Optical filters

Filter CWL shift as function of AOI and temperature

With this tool you will get an indicative effect on CWL when changing:

• angle of incidence (defined as the angle from the normal of the filter surface)
• temperature

What will CWL be for a filter at AOI and/or a temperature deviating from normal (AOI 0°, 23 °C)?

What filter should be selected to have specified CWL at a certain AOI and/or at a certain temperature?

Effects of varying angles of incident beams

The filter wavelengths on this website are listed at normal angle of incidence. If filters are intended to be used at other than normal angle of incidence, the filter´s spectral characteristics have to be considered in order to achieve the intended spectral function. All interference filters will shift to shorter wavelength as the angle of incidence of a collimated beam deviates from the normal. This effect may, to some extent, be used to tune a filter to a desired shorter wavelength. The angle effects can be approximately calculated by the following formula:

λ_θ = λ_ο (n_e² – sin² θ) ^1/2 (n_e) ^-1 | (1.1)

Typical values for the effective index for an infrared filter are in the region 2-3.5.

At high angles of incidence, 30°or more, transmission will decrease and a distortion of the filter band pass shape will eventually occur.

Effects of varying temperature

Interference filters will shift to longer wavelength with increasing temperature/ shorter wavelength with decreasing temperature. This wavelength shift is caused by thermal expansion/contraction of the coating materials. The temperature effect can be approximately calculated by the following formula:

λ (°C) = λ_ο + ΔT (Δλ_ο/1°C) | (1.2)

Definition of Common Terms