Luminous intensity is the term used to measure the strength or amount of light produced by a particular light source. Luminous intensity describes a wavelength-weighted power measure emitted by a light source.
Generally, light intensity is measured using a photometer. A photometer is a type of measuring instrument used to measure the strength of electromagnetic radiation in the range from ultraviolet to infrared, including the visible spectrum.
This device is generally a transducer that converts an electric current into a mechanical indication, for example a pointer moving across a dial (indicator).
The current source comes from a selenium cell, which generates current when light falls on it, or it may be a permanent source, such as a battery, where current passes through a cadmium sulfide cell with a resistance that varies with the amount of light falling on it.
There are 4 types of photometers that are commonly used, including:
1. Visible Light Reflectance Photometer
Visible Light Reflectance Photometer is a photometer that measures the reflectance of a surface as a function of wavelength. The surface is generally irradiated with white light, and the reflected light is measured after passing through the monochromator.
This type of measurement on a photometer is usually applied in the paint industry to characterize surface color objectively.
2. UV-Visible Photometer
UV-Visible Photometer is a type of photometer to measure the absorption of light with a certain wavelength on colored substances in solution.
In this type of photometer, Beer’s law is used to calculate the concentration of a colored substance in a solution. This tool is the main instrument used especially in the fields of biochemistry and analytical chemistry.
The wavelengths that can be received by this tool are about 240 nanometers (nm) to 750 nm. The working principle of this tool is that monochromatic light is allowed to pass through a container (which contains certain cells/components) with an optical flat window containing the solution.
Then the translucent light is detected by a light detector, which will then measure the intensity of the light and then compare it with a comparator in the form of a pure solvent liquid without containing other ingredients or components (commonly known as blanks).
There are two types of UV-Visible photometers that are commonly used:
UV-Vis Spectrophotometer
In UV-Vis spectrophotometers, a monochromator (with a prism or grating) is used to obtain monochromatic light of one particular wavelength. Spectrophotometers have advantages over filter photometers, including:
- Can be easily set to measure absorbance at different wavelengths.
- Can be used to scan the spectrum of absorbent substances.
- Provides higher optical purity of light analysis, so it is preferred for use in research.
UV-Vis Filter Photometer
In filter photometers, an optical filter is used to provide monochromatic light. Photometers for microtiter plates are referred to as filter photometers. The advantages of filter photometers over spectrophotometers include:
- Stronger
- Cheaper
- Easier to use for routine analysis.
3. Infrared Photometer
An infrared photometer or also known as an infrared spectrophotometer is a photometer used to study the structure of substances. This photometer will provide absorption at the specified wavelength. In its working process, infrared spectrophotometers do not use water, in contrast to UV-Visible spectrophotometers.
This is because water can absorb infrared light strongly in several wavelength ranges. So infrared spectrophotometry generally uses compounds in the gas phase (for volatile substances) or with substances pressed into tablets together with salts that are transparent in the infrared range.
Generally, the gas used as a solvent substitute is potassium bromide (KBr). The working principle of this tool is that first, the substance being tested is mixed with pure KBr and then pressed into a transparent tablet and placed in a light beam.
Another difference between this tool and the UV-Visible spectrophotometer is that this tool does not use a monochromator light, but uses an interferometer. To analyze the interference pattern, the Fourier transform algorithm is used.
In this way, the entire wavelength range can be analyzed simultaneously, which saves time and is also cheaper than monochromators.
The light absorbed in the infrared region generally does not correspond to the electronic excitation of the studied substance, but rather to various types of vibrational excitation. Vibratory excitation is a characteristic of different groups in a molecule.
The infrared spectrum usually has very narrow absorption lines, which makes it unsuitable for quantitative analysis but provides very detailed information about molecules.
The frequency of the different vibrational modes varies with the isotope, and therefore different isotopes give different peaks. This allows the Infrared spectrophotometer to study the isotopic composition of the sample.
4. Atomic Absorption Photometer
Atomic Absorption Photometer is a photometer used to measure the light intensity of a very hot flame.
This type of photometer is commonly used in the analysis of certain metal content. The working principle is that the solution to be analyzed is injected into the flame at a constant and known rate.
The metal in solution is present in the form of a scattering of atoms in a flame. In this photometer, monochromatic light is produced by a discharge lamp, wherein the discharge occurs in a gas with the metal to be determined. The beam then emits light with a wavelength that matches the spectral line of the metal.
The filter in this tool can be used to isolate one of the main spectral lines of the metal to be analyzed. In the process, light will be absorbed by the metal in the flame, and the absorption is used to determine the concentration of the metal in the original solution.