The quantity of light falling on a surface, expressed in lux or, in some markets, footcandles. It answers the question a lighting specification usually asks: is there enough light on this plane to do this task. It does not describe how bright a surface looks to an observer, which is luminance and requires a different instrument, and it says nothing about the colour or quality of the light.

Light striking a surface at an angle delivers less illuminance than light arriving perpendicular, in proportion to the cosine of the angle. A correctly designed sensor uses a diffuser that reproduces that behaviour. Without it, light arriving from the side is under-counted, and since real interiors are lit from many directions at once, an uncorrected meter can under-read substantially in exactly the situations where an accurate figure is needed.

Because the meter should respond to light the way the human eye does, following the standard photopic curve. A sensor that departs from it weighs some wavelengths too heavily and others too lightly, so it reads different light sources inconsistently. This has become more important with LED sources, whose spectra differ greatly from the incandescent and fluorescent lamps many older meters were effectively tuned against.

They grade a meter against defined limits for cosine response, spectral match, linearity, fatigue and temperature stability. A higher class is a tighter tolerance on all of those, and it is what separates an instrument suitable for demonstrating compliance from one suitable for a quick site check. Specifications and dispute procedures often name a minimum class, so check what is required before buying rather than comparing headline resolution figures.

On the plane the specification refers to, which for office work is normally the desk height rather than the floor or eye level. Take readings across a grid covering the area and average them rather than relying on one point, since illuminance varies considerably between and beneath luminaires. Keep yourself and the instrument's body from shadowing the sensor, which is a common source of low readings.

With the normal lighting supply isolated, so that only the emergency luminaires contribute, and after allowing the system to reach the condition the standard specifies. Readings are taken along the escape route centre line and at the points the standard identifies, typically at floor level. Because emergency levels are very low, the meter must have adequate resolution and low-light accuracy at that end of its range.

Typically annually, and always before work that may be disputed or must demonstrate compliance. Photometric sensors drift with age and exposure, and the drift is invisible from the readings themselves. Keep the traceable calibration certificate with the instrument, since a figure from an uncalibrated meter carries little weight in a contractual or regulatory context however carefully it was taken.