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otherwise would have been wasted. The radiant intensity increases in a given direction, while radiant intensity

becomes absent in other directions.

Given that the concepts of half-angle, radiant intensity, and radiant flux are related, it is sufficient to describe the

requirements for two of these concepts. It is best to examine the half-angle requirements and the radiant intensity.

For effective cost minimization, it is a matter of choosing the irLED with the minimum half-angle radiation pattern

that meets the radiant intensity requirements for the application.

One final detail is that much of the discussion on radiant power and radiant intensity thus far has been mentioned

as a property of the irLED. This is not strictly true. Making such a statement would be similar to saying that a 100-

watt bulb expends 100-watts of power when it is not in a light socket.

The reality is that for any given irLED,

When choosing an irLED, the radiant flux and radiant intensity are specified given a fixed current.

In a similar way, a 100-watt bulb does not always expend 100 watts. The purchase of a 100-watt bulb presupposes

that it is to be used in a known power system. For example, in the United States, we should expect that a 100-watt

bulb would dissipate 100 watts only when it is connected to a 120 Vrms power system.

It is possible to take that same bulb and use it in France where the power system is 220 Vrms. For a short period of

time before the filament burns out, the 100-watt bulb would actually be emitting light based on 484 watts of power

and would be much brighter for this short period of time.

The radiant intensity and radiant flux rating of any irLEDs are typically associated with a given DC current level. It is

generally permitted to drive the irLEDs with a higher peak current as long as the duty cycle is reasonably short.

Pulsing the irLED allows higher radiant intensity during the on-period of the pulse.

3.2. Calculating the irLED Half-Angle

Distance and width of coverage are used to determine the half-angle (

) of the irLED to be selected. Once you

have decided on the range required for your application it is a simple matter of trigonometry to determine the half-

angle as shown in Figure 1. For detailed information related to half-angle and LED power definitions see the

Appendix (Optical Power Primer).

Figure 1. Width of Coverage