Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Monochromator

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_354

Definition

A monochromator is an optical dispersing device that is used to select a narrow band of light (i.e., optical radiation) from a wider range of wavelengths available at the input. The Greek roots “mono-” and “chroma-” refer to “single” and “color” respectively. Ideally, a monochromator should produce a single wavelength of optical radiation at its output. Although lasers produce light that is much more monochromatic than the optical monochromators discussed here, only some lasers are easily tunable but are not as simple to use.

Monochromators are included in many optical measurement instruments and systems for applications where tunable monochromatic light is required. A monochromator combined with optical detectors can be used to obtain the spectral power distribution (SPD) of light sources, reflectance or transmittance of objects, etc.

Monochromators are important for color measurement because many color-related optical characteristics are dependent on wavelength. In color science, reflected or transmitted light from a sample is usually measured, where a monochromator can be used before or after the incident light illuminates a sample. If a monochromator is used before the incident light illuminates the sample, the incident light is monochromatic light. On the other hand, if a monochromator is used after the light illuminates the sample, the monochromator is used to convert the reflected or transmitted light to monochromatic signals for analysis. It is typically used in a spectrometer (or spectroradiometer) or a spectrophotometer.

There are different types of monochromator based on its color selection mechanisms and/or designs, e.g., prism, Czerny–Turner, holographic grating, double, etc. The principles of these monochromators are introduced in the “Overview” section below.

Overview

The main function of a monochromator is to separate the color components of a light. It can use either the optical dispersion phenomenon in a prism or that in a diffraction grating.

Figure 1 shows a scheme of a simple monochromator. Various types of monochromator have been developed, but a monochromator usually contains an entrance slit, an essential dispersing element, and a mechanism to direct the selected color to an exit slit for color selection purposes [1].
Monochromator, Fig. 1

Scheme of a simple monochromator

The Czerny–Turner monochromator is a type of monochromator using diffractive gratings as the dispersing element. A Czerny–Turner monochromator is sometimes referred to as a single monochromator because the radiant flux is only diffracted once [2]. A schematic diagram of a Czerny–Turner monochromator is shown in Fig. 2. In Fig. 2, the light under measurement (A) is incident through the entrance slit (B), reflected by a concave mirror (C) to become collimated. It then illuminates the optical grating (D), disperses light to different colors, reflected and focused by the concave mirror (E) to the exit slit (F). By rotating the optical grating (D), different colors of light can reach and exit at the exit slit (F), and such colors are “selected” by the monochromator. The orientation of the optical grating (D) and the spatial location of the exit slit (F) determine which wavelengths of the light are selected. The two concave mirrors (C and E) image the entrance slit on the exit plane. In practice, the collimating and focusing elements (C and E) are usually the same mirror, used twice [2].
Monochromator, Fig. 2

Basic structure of a Czerny–Turner monochromator

The double monochromator usually contains two Czerny–Turner monochromators used in a system. Double monochromators are known for their double dispersion and thus can significantly reduce scattered light (Fig. 4) [2].
Monochromator, Fig. 3

An example of a prism monochromator

Monochromator, Fig. 4

Double monochromator

A prism monochromator uses a prism as the dispersing element (Fig. 3). It is not necessary for the collimating and focusing elements to be concave mirrors, but they can be optical lens. Usually, reflective mirrors are preferred over optical lens because mirrors conserve more optical radiation energy during the transmission process. However, because of the higher cost and small angular dispersion, prism monochromators are rarely made today [2].

A holographic grating monochromator uses holographically constructed concave gratings to simplify the optics. This eliminates some of the lenses and mirrors that would normally be required with conventional flat plane grating monochromators.

When choosing a monochromator, the following specifications should be considered: wavelength range, wavelength accuracy, measurement speeds, spectral bandwidth, spectral scattering, slit widths, grating groove density (if applicable), mechanical and thermal stability, size and weight, stray light, and dispersion [3, 4]. For example, Table 1 shows the criteria for the selection of gratings according to the above specifications. The resolution is the minimum detectable difference between two spectral peaks provided by a monochromator. Theoretically, resolution is approximately equal to slit width (mm) × dispersion (nm/mm), where the dispersion describes how well the light spectrum is spread over the focal plane of the exit slit. However, in practice, because of aberrations in monochromators, the actual resolution is rarely equal to this. The actual resolution of a monochromator as a function of the slit width is described as the bandwidth (nm) of a monochromator.
Monochromator, Table 1

The criteria for the selection of a grating [4]

Specification of monochromators

Criterion

Groove density (or groove frequency):

The number of grooves contained on a grating surface (lines/mm)

Groove density affects the mechanical scanning range and the dispersion properties of a system. It is an important factor in determining the resolution capabilities of a monochromator. A higher groove density results in greater dispersion and a higher resolution

Select a grating that delivers the required dispersion, such as that for a charge-coupled device (CCD), array detector, or a required resolution (with an appropriate slit width) when using a monochromator

Mechanical scanning range:

The wavelength region in which an instrument can operate

Refers to the mechanical rotation capability (not the operating or optimal range) of a grating drive system with a specific grating installed

Select a grating groove density that allows operation over the desired wavelength region

Blaze wavelength:

The angle in which the grooves are formed with regard to the normal grating, often called the blaze angle

Diffraction grating efficiency plays an important role in spectrograph throughput. Efficiency at a particular wavelength is largely a function of the blaze wavelength if the grating is ruled, or a modulation if the grating is holographic

Select a blaze wavelength that encompasses the total wavelength region of the applications. More attention should be paid to covering the short wavelength of the spectrum region

Quantum wavelength range:

The wavelength region of highest efficiency for a particular grating

Normally determined by the blaze wavelength

Select a grating with maximum efficiency over the required wavelength region for the applications

When using a monochromator, some parameters need to be set up properly, as they affect the measurement results. For example, smaller slit widths produce smaller spectral bandwidths, which corresponds to a narrower spectral band at the exit slit (i.e., closer to the ideal single-wavelength output). However, smaller slit widths mean that less optical radiation power is transmitted through the monochromator, which results in lower output signals and decreases the signal-to-noise ratio of the signals measured. The bandwidth of commonly used monochromators is from the order of 10−2 to 10 nm. Detailed information on monochromators can be found in manufacturers’ user manuals.

It is important to evaluate the performance of a monochromator to reduce measurement uncertainties (i.e., gain more confidence regarding the accuracy of the results measured). To evaluate the performance of a monochromator, the following characteristics should be considered: wavelength accuracy, spectral bandwidth, stray light, system reproducibility, and system linearity.

Cross-References

References

  1. 1.
    Webster, J.G. (ed.): The Measurement, Instrumentation, and Sensors Handbook. Springer, Heidelberg (1999)Google Scholar
  2. 2.
    Christensen, R.L., Potter, R.J.: Double monochromator systems. Appl. Optics 2, 1049 (1963)ADSCrossRefGoogle Scholar
  3. 3.
    Kostkowski, H.J.: Reliable Spectroradiometry. Spectroradiometry Consulting, La Plata (1997)Google Scholar
  4. 4.
    Yoshizawa, T. (ed.): Handbook of Optical Metrology: Principles and Applications. CRC Press, Boca Raton (2009)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Center for Measurement StandardsIndustrial Technology Research InstituteHsinchuTaiwan