What is the standard unit for luminous intensity?
Candela
Understanding Luminous Intensity and Its Unit
Luminous intensity is a fundamental concept in photometry, the science of measuring light as perceived by the human eye. It describes the power emitted by a light source in a particular direction, based on the sensitivity of the human eye to different wavelengths of light.
The question asks for the standard unit used to measure luminous intensity. Let's examine the given options to identify the correct unit.
Analyzing the Options for Luminous Intensity Unit
- Tesla: The tesla ($\text{T}$) is the SI derived unit of magnetic flux density (which is sometimes called the magnetic B-field). It is not related to light measurement.
- Candela: The candela ($\text{cd}$) is the SI base unit of luminous intensity. It measures the luminous power per unit solid angle emitted by a point light source in a particular direction.
- Steradian: The steradian ($\text{sr}$) is the SI unit of solid angle. While solid angle is part of the definition of luminous intensity (luminous power per unit solid angle), the steradian itself is a unit of angle, not luminous intensity.
- Radian: The radian ($\text{rad}$) is the SI unit of plane angle. Like the steradian, it is a unit of angle and not luminous intensity.
Based on the standard definitions of SI units, the candela is the unit specifically defined for luminous intensity.
| Unit | Quantity Measured | Relevance to Question |
|---|---|---|
| Tesla ($\text{T}$) | Magnetic Flux Density | Incorrect |
| Candela ($\text{cd}$) | Luminous Intensity | Correct |
| Steradian ($\text{sr}$) | Solid Angle | Incorrect (part of related definition) |
| Radian ($\text{rad}$) | Plane Angle | Incorrect |
Therefore, the standard unit for luminous intensity is the candela.
Revision Table: Photometric Units
| Quantity | Symbol | SI Unit | SI Unit Symbol | Description |
|---|---|---|---|---|
| Luminous intensity | $I_v$ | Candela | cd | Luminous power per unit solid angle in a given direction. |
| Luminous flux | $\Phi_v$ | Lumen | lm | Total perceived power of light emitted by a source. ($1 \text{ lm} = 1 \text{ cd} \cdot \text{sr}$) |
| Luminance | $L_v$ | Candela per square meter | $\text{cd}/\text{m}^2$ | Luminous intensity per unit area of light traveling in a given direction. |
| Illuminance | $E_v$ | Lux | lx | Total luminous flux incident on a surface per unit area. ($1 \text{ lx} = 1 \text{ lm}/\text{m}^2$) |
Additional Information: The Candela Standard
The definition of the candela is based on a specific physical constant. The 26th General Conference on Weights and Measures (CGPM) in 2018 redefined the SI base units, including the candela. The candela is now defined by fixing the luminous efficacy of monochromatic radiation of frequency $540 \times 10^{12}$ Hz to be exactly 683 lumens per watt.
This definition means that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540 \times 10^{12}$ Hz and that has a radiant intensity in that direction of $(1/683)$ watt per steradian.
The frequency $540 \times 10^{12}$ Hz corresponds to a wavelength of about 555 nanometers, which is close to the peak sensitivity of the average human eye under bright conditions (photopic vision).
Important Questions from Units, Dimensions and Measurements
Which one of the following is the usual unit of measurement for Air Pressure used in India?
Which one of the following physical quantity has the same unit as that of pressure
The SI unit of acceleration is
Light year is a unit for measurement of
A light year is a unit of measurement of
