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Calculation of Central Wavelength

When interference filters are used at angles other than normal incidence, the optical path difference between the direct transmitted beam and the multiple order reflections within the cavity decreases, causing a corresponding decrease in the wavelength of peak transmission. Changes in the bandwidth and transmission characteristics are generally small.

For angular tilts of less than 30, the central wavelength can be calculated from the expression

where = Central wavelength at normal incidence

= Central wavelength at the off-normal angle,

and = Effective refractive index of the total filter.

• Change due to uncollimated light
• Change due to temperature