digital photography Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image is ...

, dark-frame subtraction is a way to reduce

image noise Image noise is random variation of brightness or color information in images. It can originate in film grain and in the unavoidable shot noise of an ideal photon detector. In digital photography is usually an aspect of electronic noise, produ ...

in photographs shot with long exposure times, at high ISO sensitivity or at high temperatures. It takes advantage of two components of image noise that remain the same from one shot to the next, dark current and

fixed-pattern noise Fixed-pattern noise (FPN) is the term given to a particular noise pattern on digital imaging sensors often noticeable during longer exposure shots where particular pixels are susceptible to giving brighter intensities above the average intensity. ...

. Noise from the

image sensor An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they refraction, pass through or reflection (physics), reflect off objects) into s ...

include

hot pixels A defective pixel or a dead pixel is a pixel on a liquid crystal display (LCD) that is not functioning properly. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have ...

, which light up more brightly than surrounding pixels. The technique works by taking a picture with the shutter closed and subtracting that electronically from the original photo exhibiting the noise. Dark frame subtraction

A dark frame is an image captured with the sensor in complete darkness (i.e. with a closed shutter or the lens and

viewfinder In photography, a viewfinder is a device on a camera that a photographer uses to determine exactly where the camera is pointed, and approximately how much of that view will be photographed. A viewfinder can be mechanical (indicating only direct ...

capped). Such a dark frame is essentially an image of noise produced by the sensor. A dark frame, or an average of several dark frames, can then be subtracted from subsequent images to correct for fixed-pattern noise. It is important that dark frames are taken at the same ISO sensitivity and

exposure time In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter is open) when taking a photograph. The amount of light that rea ...

as the original photo, because the

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

of fixed pixel noise is dependent on both. Under these circumstances, a single dark frame can be subtracted from multiple photos taken at these parameters, amounting to a time saving and allowing

noise reduction Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an u ...

for stacked star trail images, which do not allow interruption. Dark-frame subtraction is also used in digital

photogrammetry Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...

, to improve the contrast of satellite and air

photogram A photogram is a Photography, photographic image made without a camera by placing objects directly onto the surface of a light-sensitive material such as photographic paper and then exposing it to light. The usual result is a negative shadow im ...

s, and is considered a best practice, along with flat-field correction, for

astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1839, but it was no ...

References

{{Reflist Astrophotography Digital photography Image noise reduction techniques Photogrammetry Photographic techniques

See also

References