Out of the Wood BY MIKE WOOD Total lumens—revisited With simple math you can gain some perspective on which luminaires really deliver energy savings. BACK IN THE SUMMER 2007 ISSUE OF Protocol , (available on line at http://www. mikewoodconsulting.com/articles.htm and http://plasa.me/summer2007) I wrote an article about the dangers of judging lights by just looking at the centre beam illuminance, and suggested methods for how you might measure total lumens. I make no apologies for returning to the topic; I’m seeing a lot of very variable quality photometrics at the moment, particularly those published on the spec sheets for LED luminaires. When you are trying to judge if a luminaire is truly effi cient or not, it is even more important to be able to separate the elixir from the snake oil. The hope is the draft PLASA standard, E1-41, Recommendations for Measuring and Reporting Photometric Performance Data for Entertainment Luminaires Utilizing Solid State Light Sources will help, but not all manufacturers will adopt it. effi cient, then you need methods to check that this is indeed the case. Sadly, I can assure you that many of the so-called green products on the market today are actually nothing of the kind. Forget the rhetoric. The truth is in the numbers, but if a manufacturer doesn’t give you the full data, what can you do? There is something you can do if you have at least data about the illuminance at the center of a luminaire’s beam and the diameter of the area illuminated, but let’s back up and review the earlier article about what total lumens are. To start, take a look at Figure 1 . This fi gure illustrates three possible light beam profi les: A is a hypothetical perfectly fl at beam, B is a peaky distribution often seen from ellipsoidal luminaires, and C has a small dip in the middle and straight(ish) sides that is a rarer distribution but is sometimes seen in fresnel units. Each of the three beam shapes has exactly the same center level reading, as indicated by the red dotted line. This means that a light meter would read the same for each fi xture when placed in the center of the beams at points a, b, and c respectively. That center illuminance (measured in footcandles or lux) doesn’t tell us how much light in total is coming out of the unit. To do that, you have to add up the illuminance readings over the whole beam. We can make A BC c ab “ . . . it is even more important to be able to separate the elixir from the snake oil. To some extent this article forms a companion piece to “LEDs: The State of Play,” which was published in the July 2011 issues of Lighting&Sound America , and Lighting&Sound International , sister magazines to Protocol . In that article I urged users to confi rm that the output lumens, and thus the effi cacy, of luminaires are actually what they think they are. If a luminaire claims to be green and energy FALL 2011 “ Figure 1 – three possible light beam profiles 22 FALL 2011