Monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is.
Uses
A device that can produce monochromatic light has many uses in science and in optics because many optical characteristics of a material are dependent on wavelength. Although there are a number of useful ways to select a narrow band of wavelengths, there are not as many other ways to easily select any wavelength band from a wide range. See [|below] for a discussion of some of the uses of monochromators.In hard X-ray and neutron optics, crystal monochromators are used to define wave conditions on the instruments.
Techniques
A monochromator can use either the phenomenon of optical dispersion in a prism, or that of diffraction using a diffraction grating, to spatially separate the colors of light. It usually has a mechanism for directing the selected color to an exit slit. Usually the grating or the prism is used in a reflective mode. A reflective prism is made by making a right triangle prism with one side mirrored. The light enters through the hypotenuse face and is reflected back through it, being refracted twice at the same surface. The total refraction, and the total dispersion, is the same as would occur if an equilateral prism were used in transmission mode.Collimation
The dispersion or diffraction is only controllable if the light is collimated, that is if all the rays of light are parallel, or practically so. A source, like the sun, which is very far away, provides collimated light. Newton used sunlight in his famous experiments. In a practical monochromator, however, the light source is close by, and an optical system in the monochromator converts the diverging light of the source to collimated light. Although some monochromator designs do use focusing gratings that do not need separate collimators, most use collimating mirrors. Reflective optics are preferred because they do not introduce dispersive effects of their own.Geometrical design of a prism or grating monochromator
There are grating/prism configurations that offer different tradeoffs between simplicity and spectral accuracy.- Czerny–Turner
- Paschen-Runge
- Eagle
- Wadsworth
- Ebert-Fasti
- Littrow
- Pfund
Image:Monochromator.jpg|thumb|right|200px|A Littrow grating monochromator. This is similar to the Czerny–Turner but uses a common collimator/refocusing mirror.
In the common Czerny–Turner design, the broad-band illumination source is aimed at an entrance slit. The amount of light energy available for use depends on the intensity of the source in the space defined by the slit and the acceptance angle of the optical system. The slit is placed at the effective focus of a curved mirror so that the light from the slit reflected from the mirror is collimated. The collimated light is diffracted from the grating and then is collected by another mirror, which refocuses the light, now dispersed, on the exit slit. In a prism monochromator, a reflective Littrow prism takes the place of the diffraction grating, in which case the light is refracted by the prism.
At the exit slit, the colors of the light are spread out. Because each color arrives at a separate point in the exit-slit plane, there are a series of images of the entrance slit focused on the plane. Because the entrance slit is finite in width, parts of nearby images overlap. The light leaving the exit slit contains the entire image of the entrance slit of the selected color plus parts of the entrance slit images of nearby colors. A rotation of the dispersing element causes the band of colors to move relative to the exit slit, so that the desired entrance slit image is centered on the exit slit. The range of colors leaving the exit slit is a function of the width of the slits. The entrance and exit slit widths are adjusted together.
Stray light
The ideal transfer function of such a monochromator is a triangular shape. The peak of the triangle is at the nominal wavelength selected, so that the image of the selected wavelength completely fills the exit slit. The intensity of the nearby colors then decreases linearly on either side of this peak until some cutoff value is reached, where the intensity stops decreasing. This is called the stray light level. The cutoff level is typically about one thousandth of the peak value, or 0.1%.Spectral bandwidth
is defined as the width of the triangle at the points where the light has reached half the maximum value. A typical spectral bandwidth might be one nanometer; however, different values can be chosen to meet the need of analysis. A narrower bandwidth does improve the resolution, but it also decreases the signal-to-noise ratio.Dispersion
The dispersion of a monochromator is characterized as the width of the band of colors per unit of slit width, 1 nm of spectrum per mm of slit width for instance. This factor is constant for a grating, but varies with wavelength for a prism. If a scanning prism monochromator is used in a constant bandwidth mode, the slit width must change as the wavelength changes. Dispersion depends on the focal length, the grating order and grating resolving power.Wavelength range
A monochromator's adjustment range might cover the visible spectrum and some part of both or either of the nearby ultraviolet and infrared spectra, although monochromators are built for a great variety of optical ranges, and to a great many designs.Double monochromators
It is common for two monochromators to be connected in series, with their mechanical systems operating in tandem so that they both select the same color. This arrangement is not intended to improve the narrowness of the spectrum, but rather to lower the cutoff level. A double monochromator may have a cutoff about one millionth of the peak value, the product of the two cutoffs of the individual sections. The intensity of the light of other colors in the exit beam is referred to as the stray light level and is the most critical specification of a monochromator for many uses. Achieving low stray light is a large part of the art of making a practical monochromator.Diffraction gratings and blazed gratings
Grating monochromators disperse ultraviolet, visible, and infrared radiation typically using replica gratings, which are manufactured from a master grating. A master grating consists of a hard, optically flat, surface that has a large number of parallel and closely spaced grooves. The construction of a master grating is a long, expensive process because the grooves must be of identical size, exactly parallel, and equally spaced over the length of the grating. A grating for the ultraviolet and visible region typically has 300–2000 grooves/mm, however 1200–1400 grooves/mm is most common. For the infrared region, gratings usually have 10–200 grooves/mm. When a diffraction grating is used, care must be taken in the design of broadband monochromators because the diffraction pattern has overlapping orders. Sometimes broadband preselector filters are inserted in the optical path to limit the width of the diffraction orders so they do not overlap. Sometimes this is done by using a prism as one of the monochromators of a dual monochromator design.The original high-resolution diffraction gratings were ruled. The construction of high-quality ruling engines was a large undertaking, and good gratings were very expensive. The slope of the triangular groove in a ruled grating is typically adjusted to enhance the brightness of a particular diffraction order. This is called blazing a grating. Ruled gratings have imperfections that produce faint "ghost" diffraction orders that may raise the stray light level of a monochromator. A later photolithographic technique allows gratings to be created from a holographic interference pattern. Holographic gratings have sinusoidal grooves and so are not as bright, but have lower scattered light levels than blazed gratings. Almost all the gratings actually used in monochromators are carefully made replicas of ruled or holographic master gratings.