I made a light box sometime ago for the large telescope here, here is a description
Light Box
The light box needs to be light in weight and ease of use, uniformity of illumination and ultimately acceptable flat field correction.
The light box is constructed of 1/4" foam core. Each sidepiece is exactly 41.25” x 41.25" (4 pieces). The top is 40.75" x 40.75". The assembly is held together by a combination of glue and thin aluminium. The completed box is attached to the white plastic plate by glue. There are two diffusers each square is 40.00” x 40.00” positioned 2” apart within the square box. Each diffuser is bonded to a circular opening 38.5” each with the separation of 2”. The whole structure is tenaciously bonded with glue. All the edges are sealed with thin aluminium.
The whole structure is solid and incredibly light for man handling.
The illumination source and box proportions are key to a successful design with uniform illumination.
The ideal geometrical arrangement is when the most uniform illumination occurs when the distance from the inside top of the box to the LED's is equal to the separation of the LED's. The concept is that the top of the box acts as a first diffuser and by locating the illumination sources near the corners of the box, the resultant light combines to a uniform level at the plastic diffuser. The diffused light from the first diffuser projects to the next diffuser. The next diffuser smoothes out any remaining gradients. Flat field should be better than 0.5% with this design.
The design uses 8 LEDs. They are ultrabright white LED 5mm, 11000mcd. Nichia in Japan manufactures the bright white LEDs. They combine a high quality phosphor with their recently commercialized indium-gallium-nitride (InGaN) blue LED. They found that when blue light from the InGaN die passes through a thin phosphor coating, a portion of the blue light is down-converted to yellow light. This yellow light mixes with the remaining blue light to create a bright white light.
The emission spectrum of this bright white LED shows the blue emission from the InGaN die at ~460nm and the yellow (shifted light from the phosphor) peaking near 560nm. There is little blue intensity below 450nm, where as astronomical blue filters on CCD cameras extend down to about 370nm. The yellow shifted light extends to about 750nm, which covers most red astronomical filters.
Eight micro drives modules were built to power the Nichia LEDs. The driver was designed to provide maximum illumination to the LED while mimicking the light drop off of an incandescent bulb, which dims as the batteries are used up. Unlike an incandescent bulb, the driver’s current consumption drops at very low voltages, allowing usable light to be produced much longer. The optional universal power supply 3-12 Volt/800mA provides better sustainability.
A second key item were the LED holders. These holders support the LED and act as a baffle to prevent the light from directly illuminating the diffuser.
The LED's and holders are mounted near the centre of a 1" x 1" square of foam core, glued to the inside corners of the box with adhesive and located approximately above the plastic diffuser. They are located around 10.00” above the plastic. Spacing is not critical except to maintain the dimensional ratio mentioned above. Leads are through the side of the foam core and the wiring is outside of the box.
Eamonn A
www.kingslandobservatory.com
