Which of the following units is used for the measurement of Luminous Intensity?

Candela

The correct answer is option 4. Candela.

The candela (symbol: cd) is the SI unit used to measure luminous intensity, which is a measure of the amount of visible light emitted in a particular direction by a light source. 

The candela is defined as the luminous intensity in a given direction of a light source emitting monochromatic radiation of frequency \(540 × 10^{12}\) hertz and radiant intensity in that direction of \(1/683\) watt per steradian.

Luminous intensity refers to the brightness of a light source as perceived by the human eye. It takes into account the spectral sensitivity of the human eye, which varies with wavelength. Therefore, luminous intensity measurements are weighted to account for the eye's sensitivity, particularly in the green-yellow region of the spectrum.

Luminous intensity is directional, meaning it varies depending on the direction in which light is emitted from the source. For example, a flashlight may have a higher luminous intensity in the direction it is pointed compared to other directions.

The candela is commonly used in various applications related to lighting, including the specification of light sources such as incandescent bulbs, fluorescent lamps, LED lights, and other lighting fixtures. It helps designers, engineers, and manufacturers ensure that the lighting they produce meets specific brightness requirements for various purposes, such as indoor and outdoor lighting, automotive lighting, displays, and more.

The candela is one of the seven base SI units and is defined based on physical constants. This ensures uniformity and consistency in the measurement of luminous intensity worldwide.

In summary, the candela is the unit used to measure luminous intensity, providing a standardized and objective way to quantify the brightness of light sources, taking into account the sensitivity of the human eye to different wavelengths of light.