Classification of Type I to Type IV is based on the extents of isocandle diagrams traced on the surface beneath the luminaire. In fact it just considers only the 50% of maximum isocandle path.
Look at the diagram below:
Note that I said isocandle and not isolux. So we project the intensities (not the illumination) onto the road surface to make the diagram.
The road and sidewalk together are divided into a grid of 16 rectangles based on the multiples of the mounting height (often abbreviated to MH). A luminaire will have a distribution (Short, Medium, Long or Very Long) and a type (Type I to Type IV).
Here are the steps for finding out these distribution and type.
- Locate the position of the maximum isocandela point on the road. That will tell you in which distribution section you are in (S M L o VL).
- Draw the isocandela contour at 50% of the maximum and see, within the distribution zone how far across the road the contour goes. You must only follow the contour in the distribution section found in Step 1 (in the above example the S zone). The position of this restricted section of the contour tells you the type (in the above example Type 2)
(The display above also shows the LCS and BUG diagrams.)
So the type says how far across the road the light stretches, and the Very Short to Very Long things tell you how far along the road the light stretches.
Here's another example, see if you can follow the reasoning to arrive at...
... Type 2 Very Short (VS).
And another, the answer is...
Here is a 3D view of a photometric solid over a road divided into the MH (mounting heigh) based grids:
You can see the important 50% isocandela contour, as well as the maximum candela intensity point (center of the yellow circle).
And here is a closeup of the photometric solid:
The photometric solid you see above has been exported from OxyTech as a 3D AutoCAD DXF file. Remember that on the road we plot isocandelas, not isolux.
If we go back to the general diagram...
...you can see how the intensities from the photometric solid give us the contour and point used in the IES calculation.