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I am making a basic circuit to test the responsivity of a phototransistor.

I made a basic circuit using a phototransistor connected to 5V and a 22kOhm resistor. On the other side of the phototransistor is a Red/Green LED that has equal mcd ratings for both colors.

My problem is that, even when I calibrate for I_f, the detected voltage from the red LED is much higher than the green LED -- to the order of 2.5x or higher. This result does not agree with the phototransistor datasheet, which says the difference in responsivity is not that high.

For reference: I am using the PT908-7C-F phototransistor and the SSL-LX2577IGW R/G LED. However, these results seem to be independent of phototransistor and LED, as I have been able to reproduce this result with another phototransistor/LED setup in the past.

I am at a loss as to what could cause this result. Short of obtaining a laser/light power meter from my other lab to verify my results, what could be a possible explanation? mcd ratings are not reliable? Line width? Actual responsivity curve more steep than reported on datasheet?

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The two LEDs are specified to emit the same luminous intensity, measured in candela. Luminous intensity is normalized to the human eye response, so that a red LED and a green LED emitting the same luminous intensity (10 mcd, in your case) will appear to be equally bright when seen by the human eye.

The responsivity of the photodetector, on the other hand is specified in terms of irradiance (mW / cm2). There is no correction for the human eye response, and indeed your device is sensitive well into the infrared, where there is practically no response in the human eye.

The Wikipedia article Luminous Efficiency gives a curve that shows the standard assumptions about human eye response that relate these two ways of measuring brightness.

enter image description here

Given the two wavelengths involved (565 and 635 nm) it looks like the luminosity difference between these two wavelengths is pretty close to 2:1, which means your red LED must produce close to twice as much optical power to have the same perceived brightness as the green LED.

This accounts for a large part of the 2.5x difference you are seeing in the response to the two LEDs. Another factor of 1.2x or so can be attributed to the difference in spectral sensitivy shown in the phototransistor's datasheet. 2x times 1.2x is 2.4x, so that's pretty close to explaining an overall 2.5x response difference.

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  • \$\begingroup\$ Thank you so much! I didn't realize that Candela/Luminous Intensity was normalized to the human eye. I also mistakenly thought that Luminous Intensity and Irradiance were interchangeable in this application. \$\endgroup\$
    – user39532
    Commented Mar 30, 2014 at 22:22
  • \$\begingroup\$ Unfortunately, my numbers don't add up. I found actual tables of the standard luminosity curve, and the red LED should actually have closer to 5x the output power to achieve the same luminosity as the green LED. So really, if you measure a 2.5x difference there's actually about half the red light being lost somewhere. \$\endgroup\$
    – The Photon
    Commented Mar 30, 2014 at 22:26
  • \$\begingroup\$ Some of my older data with my previous setup was definitely around 4-5x. \$\endgroup\$
    – user39532
    Commented Mar 30, 2014 at 22:39

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