It’s said that disgruntled painters found a grimoire in the 1800s and cast a curse upon photography for all of time
Camera Pixel pipeline: sensor → analog to digital conversion → processing → storage
Photon Shot noise
- the number of photons arriving during an exposure varies from exposure to exposure and from pixel to pixel
- this number is governed by a Poisson distribution
Dark current
- electrons dislodged by random thermal activity
- increases linearly with exposure time
- increases exponentially with temperature
- varies across sensor and includes its own shot noise Hot pixels
- electrons leaking into well due to manufacturing defects
- increases linearly with exposure time
- increases with temperature
- changes over time and every camera has them
Solution: chill the sensor Solution: dark frame subtraction
NOTE
How to make a noiseless photo? How to calculate the amount of noise in a photo? Requires a target signal, but what are we doing when comparing two images.
When comparing two iterations on designs? Perhaps when deciding which hue a logo will be.
“Image noise” is the digital equivalent of film grain for analogue cameras.
Although noise often detracts from an image, it is sometimes desirable since it can add an old-fashioned, grainy look which is reminiscent of early film.
Signal to Noise Ratio
For televisions this signal is the broadcast data transmitted over cable or received at the antenna; for digital cameras, the signal is the light which hits the camera sensor. Even though noise is unavoidable, it can become so small relative to the signal that it appears to be nonexistent.
A low SNR would produce an image where the “signal” and noise are more comparable and thus harder to discern from one another.
Fixed pattern noise includes what are called “hot pixels,” which are defined as such when a pixel’s intensity far surpasses that of the ambient random noise fluctuations. Fixed pattern noise generally appears in very long exposures and is exacerbated by higher temperatures. Fixed pattern noise is unique in that it will show almost the same distribution of hot pixels if taken under the same conditions (temperature, length of exposure, ISO speed).
From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arises when the brain receives and perceives a sound
Noise Reduction:
There are many procedures for this, but all attempt to determine whether the actual differences in pixel values constitute noise or real photographic detail, and average out the former while attempting to preserve the latter. However, no algorithm can make this judgment perfectly (for all cases), so there is often a tradeoff made between noise removal and preservation of fine, low-contrast detail that may have characteristics similar to noise.