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15
Jul
posted by - Wednesday, 15 July 2009

milkyway

(Photo credit: Evin Grant)

Did you ever wonder what creates permanently lit, or what we often refer to as “blown”, pixels in your CCD camera’s imager? Here’s the reason:

Charged-Coupled Devices (CCD’s) are made up of an array of millions of photodiodes, which turn light into voltage. Photodiodes have two leads coming from the bottom, a cathode and an anode. When a photon of energy strikes the diode, it excites an electron, creating a mobile electron and a positively charged electron hole. Holes move toward the anode, and electrons toward the cathode, producing a photocurrent. Because photodiodes use voltage to convey information, they can be susceptible to high-energy particles such as cosmic rays.

Cosmic rays are created in the upper atmosphere during a supernova. When a star collapses, the resulting explosion and shockwave ejects nuclei into the cosmos (made up of mostly hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, or sulfur). The electrons surrounding the nuclei become stripped away during their passage through space, and many of these nuclei have been accelerated during the supernova to near light speed. The typical energy of a cosmic ray is around 1 GeV (1,000,000,000 eV). To get an idea of how much energy that is, a stream of photons with a wavelength of 532 nm (green light) would have an energy of approximately 2.33 eV. So as you can tell, cosmic rays pack a lot of punch! And to bring this all back home – when that cosmic ray punches your photodiode, the energy stream is so great that the diode gets stuck, creating a permanently lit pixel.

Cosmic rays are affected by ambient galactic magnetic fields, including the magnetic field of the Earth. And when a cosmic ray collides with a molecule, mainly oxygen or nitrogen, within the Earth’s atmosphere, it creates a shower of new particles. This is why you’ll find more cosmic rays at higher altitudes, and ultimately why your camera is more susceptible to such an effect while traveling by airplane.

Most high-end CCD cameras include ways of masking lit pixels, including the black balance function. The black balance function includes an automatic pixel restoration function, which scans each CCD block looking for pixels above a certain acceptable noise level, and turns them off. Then the camera uses the voltage values of the surrounding pixels to fill in the gap left behind.

It is important to note one last fact: there is absolutely nothing one can do to prevent cosmic ray damage. Wrapping the camera in lead or any other substance will not provide protection, as the particles will simply pass right through. The best protection is to ship your equipment on the ground, rather than in the air.

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