Absolute Blocking: The ratio of the largest peak outside the passband to the peak within the passband. Expressed as an area or %T.

Absorbance: The logarithmic function of transmittance. Sometimes used to express the degree of blocking. A = log(I_◦/I).

Angle of Incidence: The angle formed by radiation arriving (incident) at the filter surface and the perpendicular to the surface at the point of arrival.

Angstrom (Ǻ): Unit of length used to measure wavelengths of light. One tenth of a nanometer (nm). One Angstrom is equal to 1 x 10^-10 meters.

Bandpass Filter: See interference filter.

Bandwidth: Specified wavelength interval of transmitted radiation.

Blocking: The degree to which detectable radiation outside the passband is rejected. Expressed as transmittance, absorbance, optical density, scientific notation, signal-to-noise or rejection ratio. Blocking requirements are specified over a useful wavelength range.

Cavity: Basic component of an interference filter consisting of two layers of reflective stacks separated by a spacer layer. Also known as a period.

Clear Aperture (CA): The central, useable area of a filter through which radiation can be transmitted.

Central Wavelength (CWL): The mean of the two wavelengths corresponding to the half power points.

Half Power Points: Points on both sides of the passband curve of a filter, with a value 50% of the peak transmittance. Used to calculate HBW and CWL.

Half Bandwidth (HBW): The wavelength interval of the passband measured at the half power points (50% of peak transmittance). Expressed as halfbandwidth (HBW), full width half maximum (FWHM) or half power bandwidth (HPBW).

Incident Radiation (I_◦): The radiation, usually polychromatic, that impinges on a filter.

Interference Filter: A filter that, operating on the principles of constructive and destructive interference, transmits radiation in a discrete, narrow wavelength range while rejecting other radiation. Also known as a bandpass filter.

Integrated Blocking: The ratio of the total transmitted radiation (energy) outside the passband to the total transmitted radiation within the passband. Integrated blocking is influenced by the source output and detector response as functions of wavelength.

Micron (µ): Unit of length used to measure wavelengths of light. One micron is equal to 1,000 nm.

Nanometer (nm): Unit of length used to measure wavelengths of light. One nanometer is equal to 1 x 10^-9 meters.

Near Infrared (NIR): Light from the region of the electromagnetic spectrum with wavelengths between (approximately) 750 nm and 3.0 µ.

Optical Density (OD): Used to express the degree of blocking or the value of neutral density filters. Also known as Absorbance.

Passband: A wavelength interval through which incident radiation is transmitted. The first order passband is at the filter design wavelength.

Peak Transmittance: The highest transmittance value of a filter.

Peak Wavelength: The wavelength at which a filter has its peak (highest) transmittance. Period: See cavity.

Rejection Ratio: The ratio of the maximum transmittance outside the passband to the total transmittance within the passband.

Signal to Noise Ratio (S/N): The ratio of detected energy transmitted through the passband to the detected energy transmitted outside the passband. It is source and detector dependent.

Stray Light: Unwanted energy transmitted through the filter.

Transmittance (Tx): The ratio of the transmitted radiation to the incident radiation, expressed as a percent. %T = I/I_◦ x 100.

Transmitted Radiation (I): Radiation passing through a filter, either inside or outside the passband.

Ultra-Violet (UV): Light from the region of the electromagnetic spectrum with wavelengths between 150 nm and 400 nm.

Visible (VIS): Light from the region of the electromagnetic spectrum with wavelengths between 400 nm and 750 nm.