# Optical switch LED-Photodiode

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]
• 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?

• of course the optical paramters of the LED are important. You care about how much of the power is transmitted on the axis. Commented Oct 27, 2023 at 14:45
• 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. Commented Oct 27, 2023 at 14:46
• 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 Commented Oct 27, 2023 at 14:47