Understanding Illuminance and Lux
Lux (lx) quantifies how much luminous flux falls per square meter of surface. A single lux equals one lumen distributed across one square meter—roughly the light from a candle illuminating a surface one meter away. Unlike watts, which measure energy consumption, lux measures the perceptual brightness of a lit area.
The relationship between power and illuminance is not direct. Two light sources consuming the same electrical power can produce vastly different levels of lux depending on their efficiency. An LED bulb and an incandescent bulb at identical wattages will create different brightness levels on the same surface because they convert electrical energy into visible light at different rates.
Watts and Lux Are Different Quantities
Watts measure electrical energy flow, while lux measures the visible light intensity on a surface. Attempting to convert watts to lux without additional information is meaningless—like converting kilograms to meters. The calculator bridges this gap by incorporating luminous efficacy, which describes how efficiently a light source converts electricity into visible light.
- Higher efficacy: More lumens per watt means brighter illumination for the same power input.
- Lower efficacy: Fewer lumens per watt results in dimmer surfaces despite identical wattage.
- Surface area matters: The same light spread over a larger area produces lower lux values.
The Illuminance Formula
Illuminance depends on three variables: the power supplied to the light source, how efficiently that power converts to visible light, and the area receiving that light. The fundamental equation is straightforward:
Illuminance (lux) = Power (W) × Luminous Efficacy (lm/W) ÷ Surface Area (m²)
E = P × η ÷ A
E— Illuminance in lux (lx)P— Electrical power supplied to the light source in watts (W)η— Luminous efficacy of the light source in lumens per watt (lm/W)A— Surface area being illuminated in square meters (m²)
Luminous Efficacy of Common Light Sources
Different technologies convert electricity into visible light with varying efficiency. The table below shows typical efficacy ranges for common light sources:
- Incandescent bulbs: 12–18 lm/W (least efficient)
- Halogen: 16–30 lm/W
- Compact fluorescent: 45–75 lm/W
- Fluorescent tubes: 40–105 lm/W
- Metal halide: 75–100 lm/W
- LED: 80–120 lm/W (most common modern choice)
- High-pressure sodium: 85–150 lm/W
- Low-pressure sodium: 100–190 lm/W (highest efficacy)
LED technology dominates today because it offers excellent efficacy (80–120 lm/W) at a reasonable cost, making it ideal for both residential and commercial spaces seeking energy efficiency.
Practical Considerations When Using This Calculator
Account for these factors to get realistic illuminance predictions in real-world lighting design.
- Account for fixture losses and reflectance — The calculator assumes ideal conditions where all luminous flux reaches the target surface. In practice, fixtures, diffusers, and reflectors absorb or redirect some light. Reduce your calculated lux by 10–30% depending on fixture quality and room reflectance characteristics.
- Measure actual surface area accurately — A small error in surface area creates a proportional error in lux. If lighting a non-rectangular space, break it into simple shapes (rectangles, circles) and sum them. For spherical geometry, the calculator can derive radius from known area.
- Multiple sources add together — If several light sources illuminate the same area, the total illuminance is the sum of individual contributions. Calculate each source's lux separately, then add them together to find total brightness.
- Standard illuminance recommendations exist — Office spaces typically require 300–500 lux, retail areas 500–1000 lux, and outdoor security lighting 10–50 lux. Use these benchmarks to check whether your calculated illuminance meets the intended application.
Worked Example: LED in a Room
Suppose a 60 W LED bulb with 90 lm/W efficacy illuminates a 20 m² office:
E = 60 × 90 ÷ 20 = 5400 ÷ 20 = 270 lux
This provides adequate general lighting for office work (typical target is 300–500 lux). To reach 500 lux, you would need either more power, a more efficient bulb, or a smaller area. Conversely, if the same bulb lit only 10 m², the illuminance would be 540 lux—brighter than needed—suggesting the room might be over-lit and energy could be saved by reducing wattage or using a dimmer.