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Colour cameras on Mars

Waves, sound and light · level 6, lesson 6 of 6

A question to start

NASA's notes on Curiosity's sunset photo say the colours were "calibrated and white-balanced". If a camera just records light, why do the colours need adjusting?

The idea

A digital camera has millions of tiny light sensors, pixels. On their own, they only measure brightness. To record colour, each pixel sits under a tiny filter: red, green or blue. The camera combines the three to make each colour.

But the camera's filters do not exactly match human eyes. NASA notes that Curiosity's Mastcam sees colour very like people do, but is a little less sensitive to blue. And the light on Mars is itself different: dimmer, and tinted by the dust.

So scientists calibrate images. Perseverance carries a calibration target with colour and grey swatches; the cameras photograph it, and the team uses the known colours to fine-tune the settings. White balance is one adjustment: it shifts the colours so that a white object looks white, as it would under sunlight on Earth. This helps geologists compare Mars rocks with rocks on Earth.

colour filtersover the pixelseach pixel sees one colour

Worked examples

1. Mixing. A pixel area that reads strong red, medium green and weak blue makes an orange colour.

2. White balance. If a white target looks slightly yellow in a Mars image, the software turns down red and green a little, or turns up blue, until the target looks white.

3. Two versions. Scientists may make a "natural" image, close to what a person on Mars would see, and a white-balanced one, showing the rocks as they would look on Earth.

Try it

  1. Why does each pixel need a colour filter?
  2. What does white balance do?
  3. Why might a camera image of Mars look slightly different from what a person would see?
  4. Why do geologists like white-balanced images?
Answers
  1. A pixel on its own measures only brightness, not colour.
  2. It adjusts the colours so that white objects look white.
  3. The camera's filters do not exactly match human eyes; Mastcam, for example, is a little less sensitive to blue.
  4. They can compare the colours of Mars rocks with rocks on Earth.

Common mistake

"Adjusted colours are fake." Calibration corrects for the camera and the lighting. It is done carefully, with targets of known colour, to show the true colours of the rocks.

Where it leads