Spectrophotometer Principle

The spectrophotometer is an instrument which measures an amount of light that a sample absorbs. The spectrophotometer works by passing a light beam through a sample to measure the light intensity of a sample. These instruments are used in the process of measuring color and used for monitoring color accuracy throughout production. They are primarily used by researchers and manufacturers everywhere. The major Spectrophotometer Applications are limitless as they are used in practically every industrial and commercial field, however it finds its major applications in liquids, plastics, paper, metals and fabrics. This helps in ensuring that the color chosen remains consistent from its original conception to the final, finished product.

Spectrophotometer Principle

A spectrophotometer is made up of two instruments: a spectrometer and a photometer. The spectrometer is to produce light of any wavelength, while the photometer is to measure the intensity of light. The spectrophotometer is designed in a way that the liquid or a sample is placed between spectrometer and photometer. The photometer measures the amount of light that passes through the sample and delivers a voltage signal to the display. If the absorbing of light change, the voltage signal also changes. Spectrophotometers come in a variety of shapes and sizes and have multipurpose uses to them. The different types of spectrophotometers available are all different from one another, based on their application and desired functionality. The most popular spectrophotometers are 45 degree, sphere and multi-angle spectrophotometers. Another closely related concept is Spectroscopy, that simply measures the absorption of light from its source and the intensity of light as well.

The basic spectrophotometer instrument consists of a light source, a digital display, a monochromator, a wavelength sector to transmit selected wavelength, a collimator for straight light beam transmission, photoelectric detector and a cuvette to place a sample.

The intensity of light is symbolized as l0 measure the number of photons per second. When the light is passed through the blank solution, it does not absorb light and is symbolized as (l). Another important factors are Absorbance (A) and Transmittance (T).

\(T = \frac{l}{l_{0}}\)

\(A = -log_{10} \ T\)

Here, we need to measure the intensity of light that passes a blank solution, and later measure the intensity of light passing a sample. Calculate the transmittance and the absorbance. For the measurement of absorbance, we can use an isosbestic point where the absorbance and wavelength of two or more species are same.

A number of protons transmit and absorb totally depended on the length of the cuvette and the concentration of the sample.

The transmittance and absorption relation is:

\(Absorbance \ (A) = -log (T) = – log(T) = -log\left ( \frac{l_{t}}{l_{0}} \right )\)

The transmittance of an unknown sample can be calculated using the formula given below.

\(Transmittance \ (T) = \frac{l_{t}}{l_{o}}\)


\(l_{t}\) = Light intensity after passing via cuvette

\({l_{0}}\) = Light intensity before passing via cuvette

Further, there are several varieties of spectrophotometer devices such as UV Spectrometry, atomic emission spectrophotometry and atomic absorption spectrophotometry and much more. It can also be classified into two types based on the range of light source wavelengths like IR spectrophotometer and UV-visible spectrophotometer. Some of the major real life applications of spectrophotometry in various fields are laundry soap, carpeting and production of small parts such as toys or intricate machinery. The major types of spectrophotometers are categorized into 2, these are mainly portable spectrophotometers and bench spectrophotometers, they both are unique and have their own uses.

Practise This Question

Is Zeff the same in the following two cases?