Re: how to get a crisp circle of light

On Mar 28, 1:30 pm, "Boxman" <box...@xxxxxxxxxxx> wrote:
On Mar 28, 12:01 am, "rahul" <rahul...@xxxxxxxxx> wrote:

Hi,

Pardon my non-optics language - I am a product designer by profession
and need your help/guidance. Currently designing a special purpose
Flashlight based on 1W Lumiled LED (beam angle-125). My concern is to
get a very uniform spot of bright light of about 50 mm at a distance
of 420 mm. Beyond the edge of circle there should not be any light
ideally.

Can some one guide me how to go about it ? I have been experimenting
with off-the-shelf optics available for the LEDs but with no luck.

Thanks
Rahul Kaushik

There are a couple of issues with the suggested approaches using the
lens imaging/aperture techniques discussed.

1. Using an imaging lens that is focused on the LED will essentially
re-image the LED onto the imaging plane. If you look at the LED you
are using, you'll see that there is about a 1 mm square chip in the
center, with some surrounding geometry encapsulated in a dome that is
around 2.5mm radius. When this is lit up, about 60% of the light
comes from the top of the chip, but 40% comes from the area
surrounding the chip contained within the dome. The light coming from
the top of the chip is very bright compared to the surrounding 40% and
using an imaging lens to project the LED onto the plane will basically
replicate this brightness distribution on your image plane which will
make your uniformity very undesirable.

2. Since you are limited to a 30 mm diameter, and you are asking for
a half angle on your beam of approximately 3.4°, using an imaging lens
set up in this case will significantly limit your total light
collection from your LED. Even if we assume the source is essentially
a 1 mm square, your lens would need to be about 17 mm away from your
LED to keep your beam angle narrow enough to meet your beam size
requirements without an aperture and therefore only collects about
50-60% of the outgoing light from the LED. Recalling that only 60% of
the light actually comes from the 1 mm square chip, you are already
very low on your efficiency if that is a concern.

Having said that generally speaking, there are a couple of other
approaches that can be considered but will probably require
specialized software and expertise to execute. Here is a link to an
article that explains the gist of these techniques.

http://www.laserfocusworld.com/display_article/216614/12/none/none/De...

If you can use a two piece design, using an optimized metallized
reflector in combination with a faceted aspheric lens over the center
of the LED would probably be a good solution as well.

If you cannot find a suitable solution for yourself, there are several
companies that can do the custom design for you. Most of the standard
LED collimator suppliers can design custom optics for you as well and
would be a good place to start for finding someone to do your
project.

I think you misunderstand the solutions. The LED structure is not
imaged on the final screen, the aperture is. This is the same
illumination method used by slide projectors and microscopes. It
produces a very good flat illumination and sharp cutoff for very
simple optics.

As far as illuminartion is concerned, this depends on the how much of
the LED cone of light one can collect. For many LEDs there is a lens
already on the LED which reduces this cone, making the design of
ilumination systems simpler. (I don't know if the poster's LED is
lensed.)

Please visit my web site at www.richardfisher.com

.

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