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How to calculate spread and throw from isocandela diagrams

Photometric isocandle diagrams
show what you can think of as a "sphere of light" with isocandles plotted on its surface. They
are contour maps of the intensity of the luminaire.
There are many projections of the sphere, you must use the equal-area (also
called
equivalent, equiareal
or authalic)
projection to calculate throw and spread, which are, by the way, purely "graphical" calculations.

**To calculate spread:**
Draw a line from the C=0° G=90°
point and make it tangent to the 90% isocandela contour. In the above example this happens at about 17° "East" and 62° "South"
The angle at
which it hits the edge of the "sphere" is the spread. It is
the red diagonal line in the image above, so the spread is 27.2°
**To calculate throw:**
The *throw* is calculated
by finding the longitude of the maximum candle intensity,
then drawing a "vertical" (longitudinal line) from the intersection of the
90% contour with the longitude at maximim intensity. Here's the above image zoomed:

In the image
above you can see that the maximum intensity is at C=10° ("longitude"=10°),
and you can see the short red line inside the innermost 90% contour.
Now, once you have that line the throw is halfway along it, and in the first image on this pageis the long curved red line touching the edge of the sphere
at Gamma = 62.3°
To be honest I don't find spread
and throw very intuitive. Spread is considered to be how far a luminaire
gets its light across a road. While throw is supposed to give you an idea
about how far a light shines along a road. I find the newer Type 1 etc. classification much easier to understand.
Very occasionally another problem with this classification method is that the isocandle contours don't always follow the neat "all roughly elliptical and centered on a single maximum" idea. Which can lead to ambiguous calculations and/or results.

Here's another example for you to test yourself on:
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