TL/DR: It's possible to calculate LED efficiency indirectly by measuring heat output.
I measured the efficiency for some '10W' LED chip modules, this type:
For each LED the efficiency falls as the drive current increases. Don't trust the '10W' rating given to the chips, it's not necessarily the best operating point. At the maximum power most of the energy will end up as heat, which is a problem as high temperatures can degrade the LED. The maximum junction temperature for these LEDs is quite low, typically 85C, and the corresponding maximum heatsink temperature can be as low as 40-60C at max power.
All of the LEDs I tested are based on variations of gallium nitride semiconductors (combinations of aluminium, gallium, indium and nitrogen Al/Ga/In/N). The typical emission wavelength for these semiconductors is blue, but by adjusting the ratio of aluminium and indium it can be shifted all the way from UV to turqoise or emerald green. The 'Cool White' and 'PC Orange' LEDs use phosphors to convert some or all of the blue light into yellow/orange wavelengths to produce the final colour. This extra step limits the efficiency, for example the low efficiency of the 'PC Orange' LED. The white light has a high efficiency because it allows a large fraction of the directly emitted blue light to escape from the package.
The 480nm and 490nm LED chips are relatively special items, as these are direct emission LEDs (ice blue and cyan/turquoise colours). Cheaper chips will use a standard blue emitter (450-460nm) and a green phosphor (530nm) to produce a colour which appears identical, but for biological purposes there are specific uses for true 480-490nm LEDs. They are not cheap! They cost more than 10x the price of the white LED module, and have about half the efficiency. The reason for this can be seen in the photographs, it requires far larger semiconductor dies to achieve the same power output. This is due to loss mechanisms which arise when shifting the band-gap of the Ga/In/N semiconductor.
When calculating the optical power output, it generally increases as the current rises. For the orange LED however, there is almost no increase above the 3W power level! I would be curious if anyone has had the same result with fully phosphor converted LED output.
Measuring LED efficiency
Measuring LED optical power output by measuring waste heat relies on a number of approximations, it's not going to be accurate! The first is that all power input to the LED results in one of two outputs:
- Radiated power (infra-red, visible, etc)
- Conducted power (heating)
For the radiated power the following approximation is used: that all radiated power is at the desired wavelength. LEDs do have a relatively sharp emission function, and most of the optical power will be at useful wavelengths. Unlike incandescent lamps there are limited tails of energy in the IR and UV range. There will be some deep IR due to the temperature of the die, however this is limited by two effects:
- The 'hot' part of the die is quite small. In the 490nm die, there is only 9mm2 of hot surface to radiate heat.
- The system is well thermally coupled, and the peak temperature should be below 85C (compared to 2000C for incandescent). The LED module has a 1mm thick aluminium heat-plate, which is screw mounted onto a copper heat-spreader with a thin layer of thermal paste.
For conducted thermal power, the temperature change of a heatsink is used to measure the output. This works by first calibrating the thermal mass of the heatsink using standard resistors, and then using this to calculate the energy input to the heatsink during measurement runs. There are a number of potential sources of error.
- The heatsink loses heat by conduction, convection and radiation to the environment as it warms up. This is handled by first calculating the coefficient of thermal conduction to the environment when using resistive heating, and using this to compensate in measurement runs. To a first approximation it's a single linear term, it's not perfect.
- The geometry of the calibration resistors and the LED package is not identical. The thermal conduction directly from the resistors to the environment is likely to be higher than that from the LED packages.
