# How determine aspheric lens beam angle?

1. Apr 11, 2013

### spiffly

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!

2. Apr 11, 2013

### spiffly

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:-)

3. Apr 11, 2013

### Andy Resnick

LEDs have a finite size- you cannot collimate the beam below a certain angular spread. Otherwise, you can use the usual lens formulas.

4. Apr 12, 2013

### spiffly

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!

5. Apr 12, 2013

### Andy Resnick

Yes, and yes.

6. Apr 12, 2013

### spiffly

the thin lens equation will allow me to solve for f, or distance to object or image, and for the magnification equation, I can solve for magnification, and object or image height.

In my case I have the light source closer to the lens than the focal point, so these equations just give me dimensions and location of the virtual image, which as near as I can tell is not what I need. I'm simply interested in the height of the projected light at a given distance coming out of the aspheric lens.

Is there a way to use the magnification value to predict the projected light height at a given distance? It seems like it could be used as a rate of divergence?

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