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Spectrophotometry

From Academic Kids

In physics, spectrophotometry is the quantitative study of spectra. It is more specific than the general term electromagnetic spectroscopy in that spectrophotometry deals with visible light, near-ultraviolet, and near-infrared. Also, the term does not cover time-resolved spectroscopic techniques.

Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color, or more specifically, the wavelength of light. There are many kinds of spectrophotometers. Among the most important distinctions used to classify them are the wavelengths they work with, the measurement techniques they use, how they acquire a spectrum, and the sources of intensity variation they are designed to measure.

Perhaps the most common application of spectrophotometers is the measurement of light absorption, but they can be designed to measure diffuse or specular reflectance. Strictly, even the emission half of a luminescence instrument is a kind of spectrophotometer.

The two major measurement classes are sometimes called single beam and double beam spectrophotometers. A double beam spectrophotometer measures the ratio of the light intensity on two different light paths, and a single beam spectrophotometer measures the absolute light intensity. Although ratio measurements are easier, and generally stabler, single beam instruments have advantages, for instance they can have a larger dynamic range.

Contents

Visible-region spectrophotometers

Visible region 400-700nm spectrophotometry is used extensively in colorimetry science. Ink manufacturers, printing companies, textiles vendors, and many more, need the data provided through colorimetry. They usually take readings every 10 nanometers along the visible region, and produce a spectral reflectance curve. These curves can be used to test a new batch of colorant to check if it makes a match to specifications. Traditional visual region spectrophotometers cannot detect if a colorant has fluorecence. This can make it impossible to manage color issues if one or more of the printing inks is fluorescent. Where a colorant contains fluorescence, a bi-spectral fluorescent spectrophotometer is used.

Spectroradiometers

Spectrographic representation of the light output of an  (left) and a  (right)
Spectrographic representation of the light output of an incandescent bulb (left) and a fluorescent lamp (right)
Spectroradiometers, which operate almost like the visible region spectrophotometers, are designed to measure the spectral power distributions of illuminants in order to evaluate and catagorize lighting for sales by the manufacturer, or for the customers to confirm the lamp they decided to purchase is within their specifications.

UV and IR spectrophotometers

The most common spectrophotometers are used in the UV and Visible regions of the spectrum, and some of these instruments also operate into the near-infrared region as well. Spectrophotometers designed for the main infrared region are quite different because of the technical requirements of measurement in that region. One major factor is the type of photosensors that are available for different spectral regions, but infrared measurement is also challenging because virtually everything emits IR light as thermal radiation, especially at wavelengths beyond about 5 μm.

Many spectrophotometers use a monochromator to analyze the spectrum, but there are also spectrophotometers that use arrays of photosensors and, especially in the IR, there are spectrophotometers that use a Fourier transform technique to acquire the spectral information in a techique called FTIR.

The spectrophotometer measures quantitatively the fraction of light that passes through a given solution. In a spectrophotometer, a light from a pimp in a near-IR/VIS/UV spectrophotometer, typically a deuterium gas discharge lamp) is guided through a monochromator, which picks light of one particular wavelength out of the continuous spectrum. This light passes through the sample that is being measured. After the sample, the intensity of the remaining light is measured with a photodiode or other light sensor, and the transmittance for this wavelength is then calculated.

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