Spectral devices. Selection of a spectral device.

Spectral devices are optical instruments used for decomposition of light into monochromatic components. Such devices are used for qualitative and quantitative spectral analysis of the light emitted, absorbed, reflected or scattered by a sample. These studies define the properties of a sample, its chemical composition and the nature of physical processes associated with the optical radiation and its interaction with sample. Spectral devices are also used to form the predetermined spectral composition of light.

The spectral devices, offered by SOL Instruments company, are “classical” in manner of realization of the light spectral decomposition. These devices use diffractive gratings as dispersive elements to perform the spectral decomposition of light. The “classical” instruments can be divided into two groups: monochromators and spectrographs.

Monochromators are designed to select the radiation within a predetermined spectral range. The optical system of a monochromator is composed from two slits (entrance and exit), a collimator lens, a diffraction grating, and a focusing lens. The exit slit allows to select a narrow range of wavelengths. Monochromators always have the ability to wavelength scanning by rotating the grating with a special mechanism or manually.

Spectrographs are generally used to observe a relatively wide range of wavelengths. Unlike monochromators, a multi-element detector (photodiode line array, CCD, etc.) is installed in the focal plane of the focusing lens instead of the exit slit. It allows detecting the optical radiation within a defined field. Spectrographs are mainly used in the ultraviolet (UV), visible and near infrared (IR) spectral regions because multi-element radiation detectors are currently available only for wavelengths of 190 – 2600 nm.

Optical radiation occupies a vast part of the electromagnetic spectrum. We propose devices which allow operation in the spectral range from 0.18 to 60 microns. The short-wavelength is limited by the fact that the air becomes opaque to wavelengths shorter than 180 nm. Measurements at the shorter wavelengths require special vacuum spectral instruments.

The devices with mirror optics offer the most broadband spectral range. The spectral range of such devices is determined by the grating parameters and by the type of coatings of the optical elements (mirrors, gratings). The spectral range of spectral devices is mainly determined by the material from which lens optic is made.

Parameters of diffraction gratings determine the device spectral range too. We offer many types of diffraction gratings for UV (ultraviolet), visible and IR (infrared) ranges. The proper grating selection provides the best combination of high-throughput efficiency and a minimum of stray light in various spectral regions. You can select the desired diffraction grating from the list of gratings for each of our spectral devices, or consult with our experts.

Each grating has its wavelength range determined by one of the grating parameters, the blaze angle. Grating efficiency is the highest at the blaze angle (blaze wavelength), and it decreases both for the longer and shorter wavelengths. The wavelength region corresponding to at least 40 percent reflection at blaze is the operating wavelength range of the grating.

While choosing gratings, an important parameter, the “rotation angle of the grating” should be taken into consideration. The rotation angle of a grating is determined by the hardware design of Monochromators/Spectrographs. For example, the rotation angle of grating in a MS3501 monochromator-spectrograph is varied from 0° до 55°. It corresponds to 0-1290 nm in terms of wavelengths for the diffraction grating 1200 lines/mm. This parameter is different for gratings with different numbers of lines, and it is proportional to the number of lines per millimeter in the gratings. The maximum wavelength that a grating can diffract is 2 and 4 times larger for the gratings 600 lines/mm and 300 lines/mm correspondingly than for the grating 1200 lines/mm. It is 2580 nm and 5160 nm respectively. The maximum wavelength for the gratings 1800 lines/mm and 2400 lines/mm is 1.5 and 2 times less respectively, and it is 860 nm and 645 nm accordingly.

The rotation angle of the grating 1200 lines/mm in monochromator-spectrograph MS3501 is 0 – 1290 nm. The wavelength range should be from 170 to 500 nm in case of the grating with the blaze wavelength of 250 nm. It is the spectral range in which the grating efficiency is not less than 40 percent relative to the efficiency at the blaze angle. The working spectral range is 180 – 500 nm, because the air becomes opaque to wavelengths shorter than 180 nm.

The rotation angle of the diffraction grating with 1800 lines/mm in MS3501 is 0 – 860 nm. The wavelength range is 500 – 1100 nm in case of the grating with the blaze wavelength of 750 nm. The working spectral range is 500 – 860 nm because of limitation by the rotating angle of the diffraction grating.

The light throuput of a spectral device characterizes the illumination in the focal plane of a focusing objective or a light flux falling on a detector. The light energy, passing through a spectral device and reaching a detector, is determined by the relative apertures of collimator and focusing lenses. A lens with the round entrance pupil having diameter  and focal length  can be characterized by the relative aperture  or by the f-number  . A lens with the rectangular entrance pupil having dimensions  and  is characterized by the relative aperture  .

For example, if the focal length of the collimator mirror equals 380 mm and its size is 70×70 mm, the relative aperture  should be 1/4.8, and the f-number should be 4.8. The smaller value of the f-number the higher amount of light which passes through the spectral device and comes into the detector.

Therefore, if the light source is very weak, it is necessary to choose the spectral device with a smaller f- number value (with a larger value of the relative aperture). However, it is necessary to take in account, that the spectral resolution of device becomes worse with the f-number decreasing, because the level of aberrations (spherical and coma) increases.

The angular and linear dispersion are important parameters of a spectral device. The angular dispersion is a characteristic of a dispersive element (dispersive grating). This value determines its ability to deflect light with different wavelengths at different angles. If the beams of two nearest wavelengths,  and  are deflected to the angles of  and  respectively, the angular dispersion is determined as the derivative of  .

The angular dispersion of a grating is  , where  – diffraction order,  – number of lines per millimeter in the grating,  – angle of diffraction. It is obvious, the angular dispersion increases with increasing of the number of lines per millimeter in the grating (lines/mm), for a larger angle of diffraction, and also for operation at the high spectrum orders.

The linear dispersion is the characteristic of the whole device. If  is the distance between two neighboring spectral lines on the image surface,  is their wavelength difference, the linear dispersion is the  derivative. Spectral devices are often characterized with a value expressed in term of nm/mm and called the Reciprocal Linear Dispersion  :  , where  – focal length of the focusing lens,  – inclination of image surface.

The spectral resolution is an important characteristic of a device. It defines the minimum wavelength difference () between two lines of equal intensity that can be distinguished (observed separately). Resolving power is used as a quantitative characteristic of the ability to distinguish the closest spacing of two peaks. Resolving power is given by the wavelength, divided by the resolution:  . Consider the relationship between resolution and dispersion:  , where  – the shortest distance between two resolvable monochromatic lines.

Thus, the resolution of device is proportional to its linear dispersion. To increase the spectral resolution, monochromators providing additive dispersion mode (MSDD1000) or Echelle monochromators working at the higher orders of a spectrum (MS520) can be used.

Along with the light, decomposed in the spectrum, some part of stray (scattered) light of other wavelengths is always coming to the exit slit of all monochromators. This can be explained by the multiple reflections of light from the optical parts, the light scattering on the surfaces of the optical components, optical flares on their rims and the device inner walls.

Stray light reduces the accuracy of any spectrophotometric measurements, especially when brightness of light sources or the detector sensitivity are low in the studied spectral region. To reduce stray light, we use either fully blackened (with a special coating) holders and inner walls or additional light filters.

The most reliable way to eliminate the stray light is the use of double dispersion monochromators (MSDD1000), and double monochromators (DM160). The intermediate slit in double monochromators allows to decrease the stray light in three orders of magnitude thus making these devices indispensable in laser Raman spectroscopy, where measured signals are in ten orders of magnitude weaker than the laser light.

From a variety of types of aberrations in the optical systems a special attention should be paid to astigmatism , as this aberration is typical for all the “classical” spectral instruments. The reflective objectives used in the spectral devices have not any axis of symmetry, and have decentering aberration (astigmatism) in addition to the aberrations typical of the conventional centered systems.

Due to this aberration, a point source on entrance slit of a device is represented in the focal plane as a vertical line. Astigmatism is corrected through the use of special optics in most devices, produced by SOL instruments. Such devices are called imaging monochromators or imaging spectrographs.

If the multiple point light sources are placed along the entrance slit the spaced in vertical direction spectra in the focal plane will be formed. This allows to use devices with astigmatism compensation in Multi-track Spectroscopy, where many tracks of the matrix detector are needed to capture many spectra simultaneously.

Moreover, using the optical scheme with astigmatism compensation, it is possible to minimize the signal losses in case of a small size detector. It is typical for the IR detectors, the size of which is usually very small to reduce noise.

Various spectra signal detection systems (detectors) can be used with spectral devices. Depending on a detector type, it can be mounted either directly to the output port of the device or on the exit slit of it.

It is necessary to use a special conjugation unit for detectors if size of their active area is less than 5 mm. It transfers an exit slit image, with the help of a toroidal mirror, to the active area of a detector. In this case, the monochromator output signal falls on the detector active area without any losses.

We offer adapters to mount our standard detectors as well as for detectors of other manufacturers.

Most of the SOL instruments devices are highly automated what is ensured by reliable and precision mechanics and internal electronics. The processes of grating rotation, grating changeover (for models with turret), slit opening width adjustment, exit port selection, as well as control of additional devices such as a filter wheel, fast shutter and others, are fully automated. You only need to set the appropriate device parameters for the measurement using our program and the system will automatically perform all replacements and settings with high accuracy.

DevCtrl service program for control of spectral devices is used. This program is supplied with a device free of charge. The DevCtrl program controls all mechanisms and units of a spectral device, as well as additional accessories. This program also provides access to the system settings and the device calibration parameters.

We offer the powerful spectroscopic program SpectraSP, designed for simultaneous control of devices and registration systems (integral or multi-element photodetectors). With the help of the SpectraSP program, any device control, optical signal registration with various detectors, spectral data processing and their visualization are implemented simultaneously.