Science Buddies: "Ask an Expert"

For Science Fair this year I'm doing the project "The Future of LEDs Is So Bright, I've Got to Wear Shades". Here is the link...

https://www.sciencebuddies.org/science- ... p003.shtml

Now, if you scroll down on that page to Equation 2, that's where I'm having the problem...

I have to find the optical output power of the LED, in watts... this is the equation listed:

Optical output power of LED (watts) = Nlinear factor × Voltage drop across resistor (volts)
Pout = N × Vres

Where...

Pout is the optical output power in watts (W).
N is a linear factor.
Vres is the voltage drop across the resistor in the light-detection circuit in volts (V).

But the problem comes up with the 'linear factor': What is it? I need the optical output power to find the relative wall plug efficiency, so this puts a major stop in my project.

If anybody would be able to help, it would be greatly appreciated!

Your problem may not be as major as feared. I checked the project page
and the writeup is pretty clear that the linear factor N is difficult to measure
in absolute terms. Thus they recommend carrying the N factor through all
the calculations, expressing final results in terms of the N factor. This
approach is a practical way to express experimental results in a relative,
not absolute, manner. Good luck - you appear to have a good grasp
on the experimental work required!

Regards

Peter Young

The factor is simply the relationship between light intensity and the output voltage of the measuring device. This factor will be constant for a given wave length of light for a specific measuring device over some range of light intensities. Without having a set of known intensity light sources, you will not be able to measure this constant or the range over which it is linear.

Since this experiment is all based on relative intensities, you don't actually need to measure this constant.

Thank you both for your answers! Sorry I didn't respond for a long time.

My science teacher came up with the same solution for us. I guess I was just a bit stressed out because of it, so I got a bit carried away.