A typical 0.32-m focal length monochromator might have a specified resolution of 0.06 nm with a 1200-g/mm grating and 0.010-mm slits. Most spectrometers have the resolution specified when the spectrometer is in a scanning-monochromator and single-element-detector configuration. The bandpass for the case in which the slits are as narrow as practical is known as the resolution specification of the instrument, stipulated for a specific wavelength and grating density. At this point, narrowing the slit further no longer improves resolution but does cut down the amount of light. We can improve spectral resolution by narrowing the slit, but eventually (at 5 to 25 µm or so), aberrations begin to degrade performance. In other words, if two spectral features are 0.26 nm or more apart, we can positively identify them as being separate. This means that if the slit width is set at 0.1 mm and the monochromator has a linear dispersion of 2.6 nm/mm, the bandpass or spectral resolution will be 0.26 nm. In cases in which M = 1 and W o = W i, the bandpass of a two-slit spectrometer, to first-order, is The output image width is easily determined by the input slit width, W i multiplied by the magnification, M, of the spectrometer. The value is given in nm/mm and is listed as a primary instrument specification which again varies with wavelength. Reciprocal linear dispersion indicates the width of spectrum that is spread over 1-mm at the focal plane.
To calculate the BP, we require the output image width ( W o) and the reciprocal linear dispersion ( RLD). Since this limits the ability of the spectrometer to separate peaks, it is common to refer to the BP as the spectral resolution. Determining the working resolution of a spectrometer system is not as simple as reading the resolution value off the specification sheet, however.įor a digital detector to resolve two separate peaks, peak signals pixels must be separated by a pixel with lower signal.įor a spectrometer, the bandpass (BP) specifies how much spectral bandwidth is being seen for a given wavelength position. The spectral resolution is generally defined as the spectral separation between the two closest peaks that the instrument can resolve. When you're choosing a grating-based spectrometer, whether one slit or two, one of the first issues you should consider is the spectral resolution of the instrument.
0 kommentar(er)
