Q. Visible LEDs seem to be specified in millicandelas, while IR LEDs are specified in milliwatts. What are millicandelas (mcd) and how can I compare them to milliwatts?
Candelas, candles, foot-candles, lux and related visible light output units are all photometric units, meaning that they are measured using devices that mimic the human eye response. Watts are radiometric units, meaning that they are used to measure power without regard to the response characteristics of the sensor.. The human eye has its maximum sensitivity in the yellow green part of the spectrum, where it has easily 50 times more sensitivity than it does for deep red light. In other words, the sensitivity of the human visual system varies strongly as a function of wavelength. LEDs emit light over a range of wavelengths, and human eye sensitivity is different at each wavelength.
If you want to get an estimate of how many milliwatts are output by an LED with a given millicandela rating, there is a procedure for Using a triple convolution integral, it is possible to convert from mcd to mw, there's an easier way. Most silicon photodiodes convert light to current with an 'efficacy' of about 0.45Amps/Watt. So, procure a large area photodiode and connect it to a low impedance current metering device. Good quality digital multimeters work well, but cheapies won't do. You need to present nearly a dead short load to the photodiode. If you have any trouble getting a big photodiode, see about getting a single solar cell (same thing). You need a photodiode that is at least as big and preferably bigger than your red LED diameter. Fire up the LED, position it quite near to the photodiode and aimed squarely at it, and measure the current. Adjust the LED current (vary voltage, series resistance, etc.) until you get about 2.5 milliamps photocurrent. Since 2.5/0.45 ~ = 6 you know that about 6mW got converted to about 2.5mA! The large photodiode is monitoring total power.
The photodiode should not have any magnify
