AN521 Datasheet, PDF (1/26 Pages) Silicon Laboratories – IRLED SELECTION GUIDE

AN521
IRLED SELECTION GUIDE FOR Si114X PROXIMITY APPLICATIONS
1. Introduction
There are numerous factors involved in choosing the proper irLED for any given system.
This application note defines the technical requirements governing the choice of an irLED. Armed with this
information, a design engineer can choose the least expensive, most appropriate irLED suitable for the intended
application and usage. Specific irLED pricing information will not be discussed in this application note.
An important factor in an irLED choice is the industrial design of the end product. As an example, a thru-hole irLED
probably would be too tall for use inside a cell phone. Although this document occasionally touches some industrial
design topics, they are generally outside the scope of this document. From time to time, some aspects of industrial
design are mentioned, but only within the context of choosing an irLED.
There are many shapes, sizes, and footprints for LEDs that primarily are derivatives of two types, SMD and thru-
hole the standard T 1-3/4 format. In general, the following 4 categories cover typically available package types and
light emitting direction:
 Standard clear or IR pass blue, thru-hole T1-3/4 format heights 5.2–8.7mm
 SMT vertical emitting, heights 0.9–3.8 mm
 SMT side emitting, 2–5 mm
 SMT gull wing or similar, heights 2–5 mm
It is assumed that the design engineer can take the mechanical requirements implied by the industrial design and
choose the correct package type. The irLED package choice is outside the scope of this document.
This document describes the technical requirements involved in choosing an irLED for use with the Si114x active
reflectance proximity applications.
2. Terminology
In this document the following definitions apply:
Range—Distance in cm from emitter/detector to target to be detected.
Width of coverage—The usable width of illumination at RANGE measured perpendicular to the zero axis of
irradiation.
Radiant Flux—Total radiant power emitted by a source expressed in (mW).
Radiant Intensity (Ie)—Equal to the radiant flux per unit solid angle from a point light source expressed in milli-
watts per steradian (mW/sr).
Irradiance (Ee)—Power incident on a given surface at a given distance (mW/cm²).
Reflectivity()—Amount of power reflected from a surface divided by the power incident upon it expressed in (%).
Steradian (sr)—The cone of light spreading out from the source which would illuminate one square meter of the
inner surface of a sphere of 1 m radius around the source.
NIR—The spectrum of infrared radiation in the 720–1300 nm range.
Half-angle ()—The angle measured with respect to the LED's light emission center line at which the radiant
intensity falls to 50% of its max value.
Ie(0)—The peak low duty cycle pulsed radiant intensity capability of a source LED (mW/sr).
Ie(ref)—The radiant intensity expressed in (mW/sr) of the power reflected by an object.
Ee(sensor)—The amount of power incident to the sensor expressed in (mW/cm²).
Rev. 0.1 1/15
Copyright © 2015 by Silicon Laboratories
AN521