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Purple sun?

These pictures were taken on a flight from Geneva to Warsaw on 18 July 2007. Shortly, after the sun disappeared behind a bank of clouds, I was surprised to notice that the LCD display of my Lumix TZ-1 camera was showing a purple sun. As purple is not a colour not normally associated with the sun, my first thought was that my camera had been damaged by taking pictures of the sunset. However, it soon became clear that the camera really could “see” the sun – despite the fact that the sun was completely invisible to my eyes! In fact, the sun remained visible to the camera for about 2 minutes after it disappeared according to my eyes.
Notice that the sun appears to have been squashed vertically (due to atmospheric refraction).

The explanation for this phenomenon is that the CCD sensors used in digital cameras have their peak sensitivity in the infra-red – typically at a wavelength of about 1000 nm, which is well beyond visible spectrum of 400 – 700 nm. You can test the infra-red performance of your own digital camera by pointing a TV remote control at the lens of the camera from a distance of about 15 cm (6 inches). Most remote controls transmit infra-red at wavelengths of 850 – 1000 nm. Your camera viewfinder will probably show a purple light when you press a button on the remote control. The purple colour suggests that the red and blue sensors in the camera are sensitive to infra-red – but not the green sensor. Hence, the purple sun …..

Author: Philip Laven