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3. How to Choose an irLED

Choosing an irLED for Proximity Detection applications is analogous to purchasing a light bulb. The following steps

are involved in choosing an irLED:

1. Understand the concepts of half-angle, radiant intensity and radiant flux.

2. Determine the irLED half-angle for the intended application.

3. Determine the irradiance necessary at sensor.

4. Calculate the radiant intensity needed to arrive at the necessary irradiance.

5. Make adjustments based on the overlay.

3.1. Understanding Half-Angle, Radiant Intensity, and Radiant Flux

There are three important concepts that are linked together. Understanding these three key concepts is important

to the irLED decision.

the radiant intensity falls to 50% of its max value. It is an indicator of radiation pattern of the irLED.

intensity is expressed in watts per steradian.

In general, the cost of an irLED is linked to the radiant flux (power).

Note: “Radiant flux" is a power measurement and uses the measurement of watts.

To allow us to more easily relate to this concept, we can imagine making a choice between a 100-watt

incandescent bulb and a 15-watt incandescent bulb. A 100-watt bulb is generally more expensive than a 15-watt

bulb; not only in its initial cost of investment, but also in the recurring usage cost as well. The 100-watt bulb is

generally brighter than a 15-watt bulb also.

To minimize cost, the challenge is to choose the right irLED that emits the right amount of illumination (irradiation)

depending on the size and distance of the target we need to illuminate (irradiate).

The next concept is “half-angle.” The half-angle of an irLED describes the radiation pattern of an irLED. The radiant

intensity at the half-angle is ½ the radiant intensity compared to the axial direction. Refer to Figure 1.

The irLED does not radiate equally in all directions. An irLED with a narrow half-angle concentrates most of its

power at a smaller region of space. The radiant intensity becomes higher as a result.

Conceptually, imagine taking a 15-watt bulb and placing it in a parabolic reflector. Most of the light power is

redirected to a certain direction. Fundamentally, the radiant flux (power) has not changed, but, in the intended

direction, the radiant intensity increases significantly. The higher radiant intensity is achieved by redirecting the

power towards the intended illumination target.

Given an irLED with a similar half-angle radiation pattern, the irLED with higher radiant intensity in the axial

direction can only do so if the radiant flux is higher. In the same way, given an irLED with the same radiant flux

(power), an irLED with a larger half-angle would have a lower radiant intensity in the axial direction.

These three concepts of half-angle, radiant intensity and radiant flux are all linked together. It is good engineering

practice to direct all of the available light only in the direction which will illuminate the intended target. This way, it is

possible to choose the least expensive irLED which can do the intended job.

In general:

What these equations imply is that given a fixed cost (implied by the radiant flux rating of the irLED), there is a

trade-off between the radiation pattern (half-angle) and the radiant intensity of the irLED.

We can use a common reflector and a flashlight to allow us to relate to the equations stated above.

A flashlight consists of an incandescent bulb. Without the reflector, a light bulb radiates in almost all directions

(except the direction where the bulb connects to the socket). The reflectors of the flashlight direct the light that