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Abhishek K J

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- TL;DR Summary
- Procedure to calculate radiant intensity of a LED in watts/cm^2 given total radiant power of the LED or luminous intensity of the LED. Note that the target (on which radiation from LED is falling) is placed close to the LED source and is big enough to accommodate circle projected by the light cone from the LED. LED has narrow wavelength band, and assume almost uniform intensity distribution. Viewing angle and other details are accurate.

I am calculating Responsivity of a pn junction photodiode (a.k.a the target) by irradiating radiation from LED sources. For this purpose, i have two LEDs, one UV and another green LED. Note that LEDs are placed close to the target.UV LED : Manufacturer has given total radiant power to be 20mW. How can i calculate radiant intensity in watts/cm^2 ? should i just divide the power of LED by the total area of the target since all radiation from UV LED is reaching the target (since the LED is placed close to the target)?Green LED : Luminous intensity is given in mcd (as"x" mcd). I have converted mcd into lumens then finally to watts as follows :Firstly, i find total solid angle using the expression: Ω (solid angle)= 2(pi)(1-cos(θ)). here, θ is half the apex angle of the light cone. Apex angle is also equal to viewing angle of the LED mentioned in the datasheet.Now, Lumens = Ω*(x) mcd. After this, i use the luminosity efficiency function to convert lumens into watts.Now, given that i have placed the LED near the target, how to find the radiant intensity in watts/cm^2 ? Should i just divide the power by area of the target ?

**Or, should i divide the power of the LED by the area of the circle projected onto the target by the light cone?**