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Rev. 0.1

Example: If the distance to the target is 100 cm and width of coverage 100 cm, then:

irLEDs come in standard half-angles of 5, 10, 15, 20, 30, 60, etc. Choose the nearest half-angle available; in this

case an angle of 30 degrees would be appropriate to fully cover the target. An angle of 20 degrees could also work

if the designer trades off a slightly longer distance to obtain the same width of coverage (e.g. D= (100 * 0.5) ÷

tan(20) = 137cm).

3.3. Deciding Target Irradiance at the Sensor

In prior sections it was stated that an irLED decision is contingent upon knowing the half-angle and the radiant

intensity. The method of choosing the irLED radiant intensity is covered in the next section. However, before it is

possible to derive the radiant intensity of the irLED, it is important to know the minimum irradiance level needed at

the sensor. This section provides some guidelines for choosing the proper irradiance target.

is beside the Si114x, and that the target object is directly above both the irLED and the Si114x.

Fundamentally, the Si114x must be able to measure an increase in irradiance due to the irLED, compared to the

background ambient level. The Si114x internally makes two measurements. The first measurement is made with

the irLED disabled to estimate the background IR radiation. The second measurement is with the irLED enabled.

The Si114x reports the difference between these two measurements. The difference in reading is proportional to

the reflected light from the target object, illuminated by the irLED.

The required irradiance at the sensor is a function of the ambient IR noise level, the IR ambient level, the light

source type, and the ADC setting used by the Si114x.

Assuming that the sensor is operating indoors, illuminated with artificial light sources with at maximum expected

light levels of less than 450 lux incandescent or 1 klx of CFL lighting, the default Si114x Proximity ADC Setting can

For applications that need to operate under high levels of artificial lighting (but less than 6.5 klx incandescent or

14.5 klx of CFL lighting), the HSIG bit in the PS_ADC_MISC needs to be set. The minimum irradiance at the

For applications operating outdoors but not under direct sunlight, the PS_ADC_MISC HSIG bit should be set. The

For use under direct sunlight, in addition to setting the HSIG bit, the PS_ADC_MUX may also need to be

configured to use the smaller IR photodiode to avoid saturation. In this case, the minimum irradiance target is 45

Table 1. Typical Ee Design Targets

Ee (sensor)

Minimum

HSIG Bit

IR Photo

Diode

ADC

Integration

Time

Usage (no overlay assumed)

No

Large

408.6

s

Artificial Lighting CFL < 63 lx; Incandescent < 23 lx

No

Large

204.8

s

Artificial Lighting CFL < 125 lx; Incandescent < 56 lx

No

Large

102.4

s

Artificial Lighting CFL < 250 lx; Incandescent < 112 lx

No

Large

51.2

s

Artificial Lighting CFL < 500 lx; Incandescent < 225 lx

No

Large

25.6

s

Artificial Lighting CFL < 1 klx; Incandescent < 450 lx

Yes

Large

25.6

s

Artificial Lighting CFL < 2 klx; Incandescent < 900 lx



arctan 100

0.5 100







26.6 degrees

==