AES - Spectrometers - Plane Grating, Multiple Detector

Plane grating, Multiple detector

If a Czerny-Turner or Ebert-Fastie spectrometer has an exit slit in its focal plane, only a single, narrow band of wavelengths will be observed passing through the slit. If instead of a slit, one puts a CCD in the focal plane, a range of wavelengths can be observed. Suppose one is using a 1 m focal length Czerny-Turner spectrometer with a 1200 groove per millimeter grating. Using the dispersion relationship for such instruments, it can be shown (try it!) that the dispersion is approximately 0.83 nm/mm. Suppose the CCD has 25 µm pixels and is 1000 × 1000 pixels. Then each pixel will observe (nominally) 0.025 mm times 0.83 nm/mm or 0.021 nm, and the CCD as a whole will span 25 mm (physical distance) or 21 nm (spectral window). One must be a bit careful in considering the nuances of the situation. The focal plane of a Czerny-Turner system is not perfectly flat, while the CCD chip is very close to being a Euclidian plane. Field curvature, astigmatism, and coma will all perturb the line shape, making resolution poorer than a simple dispersion calculation might indicate. The image of the entrance slit will be curved in the focal plane, so that lines will appear to be circular arcs if the entrance slit is straight. If the entrance slit is illuminated only at a single point, astigmatism will render the spectrum in the tangential focus (the plane in which the spectral image is formed) as a line of non-negligible height. Just as for the Paschen-Runge mount, astigmatism spreads the light out in space, rendering signal amplitude on a single pixel lower than would be the case for perfect imaging. Examples can be found at Intevac and Bruker/Chromex.

Plane grating, Crossed with prism or grating, Multiple detector

The cross-dispersion echelle spectrometer is the state of the art in elemental analysis. Examples include Andor/Mechelle, Teledyne/Leeman, and Varian. The echelle grating, operating at a blaze angle of 75° or thereabouts, disperses the UV and visible regions of the spectrum in ~ 50th to 150th order. Cross dispersion by a non-blazed grating or prism separates the orders. The dispersion values are chosen so that all lines of interest fall on a CCD or other array detector with sufficient dispersion that physical line shape, not instrument dispersion, limits resolution. The patent on the Varian instrument is freely available: U.S. patent 5,596,407. If you have Quicktime on your computer, you can see the graphics. Alternatively, if you have a subscription to Applied Spectroscopy, go to 45(3), 334-346 (1991) for a full description of how one such system is engineered.