Optical Filter Specifications
AOI and Snell’s Law
Angle of incidence (AOI) refers to the tilt of an optical filter with respect to the incident light (Figures 1a-1c). The simplest case is 0° AOI, where the incident light is normal to the filter.
Figures 1a-1c: Diagrams showing (a) normal AOI for an optical filter, (b) 45° AOI for a dichroic filter, and (c) 45° AOI for a high-reflectivity mirror.
CHA and F-Number
Cone half angle (CHA) describes the extent to which an incident beam is converging or diverging. It is defined as the angle between the AOI of the incident beam and the most oblique marginal ray (Figures 1a-1c). Therefore, a 0˚ CHA is synonymous with collimated light, and larger cone half angles designate a pronounced convergent or divergent beam.
Figures 1a-1c: Diagrams showing uncollimated light and cone half angle for (a) an optical filter at 0° AOI (b) a dichroic filter at 45° AOI, and (c) a high-reflectivity mirror at 45° AOI.
Cut-On and Cut-Off Wavelength
Cut-on wavelength describes an optical filter edge transition where transmission increases sharply over an increasing wavelength range, such as seen with a longpass filter. Conversely, cut-off wavelength describes an edge transition that decreases over a wavelength range, as seen with a shortpass filter. They are defined as the point on each respective edge where transmission reaches 50% of the peak (Figure 1), and are also known as 50% edge points and half-power wavelengths.
Figure 1: Cut-on and cut-off wavelengths, center wavelength (CWL), and full-width at half-maximum (FWHM) for a bandpass filter.
Surface Flatness Interferograms
Surface flatness describes the deviation between the surface of an optical component and a perfectly flat reference plano surface. Optical filter surface flatness is measured using an interferometer (typically a laser Fizeau interferometer) that represents this deviation as a pattern of light and dark bands known as interference fringes. Interference fringes are a visual representation of the destructive interference that results from the difference in phase between light reflected off the optical filter and the reference flat. Once the interferogram is obtained, post-processing software can be used to create a 3D model of the surface.
Interference filter spectra are temperature dependent. Extreme temperatures result in the expansion or contraction of the thin-film layers, resulting in a red shift with increasing temperature and a blue shift with decreasing temperature. This shift can be dramatic unless the filter has been specified and designed to operate in harsh environments, and is an especially important consideration for ultra-narrowband interference filters.
Surface quality specifications refer to the type and amount of allowable imperfections on each of the coated or uncoated surfaces of an optical component. Although some surface imperfections are purely cosmetic, many can introduce unwanted scattering or make the optical filter more susceptible to laser induced damage, resulting in decreased system performance.
Clear Aperture (CA)
Clear aperture (CA) is defined as the dimensional area of an optical component over which the specifications must be met (Figure 1). It is usually specified in terms of diameter for round parts or length and width for square or rectangular parts.