What is the formula for calculating radiant intensity from an LED onto a body?

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Discussion Overview

The discussion revolves around calculating the radiant intensity from an LED onto a body, focusing on parameters such as radiant intensity, LED dimensions, beam angle, and distance to the illuminated surface. The scope includes mathematical reasoning and technical explanations related to geometry and energy calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks assistance in determining the energy radiated onto a body from an LED, providing specific parameters including radiant intensity, diameter, beam angle, wavelength, total radiant flux, and distance.
  • Another participant suggests that simple geometry and unit conversions are sufficient to solve the problem, implying that the task does not require advanced expertise.
  • A participant acknowledges the initial request for help and expresses a need for more detailed guidance, indicating that the information provided was insufficient.
  • One participant proposes that knowing the angle of the emitted cone allows for calculating the size of the illuminated spot, suggesting that treating the LED as a point source can yield a first-order approximation for the area calculation.
  • A participant shares their calculation of the beam radius based on a right-angled triangle, using the LED height and divergence angle, and attempts to derive the radiant power per unit area.
  • Another participant corrects their earlier calculation, providing a different method to determine the radiant power per unit area using the area of the spot.

Areas of Agreement / Disagreement

The discussion contains multiple competing views on the approach to calculating radiant intensity, with participants providing different methods and calculations without reaching a consensus on the correct procedure or outcome.

Contextual Notes

Participants express uncertainty regarding the calculations and methods used, with some relying on approximations and others correcting earlier claims. The discussion reflects a range of assumptions about the geometry and properties of the LED.

rayellam
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Hi,
I’m hoping some genius can help me out here. I’m trying to determine how many joules of energy per square centimeter (from an LED) are radiated onto a body given the following parameters.

LED radiant intensity = 80mW/Sr
LED diameter = 5mm
LED beam +/- 20 degrees

Wavelength = 880nM
Total radiant flux = 25mW
Distance from LED to body = 5mm

Thanks
Ray
 
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Simple geometry (and unit conversions) suffices, genius has nothing to do with it.
 
Hi Andy,
Well thanks for the help on this :-0...well I get the genius bit but unfortunately the information to my original question is a little on the ‘light’ side (pardon the pun). I was only looking for some pointers and a little help.

Ray
 
Well, since you know the angle of the emitted cone of light, you can calculate the size of the spot if the LED is 5 mm from your surface to be illuminated. Assume the LED is a point source, for simplicity; this will result in a good first-order approximation. Once you know that, basic geometry will give you the area of the spot. And since you're given the total radiant flux, you can divide one by the other to find the radiant power per unit area.

One Joule is equal to one Watt for one second.
 
OK, sorry may math is not good.
I calculated the beam radius of the point source based on a right angled triangle.

LED height = 5mm
Radiant flux = 25mW
Divergence = 20 degrees

So beam radius = (5 / tan70) = 1.82

Then
25mW/1.82mm = 13.73 mW/mm2

?

Ray
 
oops,
25/(PI R2)
=25mW/10.4 = 2.4mW/mm2
 

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