Filter wheels are used to position a selection of filters over a lens, or an array of lenses. Applications for filter wheels include microscopes, cameras, and video systems. Filter wheels are also used in machine vision and part inspection and research. Machine vision applications that use filter wheels include fluorescent microscopy, spectrophotometry, photometry, color charged couple device (CCD) photography, laboratory applications, and optical and infrared (IR) imaging.
Generally, filter wheels are available in configurations with between 3 and 12 lenses or filters. Typically, the first lens is a simple clear filter. The other lenses in the filter wheel may be of different colors, lens thickness, lens materials, or any combination thereof. Custom and specialized filter wheels with more than twelve filters are also available.
In addition to the number of filters, filter wheels may be configured in systems with more than one wheel. This allows for additional colors or views to be used, as well as the possibility of overlapping filters to allow for widely varied views. A system with two, six-filter wheels could provide 12 separate views with the potential for as many as 36 overlapping views; however, it is very rare that this many options would be available, since many lens types do not work together.
Filter wheels are available manual and motorized designs. Manual filter wheels are tuned by hand. They are generally used in situations where filter changes are infrequent. Motorized filter wheels use a controller or computer interface. Motorized via controller systems are designed with a packaged control unit for wheel indexing. Typically, these types of filter wheels are typically equipped with keypad, buttons, or other local interface. Filter wheels that are motorized via a computer interface driven systems allow the filter wheel to communicate directly with the computer. Communication is done via protocols such as RS232 or general-purpose interface bus (GPIB), run set modes, or functions within set parameters.
Neutral density (ND) filters are available with some styles of filter wheels. ND filters attenuate the light passing through the subsequent optics. These filter wheels can provide enhanced color balance and allow for aperture adjustment for depth-of-field effects. Often, sets of ND filters are graduated in degrees of light absorption.