Figure 10.

The amount of shift may be calculated using the following formula:

Lamda1 = Lamda0 (1 - Sin2i / eff2) 1/2
where: i = angle of incidence
Lamda1 = resulting wavelength at angle i
Lamda0 = central wavelength at normal incidence
eff = effective index of the filter

Some applications make use of the change of central wavelength as a function angle. Astronomers use this effect to see the doppler shift of light emitted from celestiaI bodies as they move toward or away from the observer and to measure that speed.

If the application requires that the wavelength of interest maintain its intensity within a range of wavelengths, somewhat wider bandwidth filters might be required.

Lastly, filter performance with angle and polarized light will alter the transmission and the bandwidth of the filter.

Temperature

Temperature effects on interference filters take two forms – reversible and irreversible.

Filters exposed to relatively mild changes in temperature can be extremely useful, as the filter tends to change wavelength as a function of the temperature. This is also a technique used by astronomers to measure doppler shift. As a general guideline, filters will shift .02nm per ¼C. This is a function of the number and types of layers deposited on the substrate.

If a filter is exposed to high temperatures for an extended length of time, the filter can be irreversibly changed to a shorter wavelength.

Filters regardless of bandwidth will exhibit a minor shift towards the blue over time. For most instruments, this is irrelevant; however, for narrowband filters this may shift the central wavelength of the filter from the wavelength of interest. SpectroFilm takes advantage of this natural process by heating or baking narrow band filters into position. Although this will stabilize the central wavelength of the filter, some additional aging may occur but at a rate 100-1000 times slower than without baking.

Precision filters are generally a laminated sandwich design of various elements. The epoxy bonding the filters together will be damaged if exposed to temperatures of approximately 120 °Celsius even for short periods of time. Instrument development and design should include some method of cooling the filter if it must be placed in a position of high temperature. The limits of temperature for a laminated filter can be as high as +50 °C to below -50 °C. In all cases, the rate of temperature change should be held to a maximum of 5-10 °C.

Physical

SpectroFilm is a manufacturer of custom filters and produces filters in the sizes and shapes specified by our customers. It is important to note that small filters in quantity are less expensive than larger filters.

Most instrument manufacturers place passband filters as physically close to the detector as possible and specify a size comparable to the detector size.

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