In the field of optics, photometry is defined as the term that provides the measurement of light, as recognized by the eye of a human being. This measurement deals with measurements associated with psychophysical attributes' of the energy of electromagnets that come under the visibility ranges of human eyes. However, the concept of photometry is quite different from radiology as radiology deals with the measurements associated with radiations from the optics.

In dealing with this concept of photometry, the term luminous holds much significance.

Definition of photometry

  • In simpler terms, photometry can be stated as the measure of the brightness of light that is noticed through the human eye. However, the concept of photometry arrived 2000 years ago at the time when the famous Greek astronomer, named Hipparchos was able to classify stars into six brightness.

  • In modern times, photometry is the powers of radiance that are noticed at each wavelength. This wavelength is weighted by function luminosity based on the brightness sensitivity of humans (, 2022).

  • It is clearly known that the human eye does not have the ability to perceive all the wavelengths of light that are visible and so the science of photometers initiates the possibility to measure the power of each wavelength, to show the aspects of sensitivity to the human eye.

Photometric quantities

Four physical photometric quantities are mentioned while talking about the science of photometry. The four quantities are luminous flux, illuminance, luminance and lastly, the intensity of luminous.

Luminous flux states the power measurement of visible light that is produced from the source of light or it can be form-fitting light (, 2022). This quantity is evaluated in terms of lumens (lm). However, illuminance determines the total number of luminous fluxes, per unit area, that are incident on the surface. The notion ofluminance caters to providing a proper direction to the intensity of light that is emitted from the light source.

In the year, 1942, a committee of members, named, Commission Internationale de l'Eclairage (CIE) stated the average responses by the human eye. Certain groups of people were exposed to light and measurements of light adaptation of human eyes were conducted. However, the picture given below, shows that the people responded strongly to the green colour. In addition to that, they are relatively less sensitive to the extremes of the spectrum that is to the colours red and violet.

Figure 1: Relative sensitivity of the human eye to various wavelengths

Law of Photometry

Figure 2: Law of Photometry

The laws of photometry is essential in determining the concepts of a photometer and the usage of a photometer. The laws of photometry determines the notion of intensity of illumination of a surface, which is inversely proportional to the square of the distances that are noticed between both the source and surface. In the above diagram, the source is denoted by the letter S '' and the intensity of illuminations to the surface is denoted by E (, 2022).

However, the law for a distance of photometric is evaluated by effective plotting of illuminance that determines as a function the value of reciprocal and the squares of the distances.

What is a Photometer?

The term photometer has arrived from the root word, photo that means light, and this apparatus is used for measuring the interaction that is conducted when the light interacts with materials of reflections. It device is also used to conduct several fields of studies with the notion of photometry. This device helps in measuring the strengths that are conveyed from the electromagnetic radiations ranging between infrared to ultraviolet radiations (, 2022).

These ranges also include the visible part present within the electromagnetic spectrum. Moreover, it works by converting the light energy into current electricity, with the usage of photoresistors, photodiode and photomultiplier.

However, a photometer is used to measure certain parameters, that includes, the notion of illuminance, irradiance, aspect of absorption of light, fluorescence, reflection of light, luminescence, the notion of phosphorescence and lastly, the phenomenon of scattering of light.

Figure 3: Photometer

Types of Photometer

There are two types of photometers used to measure the brightness of light, the first one is the spectrophotometer and the second one is the filter photometer. The spectrophotometer uses a monochromator, to perceive the monochromatic light that is defined by one wavelength (, 2022).

The monochromator is constructed with the help of a prism or grating. On the other hand, a filter photometer uses optical filters to provide monochromatic lights.

Principles followed by Photometer

In simpler terms, photometers are used in evaluating how with reflective materials, the light interacts and this is done by following the principles of photometry. Some instruments are there that follow principles of photometry for observing how wavelength is absorbed and is reflected by the light. However, other photometers are there that shoot white light to the surface and help in providing the measurement of the amount of light reflected by the instruments. However, it follows the principle that when light travels through a coloured solution, certain wavelengths of the light are absorbed by the solutions based on their selectivity (, 2022).

This further provides a plot for the absorption of the spectrum of light within the solution. The maximum absorption of light occurs at a particular wavelength and is known to be the absorption maximum for that solution. However, some wavelengths of light are not absorbed instead they are transmitted through and thereby provide colour to the solutions.

Applications of photometry

Photometry can be applied in several aspects that are discussed below. The highly sensitive types of photometers are used for effective evaluation of the contrast ratios associated with the tubes of cathode rays, displays of liquid crystals and also of the displays flat panels (, 2022).

Contrasting and the luminance displays of the flat panels are done with the help of the characterization by goniophotometers over the array of angles' ' that are huge in nature. Photometric devices are used in the speedy as well as accurate testing of the dashboards.


This tutorial determines the concept that lies underneath the notion of photometry that defines the aspect of the intensity of light. In simpler words, the brightness is best determined by the word luminous. However, the intensity of brightness is measured depending upon the perceiving capacity of the average human eye, which is termed photometry.

Frequently Asked Questions (FAQs)

Q1. What are the weighting functions of photometry?

There are two types of weighing functions noticed within photometry that are, photopic sensitivity functions for conditions of light and scotopic functions for conditions of darkness.

Q2. What is the basic unit of photometry?

The basic unit that is needed to measure the notions of photometry is known as a lumen. These luminosity functions are determined by CIE standards.

Q3. What is determined as radiometry?

Radiometry is often confused with photometry and is known as the investigation of the intensity of radiations.

Q4. What is the application of photometers?

Photometers are mostly used in speedy testing of cockpit displays, used in automotive dashboards.

References, (2022), Components of flame Photometer, Retrieved from: [Retrieved on, 8th June 2022], (2022), Photometer, Retrieved from: [Retrieved on, 8th June 2022], (2022), Photometry, Retrieved from: [Retrieved on, 8th June 2022], (2022), Light concepts, Retrieved from: [Retrieved on, 8th June 2022], (2022), Photometric surveys, Retrieved from: [Retrieved on, 8th June 2022], (2022), Light Optics, Retrieved from: [Retrieved on, 8th June 2022], (2022), The principle of Photometer instruments, Retrieved from: [Retrieved on, 8th June 2022], (2022), Optics, Retrieved from: [Retrieved on, 8th June 2022]


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Updated on: 13-Oct-2022


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