How Is Luminous Intensity Measured and Why Does It Matter?
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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Contents
In this article, we will learn about the following topics in detail:
luminous intensity meaning
The unit of luminous intensity
Luminous intensity definition
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 What Is Luminous Intensity in Physics?
1. What is the fundamental definition of luminous intensity in Physics?
Luminous intensity is the measure of the perceived power of light emitted by a point source in a specific direction. It is one of the seven fundamental SI base quantities and specifically quantifies the brightness of light visible to the human eye. Unlike radiant intensity, which measures the total energy emitted, luminous intensity is weighted according to the sensitivity of the human eye (the luminosity function).
2. What is the SI unit and symbol for luminous intensity?
The SI unit for luminous intensity is the candela, and its symbol is cd. The physical quantity of luminous intensity itself is often represented by the symbol Iᵥ.
3. How is luminous intensity different from luminous flux and illuminance?
These three terms describe different aspects of light and are often confused. Here’s how they differ:
- Luminous Intensity (cd): Measures the amount of light emitted in a single direction from a source. Think of it as the brightness of a laser beam or a spotlight.
- Luminous Flux (lm): Measures the total amount of visible light emitted by a source in all directions. It describes the total light output of a bulb.
- Illuminance (lux): Measures the amount of light that actually falls onto a specific surface area. It describes how brightly a surface is lit. 1 lux is equal to 1 lumen per square metre.
4. What is the formula used to calculate luminous intensity?
The formula for luminous intensity (Iᵥ) relates it to luminous flux (Φᵥ) and the solid angle (Ω) over which the light is emitted. The formula is: Iᵥ = dΦᵥ / dΩ. This means luminous intensity is the luminous flux per unit solid angle. For a source that emits light uniformly in all directions, the formula simplifies to Iᵥ = Total Luminous Flux / 4π, since a sphere has a solid angle of 4π steradians.
5. Can you explain luminous intensity with a real-world example?
Imagine you have a bare 1000-lumen light bulb. This 1000-lumen value is its luminous flux—the total light it emits. If you place this bulb in a spotlight, the spotlight's optics concentrate all that light into a narrow beam. While the total luminous flux is still 1000 lumens, the luminous intensity within that beam is now extremely high, making it appear very bright in that specific direction. Conversely, a lantern diffuses the light, resulting in low luminous intensity in any given direction.
6. Why is luminous intensity considered a measure of 'perceived' power?
Luminous intensity is a photometric quantity, not a radiometric one. This means it is adjusted to reflect how the human eye perceives brightness. The human eye is most sensitive to light in the green-yellow part of the spectrum (around 555 nm) and less sensitive to blue and red light. Therefore, a green light source with 1 watt of radiant power will have a much higher luminous intensity (more candelas) than a red or blue light source with the same 1 watt of power, because our eyes perceive it as brighter.
7. What are some important applications of measuring luminous intensity?
Measuring luminous intensity is critical in many fields. Key applications include:
- Lighting Design: For designing LEDs, lamps, and luminaires to ensure they provide the required brightness in a specific direction.
- Automotive Industry: To design headlights and taillights that meet safety standards for brightness and beam pattern.
- Display Technology: In the development of screens and projectors to ensure uniform and adequate brightness.
- Aviation: For runway lights and aircraft signals that must be visible from specific angles and distances.
8. Is the 'brightness' of a phone screen or TV measured in candelas?
Not directly. The brightness of an extended source like a screen is measured in luminance, whose unit is candela per square metre (cd/m²), often called a 'nit'. While luminance is derived from luminous intensity, it specifically accounts for the surface area from which the light is emitted. Luminous intensity (candela) is more appropriate for characterising a point-like source, such as a single LED, rather than a large, flat surface like a display.
9. Why is the unit of luminous intensity called the 'candela'?
The name 'candela' is Latin for 'candle'. The unit was originally based on the light produced by a standard physical candle of a specific composition and size. The early definition of luminous intensity, known as 'candlepower', was the light emitted by one such standard candle. While the definition is now based on a more precise monochromatic radiation source, the historical name was kept to honour its origin.
10. How does light intensity affect the growth of plants?
Light intensity is crucial for photosynthesis, the process plants use to create food. Higher light intensity generally leads to a faster rate of photosynthesis, resulting in better growth, stronger stems, and darker green leaves. Conversely, plants grown in low light intensity are often spindly and pale. Without sufficient light intensity, photosynthesis slows down, stunting plant growth even if water and CO₂ are abundant.
