Refurbished UV/Vis Spectrophotometers

Spectrophometer

A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, single, and chroma, colour, and the Latin suffix -ator, denoting an agent.

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 color. Although there are a number of useful ways to produce pure colors, there are not so many other ways to easily select any pure color in a wide range. See below for a discussion of some of the uses of monochromators.

A similar function is performed in another part of the electromagnetic spectrum by a tunable filter, such as is found in a radio receiver, that can select one frequency from a range of frequencies, but the tuning method is quite different.

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 are used in a reflective mode. A reflective prism is made by making a right triangle prism (typically, half of an equilateral 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.

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.

In the common Czerny-Turner design, the broad band illumination source (A) is aimed at an entrance slit (B). 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 (the collimator C, usually a spherical mirror) so that the light from the slit reflected from the mirror is collimated (focused at infinity). The collimated light is refracted by the prism or diffracted from the grating (D) and then is collected by another mirror (E) which refocuses the light, now dispersed, on the exit slit (F). At the exit slit, the colors of the light are spread out (in the visible this shows the colors of the rainbow). 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 (G) 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.

Spectral bandwidth

The ideal transfer function of such a monochromator is a triangular shape. The peak of the triangle is at the nominal wavelength selected. 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. The cutoff level is typically about one one thousandth of the peak value, or 0.1%. A typical spectral bandwidth might be one nanometer, which is defined as the width of the triangle at the points where the light has reached half the maximum value. Thus spectral bandwidth is defined as Full Width at Half Maximum, abbreviated FWHM.

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.

A monochromator's adjustment range might cover the visible spectrum and some part of both or either of the nearby ultraviolet (UV) and infrared (IR) 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 & Blazed gratings

When a diffraction grating is used, care must be taken in the design of broad band monochromators because the diffraction pattern has overlapping orders. Sometimes extra, broadband 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 in 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 "ghost" diffraction orders that 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 a much lower stray light level than blazed gratings, and are generally preferred. Almost all the gratings actually used in monochromators are carefully made replicas of master gratings.

Prisms

Prisms have higher dispersion in the UV region. Prism monochromators are favored in some instruments that are principally designed to work in the far UV region. Most monochromators use gratings though. Some monochromators have several gratings that can be selected for use in different spectral regions.

The narrowness of the band of colors that a monochromator can generate is related to the focal length of the monochromator collimators. Using a longer focal length optical system also unfortunately decreases the amount of light that can be accepted from the source. Very high resolution monochromators might have a focal length of 2 meters. Building such monochromators requires exceptional attention to mechanical and thermal stability. For many applications a monochromator of about 0.4 meter focal length is considered to have excellent resolution. Many monochromators have a focal length less than 0.1 meter.

The usual optical system contains optical aberrations that curve the field where the slit images come to focus, so that slits are sometimes curved instead of simply straight, to approximate the curvature of the image. This allows taller slits to be used, gathering more light, while still achieving high spectral resolution.

Monochromators are often calibrated in units of wavelength. Uniform rotation of a grating produces a linear change in wavelength, so such an instrument is easy to build. Many of the underlying physical phenomena being studied are linear in energy though, and since wavelength and energy have a reciprocal relationship, spectral patterns that are simple and predictable when plotted as a function of energy are distorted when plotted as a function of wavelength. Some monochromators are calibrated in units of reciprocal centimeters (called wavenumbers) or some other energy units, but the scale may not be linear.

A high quality monochromator can produce light of sufficient purity and intensity that a spectrophotometer can measure a narrow band of optical attenuation of about one million fold.

Monochromators are used in many optical measuring instruments and in other applications where tunable monochromatic light is wanted. Sometimes the monochromatic light is directed at a sample and the reflected or transmitted light is measured. Sometimes white light is directed at a sample and the monochromator is used to analyze the reflected or transmitted light. Two monochromators are used in many fluorometers; one monochromator is used to select the excitation wavelength and a second monochromator is used to analyze the emitted light.

An automatic scanning spectrometer includes a mechanism to change the wavelength selected by the monochromator and to record the resulting changes in the measured quantity as a function of the wavelength.

If an imaging device replaces the exit slit, the result is the basic configuration of a spectrograph. This configuration allows the simultaneous analysis of the intensities of a wide band of colors. Photographic film or an array of photodetectors can be used, for instance to collect the light. Such an instrument can record a spectral function without mechanical scanning, although there may be tradeoffs in terms of resolution or sensitivity for instance.

In the UV, visible and near IR, absorbance and reflectance spectrophotometers usually illuminate the sample with monochromatic light. In the corresponding IR instruments, the monochromator is usually used to analyze the light coming from the sample.

Monochromators are also used in optical instruments that measure other phenomena besides simple absorption or reflection, wherever the color of the light is a significant variable. Circular dichroism spectrometers contain a monochromator, for example.

Lasers produce light which is much more monochromatic than the optical monochromators discussed here, but only some lasers are easily tunable, and these lasers are not as simple to use.

Prism (optics)

If a shaft of light entering a prism is sufficiently small such that the coloured edges meet, a spectrum results

In optics, a prism is a device used to refract light, reflect it or break it up (to disperse it) into its constituent spectral colours (colours of the rainbow). The traditional geometrical shape is that of a triangular prism, with a triangular base and rectangular sides. Some types of optical prisms are not in fact in the shape of geometric prisms.

As light moves from one medium to another (for example, from air into the glass of the prism), it changes speed. As a result, the light's path is bent (refracted), and some of the light is reflected. The angle that the beam of light makes with the interface as well as the refractive indices of the two media determine how much of the light is reflected, and by how much its path is bent. In some cases total internal reflection occurs, and all of the light is reflected.

Reflective prisms are used to reflect light, for instance in binoculars, since they are easier to manufacture than mirrors. Dispersive prisms are used to break up light into its constituent spectral colours because the refractive index depends on frequency (see dispersion); the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light. There are also polarizing prisms (also known as birefringent prisms) which can split a beam of light into components of varying polarization.

Isaac Newton first thought that prisms split colours out of colourless light. Newton placed a second prism such that a separated colour would pass through it and found the colour unchanged. He concluded that prisms separate colours. He also used a lens and a second prism to recompose the rainbow into white light.

Diffraction refers to various phenomena associated with wave propagation, such as the bending, spreading and interference of waves emerging from an aperture. It occurs with any type of wave, including sound waves, water waves, electromagnetic waves such as light and radio waves, and matter displaying wave-like properties according to the wave–particle duality. While diffraction always occurs, its effects are generally only noticeable for waves where the wavelength is on the order of the feature size of the diffracting objects or apertures.

The most conceptually simple example of diffraction is single-slit diffraction in which the slit is narrow, that is, significantly smaller than a wavelength of the wave. After the wave passes through the slit, a pattern of semicircular ripples is formed, approximately equally strong in all directions, as if there were a simple wave source at the position of the slit. This semicircular wave is a diffraction pattern.

When the slit is significantly more than a wavelength wide, the wave propagates more nearly straight through, but a diffraction pattern at the edges of the wave can be seen. The center part of the wave travels through largely unaffected at short distances, but the wave forms a stable diffraction pattern at longer distances. This pattern is most easily understood and calculated as the interference pattern of a large number of simple sources spaced closely and evenly across the width of the slit.

In multiple-slit experiments, narrow enough slits can be analyzed as simple wave sources.

A slit is an opening that is infinitely extended in one dimension, which has the effect of reducing a wave problem in 3-space to a simpler problem in 2-space. All the same effects can be seen and analyzed for small round holes and other shapes, in 3D, but they're harder to describe, compute, and illustrate.

Ultraviolet

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. It can be subdivided into near UV (380–200 nm wavelength; abbrev. NUV), far or vacuum UV (200–10 nm; abbrev. FUV or VUV), and extreme UV (1–31 nm; abbrev. EUV or XUV).

Origin of term

The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelengths of visible light.

Discovery

Soon after infrared radiation had been discovered, the German physicist Johann Wilhelm Ritter began to look for radiation at the opposite end of the spectrum, at the short wavelengths beyond violet. In 1801 he used silver chloride, a light-sensitive chemical, to show that there was a type of invisible light beyond violet, which he called chemical rays. At that time, many scientists, including Ritter, concluded that light was composed of three separate components: an oxidising or calorific component (infrared), an illuminating component (visible light), and a reducing or hydrogenating component (ultraviolet). The unity of the different parts of the spectrum was not understood until about 1842, with the work of Macedonio Melloni, Alexandre-Edmond Becquerel and others. During that time, UV radiation was also called "actinic radiation".