AN523 Datasheet, PDF (1/6 Pages) Silicon Laboratories – OVERLAY CONSIDERATIONS

AN523
OVERLAY CONSIDERATIONS FOR THE Si114X SENSOR
1. Introduction
The Si1141/42/43 infrared proximity detector with integrated ambient light sensor (ALS) is a flexible, high-
performance solution for proximity-detection applications and (visible) ambient-light sensing challenges. The
sensor is most commonly used under a glass or plastic overlay whose optical properties modify both the ALS
response and the proximity-detection distance. The Si114x can also function through slotted optical ports or other
apertures. This causes a certain amount of attenuation of transmitted LED power and received proximity and ALS
light levels.
An overlay (used with or without a slot opening) has the following effects:
 Reduced incoming light for both ALS and proximity detection due to the cover material.
 A slight reduction of the Si114x's effective view angle due to the cover material's thickness (unless the cover is
spherical).
 Reduced transmitted LED power.
 Changes in the system's spectral response compared with an uncovered Si114x.
This application note gives examples of common overlay materials with various spectral characteristics and
discusses their influence on ALS and proximity detection. For other system and mechanical considerations,
including emitter-to-receiver optical isolation, refer to “AN498: Si114x Designer’s Guide”.
2. Typical Application
Typically, the Si114x is configured to detect the proximity of an object to be detected, such as a user's hand, and
may also need to report ambient light for the purpose of screen dimming. In addition to the photodiode used for
proximity detection, the Si114x makes use of two photodiodes with different spectral responses for ALS. This
scheme enables sensing of the visible light while rejecting infrared light, which is present in large amounts in
incandescent light sources and in sunlight.
Infrared light affects the response of both ALS photodiodes to different extents. Rejection is accomplished by
summing the two outputs with different coefficients so that information about visible light is obtained. The choice of
coefficients varies depending on the cover material's spectral characteristics in the visible and infrared bands.
Proximity sensing is affected by the cover material's transmission characteristics at the LED wavelength (typically
850 to 950 nm). Since ambient light affects the performance of proximity detection, proximity detection can be
enhanced by rejecting as much of the non-LED spectrum as possible; this is because more ambient light forces the
Si114x’s ADC to operate at a less sensitive gain range in order to avoid saturation. Thus, the function of an
integrated ALS and proximity sensor depends on a compromise between favoring IR light at the LED wavelength
for proximity and allowing enough visible light for proper ALS operation. This trade-off depends on the cover
material selected.
Rev. 0.5 1/15
Copyright © 2015 by Silicon Laboratories
AN523