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These systems are able to measure optical filter transmittance using a broadband light source in conjunction with monochromators that are composed of a diffraction grating and a series of slits and mirrors (Figure 3).įigure 3: Simplified diagram of a double-monochromator-based spectrophotometer.Īs broadband light strikes the first diffraction grating, it is dispersed into its component wavelengths, with each reflecting off of the grating at a slightly different angle. Most optical filter spectra are measured using a spectrophotometer or other grating-based measurement system. In these cases, the filter steepness is already dictated by the transmission and blocking/reflection specifications. However, it is generally not necessary when transitions between adjacent bands are tight or when both bands are specified as absolute. \(T_ \) ).Ĭalling out edge steepness is useful when this specification is used in conjunction with average transmission and blocking/reflection bands. An average transmission band specification, for example, means that the average transmission values over the specified wavelength range must meet the specified tolerance (e.g. When specifying transmission, blocking, and reflection bands, it is important to indicate whether the specification is absolute or average. However, the majority of optical filters and coatings used at larger angles of incidence, such as dichroic filters and high-reflectivity (HR) mirrors, are designed to split or reflect the beam within the system, so these are necessarily specified with reflection bands instead of blocking. For most 0° AOI filters, the attenuated beam is not directed to another path in the system so specifying reflectance is not necessary. Typically, 0° AOI optical filters such as longpass or shortpass edge filters, bandpass filters, and notch filters are specified with transmission in terms of percent and blocking in units of optical density (\(OD\)).īlocking is a general term that refers to wavelength-specific beam attenuation by means of reflectance, absorbance, or both. Transmission, reflection, and blocking bands are spectral regions where each respective property is required to be above, below, or within a certain level. Transmission, Reflection, and Blocking Bands
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For example, common specifications are bandwidth at 80% of peak \(T\), or 10% absolute bandwidth.