Lux, uniformity, glare: how to read the values

One lux is one lumen evenly lighting one square metre. For a playing area, a grid of evaluation points is defined. The average illuminance is derived from the values at these points.

What matters is the plane the value refers to. Horizontal illuminance Eh describes the light on the playing surface, vertical illuminance Ev the light on vertical surfaces. Ecam counts in the direction of the camera. The DFB requirement for the 3. Liga, for example, calls for an average of at least 800 lx Ecam, whereas DIN EN 12193 for football in Class II calls for 200 lx horizontal. The two figures describe different planes and cannot be compared directly.

A change between bright and dark zones overtaxes the eyes, because they have to keep readjusting. Uniformity therefore sets two illuminances in relation to each other. The closer the value is to 1, the more even the light distribution.

There are two common ratios:

• Minimum to average value, Emin/Em or U2: the football values under DIN EN 12193 refer to this ratio
• Minimum to maximum value, Emin/Emax or U1: shows the entire range, as captured by a camera for example

An example: with an average of exactly 200 lx and a uniformity Emin/Em of 0.6, no evaluation point may be below 120 lx. UEFA also points out that measured values are often lower than calculated ones and recommends staying above the minimum value in the design.

GR stands for Glare Rating, the method for assessing glare outdoors under CIE 112. It sets the brightness of the sports area in relation to the brightness of the light sources. The scale runs from 10 for no glare to 90 for unbearable glare, so a lower value is better. The standards give maximum values, usually GR 50.

GR can only be calculated for people on the playing area. The value therefore says nothing about light spill and glare for neighbours. Indoors, the UGR method applies. It was developed for offices, and its limits can only be met to a limited extent with high-bay reflector luminaires and floodlights in tall sports halls.

The standard values for average illuminance are maintained values. They must never be undercut, and once they are reached, maintenance is required. So that this is not needed immediately after commissioning, the design sets a maintenance factor that takes ageing, soiling and failures into account. Designers and operators should agree the maintenance factor and maintenance method at the start of planning.

The initial value is the maintained value divided by the maintenance factor. For 200 lx, that is around 286 lx at a factor of 0.7 and around 222 lx at 0.9. licht.de gives 0.7 for outdoors and 0.67 for indoors as a reference when no factor can be determined. UEFA uses 0.90 for LED systems in stadiums and advises against going above that under normal circumstances. In harsh weather or with a lot of dirt in the air, the factor should be lower.

For football, DIN EN 12193 gives, for example, 200 lx, uniformity 0.6 and GR 50 in Class II. For every calculation and every report, check these points:

• Plane and direction: horizontal, vertical or in the camera direction
• Average, minimum and maximum illuminance
• Which ratio is given as uniformity, Emin/Em or Emin/Emax
• Highest GR value of the outdoor system
• Maintenance factor, and whether an initial value or a maintained value is stated
• Sport, lighting class or league the calculation was made for

According to the floodlighting guide from the football associations of Saxony, Saxony-Anhalt and Thuringia, once the system is complete a measurement report follows with average, minimum and maximum values, uniformity, glare and total power, which the operator receives in writing. In the 3. Liga, the current measurement report is part of the proof to the DFB.

More light is not automatically better. Anyone who wants funding under the Kommunalrichtlinie may exceed the value of the lighting class for sports facilities by no more than 30 %.