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The spectrometer directs the entrance light onto the line detector. Two separate spectral lines can be though of as two separate pictures of this entrance light. To be able to distinguish these two lines, the intensity of the incoming signal has to be small enough to be separable into these individual pictures. The size of the slit affects the incoming signal. A small slit raises resolution, but a wider slit increases sensitivity. The preferred width of the slit depends, therefore, on the application.
The slit is manufactured using galvanics. The standard slit is 50 μm wide. The tolerance of the slit width is +3 μm / –1 μm.
Grating
The characteristics of an optical grating are determined essentially by four features: groove density, physical dimensions, method of manufacture, and blaze wavelength. Differences in the groove density in the grating result in differing observable spectral ranges. The greater the groove density, the smaller the observable wavelength range and the higher the resolution. Although the resolution corresponds to the groove density, there is a deviation from a perfectly linear correlation due to imaging errors. The spectral range reproduced also corresponds to the sensor length. The dimensions of the grating influence the efficiency and the resolution. There is a minimum number of lines which must be used in order to achieve the necessary resolution. In our high quality gratings, the line number is well over the required minimum.
Ruled Gratings
Gratings are manufactured either mechanically or optically. The mechanically produced gratings have ruled markings imposed on them. These possess an especially high efficiency. Gratings like these are called master gratings. They are very precisely manufactured, and they have a very low amount of stray light. The complexity of their manufacture makes these gratings very expensive. “Replicas“ are made from the master in a casting process. Since the replicas are manufactured in serial production, the characteristics of the gratings are almost identical. The casting process does not produce gratings of the same quality as the master gratings. They have a higher proportion of stray light.
Holographic Grating
The optical production method uses holography to manufacture gratings. This process is cheaper than producing ruled markings in the grating structure in a substrate. From these holographically manufactured master gratings, replicas are also produced. The holographic gratings are noted for the their very low proportion of stray light. The is constant over a broad spectral range. However, the efficiency is not as high as the ruled gratings.
Detectors
The sensor records the spectrally partitioned radiation from the spectrometer and transforms it into an analog electrical signal. Various sensors are suitable for detection depending on the spectral range. The following detector types have become accepted for spectroscopy: PMT, CMOS, CCD, InGaAs. There are two competing sensors for the range of visible light: CMOS and CCD.
PMT
A photomultiplier is a special electron tube which amplifies weak light signals (down to single photons) and transforms them into an electrical signal. A photomultiplier consists of a photocathode and downstream secondary electron multipliers (dynodes). The number of secondary electrons created is proportional to the number of photons radiated within the limits of a saturation threshold. As a result, the size of the pulse emitted is directly proportional to the radiated stream of photons, i.e. the intensity of the light. A photomultiplier generates a virtually noise-free gain in the range of 106 to 108. Photomultipliers are uses for measurements with very short exposure times, high frequencies, weak signals, and for those with a wide dynamic range. In optical spectroscopy, photomultipliers are used as receivers when dealing with the ultraviolet wavelength range.
Avalanche Photodiodes
An avalanche photodiode is a solid-state device that works as a photodiode and a sold-state amplifier at the same time. The photon striking the avalanche photodiode creates electron-hole pairs. This causes an internal electrical field that accelerates the electrons. These electrons on a higher energy level collide with the crystal lattice again and create more electron-hole pairs. This process cascades and amplifies the initial signal by a factor of 50 or even more. This cascading effect creates more noise than a photomultiplier, but much less than the external amplifier in the case of a typical photodiode. Therefore, the avalanche photodiode is prefered for applications with too much light for a photomultiplier, but too little to use a photodiode at a low signal-to-noise ratio.
CCD
CCD detectors have a higher quantum efficiency. Their dynamics are greater, and they have a more consistent pixel signal. In front-illuminated CCDs, the electrodes for the charge carrier display are in the direction of the radiation. As the wavelength increases, it becomes increasingly unlikely that the radiation will be converted into electrons. At approximately 1100 nm silicon becomes transparent. Since the electrodes and the photosensitive layer are made of the same material, the short wave radiation is absorbed in the electrode. Front illuminated CCDs can only provide a sufficiently large signal from approx. 350 nm.
Coated CCD
To detect a shorter wavelength, a trick can be used. The short wave light can be converted into long wave light with a greater penetration depth before it reaches the electrode. The long wave light bypasses the electrodes and is absorbed in the deeper layers of the sensor. For the conversion, suitable phosphorus layers are used which prevent the etaloning effect. Because of the monochromatic radiation interference, etaloning arises in the thin layers of the sensor, which is prevented by the irregular structure of the phosphorus. Phosphorescing the sensor is a complicated process. First the glass cover must be removed. Then the sensor is masked and put into a coating oven. The temperature has to be sufficiently high in the oven to make the phosphorus evaporate and condense on the sensor. It should not be so hot, however, that it damages the silicon parts. Finally, the glass cover is replaced with quartz glass. As a result, wavelengths as small as 200 nm can now be detected.
Back-Thinned CCD
With back-thinned CCDs, the rear surface of the sensor is ground until the photosensitive layer has been reached. The sensor is illuminated from behind by the radiation which is to be analyzed. The electrodes are, therefore, no longer in the path of the radiation. Short wave radiation is converted to a charge immediately after entering. An anti-reflective coating, which prevents the photons from being reflected off the surface of the CCD, can be applied. This process can increase the quantum yield to nearly 95%. This anti-reflective coating increases the cost of the sensor due to additional production stages.
InGaAs
The best detector for the long wave range (beyond approximately 900 nm) are those based on InGaAs (Indium-Gallium-Arsenic). They exhibit a short exposure time and a large dynamic range. Depending on how they are used, they cover a spectral range from 900 nm – 1700 nm or from 1000 – 2500 nm. InGaAs detectors have a large dark current which can be reduced by using Peltier coolers. As a result low radiation intensities can also be measured.
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