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FAQ’s - frequently asked questions about optical spectroscopy
|When, why and how do I choose and configure the grating in a spectrometer?|
Modular optical spectrographs consist of a number of components, which must be matched with each other depending on the respective application. This task involves, on the one hand, a large variety of components to choose from and, on the other hand, a large number of possibilities in the configuration of individual components.
As a rule, the grating selection depends on the desired spectral range, the desired optimum of sensitivity and the desired optical resolution, but also on stray light or the spectral efficiency of the grating. Some available spectrometer systems offer a broad election of different gratings.
Normally, spectrometers must be configured prior to being purchased, since any subsequent changes are either not possible or very expensive. The basis for this is an exact knowledge of the requirements. Our specialists will be glad to assist you with your selection.
Can I change the wavelength range?
In most miniature spectrometers the wavelength range cannot be changed subsequently. Therefore, it must be defined prior to purchase. This restriction in flexibility is offset by the advantage of sturdiness as well as a high level of long term stability of the wavelength calibration.
When is detector cooling recommended or necessary?
As a rule, detector cooling can be desirable in the following two cases:
1. In process applications requiring a high level of long term stability at changing ambient conditions. In this case, the aim often isn’t so much the cooling but rather the increase in stability of the entire system.
2. In applications requiring an improved signal-to-noise ratio in comparison to the room temperature. As a rule, a detector cooling system considerably reduces the noise. This is frequently important in applications with very low light intensity.
Cooling of array detectors is usually achieved by means of Peltier elements. In some detectors these are already integrated. However, there are spectrometers in which the entire optical bench is thermally stabilized with an external Peltier element.
When do I use a CCD, a photo diode array or a CCD matrix?
Owing to their principle, CCD sensors have a significantly higher sensitivity compared to photo diodes (approx. 2 orders of magnitude). For that reason, CCD’s are frequently the choice when there are very weak signals or the only very short measuring times are possible. CCD’s mostly have very small detector elements (typically 7Ám to 25 Ám edge length) and up to 3000 elements in one line. There are CCD lines with detectors with a height of up to 200 Ám. Due to the large number of small detectors, greater spectral ranges can also be realized at a higher resolution.
A CCD matrix can be used to increase the sensitivity of spectrometers. For that purpose, the elements of one row corresponding to the same wave length will be arranged together and added with the so-called pixel binning technique. This often takes place in the line electronic system.
Photo diode arrays have, compared to CCD, a higher level of linearity and frequently offer a very good dynamic range. They are standard equipment in many spectrometer modules. The pixels are usually significantly larger than those of CCD chips. Typical pixel sizes are (25-50) Ám x (500-2500) Ám.
What types of glass fibers are available and what are they used for?
As a rule, quartz glass fibers are used in today’s optical spectroscopy (UV/VIS and NIR). The fibers are drawn in special drawing towers from high-purity quartz glass preforms. These preforms consist of a core with a higher refractive index and a surrounding shell with a lower refractive index. For that reason, the drawn glass fiber forms a waveguide making it possible to transmit light. This structure in the glass fiber remains intact during the drawing process.
The cross section of individual fibers is usually in the range between 55 Ám and 1000 Ám. 1000 Ám fibers require very large bending radii and are quite critical, as far as handling is concerned. Very small cross sections are frequently used in fiber bundles or special fiber arrangements. Typical applications utilize individual quartz fibers between 200 Ám and 600 Ám core diameter.
The so-called LOH fiber (low OH) only has very few OH groups. This type of fiber is required for NIR applications, since otherwise the attenuation will be too high even on short distances.
HOH (high OH) fibers correspondingly have more OH groups. The transmission capability of these fibers is significantly better in the UV range. Therefore, HOH fibers are predominantly used for UV/VIS applications.
How long can glass fibers be?
Glass fibers of lengths up to several hundred meters are used in spectroscopy. This is particularly common in process applications in the NR spectroscopy, in order to transmit signals from external areas to the analyzer or, for example, to retrieve signals from underwater applications.
In case of UV applications, as of today there are still some limitations in the length of the glass fibers. The is primarily caused by the strongly increasing absorption and diffusion of the quartz glass fibers.
To what extent can a glass fiber be bent?
A rule of thumb says, that under short term loads, the bending radius of the fibers may not be less than 100 times, and in case of permanent installation, not less than 600 times the radius of the glass fiber.