Glossary
| Terminology | Symbol | Definition |
|---|---|---|
| Wavelength | λ | Stated in microns (micrometers, 10-6 metre). |
| Wavenumber | σ | Inverse of wavelength, usually specified in cm-1 |
| Centre Wavelength | CWL | Wavelength at centre of passband measured at 50% of peak transmittance. |
| Cut On / Cut Off Wavelength |  | Wavelength at which filter begins/ceases to transmit. Usually specified at 5% absolute or 50% of peak transmittance points. |
| Transmittance | T | Ratio of transmitted energy to incident energy expressed in percent. |
| Peak Transmittance |  | Highest transmittance (occurs at peak wavelength). |
| Bandwidth | FWHM, BW, HBW, HW | Width of the passband measured at half the peak transmittance. Expressed in microns or percentage of centre wavelength. |
| Cut On / Off Slope | A measure or the steepness of the transmittance curve.  Where λ80% and λ5% correspond to 80% and 5% of absolute transmittance points. | |
| Blocking or Attenuation | Ability to prevent energy at wavelengths outside the passband being transmitted. Expressed as a percentage of incident energy. Typically less than 0.1% (optical density 3.0). | |
| Reflection | R | Ratio of reflected energy to incident energy expressed as a percentage. |
| Absorptance | A | Ratio of absorbed energy to incident energy expressed as a percentage. |
| Effective Index |  | The apparent index of refraction of a substrate plus coating stack. |
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