A double beam spectrophotometer is one of the two basic types of spectrophotometer in use currently (the other type being the single beam spectrophotometer). Spectrophotometers are devices that measure the wavelength distribution of light.
These instruments are available in many shapes, configurations and sizes. A double beam instrument compares the light intensity between two light paths by splitting the light source into two separate beams. The splitting of the beam is accomplished either statically using a partially transmitting mirror or through attenuation of the beams optical devices.
One beam is used to illuminate the reference standard, while the other illuminates the sample. The instrument measures the amount of light of a specific wavelength absorbed by an analyte in a gas or liquid sample. Typically, the two beams of a double beam spectrophotometer are combined before they reach a single monochromator, but in some cases two monochromators are used. Depending on the wavelength being studies, an electrically powered ultraviolet, visible or infrared lamp can be used. A single beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is introduced.
The comparison measurements from double beam instruments are easier make and more stable but single beam instruments still are useful for certain applications, having a larger dynamic range and being optically simpler as well as more compact.
Historically, spectrophotometers use a monochromator containing a diffraction grating to produce the analytical light spectrum, though some use arrays of photo sensors instead. Still other spectrophotometers that use a Fourier transform technique to acquire spectral information more quickly, a technique called Fourier Transform InfraRed.
A double beam spectrophotometer or single beam spectrophotometer quantitatively compares the fraction of light that passes through a reference solution and a test solution. Light from the source lamp is passed through a monochromator, diffracting the light into a “rainbow” of wavelengths. The outputs are narrow bandwidths of this diffracted spectrum. Discrete frequencies are transmitted through the test sample. Then the intensity of the transmitted light is measured with a photodiode or other brightness sensor. The transmittance value for this wavelength is then compared with the transmission through a reference sample.
Spectrophotometry routines consist of shining a light source into a monochromator. A particular output wavelength is then selected and beamed at the sample. The photodetector behind the sample responds to the light stimulus and outputs an analog electronic current which is converted to a usable format. The numbers are either plotted or (as is now most commonly the case) fed into a computer for further analysis.
The main advantages of a double beam instrument over the single beam spectrophotometer are an improvement in the stability of the light source, detectors and associated electronic devices. The disadvantages include the precision required in recombining the beams prior to reaching the monochrometer, the quality of the mirrors and other optics (if used) and their coatings and the problems which can created by dust buildup on these devices.
These disadvantages can make the double beam instruments somewhat more difficult to maintain than single beam devices, though the results they can provide make them ideal for certain spectrophotometry applications.