In the International System of Units (SI), what is the unit of luminous intensity?

Introduction / Context:
Luminous intensity describes how bright a light source appears in a given direction. It is one of the seven base quantities in the International System of Units (SI). Many related quantities, such as luminous flux and illuminance, use derived units. This question asks you to identify the SI base unit specifically associated with luminous intensity and to distinguish it from nearby photometric and radiometric units.

Given Data / Assumptions:
- The quantity is luminous intensity, not luminous flux or illuminance.
- The options are lumen, lux, candela and watt.
- We assume standard SI definitions used in physics and lighting engineering.

Concept / Approach:
Candela is the SI base unit of luminous intensity. It quantifies the luminous power emitted per unit solid angle in a particular direction. Lumen is the SI unit of luminous flux, representing total visible light output, while lux measures illuminance, which is luminous flux per unit area incident on a surface. Watt is a unit of power and applies to all forms of energy, not just visible light. Therefore, the correct answer for luminous intensity is candela.

Step-by-Step Solution:
Step 1: Recall that luminous intensity is a base quantity in SI, and its unit is candela. Step 2: Distinguish between luminous intensity and luminous flux. Luminous flux uses the lumen as its unit, not candela. Step 3: Recognise that lux measures illuminance, which is lumens per square metre on a surface. Step 4: Remember that the watt is a general unit of power, not a photometric unit of perceived brightness. Step 5: Conclude that candela is the correct SI unit for luminous intensity.

Verification / Alternative check:
Standard physics textbooks and lighting standards list the seven SI base units, including metre, kilogram, second, ampere, kelvin, mole and candela. Candela is explicitly associated with luminous intensity. Lighting equipment specifications often mention luminous flux in lumens and luminous intensity in candelas, which confirms that these are different units for different quantities.

Why Other Options Are Wrong:
Lumen: Used for luminous flux, measuring total light output, not intensity in a specific direction.
Lux: Measures illuminance (lumens per square metre) on a surface, not the intrinsic luminous intensity of a source.
Watt: Measures power in joules per second and applies to electric power, thermal power and radiant power, but not specifically to luminous intensity.

Common Pitfalls:
Many learners confuse lumen and candela because both appear in lighting product descriptions. A helpful way to remember is that lumen describes “how much light” overall, while candela describes “how bright in a direction.” Lux tells you “how much light falls on a surface.” Keeping this trio clearly separated in your mind helps answer unit questions correctly.

Final Answer:
The SI unit of luminous intensity is the candela.