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I take photos with my Canon 20D on a Kaiser Reprostand of books for publication, lectures or documentation. I get sometimes uneven/gradient lightning, especially when using a white background. When taking several photos with the same setting some might be evenly, others unevenly lit. The tint happens to be going towards blue/green tinted on one side and red/yellow tinted on the other.

This is almost identical to my setting:

enter image description here

I have the same lights: 2 Kaiser RB 5000 with each having in my case two Osram L18W/12. Why does this gradient appear? Why is it not always there? How can I produce photos with a homogeneous lighting? I am focusing on solutions in production and not in postproduction.

This is an example picture I took, I just deleted a book in the center: enter image description here

SEJU
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2 Answers2

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Likely you are picking up some ambient light from the ceiling light fixtures. Try turning off the room lights. Also, do you need the images to be in color, why not switch the camera to black & white mode?

Alan Marcus
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  • Thank you very much for your input. I turn off the room lights, but will check if there are other light sources, which might interfere. I need color images. What I do not get is why, if I take 3 pictures in short sequence the lighting differs. I was wondering if this is related to the DSLR, exposure, my light sources. – SEJU Aug 17 '22 at 15:52
  • For critical color work, the exposing light source should be plugged into a constant voltage transformer. This avoids voltage fluctuations induced by other equipment being turned on and off. Get one rated 125% higher than the load (Watts) used by your lights. – Alan Marcus Aug 17 '22 at 16:16
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I see that those L18W/12 are fluorescent lamps. Probably the lifespan of the tubes is coming to an end. I recall seeing some fluorescent lamps actually having different tones across the tube.

enter image description here

So a first option would be changing the lights.

Additionally, let's try to improve a bit the uniformity of the light.

  1. Put the lights the furthest away you can.

  2. If the lights are not hot, try putting some vegetal paper to make some additional diffusion. See if you can have some separation from the lights.

  3. I do not know if you have some flickering happening there, so use a slow shutter speed, lets say 1/15 of a second or less.

Rafael
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  • The exposure time appears to be the key! I did a test run with different exposure times and with 1/20 of a second everything appears to be ok for now, while with 1/400 - 1/800 I have very uneven lighting. The lamps seamed to be ok, but I noticed a very slight "flickering" on one side of one lamp. I will make another session with 1/20 and report back. The "flickering" is very hard to notice and very light, but that might well be the cause. – SEJU Aug 19 '22 at 19:55
  • Fluorescent lights, even new ones, will flicker, usually at twice the frequency of the alternating current powering them. In countries with 60Hz AC, they'll flicker at 120Hz. In countries with 50Hz mains power, they'll flicker at 100Hz. As they age the depth of the trough in each cycle will get dimmer, as will the height of the peak. Please see What causes these inconsistent dark bands in some of my photos from an indoor event? The shorter the exposure time, the more severe the effect. – Michael C Aug 22 '22 at 10:51