Luminous Intensity

Define Luminous Intensity

Luminous Intensity is a measure of the wavelength-weighted power radiated by a light source in a particular direction per unit solid angle. It is based on the luminosity function, a standard model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela or cd, it is an SI base unit.


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In this article, we will learn about the following topics in detail:

  1. luminous intensity meaning

  2. The unit of luminous intensity

  3. Luminous intensity definition

  4. SI unit of luminous flux


Luminous Intensity

Luminous intensity is an expression that describes the amount of light power emitting from a point source within a solid angle of one steradian. 


For instance, a frequency of 540 terahertz, i.e., 540 THz or 5.40 x 1014Hz is specified. Here,  A frequency of 540 THz corresponds to a wavelength of about 555 nanometers (nm), which is in the middle of the visible light spectrum and is usually accepted and the human eye is sensitive to this wavelength. 


The quantities employed to express luminous intensity are obvious to most non-scientists.


A steradian is the standard unit of the solid angle; a sphere encloses 4 p, which is approximately equal to 12.57 steradians.


Luminous intensity can be used as a factor to determine the quantify the power emitted by any light source in a particular direction for a given unit solid angle. The luminosity function provides a standardized model to determine the sensitivity level of the human eye. In the SI unit system, luminous intensity is represented as candela (cd). It is defined as the quantity of visible light that is emitted by a source in a unit time per unit solid angle. Lumens are used to characterize the luminous flux from power a source at any second. The lumens value mentioned is used as a reference to visual sensation to light.

Unit of Light Intensity

Luminous intensity is symbolized as LV. I the SI system, the unit of  luminous intensity is Candela or Cd; however, we have two more units of luminous intensity; these are as follows:

  • Steradian

  • Hefner Kerze

  • Candlepower


Luminous Intensity Definition

For understanding the luminous intensity definition; let’s look at the history of luminous intensity:


Decades ago, the luminous intensity was measured in the form of a unit called the candle. This expression came from the fact that one candle represented on average the amount of visible radiation emitted by a candle flame. 


This was an inexact description because burning candles vary in brightness. So, for a time, a specified amount of radiation from the element, say, platinum at its freezing temperature was used as the standard. 


Then in the late 20th century, the candela was introduced and this unit was adopted as the standard unit of luminous intensity. 


One Candela

One candela or 1 cd is the magnitude of an electromagnetic field or  EM-field in a certain direction that has a power level equivalent to a visible-light field of 1/683 watt 1.46 x 10⁻³ Watt per steradian at 540 THz.


Here, we also discussed the luminous flux. So, luminous flux is the amount of electromagnetic radiation emitted by any source. The SI unit of luminous flux is Lumen.


Luminous Flux

The luminous flux tells us how much light is emitted by any light-emitting source. This refers to lights in a visible band emitted by the LED lamp in all directions. The physical unit for the luminous flux is called lumen and an acronym for lm. With the old light-emitting sources, the brightness/sharpness could be compared well on the basis of the wattage.


Unit of Luminous Intensity

  • The SI unit of luminous flux is the lumen or simply lm.

  • Till 19 May 2019, we used to define one lumen as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. 

  • Then on 20 May 2019, the lumen was defined by fixing the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz (green light) to be 683 lm W-1.

  • In other systems of units, luminous flux also has units of power.


Luminous in Photometry

In photometry, luminous flux or luminous power is the measure of the received power of light. It varies from the radiant flux, i.e., the measure of the total power of electromagnetic radiation including infrared (IR), ultraviolet (UV), and visible light, in that case, luminous flux is adjusted to reflect/emit the varying sensitivity of the human eye to varying wavelengths of light.


Luminous Efficacy

The luminous flux takes into account the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the response of the eye to different wavelengths. 


The luminous flux is a weighted total of the power at all wavelengths coming in the visible band. The light outside the visible band does not contribute to brightness. So, the ratio of the total luminous flux to the radiant flux is known as the luminous efficacy.

Physics and Its Importance

Physics plays a vital role by being a branch of science that deals with the structure of matter that helps in studying various things and how they actually work and thereby finding out how these fundamental constituents interact with various elements in the universe. It starts from the study of objects starting from the very small ones using quantum physics to the entire universe using the various general theory of relativity. It is indeed these physical processes and phenomena of a given system based on the physical characteristics, the composition of any matter that is found.

Physics plays a vital role in acting as an international enterprise, which plays a key role in the progression of humankind in a futuristic way by studying how the various components and elements of the universe actually function and interact with each other and various systems. This important aspect of Physics plays a vital role in ensuring that it adds valuable contributions to the technological infrastructure and provides trained professionals and profound scientists to the universe, necessary to create significant scientific advances and help us discover our future with a noble mission to ensure that the earth is transformed into a better place to live in.

FAQs on Luminous Intensity

1. How to measure the luminous intensity emitted from a light source?

The luminous intensity is used for characterizing the light emitted from a defined light source. It is explained as the luminous flux per unit solid angle. In the simplest case, the luminous intensity is not dependent on the direction. This means that in the case of the uniform omnidirectional emission, the luminous intensity is the luminous flux divided by a factor of 4π sr. In terms of radiometric terms, Radiant intensity is measured as watts per steradian W/sr. Luminous intensity can be measured as a photometric quantity either as lumens per steradian lm/sr or candela cd.

2. What is the importance of the intensity of light for plants?

The intensity of light plays a vital role in the manufacturing of food in plants, the length of its stem, the color of the leaves, and the flowers it bears. The plants that are grown in low light conditions or environments tend to be spindly in appearance and with leaves in light shades of green. At the same time, a similar plant grown under bright light tends to be shorter, possess better branches, and have leaves that are larger and dark green in appearance. Without having enough light, the process of photosynthesis for plants cannot be done very quickly and effectively - even if there is an abundant quantity of water and carbon dioxide and the most suitable temperature available that are essential for the survival of a plant. The rate of photosynthesis for the plant can be increased by increasing the light intensity unless some other factor becomes a limiting factor - that turns out to be in short supply.

3. How to determine brightness is luminous intensity?

Luminous intensity can be used to quantify the overall brightness from a light source. For instance, a light lamp’s brightness can be represented by the luminous intensity without regard to the area of the source from where the light is produced. Luminance indicates the degree of brightness over an area of how much the light covers, such as from a display device. The luminance factor can be derived by dividing the luminous intensity by the area lighted by the source. The luminous intensity also can also be expressed as the level of radiation coming from an object while the intensity of illumination corresponds to the actual radiation that is falling upon an object.

4. What is the difference between the terms lux and lumens?

The difference between lumen and lux is that the lux takes into account the area over which the luminous flux lumens are spread. To light up a square meter with an illuminance of 1000 Lux, a flux of 1000 lumens concentrated over an area of one square meter is required.

5. Where do we use luminous intensity?

Luminous intensity can be expressed as the perceived power per unit solid angle. For instance, if a 1-lumen bulb is used in a lamp and the lamp is configured that the optics of the lamp are set to focus the light in a distributed manner to a 1-steradian beam, then the luminous intensity of 1 candela would be available for the beam. The symbol Iν is used to represent the Luminous intensity typically represented by the symbol Iν. The ν mentioned as subscript denotes the photometric quantity. The SI-based unit is the candela. The relation between the both of them is as follows:


1 cd is equivalent to 1 lumen/steradian, where the steradian is a measure of solid angle). Candle power is an antiquated term for the candela. 

6. What does luminous flux mean in optics?

Luminous flux in lumens is a measure of the total amount of light a lamp emits. The luminous intensity (in candelas or cd) is a measure of how bright the beam in a particular direction is. 

If a lamp has a 1-lumen bulb and the optics of the lamp are set up to focus or incident the light evenly into a 1-steradian beam, the beam will have a luminous intensity of 1 candela or 1cd. If the optics were changed to concentrate/focus the beam into 1/2 steradian, then the source will have a luminous intensity of 2 candelas or 2 cd. The resulting beam is narrower and brighter; however, the luminous flux remains the same or constant.

7. What is light intensity?

Light intensity or light quantity refers to the total amount of light that plants receive for preparing the food. It is also defined as the degree of brightness or the intensity of sunlight that a plant is exposed to. In contrast to light quality, the specification of the intensity of light does not take into account or consider the wavelength or color.

The following measures of light intensity are called light intensity:

  • Radiant intensity is a radiometric quantity that is measured in watts per steradian W/sr.

  • Luminous intensity is a photometric quantity that is measured in lumens per steradian lm/sr or candela cd.

  • Irradiance is a radiometric quantity that is measured in watts per square meter W/m².

  • Radiance is basically called "intensity" in astronomy and astrophysics, its unit is W·sr⁻¹·m⁻².