Why did a red signal light photograph as white, and its black hood look red?
Asked 11/24/2011
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I photographed a train-station signal with a Canon Rebel T3i and EF-S 18-55mm IS II lens at 1/8s in Av mode, with the aperture at f/4.5. In the scene, the signal light itself looked red and the hood around it looked black. But in my photo, the light appears white and the hood appears red. Why would the colors shift like this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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The reason is that the red light is a light source, therefore it's much brighter than any other parts of the scene. The pixels showing it are overblown - meaning there was more light coming than your camera sensor could capture. The light is not pure red, it emits enough green and blue light to blow these color channels of pixels too.
The hood is just reflecting light from it. It was hard to see with a naked eye, because the light beneath it was much stronger and the hood seemed perfectly black compared to it. In photo, however, the intensity of captured light is capped by your camera sensor's dynamic range, and you can see the subtle reflection next to overblown pixels.
Unfortunately, overexposed pixels cannot be restored in post processing. You could try simply painting the pixels with solid color or slight texture. As evidenced by blowing all color channels, the color wasn't pure red, so it might take a bit of playing with colors to get it looking right. Judging by the glow, the color seems to have more blue than green.
To capture a similar scene with correctly exposed light source, you should take a second frame (from tripod in same place) with exposure reduced to a level where the light source is exposed correctly, and then either create an HDR image (might not work too well due to steam changing between frames) or simply use layer masking in Photoshop/Gimp/etc to select which parts of image should come from which frame.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
14y ago
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The signal lamp was much brighter than the rest of the scene, so it likely overexposed the sensor. When a very bright red light is intense enough, it can clip not only the red channel but also green and blue, causing the recorded light to appear white instead of red.
The hood likely looked black to your eyes because it was much darker than the glowing lamp, but it was still reflecting some of that red light. In the photo, once the bright lamp is blown out and the camera compresses the scene’s brightness into its limited dynamic range, that reflected red becomes visible, so the hood looks red.
In short: the lamp blew out to white, and the hood showed reflected red light that your eyes didn’t notice as strongly in person.
Once highlights are clipped, the original color usually can’t be recovered in post. To avoid this, expose for the bright light source (use a faster shutter speed, lower ISO, or exposure compensation) so the lamp doesn’t blow out.
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