AN521

Rev. 0.1

13

To make adjustments for the overlay transmittance,

I

e(0) = 295 * 1.2 / (0.95^2) = 392 mW/sr

At 400 mA, the radiant intensity of the SFH 4259S is 5 times the typical rated radiant intensity of 130 mW/sr which

would lead to 650 mW/sr, easily meeting the 392 mW/sr requirement.

Silicon Labs makes the following suggestions based upon general knowledge of the application and published

device performance of various LED suppliers. The information contained in this application note is intended as a

guideline only. The user of these guidelines assumes full responsibility and associated liability for final device and

supplier selection. It is expected that the user of these guidelines will perform due diligence to determine the

suitability of any device and/or manufacturer for their specific business requirements.

4.2. A Typical One-Dimensional Linear Position Sensing Application (11 cm)

In this application the objective is to detect a one-dimensional linear movement along the plane that intersects the

radiation cones of the LEDs (see Figure 7). In this section, the application will be referred to as an irSLIDER.

Each LED is turned on at different times measuring the reflected power with each sample processed by the Si114x

and host MCU. If the reflected power levels are equal, the position is directly above the Si114x detector. The actual

math involved in implementing an irSLIDER is beyond the scope of this document.

The most important concept is to recognize that the light from each irLED must reach the entire width of the

irSLIDER and that the light reflected from the extreme angles should reach the Si114x.

The assumptions are given as:

= 0.47

I

e0



0.25e

6

–

 100

4







0.84

0.5

625









 295 mW sr



=

=