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)?

**Input Data**

**Output Data**

Calculated CWL (at AOI 0° and 27°C) |
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What filter should be selected to have specified CWL at a certain AOI and/or at a certain temperature?

**Input Data**

**Output Data**

Calculated CWL (at AOI 0° and 23 °C) |
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**Disclaimer:** Depending on filter design, CWL will slightly deviate from calculated value.

**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_{e}^{2} – sin^{2} 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)