Focus or parallelize the light from an LED

[Lexinas]

Hello, everyone,

I have a LED, 620nm, with a viewing angle of 20°. I would like to bring this light in a distance of about 20cm on a lens with as many photons as possible.
My idea is to position a converging lens between the LED and the lens. To be exact, I would position the lens at the distance of the focal length to the LED. To get as many photons as possible into the lens, the lens should be as close as possible to the LED so that as much light as possible goes through the lens and that would correspond to a very short focal length.

Are my considerations correct so far?

Best regards
Lexinas

 

[Drakkith]

Yes, that's right. The closer, stronger, and larger the lens, the more light that you can intercept with it and redirect onto the 2nd lens.

 

[sophiecentaur]

A compound arrangement with two lenses could do what you want without needing a single large diameter, lens.

eBay has a selection of many different second hand SLR camera lenses you could play with and the cheapest lenses are only a few quid each. (Even the lowest spec lenses are actually pretty good.)
I do appreciate that you are not planning on building a whole Optics lab but you could find it interesting and get a result.

Do some sketches , using the dreaded 1/f = 1/u + 1/v formula. The first lens will form one virtual image behind the LED and the second lens could focus that intermediate image to a point where you want it. There are a host of possible solutions, depending on the lenses you can get hold of.