Watts (W) is a unit of power that measures the rate of energy transfer. In lighting, it represents the amount of electrical power consumed by a light source. Lux (lx) is the SI unit of illuminance, which measures the amount of light that falls on a surface area.

The conversion from watts to lux involves two steps:

- Convert watts to lumens: \[\text{Lumens} = \text{Watts} \times \text{Efficacy}\] Where efficacy is measured in lumens per watt (lm/W).
- Convert lumens to lux: \[\text{Lux} = \frac{\text{Lumens}}{\text{Surface Area}}\] Where surface area is measured in square meters (m²).

Combining these steps, we get:

\[\text{Lux} = \frac{\text{Watts} \times \text{Efficacy}}{\text{Surface Area}}\]Where:

- Lux (lx) = Illuminance
- Watts (W) = Power of the light source
- Efficacy (lm/W) = Luminous efficacy of the light source
- Surface Area (m²) = Area illuminated by the light source

- Identify the power of the light source in watts (W).
- Determine the luminous efficacy of the light source in lumens per watt (lm/W).
- Measure or calculate the surface area being illuminated in square meters (m²).
- Multiply the watts by the efficacy to get lumens.
- Divide the lumens by the surface area to get lux.

Let's calculate the illuminance for a 100W LED light with an efficacy of 100 lm/W, illuminating an area of 10 m²:

- Power = 100 W
- Efficacy = 100 lm/W
- Surface Area = 10 m²
- Lumens = 100 W × 100 lm/W = 10,000 lm
- Lux = 10,000 lm ÷ 10 m² = 1,000 lx

This diagram illustrates how a 100W LED light source with an efficacy of 100 lm/W produces an illuminance of 1,000 lux over a surface area of 10 m². The red point represents the light source, and the yellow circle shows the illuminated area.

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