What is Photometry : Photometric Quantities & Its Applications

The photometry is invented by Dmitry Lachinov and the terms used in photometric are radiant flux, luminous flux, luminous intensity and efficiency, and illuminance. The most important information that we receive about the celestial object is the amount of energy, which is called as a flux. In the form of electromagnetic radiations, the science of major flux from celestial objects is called photometry. This is an efficient way to carry out the brightness measurement of light from astronomical objects and therefore it plays a key role in the characterization of an astrophysical target. The brief explanation of photometry is discussed below.

What is Photometry?

Definition: The photometry is used to measure the light quantity, and it is the branch of optics in which we discuss the intensity emitted by a source. The differential photometry and absolute photometry are the two types of photometry. The radiant flux, luminous flux, luminous intensity and efficiency, and illuminance are the terms used in photometric. The radiant flux is defined as the total number of energy which is radiated by a source per second and it is represented by a letter ‘R’.

The luminous flux is defined as the total number of energy which is emitted by a source per second and it is represented by a symbol φ. The luminous intensity is defined as a total volume of luminous flux divided by 4Π. The luminous efficiency is defined as a ratio of luminous flux to the radiant flux and it is represented by a symbol ‘η’. The intensity is defined as a ratio of luminous flux per unit area and it is denoted by a letter ‘I’ (I=Δφ/ΔA). The illuminance (E)is the light falling on the surface of the earth.

Photometer and Electromagnetic Spectrum

The photometer is an experiment set up used to compare the illuminance of the two sources on a screen. Let consider a realistic example to understand photometer.

In the figure, there is an optical bench, where two sources A and B placed on two sides of the screen ‘S’ and two boards are placed at the two ends of the screen. On the left sideboard, there is a circular cut and the right sideboard there is a ring shape cut. When a source ‘A’ is switched on, a circular path is obtained on a screen due to the light passing through the circular cut. Similarly, when the source ‘B’ is switched on, you can see light passing through the annular region and the ring patch is obtained on the screen.

When both the sources switched on, you can see both the patches are illuminated simultaneously and you can see the different illuminance of two patches. When a source ‘A’ brought closer to the screen then you will see that the circular patch becomes more bright or you can see that the illuminance of source ‘A’ on screen increases. Similarly when a source ‘B’ brought closer to the screen then you will see that the illuminance of ring shape patch becomes more because of less distance.

Now the sources are adjusted in such a way that, there is no difference between these two sources. The illuminance on the screen due to the two sources are the same or equal. When the illumination due to the sources on the screen becomes equal, we can use

L₁/r₁² = L₂/r₂²

Where L₁ and L₂ are the illumination intensity of two sources and r₁² & r₂² are the separation of the sources from the screen. The above equation is called the principle of photometry.

The electromagnetic spectrum consists of seven regions they are a visible spectrum, infrared spectrum, radio waves, microwaves, ultraviolet spectrum, x rays, and gamma rays. The radio waves have the longest wavelength and lowest frequency when the radio waves move from left to right, the wavelength increases, frequency increases, and energy will decrease. The radio waves, microwaves, and infrared waves are the low energy electromagnetic waves. The ultraviolet, x rays and gamma rays are the high energy electromagnetic waves. The electromagnetic spectrum is shown below.

The photometry is considered only with the visible portion of the spectrum, from about 380 to 780 nanometers. In observational astronomy,  photometry is fundamental and it is an important technique.

Single Beam Photometer

The single beam photometer follows “LAMBERT LAW” to determine the concentration of the unknown samples. The absorption of light by a reference sample and an unknown sample is used to obtain the value of the unknown. The construction of the single beam photometer instrument is shown in the below figure.

The basic components of a single beam photometer are light source and absorption or an interference filter. It is called a photometer because the device that is used for isolating the wavelengths in a figure is the filter, a cuvette is used as a sample holder and a photocell or photovoltaic cell acts as a detector. The source of light generally used is a tungsten halogen lamp. When the filament-like tungsten is heated, it starts emitting radiations in the visible region, and these radiations act as a source of light for the instrument.

An intensity control circuit is used to vary the voltage supply to the tungsten filament lamp, by varying the voltage, the lamp can change the intensity. The intensity should be kept constant for the duration of the experiment. The filter can be a basic absorption filter, this filter absorbs light of a certain wavelength and allows only a particular wavelength to pass through it. The light allowed to pass mainly depends on the material color, for example, red will allow the radiations in the red region to pass and so on.

The selectivity of these filters is very low and the emission of the existing of these filters is not highly monochromatic. The other filter that is used is the interference filter, and the detectors that can be used in single beam photometry can be photovoltaic cells. The detectors give readings of the light intensity. The inverse square law and the cosine law are the two types of laws used to produce the photometric measurements.

Working of Single Beam Photometer

The light from the source falls on the solution placed in the cuvette. Here a part of light observed and the remaining part of the light is transmitted. The transmitted light falls on the detectors which produce photocurrent proportional to the light intensity. This photocurrent enters the galvanometer where readings are displayed.

The instrument is operated in the following steps

• Initially, the detector is darkened and galvanometer is adjusted mechanically to zero
• Now a reference solution kept in the sample holder
• The light is transmitted from the solution
• The intensity of the light source is adjusted by using the intensity control circuit, such that galvanometer shows 100% transmission
• Once the calibration is done, the readings for the standard sample (Q_s) and unknown sample (Q_a) are taken. The concentration of an unknown sample is found out using the below formula.

Q_a=Q_s*I_Q/I_S

Where Q_a is the concentration of the unknown sample, Q_s is the concentration of the reference sample, I_Q  is the unknown reading and  I_S is the reference reading.

Flame Photometry Instrumentation

The basic flame photometry instrumentation is shown below.

In the figure, the burner produces excited atoms and the sample solution is spread to fuel and oxidant combination. The fuel and oxidants are required to produce flame, such that the sample converts neutral atoms and get excited by heat energy. The temperature of the flame should be stable and also ideal. If the temperature is high the elements in the sample converts into ions instead of neutral atoms. If the temperature is too low then the atoms may not go to excited state, so a combination of fuel and oxidants is used.

The monochromatic is needed to isolate the light in a specific wavelength from a remaining light of the flame. The flame photometric detector is similar to that of the spectrophotometer, to read out the recording from the detectors computerized recorders are used. The main disadvantages of the flame photometry are precision is low, accuracy is low & due to the high temperature, ionic interferences are more.

Difference Between Colorimetry and Photometry

The difference between colorimetry and photometry is shown in the below table

Photometric Quantities

The photometric quantities are shown in the below table

Photometer Products

Some of the photometer products are shown in the below table

Applications

The applications of photometry are

• Chemicals
• Soils
• Agriculture
• Pharmaceuticals
• Glass and Ceramics
• Plant materials
• Water
• Microbiological Laboratories
• Biological Laboratories

FAQ’s

1). What is a photometric test?

The photometric test is required to measure light intensity and distribution.

2). What are photometric quantities?

The radiant flux, luminous flux, luminous intensity & efficiency, and illuminance are the photometric quantities.

3). What is a photometric analysis?

The analysis of photometric includes measurement of the spectrum in visible, ultraviolet and infrared regions

4). What is the difference between photometry and spectrophotometry?

The spectrometer is used to measure the concentration of solution whereas photometry measures the light intensity.

5). What is the photometric range?

The photometric range is one of the specifications in the photometer instruments, in V-730 UV-Visible Spectrophotometers the photometric range (approx) is -4~4 Abs.

In this article, the overview of Photometry, photometric quantities, flame photometry instrumentation, single beam photometer, electromagnetic spectrum, and applications are discussed. Here is a question for you what is spectrophotometry?