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Photometry & Reflectometry

Principle of photometry demonstrated with a schematic of light absorption when passing through a solution


Photometry is the measurement of light absorbed in the ultraviolet (UV) to visible (VIS) to infra-red (IR) range. This measurement is used to determine the amount of an analyte in a solution or liquid. Photometers utilize a specific light source and detectors that convert light passed through a sample solution into a proportional electric signal. These detectors may be for example photodiodes, photoresistors, or photomultipliers. Photometry uses Beer–Lambert’s law to calculate coefficients obtained from the transmittance measurement. A correlation between absorbance and analyte concentration is then established by a test-specific calibration function to achieve highly accurate measurements.



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Photometry

Photometry is a widely used quantitative analysis in research laboratories to determine the amounts of inorganic and organic compounds in solutions and other liquids. Photometry also has broad industrial applications in the determination of contaminants in drinking and wastewater, analysis of nutrients in soil, food and beverage samples, building material composition, and many other areas.

Construction and Working of Photometer

The components of a typical photometer include a light source, monochromator, sample, and detector. Light sources can be tungsten-halogen lamps (generally the source of light used for analyses in the visible light range) or LEDs. For measurements in the UV–Visible range a xenon flash lamp may be used. The monochromator filters the light radiated by the light source to allow only a very narrow spectrum to pass. The light then passes through the cuvette or the sample holding cell. Based on the amount of analyte (or a dye derived from it) that is present in the sample solution, some part of the light is absorbed by the solution and the remaining is transmitted. The transmitted light is directed towards the detectors, which produce an electric current proportional to the light intensity.

Beer–Lambert Law

Beer–Lambert law, also known as Beer’s law, states that the quantity of light absorbed by a sample is directly proportional to the concentration of the analyte present and path length of the light through the sample. “Sample” in this context means either the analyte itself (direct measurement) or a dye derived from the analyte (when using reagents or kits). The relationship is described by the formula:

A = elc

where

A = absorbance of the sample
l = optical path length (cm)
c = concentration of the analyte (M)
e = molar extinction coefficient (M-1cm-1)

The spectrophotometer measures the intensity of light before and after passing through a solution and relates it to the transmittance (T) by the following equation:

Transmittance(T)=It/Io

It and Io are the intensity of light after and before passing through the solution, respectively. The transmittance is related to the absorbance by the equation:

Absorbance(A)=−log(T)

Reflectometry

Reflectometry (also known as remission photometry) is a non-destructive analytical technique that uses the reflection of light by surfaces and interfaces to measure characteristics such as color intensity, film thickness and refractive index.

As with other photometers, the main elements of reflectometers include a light source, usually long-life LEDs of specific wavelengths that are focused onto a sample surface via a lens system and the reflected light is measured by detectors.

Reflectometers are often designed to measure physical characteristics of surfaces like color changes on a test strip. In this approach, a sample can be placed on a test strip and its remission (REM) value can be compared against appropriate controls and standards. As in photometry, the difference in intensity between emitted and reflected light allows a quantitative determination of the concentration of specific analytes.

Reflectometry is commonly used in industrial applications as a rapid, sensitive method for quantitating a wide variety of organic and inorganic parameters in water, food, beverages, and environmental samples as well as other diverse uses such as surface analysis of building materials and skin color quantification. 

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