<P> Invented by Arnold O. Beckman in 1940, the spectrophotometer was created with the aid of his colleagues at his company National Technical Laboratories founded in 1935 which would become Beckman Instrument Company and ultimately Beckman Coulter . This would come as a solution to the previously created spectrophotometers which were unable to absorb the ultraviolet correctly . He would start with the invention of Model A where a glass prism was used to absorb the UV light . It would be found that this did not give satisfactory results, therefore in Model B, there was a shift from a glass to a quartz prism which allowed for better absorbance results . From there, Model C was born with an adjustment to the wavelength resolution which ended up having three units of it produced . The last and most popular model became Model D which is better recognized now as the DU spectrophotometer which contained the instrument case, hydrogen lamp with ultraviolent continuum and a better monochromator . It was produced from 1941 to 1976 where the price for it in 1941 was US $723 (far - UV accessories were an option at additional cost). In the words of Nobel chemistry laureate Nobel chemistry laureate Bruce Merrifield said it was "probably the most important instrument ever developed towards the advancement of bioscience ." </P> <P> Once it became discontinued in 1976, another company known as Hewlett - Packard created the first commercially available diode - assay spectrophotometer in 1979 known as the HP 8450A . Diode - assay spectrophotometers differed from the original spectrophotometer created by Beckman because it was the first single - beam microprocessor - controlled spectrophotometer that scanned multiple wavelengths at a time in seconds . It irradiates the sample with polychromatic light which the sample absorbs depending on its properties . Then it is transmitted back by grating the photodiode array which detects the wavelength region of the spectrum . Since then, the creation and implementation of spectrophotometry devices has increased immensely and has become one of the most innovative instruments of our time . </P> <P> There are two major classes of devices: single beam and double beam . A double beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample . A single - beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted . Although comparison measurements from double - beam instruments are easier and more stable, single - beam instruments can have a larger dynamic range and are optically simpler and more compact . Additionally, some specialized instruments, such as spectrophotometers built onto microscopes or telescopes, are single - beam instruments due to practicality . </P> <P> Historically, spectrophotometers use a monochromator containing a diffraction grating to produce the analytical spectrum . The grating can either be movable or fixed . If a single detector, such as a photomultiplier tube or photodiode is used, the grating can be scanned stepwise (scanning spectrophotometer) so that the detector can measure the light intensity at each wavelength (which will correspond to each "step"). Arrays of detectors (array spectrophotometer), such as charge coupled devices (CCD) or photodiode arrays (PDA) can also be used . In such systems, the grating is fixed and the intensity of each wavelength of light is measured by a different detector in the array . Additionally, most modern mid-infrared spectrophotometers use a Fourier transform technique to acquire the spectral information . This technique is called Fourier transform infrared spectroscopy . </P>

Differences between single beam and double beam spectrophotometer
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