Spectrometer linear dispersion

A fluorescence spectrum shall be measured with a spectral resolution of 10 nm. The experimenter decides to use a crossed arrangement of grating spectrometer (linear dispersion 5 x 10 nm/mm) and an FPI with coatings of / = 0.98 and A = 0.3%. Estimate the optimum combination of spectrometer slit width and FPI plate separation. 

Spectrometer ARL 3400 C Quantovac 1 m Paschen-Range monuing grating ruled 1080 lines mm"1, 0.309 nm mm-1 reciprocal linear dispersion (3rd order). Primary slit width 20 pm, secondary slit 50 pm, Hamamatsu R-306 photomultiplier tube... 

These tubes have been employed with both one- and two-dimensional dispersive systems. For example, Harber and Sonnek (43) described an electronic scanning spectrometer based on an image-dissector photomultiplier in conjunction with a onedimensional dispersive system. Their system used a 12.7 cm Czerny-Turner mount with a reciprocal linear dispersion of... 

Spatial resolution is one channel which is typically 25 ym wide. Spectral resolution is the product of the channel width and the reciprocal dispersion of the spectrometer. For example, a spectrometer with a focal length of 0.25 m and grating of 152.5 grooves/mm typically produces a reciprocal linear dispersion of 25 nm/mm. Therefore a 25 ym channel will cover 0.64 nm. A 305 g/mm grating used with the same spectrometer would produce a resolution of 0.32 nm/channel. 

Spectrometers 1) Jarrell-Ash, Model 78-466, 1 meter f/8.7, Czerny-Turner mount with a 1180 g/mm grating blazed at 250 nm, reciprocal linear dispersion of 0.825 nm/mm (0.0206 nm/diode). Entrance slit width 20 um, height 2 mm. 2) McPherson, Model 2051, 1 meter, f/8.7, Czerny-Turner mount with a 1800 g/mm holographic grating blazed at 200-700 nm reciprocal linear dispersion of 0.0138 nm/photodiode, entrance slit 20 um, height 2 mm. 

Wavelength Calibration And Spectral Resolution. The wavelength calibration accuracy of the SPD depends, in principle, only on the corresponding accuracy of the spectrometer s reciprocal linear dispersion. In this study, the wavelength-to-diode calibration was proven adequately linear to determine wavelength to within +0.02 nm over the entire 20 nrn spectral window (1024 diodes) of the array, from any pair of known spectral lines. 

It is therefore essential that the user determines the effect of this "array discretness" on the accuracy of determination and accordingly set the reciprocal linear dispersion of the spectometer. Thermal insulation and/or control of the spectrometer SPD system should also be considered, because thermal expansion may reduce the accuracy of the diode-to-wavelength calibration. 

The spectral resolution of a detector is defined here as equal to its spatial resolution (in urn ) times the reciprocal linear dispersion of the spectrometer (in nm/um ). It was measured to be 1.5 - 2.5 and 2-4 times poorer for the SPD and SIT, respectively, compared to that of a PMT. All measurements were performed with the same spectometer, utilizing 20 urn slit widths. Because, the proximity focused, microchannel plate (MCP) intensi-fier broadens the line images, the spectral resolution of the ISPD was found to be significantly worse than that of the SPD. Peak widths measured at half maximum intensity were four diodes wide even when only a single diode width was illuminated. 

The spectrometer used is a modified Spectrametrics Spectraspan III echelle grating spectrometer with a quartz prism cross disperser. An echelle spectrometer was chosen because of its two dimensional format display. This format allows efficient simultaneous examination of a much wider spectral range than with a linear dispersion spectrometer when a two dimensional television camera type detector is used. 

The linear dispersion of an echelle grating dl/dX) is the focal length of the spectrometer (/) multiplied by the angular dispersion ... 

As commonly used, dispersion is the wavelength separation of two spectral lines which are a unit distance apart at the focal plane of the spectrometer. This is actually reciprocal linear dispersion or dX/f d02, where / is the distance from the grating to the focal plane of the spectrometer. Reciprocal linear dispersion has the dimensions of angstrom units per millimeter. 

Gratings produce a near-linear dispersion with wavelength. The remaining nonlinearity can be removed with software. Spectrometers incorporating gratings are said to be fuU-spectrum instruments. 

However, when a multichannel detector is used, only a limited wavelength range can be observed at any one time. This range is determined by the size of the detector, the position (or wavelength) of the spectrometer, and the linear dispersion of the grating at the detector. 

Mounted behind a spectrometer, the OMA can measure an extended spectral range simultaneously while the photomultiplier accepts only the radiation passing through the exit slit, which defines the resolution. With a spatial resolution of 30 lines per mm and a linear dispersion of 5 X/mm of the spectrometer, the spectral resolution is 0.17 %. A vidicon target with a length of 16 mm can detect a spectral range of 80 K simultaneously. 

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