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3. Optical Considerations

This section contains various light-related topics helpful in understanding the light environment and its effect on the

Si1102 and Si1120. In general, the following sections are self-contained and need not be read sequentially.

Some of the topics covered are:

3.1. Reflectance Proximity Principles

Unlike Passive Infrared Receivers (PIR), which detect object motion by sensing changes in illuminated or naturally-

emitted infrared, reflectance proximity devices actively illuminate the object with an emitter, directly measuring

reflectance. However, unlike radar or lidar, which measure reflectance transit time to determine distance,

reflectance proximity sensors use signal level and optical techniques to determine range. Because the reflectance

from an object falls off with the fourth power of the distance (as with radar), if the object's angle is less than the

beam width of the emitter or receiver, the signal level of the reflectance can provide a range estimate typically

within a factor of two since an object’s reflectance does not typically vary by more than about ten to one.

Although, this same fourth-power response severely limits range beyond about ½ meter without optical lensing or

averaging, it does mean that large or even very reflective objects more than 2x further than the proximity range will

not cause spurious detection, unlike PIR devices.

3.2. Single Port vs. Dual Port

Single-port operation, where the same window is used for both transmit and receive, creates significant receiver

overload issues (even for radar). Single port operation is particularly problematic with non-transit time reflectance-

based systems since the reflectance from a common shared window for the transmit LED and the receive

photodiode produces a very large interfering signal with no way of differentiating the reflection from the window or

object (unlike transit time systems). Without some type of optical isolation, a single window will easily have 100x

more reflectance from the window than the object being detected, even if it is only a few centimeters away. This is

also why a camera flash uses an optical path separate from the imaging path (unlike very short-range

microscopes, which use the imaging objective to illuminate the object) and why car headlights are not behind the

windshield.

Dual-Port operation will always give the best performance. The Si1102 only operates in dual-port mode requiring

very high optical isolation where the leakage coupling from the LED to the IC is always less than the reflectance

from an object. However, the Si1120 can be used in either dual-port or single-port mode. In single port mode

(PRX50H), the Si1120 requires motion detection signal processing.

For long-range proximity sensing (usually

over ½ meter), high isolation dual-port operation is normally not possible since there are normally many large

objects within proximity range, such as walls, desktops, etc. These nearby objects return a large reflectance

relative to any moving object that needs to be detected. However, for best performance, it is desirable to minimize

optical coupling between the transmitting LED and detector. Excessive coupling increases noise floor and spurious

motion detection due to ambient light modulation.