I See Dead Pixels

As part of my responsibilities at AbelCine, I handle a number of technical-support questions from our professional clients in the field. One such inquiry I recently received was in regards to a "dead pixel" on a CMOS camera sensor.

A client of ours had recently purchased a Sony PMW-F3 camera from us, and he was troubled to find a bright green spot on the video output. What the client described as a dead pixel is actually an individual photosite on a sensor that’s constantly producing a voltage and, as a result, looks like a bright speck on your image. This isn’t an uncommon problem for a camera to develop; however, it’s unusual to see on a CMOS camera like the F3. So, our client’s concern about his new camera was well understood. The good news is that we had a quick and simple fix, but first, a little background.

SUPERNOVA = DEAD PIXELS?

Millions of years ago in a galaxy far (far) away, a star collapsed in a brilliant supernova. The resulting explosion ejected nuclei (mostly hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron or sulfur) into space at nearly the speed of light. The electrons surrounding these nuclei became stripped away during their passage through space, resulting in a charged particle called a cosmic ray. The typical energy of a cosmic ray is around 1 GeV (1,000,000,000 electron volts). 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, these little guys pack a lot of punch, and they’re hitting Earth all the time. Many are deflected by Earth’s magnetic field, pushing them toward the poles and giving rise to the aurora. However, some will pass right through Earth and right through camera sensors all around the world. It’s the impact of these energetic particles on an individual pixel that makes it glow.

All sensors, both CCD and CMOS, are composed of photodiodes (pixels) that convert photons of light into voltage. The more light that hits the photodiode, the more voltage it produces. This voltage is then converted to digital data, and an image is created. Very sensitive sensors, such as those found in professional HD cameras, are designed to read even the slightest amount of voltage coming from these photosites. When a high-powered cosmic ray hits these photodiodes, it can punch a hole in their insulation, which creates a voltage leak. So, the photodiode outputs extra voltage that produces a false, bright output all the time. There’s no way of repairing these leaky pixels, but we can compensate for them.

HOT, STUCK AND DEAD

While many people often refer to these bright pixels as "dead," that’s actually not the correct term. When a small voltage leak develops on a pixel, it may not be visible at a normal gain level. However, when gain (ISO) is increased in the camera, the output is boosted, and the pixel reveals itself. We call these pixels "hot pixels" because they only cause problems at high gain. A pixel with a big voltage leak, which outputs a bright color at any gain, is called a "stuck" or "lit" pixel. And, finally, a "dead pixel" is one that produces no voltage at all and is actually the most unusual to find. A dead pixel is hard to spot because it doesn’t glow at all. A hot or lit pixel is a bright color (red, green, blue or somewhere in between) because the color we see comes from any one of three different sensors (in prism-based cameras) or from a color pattern (in single-sensor cameras). The color of the lit pixel comes from the specific chip it’s located on or where it falls in the color pattern.

PIXEL COMPENSATION

The loss of a pixel may sound quite devastating, but a 1920×1080 sensor has millions of pixels, and losing a couple is far from serious. When I started at AbelCine, I prepped new cameras for sale and serviced customers’ cameras. I would say that, on average, a brand-new camera has around 15 to 30 lit pixels out of the box; these could be from cosmic rays or just factory imperfections. In fact, just about every sensor has this type of miniscule damage, from an SLR to your cell phone’s video camera. Only in the medical industry does it create a real problem, where a lit pixel could give a false positive.

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