Photometric Versus Radiometric...??

I'd like to know how lumens or perceived brightness scales with luminous flux in some radiometric unit....would it be linear?

For example, if you measure the lumen output of an LED at different drive currents and then switch to a quantum sensor that measures number of photons.....and you plot lumens versus photons....what kind of curve would you get?

daschredder said:

I'd like to know how lumens or perceived brightness scales with luminous flux in some radiometric unit....would it be linear?

For example, if you measure the lumen output of an LED at different drive currents and then switch to a quantum sensor that measures number of photons.....and you plot lumens versus photons....what kind of curve would you get?

Click to expand...

Your example is very different from your first question. Your Example has the easier answer: Assuming that the spectral distribution of the light from the LED doesn't change as it is dimmed [in the real world it does somewhat] then yes it is a linear relationship.

For your first question, you will often see people say that it is a logrithmic relationship. This is false. This is a case of people repeating a falsehood until it is accepted as ''fact''. The actual relationship is complicated and varies with many factors including intensity, duration, wavelength, the observer [note, if observer is blind, it is linear].
Some discussion can be found here:
http://en.wikipedia.org/wiki/Stevens'_Power_Law

Thanks for your the answer....sorry for my confusion in the question.

What I want to know is a way to predict PAR (photosynthetically active radiation) values from manufacturers lumen/Watt values. So....given a Cree XRE now with a value of 70 lm/W, and knowing the PAR value from my own measurements....what will the PAR values be if the LED now has a value of 150 lm/W. According to your answer above...I'll assume that the relationship is linear.

daschredder said:

I'll assume that the relationship is linear.

Click to expand...

Only if the spectrum is the same.

I calculated the XR-E luminous efficiency from the spectrographic charts uploaded in the forum.

For a 6402K SPD, luminous efficiency is 310lm per watt of light (emited). Its correspondent PAR value is 4,4 uE (micromols of photons per second).

For a 6660K SPD, luminous efficiency is 276,5 lm/w. Its PAR value, 4,5 uE/w(emited).

To calculate this, i made a Excel sheet wich convert optical mW emited at each wl (1nm bands) by the CIE photopic curve's respective figures. This examples shows the bin tint affect luminous efficiency strongly.