Question

Lux is the unit of
A. Luminous flux
B. Illuminance
C. Density of illumination
D. Luminous efficiency

Answer 1

Hint: Luminous efficacy is a measure of how well a light source can produce visible light. There are various terms to define the intensity and efficiency of luminosity. We will study about all those terms and their measuring units to determine the correct quantity associated with the unit Lux.

Complete step by step answer:
Luminous flux is defined as a measure of the total amount of visible light emitted by a lamp. Luminous flux is basically the amount of light that the human eye senses. It essentially reflects the sensitivity of the human eye by doing weight measurement of each wavelength with a luminosity function. So, it is the weighted sum of all the wavelengths of power in the visible light band, excluding infrared and ultraviolet bands.
The SI unit of the luminous flux is the lumen $\left( lm \right)$. The lumen is defined in relation to the candela which is the unit of luminous intensity as:
$1\text{ }lm=1\text{ }cd\cdot sr$
In the photometry, Illuminance is defined as the total luminous flux of light incident per unit area.
The SI unit of the Illuminance is the Lux $\left( lx \right)$. 1 Lux is equal to one lumen per square meter.
$1\text{ }lx=1\text{ }lm\cdot {{m}^{-2}}=1\text{ }cd\cdot sr\cdot {{m}^{-2}}$
Luminous flux density or the density of illumination is defined as the photometrical weighted radiant flux density, which specifies the luminous flux per unit area at a point on a surface where the surface can either be real or imaginary.
Luminous efficiency is defined as the ratio of the total luminous flux radiated by the source to the total radiant flux from that source.
Luminous efficiency is commonly expressed in lumens per watt.
Lux is the SI unit of Illuminance or luminous intensity.

Hence, the correct option is B.

Note:
Luminous flux is different from the radiant flux. Radiation flux is the measurement of all the electromagnetic radiation emitted, including infrared, ultraviolet and visible bands, which is the total amount of objective light being emitted by the source. Luminous flux is the total amount of light that the human eye senses.
Luminous flux basically represents the total amount of light emitted by the source, while Illuminance is the total amount of light received by an object.
The relationship between the Illuminance and the luminous flux is somewhat similar to the relation between Irradiance and the radiation flux, that is, the radiation flux received by an object per unit area. However, the Illuminance is weighted as per the sensitivity of the human eye to light bands of different wavelengths, which represents the light intensity perceived by the human eye.