How Lumen Relates to Candela and Steradian in Practical Applications
The solid angle of a sphere subtended by a portion of the surface whose area is square of the sphere’s radius is called steradian. Unit of the solid angle measured in the International System of units is called steradian. Luminous flux of unit or unit of light that is the amount streaming outward using one steradian from a point that is uniform known as one candela is called Lumen. Regarding artificial lighting, Lumen is used for calculation. In the International System of Units, the luminous intensity is known as One Candela. The emission of monochromatic radiation of frequency 540 x10¹² Hertz as a luminous intensity from a given direction of source is measured.
Lumen Definition
An interesting fact to be brought into the limelight is that there are two common units for measuring light. They are lumen and candela. Both are recognized as standard international units. It includes the second(for time), kilogram(for weight), and meter(for distance). The measure of intensity is candela. The intensity of light of one candle is the intensity of the measure of one candela. A measure of luminous flux is the lumen. A standard 1500 to 1700 lumens is given off by a standard 100-watt light bulb. How much light is perceived by human eyes is indicated by luminous flux.
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Steradian
In the International System of Unit, the unit of the solid angle measured is known as steradian. When subtended by a portion of the surface whose area is equal to the square of the radius of the sphere, the solid angle is termed as steradian. Four pie is the complete surface area of the sphere of its radius. So 4 pie steradian is the total solid angle about a point. It is derived from the Greek word solid and the English word radian. In short, it is a solid radian. The angle of the circle defined as the radian of plane-angle measurement subtended by an arc equal in length to the circle’s radius is called the radian.
Candela
In the International System of Units, the luminous intensity is known as One Candela. The emission of monochromatic radiation of frequency 540*10¹² hertz as a luminous intensity from a given direction of source is measured. The intensity of radiation in the same direction of 1/683 watt/steradian is also considered. As a unit of luminous intensity of calculations involving artificial lighting, candela has replaced standard candles or lamps. Candela is also termed as New Candle. A specific point of light strength is specified by lux. For example, on 705 m one Lux is applied (10.764 lux or equal to one lumen or lux per square foot). 10.76 lux is a non-metric unit of illumination value. The life of an installation averaged over the working area is called the service value of illumination(lm ft-2 is the abbreviation). Over a surface of an area to a given point of a small sphere center is called mean spherical illumination. Mean spherical illumination is the scalar illumination.
Questions and Answers
1) What is a Lumen hour?
Ans: When the Luminous flux's lumen acting for one hour is equal to a unit of luminous energy it is termed as Lumen hour.
2) What is Lumen?
Ans: When we compare the strength of the light emitted from the light source, then its common measurement is the lumen. The flow of light in every direction is measured with the help of the lumen. The raw and actual lumen is measured as the lumen value. In a laboratory, the raw lumen is measured and from a lamp, the actual lumen is measured. Various factors like lens and reflector affect the lumen result to be negative.
3) What is Lux?
Ans: The unit of illuminance measuring luminous flux per unit area is called a flux. One lumen per square meter is equal to Lumen. 1Lux=1lm/m2. A specific point of light strength is specified by lux. For example, on 705 m one Lux is applied.
4) What is Luminous Intensity and Luminous Flux?
Ans: In any particular direction the luminous flux emitted by the sources per unit solid angle is called the luminous intensity.
From a luminous body The light energy radiated per second in the form of light waves is called luminous flux. Lumen is its measuring unit.
FAQs on Lumen Unit of Energy Measurement in Physics
1. What is a lumen and what physical quantity does it measure?
A lumen (symbol: lm) is the SI derived unit of luminous flux. It measures the total quantity of visible light emitted by a source per unit of time. Essentially, it quantifies the perceived power or brightness of light as seen by the human eye, not the total energy emitted by the source.
2. Is a lumen a unit of energy? If not, how does it differ from a watt?
No, a lumen is not a unit of energy. This is a common misconception. A lumen measures photometric power (perceived brightness), while a watt measures radiometric power (total energy output per second). The key difference is that lumens are adjusted for the varying sensitivity of the human eye to different wavelengths (colours) of light. For example, our eyes perceive green light as much brighter than deep red light, even if both have the same energy in watts.
3. How is the lumen (lm) technically defined in relation to the candela (cd)?
Technically, one lumen is the luminous flux emitted into a solid angle of one steradian by a point source having a uniform luminous intensity of one candela. The relationship is expressed as: 1 lm = 1 cd ⋅ sr. The candela (cd) is the SI base unit for luminous intensity, which measures the light emitted in a specific direction.
4. What is the practical difference between lumen and lux?
The main difference lies in what they measure:
- Lumen (lm) measures the total amount of light a source produces (its output). Think of it as the total amount of water coming out of a showerhead.
- Lux (lx) measures the amount of light that actually falls on a specific surface (illuminance). It is equal to one lumen per square metre (1 lx = 1 lm/m²). Think of it as how wet the floor gets in one specific spot under the shower.
5. What does the 'lumens per watt (lm/W)' rating on a light bulb signify?
The 'lumens per watt' (lm/W) rating measures the luminous efficacy of a light source. It tells you how efficiently the source converts electrical energy (watts) into visible light (lumens). A higher lm/W value indicates greater efficiency. For example, modern LED bulbs have a very high luminous efficacy (e.g., >100 lm/W) compared to older incandescent bulbs (e.g., ~15 lm/W), meaning they produce the same brightness using far less electricity.
6. Why is the measurement of light in lumens based on the human eye's sensitivity?
Lumens are used for practical lighting applications where the observer is human. Our eyes do not perceive all colours (wavelengths) of light equally. We are most sensitive to greenish-yellow light (around 555 nm). A unit based on total energy (like the watt) would be misleading for lighting, as a lamp emitting high-energy but invisible infrared light would have a high wattage but zero brightness. The lumen is a photometric unit designed specifically to reflect this human-centric perception of brightness.
7. How are lumens useful for comparing the brightness of different types of bulbs, like LED and incandescent?
Lumens are the most accurate way to compare the true brightness of different bulbs, regardless of their technology. In the past, people associated brightness with wattage (e.g., a 100W bulb). However, with technologies like CFL and LED, a bulb can produce a lot of light with very little power. A 15W LED bulb can produce the same brightness (approx. 1600 lumens) as a 100W incandescent bulb. Therefore, when shopping for a light source, you should compare their lumen output for brightness, not their wattage.
