Monochromator - Berthold Technologies GmbH & Co.KG

What is a monochromator?

A monochromator is an optical device that separates polychromatic light (such as sunlight or light coming from a lamp) into a range of individual wavelengths (monochromatic light) and allows a narrow band of these individual wavelengths to be selected. The light of the desired wavelength band is then directed onto a sample, a detector or other components of the optical system.

Principle of a monochromator

The separation of light into its individual wavelength components is called dispersion. An element with this property is called a dispersive element. In order to select a narrow band of these wavelengths, a slit can be used to block the unwanted wavelengths. The narrower the slit, the narrower the wavelength band. In general, a monochromator consists of a dispersive element, an entrance slit, mirrors to produce a parallel beam similar to sunlight, an exit slit, and mirrors to extract the monochromatic light.

By fixing the slit and rotating the dispersive element, the direction of the dispersed light is turned so that the colour of the resulting monochromatic light changes: as you can see in Figure 1 in the upper panel, the position of the dispersive element causes orange-red light to exit the slit, while in the lower panel the dispersive element rotates so that the light exiting the slit is cyan.

Figure 1: principle of a monochromator

The different types of monochromators

Monochromators can be divided into different types depending on the type of dispersive element used and the optical arrangement of the system:

Prism monochromators

The dispersive element in prism monochromators is a prism. Prisms have a high light utilization efficiency and do not produce higher order light and very little stray light. However, dispersion is dependent on wavelength (high for UV, low for IR) and temperature.

Figure 2: schematic representation of a Czerny-Turner monochromator

Grating monochromators

The dispersive element in grating monochromators is a reflecting diffraction grating. It provides a constant dispersion for all wavelengths and a low dependence on temperature. However, they produce relatively large amounts of scattered light and require the use of filters to block higher order light. Due to their superior dispersion properties, diffraction gratings are often used in modern instruments.

The most popular design for grating monochromators in microplate readers and spectrophotometers is the Czerny-Turner monochromator. This type of monochromator uses curved mirrors so that the light reflected from the mirror is collimated out of the slit (Figure 2).

Monochromator Applications

Monochromators are commonly used in measurement devices such as spectrometers or microplate readers. They are a popular device for wavelength selection in a range of detection technologies, such as absorption and fluorescence intensity.

Single, double and "quadruple" monochromators

An important feature of any wavelength selection system is its blocking efficiency, i.e. the ability to block other wavelengths than the desired ones. Monochromators do a very good job in selecting the desired wavelength, but not so good in blocking unwanted wavelengths: the amount of light at unwanted wavelengths passing through the monochromator is typically 1/1000 of the total. In other words, a typical (single) monochromator has a blocking efficiency of 10^-3.

This may seem like a good efficiency, but it is not when the monochromator is used in fluorescence-based applications, because the emission light is 1000 times weaker than the excitation light. This means that if 10^-3 is blocked, the light reaching the detector would consist of 50% emitted light and 50% unwanted light! Hence, measurements made with this setup would not be reliable. This problem can be solved by using two monochromators connected in series whose mechanical systems are connected in tandem so that both select the same wavelength: The combination of two single monochromators with a blocking efficiency of 10^-3 results in a so-called "double" monochromator with a blocking efficiency of 10^-6. This blocking efficiency ensures that the unwanted light reaching the detector is low enough to provide reliable results.

Some instruments use the term "quadruple" or "quad" monochromators, but this does not mean that they combine 4 single monochromators for a super-efficient blocking efficiency of 10^-12. In reality they have 2 double monochromators, one for absorption and excitation and another for emission, like the Tristar 5 does.

Alternatives to monochromators

In microplate readers, monochromators are not the only option for selecting a specific wavelength range. There are other popular alternatives:

Filters

Filter-based instruments are affordable and offer good sensitivity thanks to the high permeability and blocking properties of filters. However, they lack flexibility because filters have a fixed wavelength and bandwidth, so multiple filter sets are typically required to measure multiple wavelengths.

LED

Most instruments use tungsten or xenon lamps as light sources, which provide white light. However, it is also possible to use LEDs instead, which provide monochromatic light. But their flexibility is even more limited than that of filters, as each wavelength requires a different light source. Thus, they are usually only used in inexpensive instruments for measuring specific assays.

Laser

Lasers also produce highly monochromatic light of high intensity, but they are expensive and only some lasers are easily tunable, so that normally a different laser is needed for each wavelength desired. They are usually limited to very specific assays, such as AlphaScreen®.

Spectrograph/Polychromator

When an imaging device replaces the exit slit, the result is the basic configuration of a spectrograph. This configuration allows simultaneous analysis of the intensities of a wide range of colours by using an array of photodetectors such as a CCD. Such an instrument can record the entire spectrum without mechanical scanning (and thus quite fast), although there may be compromises in resolution or sensitivity, and is normally suitable for absorbance only.

The term polychromator is sometimes used instead of spectrograph, but they are not exactly the same: polychromators have multiple exit slits, each of which allows a different wavelength to pass through it, and a detector is placed after each slit so that the light at each wavelength is measured by a different detector; spectrographs, as explained above, do not make use of exit slits, buty a single spatially selective detector instead.

Hybrid instruments

To offer uncompromising performance, advanced microplate readers combine different wavelength selection tools, so that the best one can be selected for each assay. For example, the Tristar 5 has filters, 2 double monochromators and, in addition, features a laser for AlphaScreen® measurements.

Would you like to learn more about the use of monochromators in microplate readers and its most important specifications for absorbance and fluorescence measurements? Then read the following article:

Monochromators in Microplate Readers

Our Microplate Readers with Monochromator

Tristar 5 Multimode Reader