Our monochromator unit is a compact design, with dimensions of only 140x150x200mm. It is designed for use with an external light source, which keeps the operating temperature lower than would be possible with an internal source.
It is optimised for use with a light source of f/2 aperture, such as (surprisingly enough...) our own optosource lamphouse. This is a relatively fast aperture for a monochromator, but that is actually a very desirable situation, as explained in our monochromator efficiency considerations page.
The optical cofiguration is basically a standard (Czerny-Turner) design, but using aspheric mirrors in order to prevent the fast aperture from introducing significant aberrations. We use a custom-designed lightweight diffraction grating mounted on a high-performance scanner head with our own drive electronics for improved performance. This allows wavelength shifts of up to 200nm to be completed within 2msec, with even shorter times for smaller shifts. Ours is the fastest diffraction-grating monochromator that we know of, so please check with us for more detailed information if this particular aspect of the system performance is important to you. The latest version of the monochromator is fitted with a larger diffraction grating to optimise light collection from our new light source, and improve the overall light throughput of the system.
Bandwidth control
The Cairn monochromator offers more than just rapid wavelength-changing. In monochromator design an important tradeoff has to be made between spectral resolution and optical throughput, as explained in the section on monochromator efficiency. This means that it is useful if not essential to make the resolution variable, by providing some way of adjusting the width of the slits at the entrance to and exit from the instrument. Where present, this facility is normally provided as some sort of manual mechanical adjustment, but our design allows the option of a fast (submillisecond) electronic control of either or both slits. Not only does this allow the optical bandwidth to be changed on the same fast timescale as the centre wavelength, but the slits can also be closed completely to provide a fast shuttering facility, which is generally useful if not essential. The ability to change optical bandwidth on the same timescale as the centre wavelength allows optimum matching of the monochromator output to the excitation spectrum of the indicator, because for a dual-excitation fluorescence indicator, it is usually the case that one centre wavelength has a wider permissible bandwidth than the other. By increasing the bandwidth for that wavelength only, the time for the same total excitation is reduced, allowing faster sampling and/or improved signal-to-noise.
Monochromator options for bandwidth selection
Input Slit Options :
- Galvanometer controlled - This is always the recommended option as it gives you accurate bandwidth selection and rapid bandwidth changes on the fly. It is the only means of providing the correct bandwidth at all centre wavelengths, as the precise mechanical separation of the slits varies with wavelength.
- Manually controlled - The monochromator can be fitted with a micrometer drive control on the input slit. The Optoscan microprocessor controller will display the actual slit widths required to achieve the desired bandwidth at each wavelength chosen, and the micrometer set to the optimum value.
- Fixed width - Budget systems are fitted with a fixed input slit, which gives a nominal pre-set bandwidth. If you are using our microprocessor controller it is possible to determine the actual bandwidth, but it is important to note that serious limitations are imposed on the bandwidth control. See the monochromator technical section for details of bandwidth control. If at all possible we would recommend purchasing the fully automated monochromator.
Exit Slit Options :
- Galvanometer controlled - This is the recommended option as it allows full automated control of bandwidth and wavelength.
- Manually controlled - The exit slit can also be fitted with a micrometer drive control with the micrometer set to the optimum value determined from the controller display.