Have you ever taken a photo with a digital camera of a night-time scene and noticed that the image contains a few bright dots that shouldn't be there? Something like what is shown in the following figure?



It doesn't necessarily have to be a night-time shot. One can also see these annoying bright pixels in normal photos whenever they happen to be in a dark area of the scene:



These bright dots are called hot pixels and you can use HotPixelDetector to detect and analyse them.
So what are Hot Pixels?
Hot pixels are individual sensors on the CCD sensor of a digital camera which have higher than normal rates of charge leakage. This causes them to appear as small pixel-sized bright points of light in photos, as can be seen in the examples above. Every pixel on the CCD sensor suffers from charge leakage to some degree or other. For the majority of the pixels, this charge leakage is quite small and the effect is generally not visible (it creates the small amounts of noise present in images and which appears as 'graininess' in uniform areas). But a small number of pixels will suffer from serious amounts of charge leakage - these are the ones we call hot pixels.

What is the difference between hot pixels, stuck pixels and dead pixels?
These three terms are often used interchangeably, but there is a slight difference between them. The term hot pixel is the most well known of the three and in most cases its usage encompasses all the three together. But to be exact, a hot pixel is a pixel which reads high (that is, appears very bright) on long exposures, example, night-time scenes. A stuck pixel is a pixel that reads high on all photos taken, regardless of how long the exposure time is. A dead pixel is a pixel that reads zero (appears as black) on all photos taken. Note that there is some overlap between hot and stuck pixels. One can say that a stuck pixel is an extreme case of a hot pixel, one that has so much charge leakage.

Does the exposure time of a photo affect the presence of hot pixels?
Yes, the amount of hot pixels in an image will increase greatly as the exposure time increases. As mentioned before, each individual sensor of a CCD suffers from charge leakage. Because the rate of charge leakage is the same for a given pixel over time, the longer the exposure, the brighter the charge leakage is. If you expose long enough, any pixel would light up. For long exposures (several seconds), you will see pixels ranging from just barely visible to possibly bright hot starlike points. There might be a few bright hot pixels, more intermediate one, and lots of very faint ones - an entire spectrum of brightness. The figure below shows this progression.

1 sec    8 sec    30 sec    120 sec

Apart from exposure time, other factors affect the presence of hot pixels in an image. For example, charge leakage is worse at higher temperatures. Even a 10 degree difference in temperature can result in more visible hot pixels. Also, the higher the gain, the more pronounced the hot pixels are. Gain in a camera is the 'ISO' value and is a measure of how much the signal from a sensor has been amplified. When amplifying any signal, at some point there is no more information that can be gained by amplification, and you begin amplifying noise. A digital camera will show more hot pixels at ISO 400 than ISO 100. You can use HotPixelDetector to see how exposure time, temperature and gain affect your camera's hot pixels.

Do hot pixels change with age?
First, even a brand new camera has on average a few hot pixels. You may not see them because cameras are shipped from the factory with all hot pixels mapped out. That is, these hot pixels are ignored and not read out when an image is taken. As time passes, more hot pixels develop over time or existing ones become brighter (using the terminology above, from hot pixels they become stuck pixels). You can use HotPixelDetector to see how your camera's hot pixels change over time by saving the results of the analysis every couple of months and comparing them together.

How can hot pixels be removed from an image?
A few camera models allow the user to map out the hot pixels (for example, the Olympus E-10 and E-20, Sony 707). These masked pixels are ignored during the read step. Other camera manufacturers offer the possibility to map out your hot pixels at their repair centers, though this can be quite costly.

Another possibility is to use a software product that detects and eliminates hot pixels from your images. There are two types of products: those that use the so-called 'dark frame' technique and those that do not. The former are much more accurate, though they require a bit more work in generating and mantaining a dark frame. We, lightning cube software, offer a product called HotPixelRemover that uses the dark frame technique to detect and remove hot pixels. Please follow this link for more information.

What is a dark frame?
A dark frame is an image taken with a camera in which no light arrives at the sensor. This is usually done by covering the lens with its shutter and making sure that no light enters the camera lens in any way. The best way of taking a dark frame is to cover the lens tightly and taking the photo in a dark room. Theoretically, if no light arrives at the camera's sensor, then each and every pixel should be black (zero). In reality, due to charge leakage and noise in the sensor itself, each pixel will not be absolutely black, but have a value slightly larger than zero. For hot pixels and stuck pixels, the value can be quite high and even reach maximum. So it follows that a dark frame is the ideal way to detect hot pixels. The dark frame can also serve to clean hot pixels from images by subtracting the dark frame from the image. This is shown in the figure below: