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The measurement of the intensity of brightness of light that can be perceived by the human eye is called photometry. Meaning of photometry is different from radiometry. Radiometry measures the levels of optical radiations.

In 1924, the Commission Internationale de l’Eclairage (CIE) decided to make photometry as a part of modern science. They also defined the response of the average human eye.

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The commission experimented and measured light-adapted eyes for several people and put down the data. The curve of the data revealed that people strongly responded to green color and were less sensitive to extreme ends like red and violet.

The modern photometer meaning 'is the radiant power of every wavelength', measured by a luminosity function as per the human's sensitivity to brightness.

There are Two Weighting Functions:

  • Photopic sensitivity function (for light conditions), and 

  • Scotopic function (for dark conditions). 

The photopic sensitivity function is used in this condition, but sometimes the scotopic sensitivity function is also used in the same way.

Meaning of Photometer

A photometer is a device that is used to measure light. The root word ''photo,'' means light. For example, photosynthesis is a word that describes how plants produce their own food by using light energy. A photon is a particle of light, a photograph is an image which is made from light-sensitive film or light-sensitive device.

A photometer is a device that measures the strength of electromagnetic radiation in the range of infrared radiation to ultraviolet radiation, including the visible part of the electromagnetic spectrum.

Usually, a photometer converts light into electric current by using a photoresistor, photomultiplier, and photodiode.

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Photometers Measure the Following Parameters

  • Illuminance

  • Light absorption

  • Irradiance

  • Reflection of light

  • Scattering of light

  • Fluorescence

  • Luminescence

  • Phosphorescence

Types of Photometry

The observer will be ready to perform photometry on an image when the image bias is subtracted and flat-fielded.

To understand the meaning of photometry, one must know the types of photometry; which are:

  • Differential, and

  • Absolute

a. In Differential Photometry: The object under the study of differential photometry is seen beside other related comparison stars. The count for both objects are then compared to find the difference between them; this difference is used to derive the difference in brightness.

In this manner, the relative brightness of the object under study as compared to another object in consideration is measured.

b. Absolute Photometry: The object under study in absolute photometry is observed without reference or comparison with any nearby star, and then the ADU count is analyzed to measure the actual brightness of the object.

Calculation of the actual brightness of an object by an absolute photometry method is much more complicated as compared to differential photometry.

Principles of Photometers

Photometers examine how light interacts with reflective materials. The array of photometers describes the working principle of photometry in different fields of study. Some instruments use the principle of photometry to observe how light absorbs and/or reflects wavelengths.

Some instruments and devices measure light by converting it into electric current and measuring the intensity of electric current produced by the light.

There are also certain photometers that shoot white light at a surface to measure the amount of light that is reflected at the instrument.

When a light is passed through a colored solution, lights of certain wavelengths are selectively absorbed by the solution, which gives a plot of the absorption spectrum of the solution. The wavelength of light at which maximum absorption occurs is called an absorption maximum (λmax) of that solution. Some wavelengths of light are not absorbed by the solution and are transmitted through it, giving the solution its color.

Photometric instruments measure transmittance, and it is defined as the ratio of the intensity of emergent light to that of the intensity of incident light, mathematically:

Transmittance (T) = Intensity of the emergent (or transmitted) light / Intensity of the incident light

= le/lo

Transmittance is expressed on a range of 0 to 100%.

Photometry Applications

  • Highly sensitive photometers mark to evaluate the contrast ratios of cathode-ray tubes, flat panel displays, and liquid crystal displays.

  • Goniophotometers are used to characterize the contrast and luminance of flat panel displays over a huge array of angles.

  • Photometers are applied for speedy and accurate testing of automotive dashboards and cockpit displays. 

  • The illuminance of theater screens, transmittance of filters, uniformity of projection systems, and reflectance of paper, ceramics, and textiles are some other common uses of photometers.

Working of Single Beam Photometer

A source produces light. The light from this source is subjected to a solution. A part of the light is observed by the solution, and the remaining part of the light is transmitted. The transmitted light falls on detectors, which produce photocurrent, which is proportional to the intensity of incident light. This photocurrent is passed through a galvanometer where readings are displayed.

The instrument is operated in the following steps:

  • Initially, the detector is darkened, and galvanometer reading is mechanically adjusted to zero.

  • Now a reference solution is kept in the sample holder.

  • Light is transmitted from this reference solution.

  • An intensity control circuit is present to adjust the intensity of light emitted by the source, in such a way that the galvanometer shows 100% transmission.

  • After calibration, the readings of the standard sample (Qs) and the unknown sample (Qa) are taken. The concentration of unknown samples is found using the following formula.

Qa = Qs* IQ/IS


Q = Concentration of the unknown sample,

Qs = Concentration of the reference sample, 

IQ = Unknown reading, and  

IS = Reference reading.

FAQ (Frequently Asked Questions)

Q1. What are the Uses of Photometry and Photometers?

Ans: Photometry is used in the study of properties of liquids and solutions in chemistry. Photometers measure the mass of organic and inorganic materials in a solution or liquid.

In astronomy, photometry is used to restrict certain wavelengths of light and allow other desired wavelengths.

Photometers are an important device that allows scientists to observe and capture images of celestial bodies.

Q2. Give Suitable Differencing Between Photometry and Radiometry?

Ans: Radiometry includes the entire optical radiation spectrum, while photometry deals with only the visible part of the spectrum.

Q3. Differentiate Between Luminance and Illuminance?

Ans: Luminance is defined as the amount of light that passes through an object, whereas illuminance refers to the light's capacity that falls on a given surface area.

Q4. What is Absolute Photometry?

Ans: Absolute photometry precisely measures the lumens emitted by a specific luminaire. Relative photometry can be adjusted to measure the distribution of light by various lamps of different lumen outputs. Absolute photometry is usually used to measure LED performance.