Define Essential Parts of Photocolorimeter - Monochromators?

This is a means of selecting a sufficiently narrow waveband. Early colorimeters used glass filters that transmitted a wide segment of the spectrum (50 nm or more). Newer instruments use interference filters that consist of thin layer of magnesium fluoride crystals with a semitransparent coating of silver on each side. Interference filters have a band pass of 5-8 nm. The term band pass is defined as the width of the spectrum that will be isolated by a monochromator, i.e. it is the range of wavelength between the points at which the transmittance is equal to one half of the peak transmittance.

The inexpensive spectrophotometers commonly used in the laboratory have a band pass of 20 nm but more sophisticated instruments may have a band pass of 0.5 nm. The monochromator consists of an entrance slit to exclude unwanted or stray light followed by absorption or interference filters, prisms or diffraction grating for wavelength selection. This is preceded by a series of light focussing lenses. Instruments using filters as wavelength selector requires lenses to focus correctly the light from the source through the filter and cuvette to the detector. But in the ultraviolet range, quartz or fused silica is essential because glass does not transmit light efficiently at wavelength shorter than 340 nm. An exit slit at the end of monochromator allows only a narrow fraction of the spectrum to reach the sample cuvette. Some spectrophotometers have a nonmovable slit whereas in some the slit width varies automatically as the wavelength changes.