AN521 Datasheet, PDF (22/26 Pages) Silicon Laboratories – IRLED SELECTION GUIDE

AN521
Example: A manufacturer’s data sheet may specify that an LED has 100 mW/sr, therefore at 1 m (100 cm) the (Ee)
power/area is 100 mW/(100)² or 10 µW/cm², and at 4 m (400 cm) it is 0.625 µW/cm².
In a proximity detector usage, irradiance measured at the sensor does not have an inverse square law relationship
relative to the irLED illuminating the object. The irLED illuminates the object, and then the object reflects some of
this light to the sensor. The irradiance measurement at the sensor is a function of the object’s reflectivity, area,
distance, and its surface type.
Most objects fall into the category of a “diffuse” surface. In a proximity detector, the relationship between the irLED
source and the irradiance measured at the sensor is actually an inverse fourth-law relationship.
It can be shown that if:
 An irLED has a radiant intensity function Ie().
 Object surface is a Lambertian diffuse surface with a reflectivity () and an area A.
 The square root of A is less than 5 x D.
 The sensor and irLED are located close to each other.
Then the irradiance measured at the sensor is:
Ee = -^I---^e----^---^---^----^c---^o---^s----²---^---^----^A--
D⁴
Where:
 Ee is the irradiance (uW/cm²).
 Ie() is the radiant intensity (mW/sr).
 () is the reflectivity of the object.
 A is the area of the object.
 D is the distance from the mid-point between the sensor and the irLED.
4.3.4. Half-Angle
irLEDs emit light in a teardrop configuration similar to Figure 16. This plot is important to understand because it
relates the radiant intensity as a function of the viewing angle measured from the 0° axis. Note that the power falls
off as a function of this off-axis angle. The half-angle () is defined as that angle at which the power falls to 50% of
its 0° axis value. The zero axis power is the radiant intensity specified by the manufacturer in mW/sr.
Figure 16. Relative Radiant Intensity vs Off-Axis Angle
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