Measure illuminance at the actual task surface, not by pointing a meter toward the light source. A lux meter records incident light in lux (lumens/m²); U.S. guidance may use foot-candles (fc). Set the sensor in the plane of the work, take representative points and document the operating conditions before comparing readings with the relevant workplace requirements.
Document the measurement grid
For a desk or workbench, record work-plane height, luminaire operating state, daylight conditions and the sensor orientation. One bright point underneath a luminaire does not describe the entire occupied work area. If the acceptance criterion requires an average or minimum, perform the specified grid rather than selecting a convenient spot reading. For related guidance, see testo 770-1: Electrical Measurement Setup and Safe Use.
Lux versus foot-candles
One foot-candle equals approximately 10.764 lux. Both units describe light incident on a surface, not luminaire power or perceived glare. Some tasks need vertical illuminance—for example on a control panel—while desk tasks normally call for a horizontal work-plane reading.
Practical measurement method
- Identify the actual task, working height and applicable industry/site lighting criteria.
- Warm up lighting where required and note whether daylight is part of normal operations.
- Remove the sensor cap; orient the light-receiving surface in the measurement plane, without shading it with your hand or body.
- Take several points across the task area, including corners or known dark spots, instead of relying on a single central reading.
- Record each position, time, luminaire setting and daylight condition. Repeat at comparable conditions after changes.
Worked example: lux to foot-candles
Hypothetical: a workbench reading of 430 lux corresponds to 430 ÷ 10.764 ≈ 40.0 fc. Whether 430 lux is appropriate depends on the task and the applicable standard; do not treat it as a universal pass threshold.
Interpreting a lighting survey
| Finding | Possible explanation | Useful action |
|---|---|---|
| Center bright, edges dark | Uneven distribution | Map more points, review fixture spacing |
| Large variation by time | Daylight or dimming schedule | Compare equivalent times and settings |
| Adequate lux, complaints persist | Glare, contrast or flicker | Investigate separately; lux alone is insufficient |
Sampling strategy for larger work areas
A single lux reading can hide poor lighting uniformity. Divide the task zone into repeatable positions and report both the individual measurements and the location of the lowest value. An arithmetic mean can summarize a uniform grid, but should not conceal a hazard at one dark location. Keep desk height, sensor orientation and lighting state consistent so the survey can be repeated after maintenance.
Light meters measure illuminance, not flicker, color rendering or luminance contrast. Where employees report visual discomfort even with apparently adequate lux values, a more specialized lighting assessment may be needed. For related guidance, see Testo 480 Thermal Comfort Assessment: Probes, PMV/PPD and Sampling.
Testo instrument for spot checks
The testo 540 is a light meter intended for workplace spot measurements. The official product page specifies a visual-spectrum-weighted sensor and hold/minimum/maximum functions. Its reading is not, by itself, a certified lighting compliance assessment; meter condition and applicable measurement method matter.
OSHA has task-specific illumination provisions, including construction lighting requirements. Do not apply a construction threshold indiscriminately to a warehouse, office or cleanroom.