How can I arrange LEDs to build a uniform diffused backlight for digitizing film?
Asked 3/27/2023
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I want to build a homogeneous LED light source for scanning negatives or slides with a DSLR. The goal is an even, diffused light field behind the film. I can use individual LEDs, potentially of different wavelengths, and I can 3D-print a housing if needed.
What layout works best for uniform illumination: LEDs on the side walls of a white chamber, on the rear behind one or more diffusers, or aimed indirectly into the box? Does the chamber shape matter much (box vs sphere), and is a highly diffused integrating chamber a better approach than trying for collimated light?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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Likely the best lamphouse design would be collimated light. Meaning the light rays from the light source arrive at the negative / slide as parallel rays that uniformly illuminate. The design of such a lamphouse mimics the condenser enlarger. These utilize two plano convex “condenser lenses”.
Such a design is likely best as the parallel rays deliver an image that is tack sharp and the image has high contrast.
Such a lamphouse has the undesirable trait of showing every speck of settled dust. Best is a lamphouse that suppresses dust spots.
The best design for dust suppression is a lamphouse that delivers a uniform light that is highly diffused. This will be an integrating chamber. This can be a box, or a sphere painted flat white. Light is allowed to enter the integrating chamber via holes in its walls. The light reverberates about and arrives at the negative / slide with no apparent direction.
A mirror box integrating chamber is often used for this application. Four mirrors are mounted together at the edges to form a box or tube. The light is directed into this chamber, it reverberates about. In the process it arrives at the negative / slide totally diffused.
Such integrating chambers cannot eliminate dust spots, but they totally diffused light acts to suppress the visibility of dust on photo materials. Let me add that the end of the mirror tunnel, next to the negative / slide is covered with milk glass or milk plastic (white translucent diffuser).
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
3y ago
0
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For film digitizing, a highly diffused light source is generally the safer choice if your goal is uniformity and reduced visibility of dust. A collimated source can give very sharp, contrasty results, but it also tends to reveal every dust speck.
The usual solution for diffuse, even illumination is an integrating chamber: a box or sphere with a matte white interior. Instead of pointing LEDs straight at the film, let the LEDs shine into the chamber so the light reflects multiple times before reaching the diffuser/output area. That mixing action is what evens out hotspots and blends different LEDs.
So, a practical approach is:
- use a white reflective chamber,
- place LEDs on the sides or otherwise out of direct view of the film,
- aim them into the chamber rather than directly at the diffuser,
- use a diffuser at the output.
Shape matters less than good internal mixing. A sphere is ideal in principle, but a well-designed white box can work well too. The key is preventing direct LED images from reaching the film and giving the light enough space and reflection to homogenize before it exits.
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