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Hot Pixels

By Mike Pasini, Editor
Imaging Resource Newsletter

You just bought a thousand dollar digicam with full manual control and you want to show off. You take some long exposure night shots and are horrified to see a bright spot in the same part of every picture -- and it isn't a star. It's the infamous hot pixel.

"More than a defect, hot pixels are a fact of life."

Is your CCD defective?

Actually, no. More than a defect, hot pixels are a fact of life. Understanding them will make you feel better, certainly, and may even suggest when you should return your digicam. So let's take an in-depth look at them.

Your CCD is a grid of elements, each of which is sensitive to brightness.

A hot pixel is created by an element with a higher rate of charge leakage than its neighbors which, on a long exposure, may cross the threshold of an exposed value. Many digicams don't permit exposures longer than a quarter second, which effectively eliminates the chance that any element with a dark current, so to speak, will consistently report an exposure value.

In fact, with a long enough exposure to darkness, a disturbing pattern of exposure will appear from any CCD, because CCD elements tend to leak current. They may not all produce a bright white spot, but they are all -- with a long enough exposure -- capable of reporting exposure in darkness.

In addition to dark current, temperature is also a factor in creating hot pixels. The higher the temperature, the higher the charge leakage. A 10 degree change in temperature can noticeably change what the CCD reports.

Yet another factor is the ISO rating of your CCD. At ISO 400 you'll notice more hot pixels than at ISO 100, simply because the signal is amplified.

A hot pixel can range from bright white to something just barely distinguishable from black. Or, we might say, from an artifact to noise.

Astronomers who use CCDs to record the night life of celestial bodies have long dealt with this phenomenon. If you think you have problems, see http://ecf.hq.eso.org/newsletter/stecf-nl-22/freudling/freudling.html for example. They typically take very long exposures, and are concerned that dark current and hot pixels, producing measurable noise in repeatable patterns, are not confused with actual data. So they've developed techniques for masking them out of the picture or the data.

Some of these are applicable to digicam night photography. Rob Galbraith's site for digital photojournalists (http://www.robgalbraith.com/) proposes one technique that is helpful in shots of fireworks (http://www.robgalbraith.com/reports/shooting_fireworks.html). Galbraith's technique relies on the Quantum Mechanic Photoshop plug-in at http://www.camerabits.com/QM.html.

"A hot pixel can range from bright white to something just barely distinguishable from black."

And don't forget Mike Chaney's Qimage Pro program for Windows (http://www.charm.net/~mchaney/imaging/), which can remove image noise without significantly affecting detail.

But with exposures of a quarter second or less, hot pixels just shouldn't appear in your images. If they do, you've got a returnable product.

To see your camera's hot pixels, set the camera to do no image manipulation or enhancement. Use ISO 100 (if you have variable ISO settings), turn off any sharpening, contrast, or brightness settings. Assuming your indoor temperature is less than 76° F, go into a dark room, turn off the LCD monitor, cover the lens, and make exposures of 1/4, 1/2, 1, 2, and 4 seconds.

Then take a look at these "dark frame" images. Higher magnification (say, 400 percent) makes it easier to see noise. You should expect to see a gradual increase in noise relative to the exposure length. If you find one bright pixel at every exposure setting, repeat the test with shorter exposures. You may find it does not disappear at any exposure. If that's the case, save your results to document the problem for the manufacturer and try the test again.

Given that any CCD is going to exhibit some noise in a dark frame image, consider whether what you have is something you can live with. If you typically take long night exposures, it may not be acceptable. But remember, there's no escape. Astronomers have learned to live with the devil they know, rather than look for that elusive, perfect CCD.

This article is reprinted from The Imaging Resource Digital Photography Newsletter,
Advanced Mode Column, published August 11, 2000

 

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