Calculating the Optical Output Power of LEDs.

[Rai]

I am doing this project: "The Future of LEDs is so Bright, I've got to wear shades." https://www.sciencebuddies.org/science- ... p003.shtml

In the project, I need to calculate the Optical Output Power of the LED.

I understand that the Optical Output Power = N(linear factor) × Voltage drop across resistor (volts)
Pout = N × Vres

However, I am confused on what the linear factor, "N" is.

[barretttomlinson]

Hi,

I agree with you that the experimental writeup is silent on how to determine the value of N, which is a conversion factor from voltage drop across the light to voltage device to output light power of the lamp in watts. I would advise reporting output power as N*(value of Vres)., and note N is a constant, but the value is not known. Since you are instructed to keep the geometry and everything else constant when testing all devices, and since you are trying to measure only relative efficiencies between devices, doing this will not change the relative results. In principle you could measure N for your particular setup - to do so you would use a source with a known optical output power in place of your LED in your test rig. You could then determine N by taking the ratio of the known optical output power to the measured voltage output of your light detector. In practice it is very difficult to do this very accurately.

This looks like a fun project that could have great practical importance, Have fun with it!

Best wishes,

Barrett Tomlinson

[Craig_Bridge]

which is a conversion factor from voltage drop across the light to voltage device to output light power of the lamp in watts.

The output light power is typically given in milli-candles (mcd) or candles. The electrical input power to produce the light is typically given in milli-watts or watts. See http://www.brillianz.co.uk/data/documents/Lumen.pdf for a brief description of light intensity (lumens, lux, etc.) and how the geometry affects these. The original scale for preceived brightness was to compare the illumination to that of a candle. The other aspect of this "N constant" is that the relationship is probably not linear over the entire range. At very low current levels leakage current and thermal affects will likely cause less light to be produced per mW. At very high current levels, saturation will likely occur caussing less light to be emitted per mW. Effectively this equation is a simplification that is only accurate for the "linear" range of the device.

The typical way that light intensities from two point light sources are compared is by viewing shadows of an object cast at 45 degrees on a uniform reflectance wall and adjusting the intensity of one of the sources by the use of distance, aperture, or neutral density filters until the two shadows appeared to be the same. The optical factor from the distance, aperture, or neutral density filter then determines relatively how much weaker the non-filtered light source is. To be accurate, the remainder of the room or test fixture needs to be a low reflectance surface, typically painted flat black. If the standard light source is a wax candle, then the result is a comparison to a standard 1 candle power source.