Luminous Intensity

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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.

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.

So, what is luminous flux?

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 in brightness. So, the ratio of the total luminous flux to the radiant flux is known as the luminous efficacy.

FAQ (Frequently Asked Questions)

Q1: What Does Luminous Flux Mean in Optics?

Ans: 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.

Q2: What is Light Intensity?

Ans: 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.