Lux is the unit of illuminance — the amount of light falling on a surface per unit area. Measuring it correctly tells you whether a workspace is properly lit for the task, whether a lighting installation meets legal requirements, and whether LED retrofits are delivering the right output. This guide covers the physics, correct measurement technique, and reference values for every common environment.
What Is Lux? The Physics Explained
Figure — Illuminance measured at the work surface, sensor parallel to the plane
Lux Requirements by Environment
| Environment / Task | Recommended Lux | Standard |
|---|---|---|
| Emergency staircase / walkway | 10–50 lx | IS 3646 |
| General storage / warehouse | 100–200 lx | IS 3646 |
| Office — general work | 300–500 lx | IS 3646, NBC 2016 |
| Reading, computer work | 500 lx | IS 3646 |
| Drawing office / detailed work | 750 lx | IS 3646 |
| Operating theatre, precision assembly | 1000+ lx | NBC 2016 |
| Outdoor — car parks | 20–50 lx | IS 1944 |
| Outdoor — sports facilities (broadcast) | 1400+ lx | CIE 169 |
How to Measure Lux Correctly
Sensor position is everything
A lux meter only measures light falling on the sensor. If the sensor is angled, shaded by your hand, or at the wrong height, the reading is wrong. Standard task-plane measurements are taken at 0.85 m (desk height) facing upward.
Allow the meter to adapt
Bring the meter from bright to dark (or vice versa) and wait 5 minutes before measuring. Photosensors have a settling time, especially moving from high to low light.
Set the correct measurement range
Auto-ranging meters handle this automatically. On manual meters, start at the highest range and step down until you are in the upper half of the range for best accuracy.
Position the sensor at the correct height
Task plane (desk, workbench): 0.85 m above floor. Walking areas: 0.0 m (floor level). Always note the measurement height in your records.
Hold sensor parallel to the measured surface
Horizontal surfaces: sensor faces up. Vertical surfaces (display screens, walls): sensor faces the light source. Do not tilt — cosine error is significant.
Keep your body shadow off the sensor
Extend the meter at arm's length or use a tripod mount. Your body blocks light and can reduce the reading by 20–40% in small spaces.
Take multiple readings and average
Take readings at a grid of points across the area (minimum 4 points for a small room, 9+ for a large space). Average them for the maintained illuminance (Em) value.
Key Features to Look for in a Lux Meter
CIE supplies Vartech lux meters for building services, electrical contractors, and lighting designers. Contact us to discuss your measurement range and accuracy requirements.