Today’s theme is about TM15 and we will, among other things, explain what this classification brings to our industry. Most of you probably already know that the old cutoff classification has not much improved for a few decades now.
The purpose of this cutoff classification was mainly to provide users with basic information concerning the performance of an outdoor fixture. This important knowledge then allows us to evaluate where the light was directed and also the intensity it produced.
The cutoff classification gave photometric users three zones to evaluate which are the following: forward light, back light and uplight. Figure 1
The photometric software gave us the total efficacy for each zone. It also allowed us to know the cutoff classification of an outdoor fixture.
Hence, with the help of photometric software, we were able to get the percentage of light going behind the fixture location, from 0° vertical to 90° vertical. The problem was that having 20 % of light going behind the fixture in question from 0° vertical to 15° vertical (on the sidewalk) is acceptable; but having this percentage going from 60° to 75° vertical (in bedroom windows for instance) becomes a problem that cities want to avoid because they will end up with complaints from residents affected by such a light.
Concerning the forward light, the same issues came up. The following example illustrates well my point. Light at 65° vertical and at 80° horizontal (on the road) is needed, but the same percentage at 65° vertical and at 0° horizontal (front neighbors’ bedroom windows) is, again, not a good thing at all. Figure 2
The only zone that was pretty straight forward was the uplight zone: the percentage of uplight would give us the cutoff classification (none cutoff / semi-cutoff / cutoff / full-cutoff).
Let us also specify here that some software did also give us the percentage of light between 80° and 90° vertical. This data was necessary to evaluate the glare provided by the fixture. It also helped to establish, with the uplight component, the cutoff classification.
So in 2007, IESNA TM15 was revised and gave us more zones within the three main zones explained above.
In the case of the backlight, we can now know precisely where the light is going: on the sidewalk only or in a bedroom window rather than only on the sidewalk. Figure 3
Same thing goes for the forward light. Figure 4
The uplight was broken down into two zones in order to differentiate the light going to the horizon compared to the light going through the atmosphere. Figure 5
This is what is called the BUG rating and it will help you to evaluate different fixtures with the same wattage. Please do keep in mind that these last two words are very important since the BUG rating is not in percentage as is the old cutoff classification. The BUG rating limits are set depending on the lighting zones in lumens. For example, 10 % of back light from a 150w in lumens is different than a 400w lamp. This was done on purpose to limit the amount of light going in a bedroom window. I am sure you do realize that there will be a lot more lumens in windows with a 400w with a 10 % back light than 10 % of a 150w.
In the June 2011 issue of LD+A, an article shows an application with two different propositions to light a parking lot from the middle. We can see that the BUG rating is higher for the 400w than is the 150w proposition. I do agree with the studies, since going to a photometric application evaluation will give you an exact result of the total installation light trespass. So the photometric application is the way to go if you ask me what we should use between the BUG rating and the photometric study.
With all this being explained, do we really need the BUG rating then? The answer is yes. When should we use it? Whenever there is no lighting designer doing a photometric study. A lot of small parking lots will be built without having a lighting designer making sure the proposed solution will be efficient as well as meeting the required standards.
With the BUG rating, it is possible to have the manufacturer publish the BUG rating for each wattage of the fixtures. A city regulation stating what is the maximum BUG rating allowed in the different zoning of the city will also be of great help. This way, residents can be assured that a 1000w shoebox will not get installed two feet from their property line.
This is the purpose of the BUG rating: it is an easy and fast way to evaluate fixtures in order to find out their photometric control and performance.
So when a parking lot owner finds this situation too restrictive, he will imperatively need to get a photometric study in order to find the best lighting solution that will suit his needs at the same time as meeting the city BUG rating limitations.
As far as I’m concerned, BUG rating is a big improvement over the old cutoff classification. It gives us more precise information on where the light is going and how much it represents in lumens instead of in percentage.