Skip to main content

Photon etc. presents the following glossary of words and terms specific to the technologies handeled by the company. It aims at giving insights on specific and technical words the one might encounter navigating the website. It also serves to clarify Photon etc.’s definitions of some terms for which there are no clear consensus in the community.


Blooming and Smear

Blooming-type glare is an optical aberration related to the overflow of the electron wells that make up a CCD or CMOS sensor. When a pixel’s electrical charge reaches its limit value, the excess charges will overflow to adjacent pixels. A small, sharp, extremely bright point in the image will therefore appear as a large, diffuse point. Blooming can be controlled with shutter time, but becomes problematic for highly heterogeneous scenes in illumination.

Smear is a similar aberration, but is related to the scattering of charges, or photons, to the shift register of a CCD sensor. In this register, the charges are more mobile in the vertical axis of the image. The smear therefore appears as a long vertical line in the pixel column that contains the shiny element. Since CMOS has no shift register, it is not subject to smear. Smear can be mitigated by adequately protecting the register from incident photons.


Cold Shield

The cold shield protects the infrared optical sensor from thermal stray radiations outside its optical field of view. The shield is usually cooled to the same temperature as the detector. The portion of the undesired radiation impinging upon the detector defines the cold shield efficiency. The cold shield becomes a cold stop when its efficiency reaches a 100% because in that case the cold shield has become the aperture stop of the system.

Cold Stop

The cold stop is a special kind of stop used in infrared cameras. It protects the detector so it sees only radiation from the object space. The cold stop is the aperture stop mounted inside the Dewar package (see image). In this illustrated example, the cold stop is also the exit pupil.


Dark Field

Dark Field

Dark-field microscopy is a visualization method applicable for both light and electron microscopy. It consists in the exclusion of the unscattered beam from the image. As a result, the field around the specimen is generally dark, almost black, and the specimen appears brightly and distinctively. This way, we can achieve contrasted and well-resolved images from transparent unstained samples. Dark-field microscopy relies on the Tyndall effect applied to small particles, i.e. light scattered by objects. An example of this effect is observable when a beam of light enter in a dark room and the dust in the air becomes visible in the incident ray of light.

Dark Noise

Dark noise, also called dark current noise, is statistical variation of the dark current, and is the electron equivalent of photon shot noise. Dark current can be subtracted from an image, while dark noise remains. Dark current noise is temperature and time dependent, hence the shorter the exposure, the less dark current noise will be present in the image. It is typically expressed in electrons per second at a given temperature and is calculated from the dark current such as:

Dark Noise = sqrt[(Dark current)*(integration time)]


Quantum Efficiency

Quantum efficiency (EQ) corresponds to the probability that an incident photon reaching the photosensitive surface to generate an electron on one of its pixels. It depends on the energy of the photon, the nature of the sensor and the elements that protect it (glasses, films, etc.). It is therefore significantly dependent on the wavelength of the incident photon. Quantum efficiency is therefore a curve, ranging from 0 to 100%, covering a spectral range defining the sensitivity range of the sensor. Quantum efficiency is sometimes called IPCE (Incident-Photon-to-electron Conversion Efficiency). It is important for very low luminosity images and for the intensity calibration of wide spectral band images.


Readout Noise

The readout noise, also called preamplifier noise, is the main noise component that needs to be considered when ​choosing a camera. It is a combination of noise from the pixel and from the electronics that amplifies and digitizes the charge signal in the CCD readout. It basically determines the contrast resolution that the camera is able to achieve. The lower the readout noise level, the lower the minimum number of signal electrons that can be detected and higher the sensitivity of the sensor. A higher sensitivity allows for shorter exposure times which helps to limit dark noise and to see smaller changes in signal amplitude, thus detecting details with smaller contrast differences. Therefore, you need a sensor with a low readout noise to observe “dark scenes”.


Shot Noise

The shot noise is a quantum-limited intensity noise that originates from the discrete nature of electrons, hence its alternative name quantic noise. It is caused by the arrival process of light photons on the sensor. Consider the following example: imagine standing at an overpass above a highway and counting the number of cars passing by in one minute. The next minute, and the next, and the amount counted is probably not the same. The resulting measurement varies from minute to minute, following a Poisson distribution, hence its alternative name Poisson noise. In the electron domain, this is similar: the standard deviation of the amount of captured electrons in a pixel is the square root of the mean signal level.


Thermoelectric Cooling (TEC)

Thermoelectric cooling uses the Peltier effect to electrically pump heat from a cold surface to a hot surface. Thermoelectric modules, TEC, often appear as thin rectangles of ceramics with varying surfaces. They are appreciated in electronics for the absence of moving parts (favoring a longer life and a total absence of vibration) and refrigerant gases, making their implementation in circuits simpler. They allow very low temperatures to be reached, but their heat dissipation efficiency is low. To increase this efficiency, the hot surface must be actively cooled by another method, eg. air flow, water, etc. To achieve large temperature gradients, TECs can be stepped, at the cost of a rapid decrease in their heat dissipation efficiency.