Many applications such as astronomical observations, television monitoring systems and high resolution photography require image quality components for optimum system performance. A large amount of the filters produced today, including Andover standard products, are classified as commercial quality or instrumentation grade and are not acceptable for use in imaging systems. In response to the need for high quality filters, Andover has developed the "Image Quality Filter". Our image quality filters utilize high-quality optical glass, typically Schott® BK-7 and Schott® filter glass. The surfaces of this material are ground and polished to a flatness of 1/4 wave per inch and parallel to 30 arc seconds or better. If necessary, a higher degree of optical quality is available for the more critical applications. To prevent fringe patterns and multiple images, all coatings are deposited onto a single surface and all exterior surfaces are broadband anti-reflection coated for an average reflection of <=0.5%.

Cut-away view of the construction of an Image Quality filter

Interferogram of an Image Quality filter  Our in-house tunable interferometer allows us to look through filters that would be opaque to the typical laser interferometer

Wavelength Shift With Time One of the major problems with interference filters is that their center wavelength tends to drift towards the shorter wavelengths with time. This instability can be attributed to many factors such as the thermal environment and the type of illumination they receive. Andover's solution to this problem has been the development of a proprietary method of pre-aging and stabilizing these filters. All of our narrowband and image quality filters undergo this lengthy pre-aging process. The end result is a very stable filter which displays a minimal drift in central wavelength with time. Several pre-aged filters were monitored for wavelength drift with time, and after eight months, have shown no change in their central wavelength. A normal non-stabilized filter would have shifted 0.2nm to 0.3nm over this same period.

Wavelength Shift With Temperature Change One very important factor to note is that all filters are sensitive to wavelength shift with changes in the ambient temperature. Filters centered within the wavelength range of 400nm to 1550nm will shift from 0.015nm to 0.035nm/°C depending upon their location within this range. Due to this phenomenon, one can see that it is very critical to control the ambient temperature of a narrowband filter when in use. In many instances this may be impossible to do which is why Andover has developed a series of filter ovens, more commonly referred to as temperature controllers. These ovens/controllers can be used to control and tune the central wavelength of a filter irrespective of the ambient temperature. For more detailed information please refer to Andover Temperature Controllers.

Interference filters will exhibit a shift in wavelength with a change in temperature. The wavelength shift will be in the direction of the temperature change as long as one stays within the temperature limits of the filter (-50°C to +70°C). The above temperature coefficient chart gives a good approximation of the shift in wavelength for a given temperature change. When ordering custom filters, always state the operating temperature or temperature range.

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