Unit of Light: Candela, Lumen, Lux, Measurement and Examples

Unit of Light Overview

The unit of light is expressed through various physical units, such as Watts (W), Lumens (lm), Watts per steradian (W/sr), Candela (cd), Watts per square meter (W/m²), and Candela per square meter (cd/m²). Despite light not being a physical quantity, it is still possible to measure it using these units.

1. But light has a number of physical properties that can be measured in terms of lux, lumens, candela, etc., such as wavelength, intensity, and speed.
2. As a result, one can calculate the size of light using its physical characteristics.
3. Light is a type of electromagnetic radiation that is visible to the human eye, according to physics.
4. In addition to radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays, and gamma rays, it is an essential component of the electromagnetic spectrum.
5. Both particle and wave theories' tenets apply to how light operates as it travels through space as both a particle (known as a photon) and a wave.

What is Light?

Light, also known as visible light, is a segment of the electromagnetic spectrum that lies between radio waves and gamma radiation. 

1. Light is a type of electromagnetic radiation that can be seen by human eyes.
2. As waves with a specific frequency and wavelength, light is a type of energy that travels through space.
3. In a vacuum, light can pass through.
4. Light can be seen, but it can also be detected by other means, like the use of cameras, telescopes, and other scientific equipment.
5. It has many different uses, from lighting and communications to medicine and materials science, and it has an important place in the natural world.
6. Magnetic and electric fields oscillate, moving energy from one place to another. This is what is known as electromagnetic radiation.
7. Except that it can be seen with the unaided eye, visible light is not all that different from other electromagnetic spectrum components.
8. The electromagnetic spectrum is made up of a stream of photons, which are unweighted, wavelike particles moving at the speed of light.
9. The smallest unit of energy that can be moved is a photon.
10. Between the infrared waves, which have longer wavelengths, and the UV rays, which have shorter wavelengths, visible light has wavelengths that range from 400-700 nm (nanometres), or 4.00 x 10-7m to 7.00 x 10-7 m.
11. A 400-700 nm wavelength corresponds to a frequency range of approximately 430-750 terahertz. (THz).

Lights Unit Table

Unit of Light Properties

The unit of light is typically measured using the unit of radiant flux, which is the amount of energy emitted, reflected, transmitted, or received per unit time. The standard unit for radiant flux is the watt (W), which represents one joule per second (J/s).

Some properties of light that are related to its measurement in units of radiant flux include:

1. Intensity: The intensity of light refers to the amount of radiant flux per unit area. It is measured in units of watts per square meter (W/m²). The intensity of light decreases with distance from the source, according to the inverse square law.
2. Luminous flux: Luminous flux is the measure of the total amount of visible light emitted by a source per unit time. It is measured in lumens (lm), and is determined by the spectral sensitivity of the human eye.
3. Spectral distribution: The spectral distribution of light refers to the relative amount of radiant flux emitted at each wavelength in the visible spectrum. It is typically measured using a spectrometer and can be used to determine the color of light.
4. Polarization: Polarization refers to the orientation of the electric field vector of light. It is typically measured using a polarimeter and can be used to determine the degree and angle of polarization.
5. Speed: The speed of light in a vacuum is approximately 299,792,458 meters per second (m/s). The speed of light changes when it passes through a medium with a different refractive index, resulting in phenomena such as refraction and reflection.

Measuring of Light

Two sets of units are used to measure light, and they are as follows:

1. Radiometry:It places a strong emphasis on measuring light power across all wavelengths.
2. Photometry:It is the measurement of light using a wavelength weighted model of human brightness perception. It is helpful in quantifying lighting (illumination) that is meant for human use.

Unit of Light Types

Candela: To measure Luminous intensity 

The candela (cd) is the SI unit of luminous intensity, which quantifies the amount of light that a light source emits in a specific direction. In simpler terms, the candela measures the brightness of light in a specific direction, taking into account the human eye's sensitivity to different colors of light.

1. The SI unit for luminous intensity is the candela.
2. It is the amount of light a point light source emits in a given direction per unit solid angle.
3. In terms of cd/m2, candela is described.
4. Radiant intensity is comparable to luminance.
5. Instead of adding up the contributions of each wavelength of light in the spectrum of the sources, the standard luminosity function weighs the contributions of each wavelength.

The frequency chosen in the visible spectrum corresponds to a wavelength of about 555 nm. If it is configured for bright lighting, the human eye is most sensitive in the vicinity of this particular frequency. The human eye's frequency response, however, indicates that at other frequencies a significantly higher radiant intensity is necessary to achieve the same luminous intensity. The luminous intensity of light with wavelength λ is given as follows:

Iv (λ) = 683.002 lm/W. ӯ (λ). Ie (λ)

Where, 

• Iv (λ) = luminous intensity
• ӯ (λ) = photopic luminosity function
• Ie (λ) = radiant intensity

If there is more than one wavelength, then one must integrate the entire wavelength spectrum to achieve the total luminous intensity.

For Example:

• In general, a candle emits light with a luminosity of about 1cd.
• If the 1700 lumens produced by a 25 W fluorescent light bulb were evenly distributed, it would have the following luminous intensity:
• Iv = 1700 lm/4 sr = 135 lm/sr, which equals 135 cd.
• The same light bulb would have an intensity of about 18,000 cd if focused on a 20°0 beam.

Lumen: To measure Luminous Flux

The lumen is the SI unit that measures the amount of visible light emitted from a source, also known as luminous flux. Luminous flux is distinct from radiant flux (power) because radiant flux accounts for all electromagnetic waves emitted by a source, while luminous flux is weighted by the sensitivity of the human eye to various wavelengths of visible light.

The lumen, in relation to the candela, is defined as:

1 lm = 1 cd x sr

Where, 

• Lm → Lumen
• cd → Candela
• sr → Steradian 

A complete sphere has an angle of 4π steradians, a light source that radiates one candela in all directions holds a total luminous flux of 1 cd x 4 π sr = 4 π cd x sr = 12.57 lumens

For Example:

• A 5000-lumen projector is capable of displaying a bright and clear image on a large screen or in a well-lit room, making it ideal for use in presentations, classrooms, and home theatres.
• A 1000 lumen bike light provides a bright and focused beam of light for cyclists riding in low light conditions or in the dark. This helps improve visibility, making it safer to ride on the road or off-road.

Lux: To measure Illuminance

The lux is the SI unit of illuminance and luminous emittance, which quantifies the amount of luminous flux per unit area. One lux is equivalent to one lumen of light per square meter of surface area.

1. The lux is a unit of measurement for the amount of light that strikes or passes through a surface in photometry.
2. A given amount of light will illuminate a surface more dimly when it is dispersed over a relatively larger area.
3. Therefore, if the luminous flux is maintained constant, we can state that the illuminance is inversely proportional to the area.
4. One lux is equal to one lumen per square meter, and the relationship between the two is given as:1 lx = 1 lm/m2 = 1 cd x sr/m2.

For Example:

1. A well-lit office environment typically requires an illuminance level of around 500 lux, which provides adequate lighting for tasks such as reading, writing, and working on a computer.
2. A surgical room requires an illuminance level of around 2000 lux, as it requires a high level of precision and detail. This ensures that medical professionals have adequate lighting for procedures that require a high level of accuracy, such as surgeries or other medical procedures.

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Unit of Light: Electromagnetic Energy

1. Light is a form of radiation that travels through space in the form of electromagnetic waves. Animals such as humans are able to perceive this radiation through their eyes, enabling them to see and understand the world around them. Although it may seem special, light is just one type of electromagnetic energy, similar to microwaves and radio waves.
2. Light travels in straight line rays and reflects and refracts in precise ways as it moves through the world. The light we see with our eyes is often weak because it has already been refracted by objects in its path. However, light beams produced by lasers can be super concentrated and powerful enough to slice through metals.
3. Light can also be thought of as consisting of particles or packets of energy called photons. Visible light is only a small part of the entire electromagnetic spectrum. White light from the sun contains all the visible wavelengths of radiation, which can be separated using a prism.
4. Different parts of the electromagnetic spectrum are produced in different ways. For example, visible light and infrared radiation can be generated by vibrating particles of warm or hot objects. This phenomenon is known as incandescent light. Light can also be produced by fluorescence, a process in which electrons gain and lose energy within atoms.

Read more about EMF Equation of DC Generator.

Unit of Light: Things to Remember

1. Light can be measured in a variety of units based on the quantity of light being measured and the context in which it is measured.
2. Humans sense light in the visible spectrum, which ranges in wavelength from 400 nanometres to 700 nanometres.
3. Light moves at a pace of 3 x 108 meters per second. The fact that photons, the quantum particles that make up light, have a mass of zero is the only explanation for why light travels at the pace it does.
4. Light does not have a standard term of measurement. But one can quantify its physical characteristics.
5. The SI unit for luminous intensity is the candela. The unit of measurement is in cd/m2.
6. The SI unit for luminous flux is the lumen. It's given by disc x sr.
7. The SI unit for illuminance and luminous emittance is called lux. It is equivalent to one lumen per square meter and measures flux per unit area.

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Unit of Light: Previous Year Questions

1. What is the SI unit of luminous intensity and how is it defined? (GRE Physics, November 2020)
2. A light bulb emits 1200 lumens of light uniformly in all directions. What is the illuminance on a surface 2 meters away from the bulb? (SAT Physics, May 2021)
3. Define the terms luminous flux and illuminance and explain how they are related to each other. (AP Physics 2, May 2019)
4. A laser beam emits 5000 candelas of light in a narrow beam. What is the luminous flux of the beam if it has a solid angle of 0.05 steradians? (MCAT Physics and Math, January 2022)
5. What is the difference between a light source that emits white light and one that emits colored light? (IB Physics, November 2020)