The key parameters to consider when selecting a grating for a given application are the required spectral resolution (in order to identify efficiently chemical signatures or monitor subtle spectral features behaviour changes), the wavelength range of interest, the simultaneous bandpass (the wavelength range projected at the focal plane of the spectrograph during a single detector acquisition), the incoming signal polarization and the spectrograph F/#. Selecting a Grating for a Czerny-Turner Spectrograph Usually the first order lines (n=1 or n=-1) are the most intense. n = -1, -2 etc.) Higher orders may also appear, but these decrease in intensity. As well as positive orders, light can also be diffracted in the opposite direction (i.e. The diagram above shows the orders of the diffracted wavelength. Where: n is the order of diffraction, λ is the diffracted wavelength d is the grating constant (the distance between successive grooves) θ i is the angle of incidence measured from the normal and θ d is the angle of diffraction measured from the normal. The dispersion of a grating is governed by the grating equation, usually written as: Gratings are generally better than prisms - they are more efficient, they provide a linear dispersion of wavelengths and do not suffer from the absorption effects that prisms have which limits their useful wavelength range. The dispersion and efficiency of a grating are dependant on the distance between adjacent grooves and the groove angle. The shape of the grooves (blaze angle) influences what wavelength range the grating is best optimised for. The shape of the grooves (blaze angle) influences what wavelength range the grating is best optimised for. Gratings consist of equally spaced parallel grooves, formed on a reflective coating and deposited on a substrate. The dispersed light is then re-imaged by the spectrograph and the required wavelength range is directed to a detection system. The dispersion arises from the wavefront division and interference of the incident radiation from the periodic structure of the grating. The polychromatic light incident on the grating is dispersed so that each wavelength is reflected from the grating at a slightly different angle. What are Diffraction Gratings & Diffraction Uses?Ī diffraction grating is an optical element, which separates (disperses) polychromatic light into its constituent wavelengths (colors).
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