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.
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:
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.
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.
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.
1 measured 10 nm from 632.8 nm (HeNe laser line). 2measured 20 nm from 1265.6 nm (second order HeNe laser line).
NOTE: Wavelength accuracy is given as a percentage of wavelength. This means that at 400 nm, the accuracy would be 400 nm ± 0.2% or 400 nm ± 0.8 nm. At 800 nm, the accuracy in the same Mini- Chrom would be 800 nm ± 0.2% or 800 nm ± 1.6 nm.
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
SLIT SETS Your choice of a pair of slits are included with each Mini-Chrom. Additional slit sets can be purchased to optimize resolution or throughput. Slits are available in 50, 100, 150, 300, 600 microns and 1mm widths, all 4 mm in height.
HIGHER ORDER BLOCKING FILTER ASSEMBLY Required to block higher order (λ/2, λ/3, λ/4, etc.) radiation. Higher order blocking filters are Optometrics’ Long Pass Cut-On filters in an assembly which includes the monochromator slit. Plugs directly into the Mini-Chrom monochromator.
MONOCHROMATOR APERTURE ADAPTER Adapter converts slit(s) aperture to SMA connector compatible with Amphenol 905 series fiber optic connectors.
HOLMIUM CALIBRATION FILTER Comprised of 2.5 mm thick holmium oxide glass epoxied in a metal holder, the cell holder fits in all spectrophotometers that accept standard 1 cm square cells. A reference scan showing calibration wavelengths from 350 nm to 800 nm is included. A comparison between the reference scan and the indicated wave- length takes only a few seconds.
FIBER OPTIC CABLE The F. O. Cable plugs directly into the Mini-Chrom, functionally replacing the exit slit. It is supplied with a 600 micron slit assembly for use on the entrance side. The other end of the cable terminates in a conventional cir- cular bundle. The cable is 152 cm (5 feet) long with a rectangular (4.0 x 0.6 mm) fiber array and slit adapter on one end and a 2.0 mm diameter bundle on the other. Non-standard cable configurations are available on request.
For applications below 400 nm, quartz fibers are available as special order items.
Typical transmission for 1/8” bundles of glass fibers in 12”, 36”, 60” and 156” lengths
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.
The F. O. Cable plugs directly into the Mini-Chrom, functionally replacing the exit slit. It is supplied with a 600 micron slit assembly for use on the entrance side. The other end of the cable terminates in a conventional cir- cular bundle. The cable is 152 cm (5 feet) long with a rectangular (4.0 x 0.6 mm) fiber array and slit adapter on one end and a 2.0 mm diameter bundle on the other. Non-standard cable configurations are available on request.
