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For many years, the most common way to gather information for each wavelength was to move the dispersive element and illuminate the detector with the light from that wavelength only. This design made spectrometers slow, fragile, and expensive. Today, the multi-element optical detectors can scan the whole spectrum without any moving elements. This innovation makes modern spectrum capture much faster and easier and improves the durability of the spectrometer. Since spectral equipment is mostly used to analyze wide spectral ranges, the spectral equipment should exhibit very few or no color errors. For this reason reflecting parts are preferable. As a consequence, the light path must be folded, and light interacts with the components at an angle. This causes imaging errors like astigmatism and coma, which leads to poor resolution and low efficiency, but imaging errors can be minimized by using an appropriate configuration.
Monochromators
The first devices for spectral analysis which could convert spectral information into electrical signals were monochromators. At that time, the photosensitive detectors were large and costly. It was necessary to run the spectrum past the sensor using a revolving grating. Only when CCDs were introduced did it become possible to record a large spectral range all at once and hence to dispense with the mechanically unreliable revolving gratings. Monochromators are still used as tunable monochromatic light sources or in measuring applications with a low number of light signals where a photomultiplier is used.
Polychromator
Since the introduction of the CCD line detector, monochromators are gradually being replaced by spectrometers. The advantages of the spectrometer over the monochromators are the following: - Recording a broad spectral range simultaneously
- Robust construction and reduced error rate due to the elimination of moving parts
- The need for only one unique wavelength calibration
- Low-cost parts
- Compact construction
- High measuring frequencies
Asymmetric Czerny-Turner
There are different designs of polychromators have different advantages and disadvantages. The asymmetrical Czerny-Turner takes advantage of the fact that the focal length and the angle of the mirror can be freely adjusted. This means that the coma errors occurring in a spectrometer can be minimized.
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