In lighting and photometry, the SI unit called lux (lx) is used to measure which of the following quantities?

Introduction / Context:
This question tests your understanding of basic photometric units used in lighting engineering and physics. Modern lighting design uses several related quantities: luminous flux, luminous intensity, illuminance and luminance. Each has its own SI unit and physical meaning. Lux is a common unit seen in lighting specifications for rooms, offices and sports grounds. Knowing exactly what lux measures helps you interpret these specifications correctly and answer general knowledge questions.

Given Data / Assumptions:

• The unit mentioned is lux (symbol lx).
• The options refer to luminous flux, luminous efficiency, intensity of illumination and luminous intensity.
• We assume standard SI definitions used in photometry.
• Lighting levels in rooms are often specified in lux.

Concept / Approach:
Illuminance, or intensity of illumination on a surface, is defined as the luminous flux incident per unit area. Its SI unit is lux, where 1 lux equals 1 lumen per square metre. Luminous flux is the total perceived power of light emitted by a source and is measured in lumen. Luminous intensity describes light power emitted in a particular direction and is measured in candela. Luminous efficiency is a dimensionless ratio relating light output to power input. Therefore, whenever you see lux, you are dealing with illuminance on a surface, not with flux or source intensity alone.

Step-by-Step Solution:
Step 1: Recall that illuminance E is defined as E = luminous flux / area, and describes how bright a surface appears due to incident light. Step 2: Note that the SI unit of luminous flux is lumen, and that of area is square metre. Step 3: Thus, the SI unit of illuminance is lumen per square metre, given a special name lux (lx). Step 4: Recognise that luminous intensity I refers to light power per unit solid angle and has the SI unit candela (cd), not lux. Step 5: Conclude that lux measures intensity of illumination (illuminance) on a surface and select the corresponding option.

Verification / Alternative check:
As a quick check, consider typical lighting recommendations: for reading, a desk surface may require around 300 to 500 lux; for detailed technical drawing, even higher values are suggested. These recommendations refer to how brightly the working surface is illuminated, not to the total light emitted by the bulb or to its luminous intensity in a specific direction. Lighting designers calculate illuminance in lux when planning the number and placement of fixtures in a room, which confirms that lux is the unit of illuminance.

Why Other Options Are Wrong:
Luminous flux is measured in lumen, not in lux; it describes total light output of a source, not the light falling on a surface. Luminous efficiency is a ratio comparing light output to power input and does not use lux as its SI unit. Luminous intensity of a source is measured in candela, which is related but distinct from illuminance measured in lux.

Common Pitfalls:
Students often confuse luminous intensity and illuminance because both involve brightness concepts. A helpful way to separate them is to remember that candela describes the source itself, while lux describes the effect of that source on a surface. Also, the word efficiency may tempt some to choose luminous efficiency, but lux always refers to illuminance. Keeping these distinctions clear will help you avoid mistakes in exam questions related to photometry.

Final Answer:
Lux (lx) is the SI unit of intensity of illumination (illuminance) on a surface.