There are two overall designs commonly used in UV/Vis spectrometers: 1) selection of the measurement wavelength prior to passing the light beam through the sample and 2) passing white light through the sample and measuring the absorbance of all wavelengths simultaneously.
In the first case, we start with a broadband light source and select a single wavelength to pass through the sample using a monochromator:
The classic spectrometer design utilizes a deuterium lamp source, a Czerny-Turner monochromator and a photomultiplier tube (PMT) detector. Note that if we wish to take measurements at multiple wavelengths, then the monochromator must be tunable. This is often done mechanically by adjusting the position of one or more optical components.
The second type of spectrometer design passes “white” light through the sample and measures the entire spectrum at once using an array detector:
By the time it reaches the array detector, the spectrum has been separated into a line by the optical components. Each point along that line corresponds to a single wavelength, the intensity of which is a function of the absorbance of the sample at that wavelength (the greater the absorbance the lower the intensity). Typically, a linear CCD array is used as the sensor:
These sensors are very common in industry and have application in bar code readers, flatbed scanners and fax machines.
There are advantages and disadvantages to each type of spectrometer discussed above. The main advantages of the monochromator-based spectrometer are low noise and high sensitivity. The main advantages of the array spectrometer are speed of spectrum acquisition (essentially instantaneous) and low cost. However, there are a lot of tradeoffs that go into the design of a spectrometer, and the advantages and disadvantages of each type can overlap depending on the components used.
Our spectrometer design will be of the array type, using a readily-available CCD sensor.