Calcium Fluoride: A material sometimes used for optical elements such as windows or lenses for NIR or MWIR imagers in certain situations. This material is visibly transparent, and as such is a common choice for systems in which visible access is required, in addition to MWIR transmission. However, this material tends to fog in the presence of water vapor, so it does not work well for external window applications.
Calorie (also, gram-calorie): Not to be confused with the nutrionalist’s definition of calories, the gram-calorie (also referred to as the calorie) is the amount of heat required to raise the temperature of one gram of water by one degree Celsius. This is equivalent to 4.186 Joules of energy in the SI measuring system. The ‘nutritional’ calorie is equal to one kilogram-calorie, i.e., the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius.
CCD: Acronym for Charge-Coupled Device. This is an imaging array commonly used in visual video cameras, though they are steadily being replaced by CMOS-type cameras.
Celsius: The unit of temperature in the SI measuring system. Celsius is the temperature system referenced to the freezing & boiling points of water at sea level; 0 degrees C is freezing, and 100 degrees C is boiling (for water). One degree C is equal to one Kelvin (K), but the Kelvin units are referenced to absolute zero (note that ‘Kelvin’ units do not use the term ‘degree’). 1 degree C is approximately equal to 0.556 degrees F.
Centigrade: See “Celsius”.
CMOS: Acronym for Complimentary Metal Semiconductor. This is an imaging device commonly used in modern video cameras.
Cold Shield: This is a device used to shield a focal plane array (FPA) by limiting the angle at which stray (unwanted) radiation can enter the FPA from inside the camera housing.
Conservation of Energy: As it applies to infrared imaging, this means that the sum (in terms of percentages) of the energy transmitted, reflected, and absorbed by an object must add up to 100%, or: τ + ρ + α = 1 Within the assumptions of Kirchoff’s Law, the energy absorbed is equal to the energy emitted (α = ε.).
Contrast: Most IR cameras can manipulate the image to provide better ‘contrast’ between brighter or darker objects. Increasing the contrast to improve the difference between such targets can cause the image to become saturated if a brighter target then comes into view. Many camera systems use the terminology ‘gain’, instead of contrast. While these terms are not strictly synonymous, their effect on the image is similar.
Convection: One of the three possible modes of heat transfer (the others are conduction and radiation), this refers to heat transfer when a moving fluid (can be liquid or gas) is present; the heat is transferred by the bulk motion of the fluid.
Cooled Detectors: This typically refers to thermal infrared detectors that must be actively cooled to cryogenic temperatures (often, around 70-80 K) to operate.
CRT: Acronym for Cathode Ray Tube, an older type of monitor used in earlier generation television sets and computer monitors.
Cryocooler: A small cooling unit, typically attached to an imaging detector, used to cool the detector down to cryogenic temperatures (generally, ~77K). Some detectors require cooling to such low temperatures to operate properly.
