— Monochromator Design & Operation — Operation — Beam Geometry & Alignment — Resolution — Changing Slits — Optical Coatings — General Specifications — Models Available and Resolution

Optometrics Corporation has, for more than thirty years, designed and manufactured optical components and instruments for university, industrial and government laboratories and the OEM markets.

Optometrics Corp. has manufactured over 20,000 Mini-Chrom monochromators since their initial introduction in 1978. The majority have been incorporated into a variety of analytical and biomedical instruments made and marketed by leaders in their respective fields.

Mini-Chrom monochromators are currently being used as the principal optical assembly in numerous types of instruments and analyzers. When not integrated into other manufacturers’ products, Mini-Chroms are used in basic and applied research projects in industrial, government and university laboratories. The combination of excellent performance, low cost and small size has contributed to the widespread use of the Mini-Chrom.

MONOCHROMATOR DESIGN & OPERATION
Mini-Chroms are compact, in-line Fastie-Ebert monochromators with a 74 mm pathlength, applicable for general spectroscopy or for use as a component in a system. All incorporate one of a wide selection of replicated gratings from the UV to IR (ruled or holographic) and gratings are also available with aluminum or gold coatings depending upon application. All monochromators also include a set of fixed, interchangeable entrance and exit slits. Optional sets of slits are available to optimize either resolution or throughput.

The small size of the Mini-Chrom still results in resolution comparable to that of many larger, more costly, conventional monochromators.

Mini-Chroms are available in four types: Standard, Digital, Scanning and Scanning Digital. The primary differences in the four types relate to how the wavelength is selected and displayed. Each type is available in several wavelength ranges from the ultraviolet to the near infrared.

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OPERATION
All Mini-Chroms are optically identical Fastie-Ebert in-line monochromators with an effective aperture of f/3.9 and 74 mm focal length. As shown in the optical diagram, polychromatic radiation is focused at the entrance slit and reflected by a folding mirror onto a spherical collimating/ focusing mirror. This mirror collimates the radiation and directs it onto the grating, where it is diffracted. Once separated into a spectrum, the radiation is directed back to the collimating/focusing mirror, after which a segment of the dispersed radiation is focused at the exit slit via a second folding mirror. The wavelength of monochromatic radiation exiting the instrument is dependent upon the angular position of the grating. A sine drive mechanism is used to rotate the grating, either manually or via a stepping motor, so that discrete wavelengths are sequentially focused at the exit slit in a linear fashion.

Optimal throughput and wavelength accuracy are attained only if the Mini-Chrom is operated under the following guidelines:

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BEAM GEOMETRY AND ALIGNMENT
To ensure maximum wavelength accuracy and system throughput, the effective aperture of the input beam must be f/3.9 or greater. If the input radiation has a faster (less than f/3.9) effective aperture, the input folding mirror will be overfilled and stray light will increase significantly. In addition, the converging (input) beam must be normal (perpendicular) to the plane of the entrance slit. Failure to align the beam properly with the entrance slit will adversely affect both throughput and resolution.

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RESOLUTION
Resolution is a quantifiable indicator of the spectral purity of radiation exiting the monochromator. It is a function of the focal length of the monochromator, the dispersion of the grating and the width of the entrance and exit slits. In Mini-Chroms, only the slit widths are variable.

Resolution is inversely proportional to slit width, i.e. as slit width decreases, resolution increases. Throughput, however, varies directly with the square of the slit width. Halving the width of a slit will therefore decrease throughput by a factor of four. Resolution is also affected by wavelength, but to a much lesser extent than changing the slits.

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CHANGING SLITS
Changing the slit assemblies in any Mini-Chrom takes only a few seconds and no tools. The slit assembly consists of a precision slit photo etched in a black oxide coated brass disc, a slit spacer, slit cover and two banana plugs. The banana plugs allow the assembly to be easily inserted or removed while assuring alignment of the slit with the monochromator. Note: Slits should always be changed in pairs.

OPTICAL COATINGS
All optical surfaces in the Mini-Chroms are coated with aluminum which has a high reflectance throughout the UV- VIS-NIR spectral range. Aluminum does, however, exhibit a decrease in reflectance at approximately 850 nm. If your application requires maximum efficiency in this area, an optional gold coating on all optical surfaces may be required. Note that the reflectance of gold falls to very low levels below 600 nm.
 

REFLECTANCE VS. WAVELENGTH OF ALUMINUM AND GOLD COATED OPTICS

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General Specifications - Monochromators Specifications that apply only to a specific type of Mini- Chrom are listed following the description of each type.       f Number - 3.9 Focal Length - 74 mm Grating - 2 cm square       Stray Light:    Models with holographic gratings1   ≤ 0.003%    Models with ruled gratings2 - ≤ 0.02% Wavelength Accuracy (as a % of wavelength) - ± 0.2% Wavelength Reproducibility   ± 0.15% Wavelength Readability - 0.2 nm Dimensions - Type dependent

¹ measured 10 nm from 632.8 nm (HeNe laser line).
²measured 20 nm from 1265.6 nm (second order HeNe laser line).

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