Glossary

Glossary
 

Term

Symbol

Definition

Wavelength Usually stated in nanometers (1 nm = 10-9 m) or microns (1 碌m = 10-6 m).
Wavenumber Reciprocal of wavelength, usually expressed in cm-1 (1 碌m = 10,000 wavenumbers (cm-1)).
Central Wavelength CWL Wavelength at the centre of the pass band measured at the half of the peak transmittance point.
Transmittance T Ratio of the transmitted energy to the incident energy, usually expressed in %.
Peak Transmittance T peak Maximum of the transmittance.
Average Transmittance T avg Average of the transmittance.
Peak Wavelength peak Wavelength at the maximum of the transmittance.
Bandwidth HW Full width of the pass band measured at half the maximum of the peak transmittance.
Blocking Region   The region outside the pass band where the filter is designed not to transmit light. Usually expressed as a percentage of the incident energy, blocking is typical less than 0.1 % transmittance.
Cuton / Cutoff Wavelength 位c Wavelength at which a filter begins/ ceases to transmit. Specified at 5% of the absolute transmittance point.
Cuton / Cutoff Slopes   A measure of the transmittance cuton or cutoff slope.
Defined as:

%Slope =
位 (80% of Tpeak) – 位c
位c
x 100
Half Power Point 1 HP1 Wavelength at which a broad band pass filter begins to transmit. Specified at 50% of the peak transmittance.
Half Power Point 2 HP2 Wavelength at which a broad band pass filter ceases to transmit. Specified at 50% of the peak transmittance.
Band Pass Filter BP A filter which transmits energy in a specified wavelength band but rejects energy above and below. HW > 3% of CWL.
Narrow Band Pass Filter NB A filter which transmits energy in a specified wavelength band but rejects energy above and below. HW < 3% of CWL.
Broad Band Pass Filter BBP A filter which transmits energy in a specified wavelength band but rejects energy above and below.
Shortwave Pass Filter SP A filter which transmits energy at wavelengths below its cutoff point. Sometimes called a low pass filter.
Longwave Pass Filter LP A filter which transmits energy at wavelengths above its cuton point. Sometimes called a high pass filter.
Neutral Density Filter ND A filter which attenuates energy levels accurately in a defined wavelength range.

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