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, input spectral 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 and then directed 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.
Beam Geometry and Alignment
To ensure maximum wavelength accuracy and system throughput, the effective aperture of the input beam should 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 throughput, resolution, and wavelength accuracy. See the Tungsten Source Module for a pre-aligned visible source.
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 interchangeable entrance and exit slits. Resolution is inversely proportional to slit width, i.e. as slit width decreases, resolution increases.
|Resolution (nm) for Slit Widths of:|
|Linear Dispersion||Wavelength Range||50μ||100μ||150μ||300μ (included)||600μ||1mm|
A 300 micron 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 and 1000 micron widths, all 4mm in height.
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.
All optical surfaces in the standard Mini-Chroms are coated with aluminum which has a high reflectance throughout the UV-VISNIR spectral range. Aluminum does, however, exhibit a decrease in reflectance at approximately 850 nm. If your application requires optimal efficiency above 625 nm, and does not require efficiency below 600 nm, an optional gold coating on all optical surfaces is available. The 05G and 06G models feature all gold coated optics.
Tungsten Source Module
The Tungsten Source Module includes a 20W tungsten halogen lamp in a quartz envelope, a lamp housing, shutter assembly and variable aperture assembly on a base plate and a regulated 12V DC power supply. Halogen compounds in the lamp recycle tungsten deposited on the inside of the envelope back to the filament. This cycling of tungsten prevents the gradual degradation of the lamp output, particularly in the ultraviolet and increases the life of the lamp. The module can be used as a visible and near infrared source (340 nm to 3 μ) or as a building block to construct a variety of spectrophotometric systems. The module has provisions for attaching a Mini-Chrom monochromator in a pinned location for easy alignment. The addition of a Mini-Chrom converts the Tungsten Halogen Source Module to a compact and versatile monochromatic light source. A quartz lens in the lamp housing focuses radiation from the tungsten lamp onto the entrance slit of the Mini-Chrom, obviating the need for additional optics and time consuming Alignment.