There are really two different functions of the IES Roadway classifications.
The first is to serve as a tool to help select luminaires to try in your layout. This is seen in the below
excerpt from the document found at : http://www.saskpower.com/wpcontent/uploads/residential_streetlight_engineering_practices.pdf
The second, way that Roadway Classifications are used, is for more precise typification based on the IES
photometric file. The precise way that this is calculated is to use the maximum candlepower, and map it
onto a grid which will tell you whether it should type out to a short, medium or wide distribution. This
can be seen on the graph below. The cutoff distances are 1x mounting height, 2.25 x mounting height,
or 3.75 x mounting height. Over 6 x mounting height is considered very long, but I would call it very
glary.
The next metric is measured in a similar manner, and is determined not by the maximum candlepower,
but by a trace of the 50% footcandle line. In other words, if the maximum is 5,000 candlepower, then
you would find out all of the locations that have exactly 2,500 lumens and you create a shape and map it
onto the same type of grid. See the example below:
.
The Roadway Type, is then determined by the furthest extent of that 50% candlepower curve. With HID
where optics are not infinitesimally aimable, this classification system does a really good job of
approximating the real distribution of the luminaire. This is why it has been used for such a long
time. However now with the aimability and uniformity of the optical distributions of LED luminaires, it
is very easy to provide a nonsensical Roadway Classification system, that really has no bearing on the
actual distribution of the luminaire. For example there could be a maximum candlepower in a single
spot very close to nadir (directly under the pole) but have five different 50% candlepower curves. You
would take the furthest, even though it might have almost no bearing on the amount of light in that
area due to fact that light falls off at the square of the distance.
On the other hand, I could put a very high angle max candela making it a very wide but only a spot of it
with all the rest of the light falling directly underneath the luminaire. These are just extreme examples
but show that the historical Roadway Classification system falls apart when it comes to LED lighting. For
this reason, we have decided to lean towards the first example, where we are trying to relay the “spirit”
of the distribution so that the designers can easily pick a distribution that matches what they are trying
to do. EVEN THOUGH IT MIGHT NOT NECESSARILY MATCH THE EXACT CLASSIFICATION THAT IS
COMPUTED.
For this reason we will end up with catalog nomenclature which is intended to help the designer pick the
appropriate product, which is different than the calculated information which is what we see in the two
examples you attached. This is nothing to be alarmed about.
It is not helpful if we have three optics that are all called Type III Medium because nobody can tell them
apart even though they might all classify as Type III Medium optics. If one is very narrow and barely
makes it to Type III, we might call that one a Type II Medium. We might call the one that better mimics
the HID Type IV as a Type IV even though it might actually be a Type III Medium.
The point is that we are trying to help the designer pick the right product. The fact that our
nomenclature does not precisely match what the photometry types out as is really not a big concern and
should be easily set aside. If someone wants the precise Types, they have it available on the .ies file and
photometric report.