Grating Spectrometer

Content
1. What is grating spectrometer?
    1.1 What is grating?
    1.2 How to select grating?
    1.3 Grating spectrometer major parameters
    1.4 Application of grating spectrometer
2. Use of grating spectrometer
    2.1 Use of diffraction grating in spectrophotometer
    2.2 Precaution of diffraction grating spectrometer
3. How to buy grating spectrometer?

Grating spectrometer is a scientific instrument that breaks down the light of complex composition into spectral lines. The light information is captured by the spectrometer, displayed, and analyzed by a photographic negative or computerized automatic display of numerical values to determine which elements are present in an object. Grating spectrometers are widely used for color measurement, concentration measurement of chemical components or radiometric analysis, film thickness measurement, gas composition analysis, etc.

As an important analytical tool, spectroscopy methods play a great role in scientific research, production, and quality control. It is an indispensable means for how to obtain single wavelength radiation, whether it is penetration absorption spectroscopy, fluorescence spectroscopy, or Raman spectroscopy. Since modern monochromators have a wide spectral range (UV-IR), high spectral resolution (to 0.001 nm), automatic wavelength scanning, and complete computer control functions that can easily be integrated with other peripheral equipment into a high-performance automatic test system, the use of computerized automatic scanning multi-grating monochromator has become the first choice for spectroscopic research.

When a beam of composite light enters the incident slit of the monochromator, it is first converted into parallel light by the optical collimator, and then disperse into separate wavelengths (colors) by diffraction grating. Using each wavelength to leave the grating at different angles, by the focusing reflector and then imaging out of the slit. The outgoing wavelength can be precisely changed by computer control.

Grating, as an important spectral device, its selection and performance directly affect the overall system performance. Grating is divided into scribing grating, duplicating grating, holographic grating, and so on.

Scribed gratings are mechanically scribed on a thin coated metal surface with a diamond scoring knife, while replicated gratings are replicated with a master grating. The grooves of typical scribed and duplicated gratings are triangular. Holographic gratings are produced by laser interferometric stripe lithography and usually include sine grooves. Scribed gratings have high diffraction efficiency, and holographic gratings have a wide spectral range, and low stray light, and can be used with high spectral resolution.

The selection of grating needs to consider the following factors.

The number of grating lines is directly related to the spectral resolution. More inscribed lines have high spectral resolution and fewer inscribed lines have wide spectral coverage. Both to choose flexibly according to the experiment.

The shine wavelength is the maximum diffraction efficiency point of the grating. When choosing a grating, you should try to choose the flash wavelength near the wavelength needed for the experiment. For example, in experiments for the visible range, you can choose the flash wavelength of 500nm.

The lower limit of grating usage can usually be considered as half of the grating flash wavelength, while the upper limit can be considered as two times the grating flash wavelength. The actual reference can be a grating efficiency graph.

The grating efficiency is the ratio of the diffracted monochromatic light to the incident monochromatic light for a given stage. The higher the grating efficiency, the smaller the signal loss. To improve this efficiency, a special coating is also used to improve the reflection efficiency.

The resolution R of a grating spectrometer is a measure of the ability to separate two adjacent spectral lines, according to the Roland criterion as

R=λ/Δλ

A practically meaningful definition of a grating spectrometer is the measurement of the width at half height (FWHM) of a single spectral line. In practice, the resolution depends on the resolving power of the grating, the effective focal length of the system, the set slit width, the optical aberration of the system, and other parameters.

R∝ M-F/W

M - number of grating lines
F - focal length of the spectrometer
W - slit width

The dispersion of a grating spectrometer determines its ability to separate wavelengths. The inverse dispersion of a spectrometer can be calculated as follows: changing the distance χ along the focal plane of the monochromator causes a change in wavelength λ, i.e.

Δλ/Δχ=dcosβ/mF

Here d, β, and F refer to the spacing of grating grooves, diffraction angle, and effective focal length of the system respectively, while m refers to the diffraction level.

As seen from the equation, the cepstrum dispersion is not constant, it varies with wavelength. In the used wavelength range, the variation may be more than 2 times.

Bandwidth is the width of the wavelength output from the spectrometer at a given wavelength, ignoring optical aberration, diffraction, scanning method, detector pixel width, slit height, and illumination uniformity. It is the product of the inverse line dispersion and the slit width. For example, if the monochromator slit is 0.2mm and the grating cepstrum dispersion is 2.7nm/mm, the bandwidth is 2.7×0.2=0.54nm.

Wavelength accuracy is the scale level of the spectrometer to determine the wavelength in nm. Usually, the wavelength accuracy varies with the wavelength.

Wavelength repeatability is the ability of a spectrometer to return to the original wavelength. This reflects the stability of the wavelength drive mechanism and the instrument as a whole. The wavelength drive and mechanical stability of Chorley Hanlight's spectrometers are excellent, and their repeatability exceeds wavelength accuracy.

F/# is defined as the ratio of the focal length of the spectrometer to the diameter of the collimated concave reflector. The light passage efficiency is inversely proportional to the square of F/#. The smaller the F/#, the higher the light passage rate.

In the past, due to the limitations of the conditions, it was not enough to detect the structure of compounds and it was very difficult to do so, but nowadays the grating spectrometer can greatly improve the efficiency and performance of the detection. People can use the advanced grating spectrometer to smoothly determine the types of different chemical bonds so that the chemical bonds contained in the compound and the structure of the compound can be effectively determined.

The emergence of grating spectrometer can eliminate the complex steps of detecting the content of compounds in the past, and no longer need to complete the determination work by chemical reaction with the compound. And grating spectrometer detection mixture can be quickly analyzed after the content of the compounds contained in it, greatly saving time and cost and improving the rate.

The grating spectrometer has an excellent performance in detecting functional groups of organic substances. It is possible to determine the type of functional groups of organic substances in a short period. In this way, grating spectrometers play a pivotal role in scientific research for the determination of functional groups.

a. Please check carefully whether all parts of the grating spectrometer (monochromator mainframe, electric control box, receiver unit, computer,) are connected correctly to ensure accuracy.

b. To ensure the performance index and lifetime of the instrument, the incident slit width, and the exit slit width need to be adjusted to about 0.1mm after each use.

c. After the instrument system is reset, the incident slit width and the outgoing slit width need to be adjusted to the appropriate width according to the requirements of the test and experiment, respectively.

Note that if the photomultiplier tube is used as the receiving unit, it is not necessary to expose the photomultiplier tube to strong light (including natural light) with negative high voltage added to it. At the end of use, be sure to pay attention to adjusting the negative high voltage knob to make the negative high voltage to zero, and then close the electric control box.

The incoming slit and outgoing slit of the instrument are straight slits with a continuously adjustable width range of 0~2mm. Clockwise rotation increases the slit width and vice versa. The slit width changes by 0.5mm per rotation week, and the maximum adjustment width is 2mm.

To prolong the service life, the slit width adjustment should be careful not to exceed 2mm at the maximum. It is better to adjust the slit about 0.1mm-0.5mm when the instrument is finished measuring or not normally used.

The electronic control box includes the power supply, signal amplification, control system, and light source system. Before running the instrument operating software, you must make sure that all cables are properly connected and that the switch to the electrical control box has been turned on.

The wavelength accuracy of the grating spectrometer may deviate due to various reasons such as vibration during transportation, so the wavelength accuracy of the instrument is calibrated with a known spectral line before the first use. In daily use, the wavelength accuracy of the instrument should also be checked periodically.

In checking the wavelength accuracy of the instrument, deuterium lamps, sodium lamps (standard values of 589.0 nm and 589.6 nm), mercury lamps, and other known sources of spectral lines can be used.

The wavelength values of the two spectral lines of the deuterium lamp (standard values of 486.0 nm and 656.0 nm) are used to calibrate the instrument. Adjust the incident slit and the exit slit manually according to the energy signal size and scan the deuterium lamp spectrum. If there is a deviation in wavelength, use the machine button for correction.

The wavelength values of the two spectral lines of the sodium lamp (standard values of 589.0 nm and 589.6 nm) are used to calibrate the instrument. The incident slit and the exit slit are manually adjusted according to the energy signal size, and the sodium lamp spectrum is scanned. If there is a deviation in wavelength, use the machine button for correction.

The wavelength values of the five spectral lines of the mercury lamp (the standard values are 404.7nm, 435.8nm, 546.1nm, 577.0nm, and 579.0nm) are used to calibrate the instrument. The incident slit and the exit slit were manually adjusted according to the energy signal magnitude and the mercury lamp spectrum was scanned. If there is a deviation in wavelength, use the machine button for correction.

The grating spectrometer uses high-pressure mercury as the light source, which contains strong ultraviolet light, so do not look directly at the high-pressure mercury lamp with the naked eye on the spectrometer when the wavelength of the emitted light is not within the normal range. Be careful not to exceed the normal range when measuring. If it exceeds, you can adjust the photomultiplier voltage to adjust.

Before using the grating spectrometer, it is important to carefully check all parts of it, especially to see if the connection lines between them are correctly interconnected and accurate. The operator must adjust the incident slit width and the outgoing slit width to the correct value after the operation. If the photomultiplier tube is used as the receiving unit, it is not necessary to do the photomultiplier tube with negative high voltage, so that it can be naturally exposed to bright light. Finally, adjust the negative high-pressure spin to let the negative high pressure to zero and then close the electric control box.

The slit of the grating spectrometer is straight, so it needs to be adjusted within the correct range, and the slit width should be adjusted to the noted width to prolong its service life.

If you are interested in our grating spectrometer or have any questions, please write an e-mail to info@antiteck.com, we will reply to you as soon as possible.