Okay, so this week has a real practical, need it for a job purpose. I needed a neat light source with a narrow <10 degree beam pattern, reasonably uniform across its output, and around 500 Fc intensity on the target surface. I also needed it to have no appreciable spill light, and in a configuration that allowed me some latitude to shield any stray light from a light sensors location. The application is for measuring the transmission and reflective properties of lens and surface materials. While there are lab sources around, between the cost and the fuss, I decided to have a go at one for myself.
This design is based on the idea that if you take the center beam energy from a narrow optic reflector, the inner most 10 degree zone is pretty uniform. All I needed to do was find one, place it on the smallest point source I had with the necessary intensity, and trim away the part of the light distribution I didn’t want. In this case, I found a Lamina MR16, which had a star board style LED mounted inside it (after you pry off its optic – which was too wide for my use.) I used an off-shelf narrow spot reflector, which generally produces a 15 degree pattern, although it has a fat field area around it. I used the Lamina LED, as it has good light output and a decent heat sink design. In front of this, I created the baffle components to produce the finished output I wanted.
While one could argue that I could have simply cut a hole in a piece of black cardboard and accomplished the same result, one would be wrong. First, a large flat board with a hole in it is a pain in the tail to work around. Second, the reflected light from the back of the board just scatters everywhere, making control of it difficult. I am speaking from experience here, as that was the first thing I tried, along with black foil and other cheesy messes of gaffer tape and foam core.
This little gadget first uses a short snoot to hold the reflector over the LED and to provide cutoff to about 80 degrees (full width). The first cone strips the beam down to 20 degrees. The second cone cuts the beam down to its final 10 degrees included. The geometry of each outer edge of each cone provides complete cutoff from the perimeter light of the layer below it. The aperture edges are kept as thin as practical to eliminate any potential scatter, resulting in a nice tight pattern.
While there is no way to absorb 100% of the light inside the baffle or from the rear of each cone, the remaining light is emitted from within the confines of the gadget, so shielding it from the light sensor’s view is very easy to accomplish. I use either a baffle panel, some foil and a little tape, or just increase the distance from the background so any light reflected is so diminished by the time it reaches the sensor it is within the noise of the target reflectance and scatter, so is irrelevant. It’s easy enough to test how much is being contributed by simply placing a little tab of tape over the final aperture and taking a reading. This can be subtracted from final readings where it is found to be of importance.
The whole assembly snaps together, so I can change the configuration. For example removing the forward cone produces a larger 20 degree pattern. I can make custom cones, add cones, make inserts to reduce the aperture size, or add mounting features where they might be handy. The base of the unit has a 1/4 x 20 threaded boss for mounting to a small tripod. With this and a couple of first-surface mirrors, I can complete a wide range of tests, from transmission of materials to reflectance of surfaces, including specular reflection testing. If I find I need a little change here or there, it’s pretty simple to make a new part and snap it on where its needed. I can also swap the LEDs out, and the optic, to produce more light, test single color responses, or test an LED against another using the device as a convenient holder/beam angle trimmer.
Not the most exciting thing in the world, but its simplicity belies its usefulness and utility. Works for me…
More details can be found at Lumenique 52 in 52 D30. The web site also has a few images of configurations I have used, or intend to use this light source in to complete some design testing in process.