How Much Power Should an LED Emit to Be Visible from 5 Meters?

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Homework Help Overview

The discussion revolves around determining the power output of an LED that would make it visible from a distance of 5 meters, based on the number of photons required for human perception under ideal conditions.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the energy of photons and the necessary conditions for visibility. Questions arise about the assumptions regarding photon interaction with the eye and the geometry involved in calculating the solid angle.

Discussion Status

Participants are actively discussing the parameters needed to calculate the LED's power output. Some have provided insights into the energy of photons and the geometry of the eye, while others are questioning the assumptions made about photon incidence and distribution.

Contextual Notes

There is an assumption that all photons emitted by the LED reach the pupil, and the diameter of the eye is noted. The discussion also hints at the need to consider the solid angle for the calculations.

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Homework Statement


In ideal conditions human eye can feel something if 100 photons with wavelengh of 5500A fall directly tu human eye in one seconds time.LED(lambda=5500A) is placed 5m away from human eye.


Homework Equations



Calculate the power of LED to achieve that the human eye can see it?

The Attempt at a Solution

 
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What is your difficulty?
 
I even don't know where to start here.
Some kind of idea would be helpfull.
 
Each photon carries energy. How much?
 
I know that much
E=hv and v=c/lambda so E=hc/lambda
 
So you should be able to solve the problem.
 
You also need to know how many photons miss or hit the pupil. Perhaps they all bit because of lenses. Or not.
 
Ok I know the energy which is needed to feel something.
I assume that all hit the pupil and that diameter of an eye is d=15mm.
So i can calculate the solid angle.

But still how can I calculate power of LED ?
 
I think you can assume that the LED radiates in all directions. You have a particular solid angle where particular energy must be radiated to be be detectable. What else do you need?
 
  • #10
OK,thanks.
I think i got it.
 

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