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A lens array will not work for situations like this:
In addition, for the emcal, photon statistics (=light collection efficiency) is less important than uniformity. | 
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| Try this: a Fresnel lens (light green line) followed by this funny collector. The Fresnel lens tips the light cones near the edges inward. The (green) collector is a solid lightguide where the tip matches the SiPD area, and the center of the radius of curvature of the front is at the tip. The curved front (partially) focuses the cones of rays. You can tune the curvature of this surface to increase the focusing of light from the edges at the expense of those in the center, thereby making the response flat. | 
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| First things first: just the Fresnel lens. On the optical bench, use a quartz desklamp to make a difuse lightsource to illuminate the back of a tungsten/fiber module. | 
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| For readout I used a handheld light level meter. ( Center 337 Mini Light Meter), borrowed from the ESH group. The sensitive white spot measures xxx mm diameter. It slips into this custom holder. | 
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| From right to left, the diffuser, square module, fresnel lens, light meter.
The module is doubly tapered, 1x1" on the input side, 7/8" square on the side of the fresnel lens. The lens has a focal length of 1.5". | 
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| This is the actual alignment: the lens is touching the end of the
module, and the lightmeter is placed at the focal distance, 1.5".
On the upstream side of the module is a movable vertical slit, 1.5mm wide, which can be moved in 1mm increments. The procedure (with the room light out) is to position the slit, press HOLD on the meter, slip it out so you can read it, put it back and unHOLD for the next reading. | 
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| This is a plot of the readings. You can see the fall-off at the edges. There are 3 runs: one with just the lens, one without the lens, one with the light guide, cast from resin and polished. Values are in (Lumens?/Candles?) |
Hubert Van Hecke Last modified: Mon Sep 12 10:07:00 EDT 2016