Sapphire: A material sometimes used as an optical element such as windows or lenses for NIR or MWIR systems. This material transmits well in the visible spectrum, but not at all in the LWIR region.
Signal to Noise Ratio: A measurement of how strong the desired target signal is, compared to the level of background noise.
Single Mode Fiber: A type of fiber-optic cable, Single-Mode Fiber (SMF) is used for long distance data transmission (compared to multi mode). SMF are better at retaining the fidelity of the individual light pulses than multi-mode fibers (MMF), allowing longer transmission distances.
Solar Loading: Heating of objects by solar radiation. Solar loading will cause background objects (for example, trees or roadways) to appear warmer in the day than at night.
Spatial Resolution: The smallest spatial detail in an image that can be resolved. Spatial resolution is described in a variety of ways, including the IFOV or the Airy disk size (both previously defined). As a broad generalization, spatial resolution is sometimes described in terms of the number of pixels in the imager array (angular FOV of the lens and the range to the target must also be considered).
Specular Surface: A surface (typically a smooth, shiny surface) from which incoming light is reflected into a single direction. The angle of the incoming light, with respect to the direction normal (perpendicular) to the surface, is the same as the angle of the outgoing reflection, also with respect to the normal direction.
Stefan-Boltzmann Law: Describes the total rate of energy emission per unit surface area. The Stefan-Boltzmann Law is the spectrally integrated form of Planck’s Law (integrated over all wavelengths). This result takes the form: E=σT4 where E is the rate of energy emitted per unit area, T is the absolute temperature in Kelvin, and σ is the Stefan-Boltzmann constant.
Steradian: A three-dimensional unit of angular measurement defined as 0.08 of total solid angle.
Stirling Cooler: A miniature cooling device based on the Stirling thermodynamic cycle, often used to cryogenically cool the focal plane array of cooled thermal imaging systems.
SWIR: Acronym for Short Wave Infrared, generally defined as the spectral range between about 0.8 to 3 microns. Most night vision devices work within a subset of this spectral range, and many infrared (not thermal infrared) devices work in this spectral range. Nearly all devices within this spectral range require either ambient light or active illumination to work effectively.
