Glossary in Optics & Photonics

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.

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.

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 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)]

NIR: wavelengths ranging from approximately 750 to 1,400 nm and is commonly used in applications such as spectroscopy, remote sensing, and medical imaging.

SWIR: wavelengths ranging from approximately 900 to 1,700 nm but can also be classified from 900 to 2,500 nm and is used in applications such as moisture detection, material sorting, and telecommunications. Indium gallium arsenide (InGaAs) sensors are the primary sensors used in SWIR imaging, and mercury cadmium telluride (MCT, HgCdTe) cover the other portion of the spectrum.

eSWIR: wavelengths ranging from approximately 2,500 to 3,000 nm and is used for applications such as remote sensing, chemical detection, and temperature sensing.

MWIR: wavelengths ranging from approximately 3,000 to 8,000 nm and is used in applications such as thermal imaging, and gas sensing.

LWIR: wavelengths ranging from approximately 8,000 to 15,000 nm and is used in applications such as night vision, surveillance, and thermal imaging.

Each of these spectral ranges has different properties that make them suitable for different applications. Understanding their differences is critical in selecting the appropriate technology for a given task.

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.

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”.

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 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.