How is a double beam spectrophotometer different from a single beam instrument?
As you can see in the picture above, a double beam instrument looks like its single beam counterpart until just before the sample. In this type of instrument, the beam coming out of the monochromator is either split (using a 50-50 beam splitter optic) or chopped (using a rotating segmented wheel) into two beams that enter the sample compartment. In the instrument design above, the beam is split in two via a half- silvered quartz (to allow UV as well as visible light to pass through it) mirror which acts as an optical beam splitter. These more complex double beam instruments now have various mirrors to direct the light beam through the instrument. These mirrors are special “front coated” aluminum mirrors that have a highly reflective coating of aluminum on their front side (unlike your bathroom mirror that has the aluminum coating on the back side of the glass).
In the chopper version of a double beam instrument the above beam splitter is replaced by a rotating segmented wheel. The chopper disk (above) is made up of a number of different segments. Those in the instrument we are describing have three different sections, other designs may have a different number. The light coming from the diffraction grating and slit assembly will strike the rotating disk and one of three things can happen.