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Measuring luminous radiation: photometer, spectrometer, light sensor

Measuring luminous radiation
Measuring luminous radiation

Photometry is the science that studies light radiation as it is perceived by human vision. 

Spectrometry is the experimental study of the spectrum of a physical phenomenon, i.e. its decomposition on an energy scale: frequency, wavelength, etc. Today, this principle is available in a multitude of specialized techniques that have applications in almost all areas of physics: astronomy, biophysics, chemistry, atomic physics, nuclear physics, solid-state physics, mechanics, acoustics, etc. Spectroscopy analyses not only visible light, but also electromagnetic radiation in all frequency ranges, elastic waves such as sound or seismic waves.

Here are the main instruments that measure light radiation:

Photometer

A photometer is a device used to measure the light quantities according to the curve of the eye's sensitivity. It therefore differs from the radiometer that measures electromagnetic radiation unrelated to human perception.

In meteorology, it is used to measure the radiation of the sun or the sky.

The device features either a photoelectric cell (device composed of a photosensitive sensor, in which the electrical properties (voltage, resistance...) vary according to the intensity of the radiation of the captured light), or CCD sensors (a light-sensitive electronic component used to convert electromagnetic radiation (UV, infrared, etc.) into an analog electric signal). In both cases, the sensors convert the received light into a measurable electric current.

Photometers have several applications in astronomy. In the case of the stars, for example, it can be used to determine their temperature, their distance, and even their age. You can also use it to try and find out if there are planets orbiting around certain stars.

Spectrometer

A spectrometer is a measuring device that breaks down a beam of light into simple elements that make up its spectrum. In optics, it is used to obtain specific wavelengths that make up light beams (electromagnetic spectrum).

In practice wavelengths are observed in the form of spectral lines. Usually a device will only work on a small part of the spectrum due to the variety of techniques used to measure each band of the spectrum.

Fourier’s Transformed Infrared Spectroscopy (FTIR)

Infrared spectroscopy to transform Fourier or FTIR spectroscopy (or even FTIR from English Fourier Transform Infrared spectroscopy) is a technique based on the absorption of infrared radiation by the analysed material. The chemical components can be identified through the detection of characteristic vibrations. This way the molecular structure of the analysed material can be accessed directly. The different available mountings allow for the analysis of virtually any type of materials.

The purpose of the all absorption spectroscopy is to measure how much light a sample absorbs depending on the wavelength.

Applications are numerous and include the identification of products, materials, localized analysis in microscopes or even the analysis of powder. The fields of application are mainly electronics, aerospace, and automobile.

Fluorescence spectrometer

Fluorescence spectroscopy analyses the fluorescence of a sample. This involves using a beam of light which excites the electrons in molecules of certain compounds and causes them to emit light. The fluorescence reader allows for the excitement of samples containing the fluorophores in accurate wavelengths and measures the intensity of the emitted fluorescence. Tools used to measure fluorescence are called fluorometers or fluorometers.

We can distinguish different types of fluorometers according to the source light exciter, the type of filter, the presence or absence of monochromator and the sensor type. Both types operate on the following principle: the light from an excited source passes through a filter or monochromator, and strikes the sample.

Fluorescence spectroscopy is a common technique of analysis in organic chemistry and biochemistry.

Light Sensor

Light sensors are intended to provide protection against light, to optimize clarification of different locations, or even to measure brightness. Solutions dedicated to the light sensors: digital light meters, solar sensors, diodes detectors, LED testing, photon sensors.

The brightness sensor consists of a solar panel and on the basis of the amount of light received it produces more or less of energy. The sun sends electromagnetic waves (light) to the Earth. Light is composed of photons containing energy. When a photovoltaic panel receives such energy, it frees electrons, and can then turn that into an electrical signal.

Conclusion: photometry, spectrometry, spectrophotometry…what are the differences?

Photometry is the measurement of the light received, without trying to find out if we want to separate the colours or not.


Spectroscopy studies the decomposition of light and different colors. Optical spectroscopy makes it possible to know whether, for example, light of a certain color contains other colors, something that human vision does not have the capacity to do. It also studies why it is that such light is red or blue.


Spectrophotometry goes even further. It not only studies the separation, but determines the intensity of each constituent of the light. It also measures the proportion of the various components of the light. 

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