Monochromators

In this volume, we will describe the monochromator, an important part of the spectrophotometer that was explained in UV TALK LETTER Vol. 2 "The Structure of a Spectrophotometer".

The prism and diffraction grating are typical dispersive elements. Table 1 shows their respective features. Due to their superior dispersion properties, diffraction gratings are often used in modern spectrophotometers. The prism achieves dispersion due to the difference in the material refractive index according to the wavelength. However, the diffraction grating uses the difference in diffraction direction for each wavelength due to interference. The reflective blazed diffraction grating that is commonly used in spectrophotometers is described below.

Table 1 Comparison of Prism and Grating
(✓ : advantageous for spectrophotometer)

The diffraction gratings we study at high school are often a row of slits, as shown in Fig. 4. However, the reflective blazed diffraction grating has a sawtooth cross-section, as shown in Fig. 5. As light that passed through an adequately fine slit is diffracted, light reflected from an adequately fine sawtooth surface is also diffracted. There are 500 to 2000 serrations per millimeter.

The sawtooth face of a commercially produced diffraction grating is the replica of a master grating. A thin synthetic-resin replica is stuck onto a glass sheet and coated with aluminum. The master was traditionally produced using a machine tool, but now the surface is formed by an ion beam or using laser beam photolithography. A smooth surface reduces stray light (light at unwanted wavelengths).

Expression (1) indicates the presence of higher-order light. If d, i, and λ are fixed in expression (1), a different value of m results in a different value of θ. This indicates that light of wavelength λ diffracts in multiple angles θ, as shown in Fig. 7. These light directions are named using a combination of the m value and the + or - sign, such as +1st-order light or -1st-order light. Incidentally, the light when m=0 is known as zero-order light, for which the diffraction angle θ is equal to the angle of incidence i. This is reflected as white light, equivalent to normal specular reflection.

The various light orders of a diffraction grating result in dispersion of the energy and a reduction in light utilization efficiency. However, the diffracted light energy from a diffraction grating with a fine sawtooth profile is concentrated in the direction of the specular reflection, as shown in Fig. 8. This wavelength is known as the "blaze wavelength." The diffraction grating in a spectrophotometer is normally used near the blaze wavelength.

Once the monochromator elements and their positions are fixed, the resolution is determined by the slit width. As the light disperses as a rainbow, increasing the exit slit width reduces the resolution. A wider entrance slit results in a larger image at the exit slit position, such that the image for the wavelengths adjacent to the target wavelength enters the exit slit and reduces the resolution.
A diffraction grating has its own inherent resolution, which is determined by the diffraction principle according to the number of gratings. The improvement in monochromator resolution possible by reducing the slit width is limited due to this diffraction grating resolution, the aberration of the overall optical system, and mirror imperfections.
The monochromator slit width used in a spectrophotometer is expressed not as the slit width dimension but as the value of the resolution achieved. Setting the slit width to 1 nm, sets the monochromator resolution to 1 nm, such that 1 nm-bandwidth light shines onto the sample.


For measurements by spectrophotometer, the optimal resolution is determined by the spectral shape of the sample. A slightly larger slit width increases the light intensity reaching the detector and reduces the data noise but results in poorer resolution. Originally sharp spectral peaks broaden as shown in Fig. 14. A narrow slit width achieves a spectrum shape closer to the original spectrum. For example, if the original spectrum has a peak waveform, setting the slit width to between 1/8 and 1/10 the PWHM results in a measured peak with at least 99% the original height.1)

However, if the aim is not to determine the spectrum shape itself but to conduct concentration measurements using a calibration curve, these measurements are possible if the waveform is slightly imperfect. If noise detracts from measurement accuracy with the slit width set at 1/8 the PWHM, a slightly larger slit width may be appropriate. Shimadzu spectrophotometers normally achieve satisfactory resolution and light intensity for concentration measurements on solutions when the slit width is set between 1 nm and 2 nm. The slit width is normally set to 5 nm or above for measurements of solids using the integrating sphere. A larger slit width is set to reduce noise due to light losses in the integrating sphere, as high resolution is often not required when measuring solids.

We explained the monochromator, which extracts monochromatic light from polychromatic light. However, when monochromatic light enters the monochromator, it exits only in a direction specific to the diffraction grating. This property can be used to determine the wavelength of the monochromatic light.
This is the method normally used when the monochromator is used alone. A monochromator is incorporated into fluorescence spectrophotometers and emission spectrometers to determine the wavelength of fluorescence lines or emission lines emitted from the sample. In this case, the monochromator is located between the sample compartment and detector. The detector will be described in the next UV TALK LETTER. We hope you will continue to enjoy reading the UV TALK LETTERS.