I was designing an optical switch based on a LED photodiode couple to detect the presence of an object between them. What i know is, for the LED:

  • Power dissipation [mW]
  • Forward current [mA] and voltage [V]
  • Total radiate power [mW]
  • Radiant intensity [mW/sr]
  • Relative radiant intesity - Forward current GRAPH
  • Optical parameters (emission pattern, half angle,...) but now I think are not important since I am assuming they are on axis.

While for the photodiode:

  • Active Area[mm^2]
  • Forward voltage [V]
  • DC Responsivity [A/W]

Also the distance between them is known (d). What i need to know is to determine the photocurrent of the photodiode, when i supply a certain amount of power/current/voltage to the LED.

What i have thought is to:

  1. Solid_angle = Active_area / d^2
  2. Photocurrent = DC Responsivity * Radiant intensity * Solid_angle

The problem is that in these calculation the radiant intensity is the one of the datasheet, but how it changes based on the way i feed the LED? And how i need to use that graph? What is the relative radiant intensity?

  • \$\begingroup\$ of course the optical paramters of the LED are important. You care about how much of the power is transmitted on the axis. \$\endgroup\$ Commented Oct 27, 2023 at 14:45
  • \$\begingroup\$ If things are as they should be, then is a data line in the datasheet somewhere for intensity to represent 100% LED intensity for the graph. \$\endgroup\$
    – DKNguyen
    Commented Oct 27, 2023 at 14:46
  • \$\begingroup\$ You are right, i said that because along the axis the the relative radiant intensity is 1 (from datasheet) so i thought that i do not have to consider a scale factor \$\endgroup\$ Commented Oct 27, 2023 at 14:47

1 Answer 1


how [radiant intensity] changes based on the way i feed the LED?

It is very close to linear with the LED current.

What is the relative radiant intensity?

It's the radiant intensity relative to the peak intensity which normally is found directly "above" the center of the device. That is, the graph is just scaled to have a peak level of "1" instead of the actual value in W/sr.

And how i need to use that graph?

If you are putting the detector directly above the top of the LED, you don't really need this graph. It tells you roughly how much you could expect the signal to be reduced if you don't align the LED to the photodiode exactly right.


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