So, you just bought that thousand-dollar digital camera with full manual control to unlock the artist within. You play with the new features, and take some long exposure night shots and are horrified to see a bright spot in the same locaiton on every picture. No, you haven’t discovered a new star… it’s the infamous hot pixel, and unfortunately, it’s a fact of life.
Don’t panic, and don’t return your new camera to the store. Your CCD is not defective, and understanding what they are will help you cope. Nearly every camera I own has a couple, and the ones that don’t eventually will at some point. They really aren’t a problem unless it’s smack dab in the middle of the photo, and they appear no matter what your ISO rating is.
Your CCD is a grid of elements, each of which is sensitive to brightness (light). A hot pixel is created when one of the elements has a higher rate of charge leakage than its neighbors. During a long exposure, this leakage may cross a threshold of an exposed value. Cheaper digital cameras don’t permit an exposure longer than 1/4 second (or they lSO settings), which effectively eliminates the chance of revealing the hot pixels that probably exist.
All CCD elements leak current, and given a long enough exposure will show hot pixels, even in complete darkness. Temperature is also a factor in creating hot pixels. Generally the higher the temperature, the higher the charge leakage. So that hot pixel you see during those summer landscapes may disappear in the winter.
Another factor is the ISO rating of your CCD. When the ISO is increased, you are essentially turning up the gain, similar to turning up the gain on a microphone. You’ll notice a lot more hot pixels at ISO 1600 than you will at ISO 100, simply because the signal is amplified.
Normally, hot pixles are practically invisible unless you go looking for them every time. You really won’t see them unless you enlarge your photo to 200% or more, and with today’s high mega-pixel cameras you are more likely to shrink the photo down than enlarge it.
Hot pixels (or sparkles) appear as a single (or few) unnaturally bright individual pixels. They will be exactly at the same pixel-location in every frame. They are locked in on the sensor, and they will be much sharper than anything else on the image. Some cameras show these pixels as tiny little crosses, which is due to Bayer interpolation. Bayer interpolation is also what makes the pixels a high contrast pixel red, green, or blue, and is the direct result of the color-striped mask over the sensor.
The dirty little trick about our “digital” cameras is that the sensor is analog! Sensors collect photon in microscopic wells (called pixels). Sensors assign electric charges to these photons, which are read as analog voltages. These voltages are sampled and quantized to make them into digital values, which then go through more digital processing before the picture is written.
Leakage currents are electric charges which leak into sensor wells, and this excess electric charge increases the voltage in the well and make it look brighter than it should. Manufacturing variations will cause some pixels to have more leakage than others, and there really is no way to make a “perfect” CCD. If there were, then the yield of the chips would be so low that digital cameras would be exhorbadantly expensive.