.7 Table 3. TSL2550 Sensor Output Summary From Figure 3, the correlated color temperature of the ambient light source is roughly proportional to Ch0/Ch1 (i.e. the inverse of the ratio described in Table 3). This information may be useful in certain applications such as computer displays and television screens to perform color balance.
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