Why does the camera output an image with multi-colors not shown by the naked eye under the projector?

Question

I was shooting in a gallery that used what appears to be a multi-colored projector and had difficulty shooting in that type of scene as the color was not even across the scene. The image shown below is what I mean, you see traces of green, blue, and yellow colors in the picture when it's supposed to be all white/gray as seen by the naked eye.

Why does this occur to the camera's output (DSLR or mirrorless) when shooting a scene that's lit by a projector? Is it the type of projector used? Is there a way to fix this or camera settings wise?

Answer 1

It's the difference between how your camera and your eye/brain see the scene.

Many projectors scan each color channel in sequence very rapidly. It's too fast for our eyes to pick it up, as our brains average the sequence of each color channel. But if our camera is using a shutter time that is shorter than the refresh rate of the projector, it only sees part of the color sequence.

This is particularly true of most consumer cameras that use either a mechanical shutter that starts exposure on one side of the sensor and ends on the other or an electronic shutter that scans each line on the sensor in sequence from one side to the other (or from top to bottom). With the mechanical shutter, faster shutter times are exposed with the second curtain chasing the first curtain across the sensor. Only a narrow slit is being exposed at any one time. As the slit between the two shutter transitions the face of the sensor, the projector is flickering between each of the color channels it uses. Even with a shutter time of 1/8000 second, it still takes about 1/250-1/400 second for the slit between the two shutter curtains to transit the entire sensor. Electronic shutter is similar as each line of the sensor is read in sequence. When the last line is read then the camera starts reading at the beginning again.

The solution is to increase exposure time long enough to cover a full cycle of the projector's sequence as it goes through the color channels. In order to prevent overexposure, you may need to reduce ISO, close down the camera's aperture, or even use a neutral density filter.

Answer 2

If multiple projectors were used, the white balance may not have been consistent among all of them. They may have initially been set to match, but the color temperature can change as the lamps age. We don't normally perceive the differences because our brains compensate. The differences among light sources can also be exaggerated by increased saturation settings.

You can try decreasing saturation. Use layer masks to limit the adjustment to the affected areas.

Answer 3

My guess is the projectors are not projecting the image in one hit like a film projector, but are scanning or interlacing red/green/blue much the same as TV's do, and your camera is faster than your eye.

If you slow the shutter speed to more than 1/10s it might "fix" the problem as your shutter is open for long enough to capture at least one full frame.

You can google how projectors & TV's work if you need a bit more explanation.

Answer 4

Projected images are designed to be observed by human eyes. Projectors and screens apply additive color theory to match colors in the real world. When you take a photograph of a real scene. That scene contains the full spectrum of light typically. A screen instead will only contain a set of primaries which are mixed together to create what will appear to be the pure wavelength of those real colors. But only to a human observer.

For example:

In real life if you photograph a banana, it's spectral reflection will be real yellow light at around 580nm. Where if you photograph a banana on a screen, the screen will transmit a metameric combination of red (650nm) and green (550nm) light which produces that same 580nm color. But the problem is that the combination only produces a color that appears to match 580nm to a human observer.

Here's a bit of basic color theory.

Spectral power distribution of light source(λ) * Spectral reflectance / transmittance of object(λ) = Color stimulus(λ)

Color stimulus(λ) * CIE 1931 color matching functions(λ)(c) = XYZ tristimulus values

The tristimulus values define the spectral responsivities of the human eye when viewing a particular color stimulus. The color matching functions represent the chromatic response for each cone in the human eye. What's important to note here is that 580nm light and a combination of 650nm and 550nm light will produce the same color to a human observer if the tristimulus values are identical.

Since projectors are designed to metamerically match to a human observer, digital camera sensors on the other hand have their own set of spectral sensitivities determined by the quantum efficiency for each rgb filtered photosite.

Color stimulus(λ) * Spectral sensitivity of camera sensor(λ)(c) = camera spectral responsivities(λ)(c)

Those camera spectral responsivities are different to the spectral responsivities (tristimulus values) of a human observer. Thus the combination of RGB light that was used on the projector to produce white to the eye, does not produce white on camera.