 All Jarrell Ash monochromators and spectrographs use a diffraction grating as the dispersive element. Choosing a grating for a monochromator or spectrograph should be a function of the overall performance expected and the spectral region to be studied. Here is a brief description of gratings and what factors to consider when selecting a grating. A grating a system of close, straight, equidistant, and parallel lines or bars positioned on a polished surface to produce spectra by diffraction. Classically ruled gratings are produced mechanically by burnishing groove after groove into a layer of aluminum which has been deposited evenly on a quartz substrate with a diamond tool. Gratings can also be produced by using laser beams to produced interference fringes. Such gratings are called holographic gratings. Gratings are characterized by a blaze wavelength and a groove density. The blaze wavelength is the wavelength at which the grating is at its maximum efficiency. Groove density is the number of grooves per mm on the grating surface. The useful range of a grating can be conveniently described by the 2/3 - 3/2 rule which simply gives the range of a grating to be lower limit = 2/3 Blaze; upper limit = 3/2 Blaze. Thus, for a grating with a blaze of 400 nm, its useful range is 266 to 600 nm. It is not unusual to be able to operate the grating with reasonable efficiency above the "magic'' 3/2 value. However, this is not suggested on the short wavelength side below the "magic'' 2/3 value.
Grating performance is affected by aberrations created in the ruling process and by the optical design of the instruments in which the gratings are used. The most common aberration is due to periodic imperfections in the groove spacing of the grating. These imperfections arise from the mechanical ruling process, and are commonly called "ghosts.'' Current ruling processes reduce "ghosts'' to a minimum in all but the most critical experiments. The holographic process produces gratings virtually free of spacing errors, thus no "ghosts". Until recently holographic gratings had very poor efficiencies-especially when compared to classically ruled gratings. However, modern holographic gratings have been produced with efficiencies very near those of classically ruled gratings. A holographic grating may be better suited for applications requiring low stray light and or dense groove spacing. Ruled gratings are normally chosen when efficiency and overall throughput of the optical system are of primary concern. They are ideal for use in the IR spectrum. Thermo Vision Colorado offers two types of gratings: "Certified Precision,'' and "Quality Controlled.'' Certified Precision gratings are high quality gratings in which the irregularity in the 1st order is better than 0.125 waves. Quality controlled gratings are typically one wave irregular. Thermo Vision Colorado offers the gratings listed in the table below. Plane Diffraction Gratings
Sizes Available: Minimum 10 x 10 mm (width x length); maximum 110 x 110 mm (width x length), thickness 3, 5, 10, 16 or 22 mm. Other odd sizes are also available. Please ask! <back to top> |
