# how determine aspheric lens beam angle?

by spiffly
Tags: angle, aspheric, beam, determine, lens
 P: 4 Hi, I'm using an aspheric lens with a LED light source for a DIY projector. As I understand it, if I place the led light source at the focal length of the aspheric lens, the light should be collimated(parallel) coming out the other side. As I decrease the distance between the led and the lens, the light coming out should have an increasingly wide beam angle, correct? Since most LED sources have a rough beam angle of 120 degrees, I want to place the lens as close as possible, but I need to make sure that the beam angle projects the light uniformly onto the fresnel lens, and take into account also the fresnel lens fl as well. Does anyone know what formula I can use to calculate beam angle, given the light source distance from the lens? It seems that the focal length and the led distance should be enough to calculate this? Thanks in advance for any help!
 P: 4 Hi, Can anyone help me on this? Stated very simply, if I have an aspheric lens for which I know the focal length, and I place a point light source at a given distance from the lens, how can I calculate the resulting beam angle coming out of the lens? Perhaps the formula would be the same for a simple convex lens? That's what I'm not sure of, any help appreciated:-)
 Sci Advisor P: 5,401 LEDs have a finite size- you cannot collimate the beam below a certain angular spread. Otherwise, you can use the usual lens formulas.
P: 4

## how determine aspheric lens beam angle?

Hi Andy,

Thanks for your response. I do understand that because the LED isn't a point light, even if I place the light source at the focal point, it will slowly diverge; is this what you're talking about by saying it's finite?

So it sounds like I can use the thin lens equation (1/f = 1/o + 1/i) for an aspheric lens to get a rough approximation. Please correct me if I'm wrong!