Infra red LED intensity

[dr_when]

Hi,

Can anyone recommend an inexpensive radiometer or photometer that can be used to measure (even relatively) the intensity of an infra red LED. I need to see if the LEDS we are getting are varying in intensity according to manufacturers batch, etc. I know I can make something if I had to but just wondering if someone has used anything off-the-shelf.

Thanks,
Bob

[rshayes]

If all you need is a relative reading, then a solar cell and a digital multimeter may be good enough for your purposes.

The responsivity of a silicon detector is about .6 amperes/watt at the response peak, which is near 900 nanometers. A 1 milliwatt LED would generate about 600 microamps. It should be possible to read this on the 2 milliamp scale of a digital multimeter. Since the voltage drop across the DMM will only be 60 millivolts, the response should be fairly linear, since the solar cell will be operating close to a short circuited condition.

Higher powers might require higher scales on the DMM, but the voltage drop of the DMM can be kept below 100 millivolts by switching to higher current ranges.

[cato]

....another route is to "look" at them with a digital camera ...like a quickcam internet/webcam. The cameras are cheap and "see" into the infrared. If they look brighter or dimmer on the computer, they probably are.


If you want numbers, then a photo diode and an oscilliscope should do the trick.

Of course, the point or viewing angle/distance will effect your readings.

[Chris Smith]

The best way to consider IR Leds as being all the same is to use two ammeters, a ten turn pot, and a solar cell with sub IR filter.

Place the IR filter on top of the Cad cell so that it blocks all light except IR it self. The cut off frequency of the filter should be around 650nm and below.

The filter cut off should block all other frequencies of light, except per the frequency you require.

Have all the Leds and the solar cell placed at a equal distance from each other for comparison, and have a 10 turn pot feeding the IR led with a digital readout, and the Cad cell also with a digital ammeter read out.

Using the LED in question, you crank it up to the given value and mark how many milli amps it took to achieve this goal. Then each led can be fitted with an individual resistor to get the same brightness all at their individual amperages.

All reading will be arbitrary but you can set the limit below the specs, and read all others from there.

The more expensive and accurate way is to use something like the hand held Bauch & Lomb colorimeter, which uses the prism and light gauge, but this method is far more expensive is accurate down to the NM while the percentage scale is far more accurate than need be for most applications.