How to calculate the glare index (D0 ... D6) of UNI EN 13201 standard.
Appendix A.2 of the standard explains how to do the calculation, but with no examples and no drawings, so here I'll present it with drawings and examples.
The glare index goes from D0 (high glare) to D6 (very low glare). So D6 is the best result for your luminaire. At D0 you have a very small visible area and/or high luminous intensity, at D6 you have a large area and/or a lower intensity. You can imagine that high intensity from a small luminous area can be unpleasant as you are driving along a dark road.
By the way, if you need an introduction to lighting terms and calculations then have a look at the book Candelas Lumens and Lux:
Back to the UNI EN 13201 standard. All the calculation is done using the maximum intensity in the conical surface formed by the Gamma=85° angle:
You can see the curve (viewed from above the luminaire's photometric solid) here:
I imagine that 85° was chosen because it is the angle at which a distant road luminaire would be visibile.
The next step is to find out the apparent area of the luminous area at the given angle. In the above example the C-Plane is at 20°, and the gamma angle is, of course, 85°.
Luckily, with PhotoView, the program does the calculation of the apparent area using the data from the actual area specified in the photometric file:
The apparent area (from C=20° G=85°) in this case is 0.0494 square meters.
For the 13201 standard glare is calculated like this:
So in our case we have Glare = 349.09/sqrt(0.0494) = 1570.
Now, the final step is to look up 1570 in the A.2 table, Glare Index Classes:
Our 1570 value is greated than 500, and 1000, but is less than 2000, so the index is D4.
Hence this luminaire has a glare index of D4, as shown by the automatic calculation in PhotoView in the Road Classification tab:
Which basically means that our luminaire, from a distance, will not disturb the sight of nightime drivers too much.
Again, if you want a general introduction to glare have a look in the Glare chapter of "Candelas Lumens and Lux":