What is the difference between hot, stuck, and dead pixels with regards to a camera's image sensor? What can cause each to occur? What can be done to reduce their influence on a photo?
Excitingly, these terms mean different things to different people. I think the most useful distinction is like this:
- Stuck pixels are always completely bright, as if they're fully overexposed
- Dead pixels are always off, as if receiving no light (these are usually less obvious)
- Hot pixels are not permanently stuck, but show up during long exposures (as the sensor heats up). These usually are defective pixels to some degree, and the same sensor will usually have the same hot pixels in the same conditions.
Some people use "hot" and "stuck" interchangeably, for either one or both of the different situations. I think it's more useful to make a distinction, but be aware that not everyone does — it's most helpful to be a little more verbose and explain what exactly you mean.
"Dark frame subtraction" is an effective way to deal with hot pixels. The same can be done for stuck (or dead) pixels, but generally the thing to do is have them mapped out permanently. Many cameras have a function in the menu to scan for these defects and do the mapping; otherwise, it's usually covered under warranty service. (This may or may not also get some or all of the hot pixels.)
In a digital sensor, all of these usually refer to a single photosite, with its corresponding color filter, so you might have a bright spot of red, green, or blue. This may also "bleed" into nearby pixels during the demosasicing process (but only as an artifact, not from electrical leakage or anything), resulting in
Hot pixels are pixels that read out at full voltage/saturation as a result of heat in the sensor. Once the sensor is allowed to cool they will usually return to their proper state and read out a voltage value based on the amount of light that has struck the pixel well the next time they are used. They most often present an issue during long and/or high ISO exposures. Ambient temperature can also influence the number and severity of hot pixels in a photo. The same conditions that typically produce more digital noise also typically produce more hot pixels.
Often the best way to deal with hot pixels in a captured file is to use dark frame subtraction. One can also manually use a dust repair tool or healing brush tool to edit out hot pixels. In order to prevent occurrence in future captures one can allow the camera and its sensor to cool.
Stuck pixels are similar to hot pixels in that they read out at full voltage/saturation but they do it all of the time. That is, they are permanently stuck in the full "on" position.
Dark frame subtraction can also deal with stuck pixels, but a more effective and convenient way is to use pixel mapping. Some cameras do pixel mapping automatically. But if pixel mapping is done when the sensor is warm or hot, then hot pixels can be mistaken for stuck pixels and be mapped out of subsequent shots even after the sensor has cooled. In Canon cameras, for instance, the sensor is remapped each time the firmware is updated and each time the shutter is held open for at least 30 seconds using the manual cleaning function. Recording dust delete data is an effective way to map out stuck pixels with some cameras.
Hot or stuck pixels can often influence the values of the surrounding pixels as well. When the camera or RAW converter program converts the information from the sensor, the high value of that one pixel can also cause the surrounding pixels to have artificially high color values (for the color of the hot pixel) assigned by the interpolation of the demosaicing algorithms.
Dead pixels are the opposite of stuck pixels. They are permanently stuck in the full "off" position. Cameras will often interpolate values for dead pixels from the surrounding pixels in much the same way they interpolate color values for all pixels from the monochromatic values in the raw data.
What can be done over the long term to reduce the probability of hot, stuck, or dead pixels?
Only energize and expose the sensor to light when necessary. The more light and electrical energy a sensor is exposed to for a longer period of time the greater the number of pixels that develop defects. The number of stuck, hot, or dead pixels increase over the life of any sensor. Leaving the mechanical shutter open and the sensor energized and exposed to light over the course of an entire shooting session will reduce the life expectancy of the sensor by a considerable amount compared to using a mechanical shutter and only energizing and exposing the sensor to light when actually recording an image.
What is the difference between hot, stuck, and dead pixels?
Dead pixels are stuck in the off state (totally dark), while hot pixels are stuck in the on state (they stay on).
What can cause each to occur?
Hardware problems. You could think of a dead pixel on a video screen like a blown light bulb -- it just doesn't work any more. Image sensors are sort of the opposite of video screens in that they absorb light and report the brightness instead of converting some brightness value to light, but it's the same idea: some photosites will just always report a brightness value of 0 (dead), others will always report a different value like 255 (hot).
What can be done to reduce their influence on a photo?
Many cameras can map problem pixels and compensate for them. Check the user manual that came with your camera for the particular procedure, but it's basically like dust mapping: you take a photo of an evenly lit surface like a wall and let the camera find the pixels that stand out. Once it knows where the stuck pixels are, the camera can ignore them and use values from neighboring pixels.