Dark frame subtraction
Encyclopedia
In digital photography
, dark-frame subtraction is a way to minimize image noise
for pictures taken with long exposure times. It takes advantage of the fact that a component of image noise, known as fixed-pattern noise
, is the same from shot to shot: noise from the sensor, dead or hot pixels. It works by taking a picture with the shutter closed.
A dark frame is an image captured with the sensor in the dark, essentially just an image of noise
in an image sensor
. A dark frame, or an average of several dark frames, can then be subtracted from subsequent images to correct for fixed-pattern noise
such as that caused by dark current
. Dark-frame subtraction has been done for some time in scientific imaging; many newer consumer digital cameras offer it as an option, or may do it automatically for exposures beyond a certain time.
Visible fixed-pattern noise is often caused by hot pixels
– pixel sensors with higher than normal dark current. On long exposure, they can appear as bright pixels. Sensors on the CCD that always appears as brighter pixels are called stuck pixels while sensors that only brighten up after long exposure are called hot pixels.
The dark-frame-subtraction technique is also used in digital photogrammetry
, to improve the contrast of satellite and air photogram
s, and is considered part of "best practice", along with flat-field correction
, for astrophotography
.
Digital photography
Digital photography is a form of photography that uses an array of light sensitive sensors to capture the image focused by the lens, as opposed to an exposure on light sensitive film...
, dark-frame subtraction is a way to minimize image noise
Image noise
Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the sensor and circuitry of a scanner or digital camera...
for pictures taken with long exposure times. It takes advantage of the fact that a component of image noise, known as fixed-pattern noise
Fixed-pattern noise
Fixed pattern noise 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 general background noise....
, is the same from shot to shot: noise from the sensor, dead or hot pixels. It works by taking a picture with the shutter closed.
A dark frame is an image captured with the sensor in the dark, essentially just an image of noise
Image noise
Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the sensor and circuitry of a scanner or digital camera...
in an image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
. A dark frame, or an average of several dark frames, can then be subtracted from subsequent images to correct for fixed-pattern noise
Fixed-pattern noise
Fixed pattern noise 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 general background noise....
such as that caused by dark current
Dark current
Dark current is the constant response exhibited by a receptor of radiation during periods when it is not actively being exposed to light. It may refer to:...
. Dark-frame subtraction has been done for some time in scientific imaging; many newer consumer digital cameras offer it as an option, or may do it automatically for exposures beyond a certain time.
Visible fixed-pattern noise is often caused by hot pixels
Defective pixel
Defective pixels are pixels on a liquid crystal display that are not performing as expected. The ISO standard ISO 13406-2 distinguishes between three different types of defective pixels, while hardware companies tend to have further distinguishing types. Similar defects can also occur on CCD or...
– pixel sensors with higher than normal dark current. On long exposure, they can appear as bright pixels. Sensors on the CCD that always appears as brighter pixels are called stuck pixels while sensors that only brighten up after long exposure are called hot pixels.
The dark-frame-subtraction technique is also used in digital photogrammetry
Photogrammetry
Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....
, to improve the contrast of satellite and air photogram
Photogram
A photogram is a photographic image made without a camera by placing objects directly onto the surface of a photo-sensitive material such as photographic paper and then exposing it to light. The result is a negative shadow image varying in tone, depending on the transparency of the objects used...
s, and is considered part of "best practice", along with flat-field correction
Flat-field correction
thumb|210px|The brightness variation due to [[vignetting]], as shown here, can be corrected by selectively brightening the perimeter of the image.Flat-field correction is a technique used to improve quality in digital imaging...
, for astrophotography
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
.
See also
- Bias frameBias frameIn digital photography, a bias frame is an image obtained from an opto-electronic image sensor, with no actual exposure time. The image so obtained only contains noise due to the electronics that elaborate the sensor data, and not noise from charge accumulation In digital photography, a bias frame...
- Noise reduction
External links
- "Dark Frame Subtraction using Adobe Photoshop" by Chris Limone