By Sam Massa
Firefighters often get a bad rap in technical communities as being more like cavemen than scholarly “academic” types.
After all, our group does run into burning buildings when everyone else is running out. What most don’t understand, however, is that firefighters do not typically run into structures blindly without a very calculated assessment of the situation and application of a finely honed set of skills that they have mastered during their training.
Here are a few things a certified firefighter is required to know: reading smoke, understanding pyrolysis and thermochemical decomposition, knowing advanced hydraulic theory and on-the-fly calculation of friction loss, and a variety of topics in the emergency medical field, just to name a few. It would be reasonable to assume the profession is more akin to rocket science than “merely” putting out fires. Many member representatives of the Fire Apparatus Manufacturers’ Association (FAMA) are firefighters and understand the complexities the industry faces.
It’s amazing to me how a certified firefighter will-despite all his knowledge of specialized technical topics-go completely cross-eyed when the topic of lighting a scene is brought up while we are working on a truck spec. I mention “lumens,” “lux,” and “efficacy,” and it’s as though I’m speaking a rare foreign language to my group. Yet, these concepts are critical to understanding scene lighting and being able to make informed decisions during the apparatus specification process.
After five years of attempting to explain these three topics to firefighters, I had a revelation: Lighting theory and the hydraulics behind spraying water from a fire hose are virtually identical.
Lumens
Lumens is the measurement of total comprehensive output from a light source. This measurement refers to light at the source-at the fixture itself-and does not help the user gain any sense of how well the fixture will illuminate the scene. It’s a “total power” type of rating and does not measure how much light is on the target.
The firefighting analogy for lumens would be measuring the flow of water directly at the nozzle without regard to pressure or pattern. A value of lumens will tell you how much light is leaving the fixture but nothing about how it is concentrated or dispersed.
Lux/Foot-Candles
Lux and foot-candles are two means to express the measurement of how much usable light is hitting what the fixture is aimed at. Lux is a metric measurement using units of lumens per square meter; foot-candles is expressed in lumens per square foot. Typically, because this is such a finite measurement, the measurement is coupled with an “area.” For instance, “Light A” will illuminate an area on the ground 30 feet away from the fixture that is 10 feet wide by 10 feet long to 50 lux. A fixture cannot be rated in lux like it can be in lumens because lux is measured on whatever surface the fixture is shining on, whereas lumens is measured at the source.
In firefighting terms, think of a hand-line flowing 250 gallons per minute (gpm) of water. If the appliance is an adjustable fog nozzle, changing the pattern from straight stream to fog does not change the gpm or flow rate, just the spread and direction of the water. Lumens can be equated to the gpm-the optics are the nozzle pattern, but the amount of water per square area hitting any given point is likened to the lux.
In straight-stream mode, you can get either a lot of distance or a high intensity on something very close. When you open the pattern up, the fog pattern disperses that water into tiny droplets that cover a larger area with less intensity in an