Copyright © LightForm Inc, 2012 LightForm Inc: PARISS® Analytical Spectral and Hyperspectral Imaging To Determine Bandpass and Resolution We Need to Know The Linear Dispersion At The Wavelength of Interest and The Entrance Slit Width From equation (3) in the previous slide Linear dispersion  Linear Dispersion is NOT a constant: because the angle of diffraction or refraction varies with wavelength,  linear dispersion also varies with wavelength.  For angles of diffraction or refraction less than ~20 degrees beta approximates to unity. For a diffraction grating, this means that linear dispersion is greatest when wavelengths are longest For a prism, this means that linear dispersion is greatest when wavelengths are shortest Then: Bandpass = entrance slit width or exit slit width (whichever is greater) multiplied by linear dispersion at the wavelength of interest.  Bandpass and resolution are given by the Full Width at Half Maximum (FWHM) of an “infinitely narrow” monochromatic emission line. For most spectrometers the emission lines of a low pressure Hg lamp satisfies this need. Resolution = Bandpass when reducing the entrance slit-width no longer results in a decrease in FWHM. Therefore, Resolution is the instrumental limit of bandpass If a CCD is used as a detector then the “width of the exit slit” is the total width of the number of pixels that corresponds to the FWHM of the image of the entrance slit.  The optimum FWHM is three pixels and this should be used to determine the nominal width of the entrance slit. Mini-tutorial Grating and Prism Characteristics