The SI unit of luminous intensity is the candela (cd), an SI base unit. Photometry deals with the measurement of visible light as perceived by human eyes.

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Luminous intensity is a physical quantity that describes the amount of light emitted or reflected in a particular direction. It is a measure of the power of light, specifically in terms of its perceived brightness. The SI unit of luminous intensity is the candela (cd).

Explanation:
1. Luminous Intensity:
- Luminous intensity refers to the amount of light emitted or reflected in a specific direction.
- It is a measure of the power of light, taking into account the sensitivity of the human eye to different wavelengths of light.
- Luminous intensity is commonly used to describe the brightness of light sources such as light bulbs or LEDs.

2. SI Unit of Luminous Intensity:
- The International System of Units (SI) is an internationally recognized system of measurement.
- The SI unit of luminous intensity is the candela (cd).
- The candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 terahertz (THz) and has a radiant intensity in that direction of 1/683 watt per steradian.
- In simpler terms, 1 candela is equal to the luminous intensity of a light source that emits 1/683 watt of power in a specific direction.
- The candela is named after the Latin word for candle, as it was historically defined in relation to the brightness of a candle flame.

3. Comparison with Other Units:
- The lumen (lm) is another unit related to light measurement, but it quantifies the total amount of light emitted by a source in all directions.
- The lux (lx) is a unit of illuminance, which measures the amount of light falling on a surface.
- The watt (W) is a unit of power, but it is not specific to light and can be used to measure various types of energy.
- While all of these units are related to light, the candela is specifically used to measure the brightness or intensity of light in a specific direction.

In conclusion, the SI unit of luminous intensity is the candela (cd). It is a measure of the power of light in terms of its perceived brightness.