The light of the desired wavelength band is then directed onto a sample, a detector or other components of the optical system.
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.
Monochromators can be divided into different types depending on the type of dispersive element used and the optical arrangement of the system:. 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. 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.
You need a spectrometer to produce a variety of wavelengths because different compounds absorb best at different wavelengths. For example, p-nitrophenol acid form has the maximum absorbance at approximately nm and p-nitrophenolate basic form absorb best at nm, as shown in Figure 3.
Looking at the graph that measures absorbance and wavelength, an isosbestic point can also be observed. An isosbestic point is the wavelength in which the absorbance of two or more species are the same.
The appearance of an isosbestic point in a reaction demonstrates that an intermediate is NOT required to form a product from a reactant. Figure 4 shows an example of an isosbestic point. Referring back to Figure 1 and Figure 5 , the amount of photons that goes through the cuvette and into the detector is dependent on the length of the cuvette and the concentration of the sample.
Once you know the intensity of light after it passes through the cuvette, you can relate it to transmittance T. Transmittance is the fraction of light that passes through the sample.
This can be calculated using the equation:. Where I t is the light intensity after the beam of light passes through the cuvette and I o is the light intensity before the beam of light passes through the cuvette. Transmittance is related to absorption by the expression:. Where absorbance stands for the amount of photons that is absorbed. With the amount of absorbance known from the above equation, you can determine the unknown concentration of the sample by using Beer-Lambert Law.
Figure 5 illustrates transmittance of light through a sample. Beer-Lambert Law also known as Beer's Law states that there is a linear relationship between the absorbance and the concentration of a sample.
For this reason, Beer's Law can only be applied when there is a linear relationship. Beer's Law is written as:. The molar extinction coefficient is given as a constant and varies for each molecule. Expression 1 indicates the presence of higher-order light. 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.
This wavelength is known as the "blaze wavelength. However, multiple diffraction gratings can be used separately to increase the efficiency over a wide range of wavelength. The basic elements of a monochromator are 1 entrance slit, 2 collimating mirror to form a parallel beam after the slit , 3 diffraction grating dispersive element , 4 camera mirror focuses light from the dispersive element onto the exit slit , and 5 exit slit see Fig.
In Fig. A camera mirror is required in an actual monochromator, however, as light is incident over the entire surface of the dispersive element. This involves refocusing the image of the 1 entrance slit at the position of 5 exit slit at the wavelength to be extracted. The other wavelengths either miss 4 camera mirror or focus at some position away from 5 exit slit. Typical mountings used in spectrophotometers are the Littrow mount, Czerny-Turner mount, and concave mounts such as the Seya-Namioka mount.
As shown in Fig. The Czerny-Turner mount uses two symmetrically arranged spherical mirrors as the collimating mirror and camera mirror, as shown in Fig.
A concave mount uses a curved diffraction grating that offers both dispersion and focusing functions to simplify the construction, as shown in Fig. This mount is used to reduce the number of mirrors where extreme resolution is not required. We described above how a monochromator acts to product monochromatic single-wavelength light from white light. However, while it is called single-wavelength light, it covers a certain range of wavelengths. For example, nm light may extend from Consequently, when this light is used for measurements, information for the range from The OPD due to spherical aberration varies with the fourth power of the numerical aperture and cannot be corrected without the use of aspheric optics.
Astigmatism is characteristic of off-axis geometry. In this case, a spherical mirror illuminated by a plane wave incident at an angle to the normal such as mirror M2 in Fig. Astigmatism has the effect of taking a point at the entrance slit and imaging it as a line perpendicular to the dispersion plane at the exit see Fig. The OPD due to astigmatism varies with the square of numerical aperture and the square of the off-axis angle, and cannot be corrected without employing aspheric optics.
A toroidal mirror corrects for astigmatism, allowing the tangential resolution optimized and sagittal imaging optimized focal planes to cross at the center of the focal plane. This provides the flexibility to choose between imaging and resolution optimization with a CCD detector by selecting the desired detection angle. It will make the spectrograph having the largest flat fields available in an imaging spectrograph.
Recent advances in holographic grating technology now permit complete correction of ALL aberrations present in a spherical mirror-based CZ spectrometer at one wavelength, with excellent mitigation over a wide wavelength range Both monochromators and spectrographs of this type use a single holographic grating with no ancillary optics.
With only one optic in their design, these devices are inexpensive and compact. L H - Perpendicular distance from spectral plane to grating. See Table 3 for worked examples. Note : In practice the highest wavelength attainable is limited by the mechanical rotation of the grating. This means that doubling the groove density of the grating will halve the spectral range.
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