"Candelas Lumens and Lux" introduction to lighting  terms and calculations
"Candelas Lumens and Lux" introduction to lighting  terms and calculations




The IES classification can be hard to understand if you don't have a clear idea of what is being calculated in three dimensions. Here is a brief explanation, you can find more examples in the book "Candelas Lumens and Lux":

The area under the road luminaire is divided up into a grid, and where the 50% isocandela line, projected onto the road and sidewalk surfaces determine the long / short / very short etc. classification. Below you can see the luminaire placed on the grid:

IES classification, max point and and 50% isocandela

You can see the 50% icocandela projected onto the road. The two red lines help you see how the isocandela is formed. Note we are talking candelas here (intensity), not lux (illumination).

Below is a closeup of the Photometric solid. This has been exported from PhotoView as a 3D dxf and I have added the lines which are used to calculated the IES classification. The two red lines are the isocandle limits for 50% of the maximum intensity. The yellow line line is the maximum intensity positon for the luminaire:

Isocandles and IES

If we go back to the general diagram...

...you can see how these things projected give us the contour and point used in the IES calculation.

The graphical explanation above helps you to understand the textual explanation below:

The lateral classification describes the lateral light distribution with regards to the lighted area width (described as multiples of the mounting height - MH).

The width of the half-maximum candela trace within the longitudinal distribution range (Very Short, Short, Medium or Long, the first three of which are shown in the diagram above) is used.

The boundaries for each classification in terms of Longitudinal Roadway Lines (LRL, running along the roadway) are:

  • Type I - Half-maximum candela trace falls between 1 MH LRL on the House side and 1 MH LRL on the Street side
  • Type II - Half-maximum candela trace on the Street side is beyond the 1 MH LRL but not beyond the 1.75 MH LRL
  • Type III - Half-maximum candela trace on the Street side is beyond the 1.75 MH LRL but not beyond the 2.75 MH LRL
  • Type IV - Half-maximum candela trace on the Street side is beyond the 2.75 MH LRL

Type V is a little special. It has circular symmetry being essentially the same at all lateral angles around the luminaire.

Another way of explaining the types is that you first have to find in which section the maximum candle power point falls (in the Short section in this example), and then where does the 50% isocandle curve go while in that section. In this example we are on the edge, but it looks like a Type I to me.

PhotoView (used to create the DXF photometric solid) is a photometric software program for analysing photometric files as well as creating images and reports, even in batch mode.

For more details about the terms Candelas, Lumens, and Lux have a look at this book:

Book on lighting



(c) 2016
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