In order to measure illuminance and luminance on the reference surface in question, it is divided up into rectangular, ideally square measuring fields of the same size according to figure whose dimensions depend on the size of the entire reference surface, mounting height, light distribution of the luminaires and their arrangement as well as the desired precision of the evaluation. The aspect ratio of a measuring field must not exceed 2:1. Measurements are executed at the centre of the measuring fields (matrix dots in figure). The grid dimension of the measuring fields must not correspond to the grid dimension of the luminaire arrangement longitudinally and transversely. In such cases, the number of measuring fields must be increased.
The distance P between measuring points and the number of calculation points M depends on the length D (in m) of the longest side of the reference surface and can be calculated according to the following formula in cases where the aspect ratio of the reference field D/B (see figure) is smaller than 2: 1:
In cases where D/B exceeds 2: 1, the formula contains the length of the shorter side of the reference field B instead of D. A graphic representation of the formula is depicted in figure (it is also contained in EN 12193 "Light and lighting – Sports lighting").
The corresponding minimum number M of measuring points towards the longer side of the reference surface is determined by the next higher whole odd number of the ratio M = D/P.
The minimum number of measuring points towards the shorter side of the reference surface results from the next higher whole number N = M · B/D, B representing the length of the shorter side of the reference surface.
EN 12193 specifies particular measuring grids for sports facilities, e.g. tracks. Accordingly, reference surfaces and number of grid points in both directions of the reference surface are specified for various sports.
Example: Measuring field 20 m x 30 m. Image results in a point distance of P= 2,16 and M ≥ D/P = 14 for D = 30 m. The next odd number is 15. N is calculated as N ≥ M · B/D = 15 · 20 m / 30 m = 10. The next odd number is 11. The distances of the measuring points is D/M = 30/15 = 2 m towards the longer side of the reference field and B/N = 20/11= 1,8 m in the other direction.