Viewing distance of escape route signs — the Z-factor (100/200) of NEN-EN 1838

The guide on §560 — emergency lighting covers the minimum illuminance of the escape route itself (1 lux across the full width) and the transition from the withdrawn NEN 6088 sign to NEN-EN-ISO 7010. This article covers a separate, often overlooked requirement from the same standard: how far away an escape route sign itself must still be recognizable, and how that distance is calculated.

The formula: d = h × Z

NEN-EN 1838 gives the following relationship for the maximum viewing distance of an illuminated escape route sign (a "safety sign", for example the NEN-EN-ISO 7010 running-figure pictogram toward a door):

d = h × Z

d is the maximum distance (in metres) at which the sign must still be clearly recognizable.
h is the height of the pictogram itself (in metres) — that is, the height of the depicted symbol, not the height of the entire sign including its frame or any additional text.
Z is a factor that depends on the sign's illumination type.

The Z-factor: externally versus internally illuminated

Z = 200 for an externally illuminated (front-lit) sign: the pictogram is lit from outside by a separate light source, for example a luminaire aimed at the sign.
Z = 100 for an internally illuminated (back-lit) sign: the pictogram is itself lit from within, for example a light box with a luminous pictogram.

At the same pictogram height, an internally illuminated sign therefore has half the maximum viewing distance of an externally illuminated sign — the standard assumes that back-lighting typically produces a lower contrast/luminance ratio than targeted external illumination, and compensates for that with the lower factor.

Why this is a separate requirement, distinct from the 1 lux illuminance

The general illuminance requirement in §560 (minimum 1 lux across the full width of the escape route, see the §560 guide) describes how well the floor and route itself must be lit so that someone can walk safely. The Z-factor formula addresses an entirely different question: how far away someone can still recognize the sign to determine the correct escape direction. An escape route can meet the 1 lux requirement while its signs are too small for the route's walking distance, or vice versa. Both requirements must be checked separately.

Practical worked example

For an escape route sign with a symbol height of h = 0.15 m (15 cm, a common size for a sign above a door):

Externally illuminated (Z = 200): d = 0.15 × 200 = 30 m maximum viewing distance.
Internally illuminated (Z = 100): d = 0.15 × 100 = 15 m maximum viewing distance.

In a long, straight corridor of, say, 40 metres with no change of direction, a single internally illuminated sign with a 15 cm symbol height at the start of the corridor is therefore insufficient: halfway down the corridor (20 m) the walking route already falls outside the 15 m viewing distance. Either a larger pictogram must be used, or additional intermediate signs must be placed so that the distance to the next visible sign stays within the calculated limit.

Practical relevance

When designing or assessing escape route signage in a long corridor, a large warehouse, or a building with multiple changes of direction, the sign size (and its internal or external illumination type) must be matched to the actual walking distance between two successive viewing points — not automatically the smallest, cheapest standard size. During a NEN 3140 or building inspection, it is worth checking whether the applied sign size and illumination type actually correspond to the walking distance of the relevant escape route.

Common mistakes

Assuming the §560 1 lux illuminance requirement also governs the visibility of the signs themselves — these are two separate requirements within the same standard.
Using the same sign size regardless of the escape route's walking distance — a long, straight corridor requires a larger sign or additional intermediate signs compared with a short corridor.
Mixing externally and internally illuminated signs when calculating viewing distance — the Z-factor differs by a factor of two (200 versus 100) between the two types.
Entering the height of the entire sign (including frame or text line) as "h" instead of only the height of the pictogram symbol itself, which leads to an overly optimistic viewing distance.