Controlling the Scene Jim Stopa

In my article “Debunking Myths About Lumens” (October 2022), I made some bold statements about how the number of lumens doesn’t impact how effective a scene light will be at ground illumination.

In this article, I’ll talk about what you should look for in effective scene lighting. Determining the differences between scene lights and deciding which one is right for your application can be simplified by looking at three factors: Zone Total, Effective Ground Coverage, and Uniformity Ratio.

ZONE TOTAL

Zone Total is an important consideration because it tells the light’s effective usable brightness. To determine Zone Total, you must first identify the ground illumination area, or the square footage on the ground that needs to be illuminated, and then measure the light’s foot candles (fc). The current standards, National Fire Protection Association (NFPA) 1901/1906 and AMD-024, use a 10-foot by 10-foot area, divided up into multiple squares, with the center of each square being the target to measure illumination in fc. There are discussions among some in the industry to expand these standards to include a 20-foot by 20-foot area or even a 40-foot by 40-foot area divided into 2.5-foot squares. The thinking is that these larger areas would more accurately resemble what is typically used around apparatus at an emergency scene (Figure 1). While the Ambulance Manufacturers Division (AMD) test method is an ambulance document not applicable to fire apparatus, I believe it is relevant to this article.

Once you’ve identified the area, the most common way to figure out a light’s fc is to measure it with a Foot Candle/Lux Meter. First, the light is set up outside at night, in a dark area, and the ground area squares are constructed as shown in Figure 1. Then the meter is positioned in the center of each square and records the value in fc. When the fc values of all the squares in the area have been measured, the maximum and minimum values are noted. The current NFPA and AMD standards don’t include maximum fc values for scene lights but do set minimums of 3 fc and 1 fc, respectively.

Adding up the fc values of all the squares in the ground area gives you the light’s Zone Total. The larger the number, the brighter the light. Zone Total is especially important because it only measures the light being used and not the wasted light that is projected into the sky.

EFFECTIVE GROUND COVERAGE

The next factor to consider when choosing the right scene light for your needs is Effective Ground Coverage, which is displayed as a percentage and is included in NFPA and AMD standards. It is calculated by dividing the number of squares that have values above the light’s minimum fc by the total number of squares. It’s interesting to note that the standards do differ on this percentage. You only need light to the rear of the apparatus with a value of 80% to follow the NFPA standard, while AMD requires lights to the sides and rear of the ambulance and 100% of the squares’ values must be above the light’s minimum fc to comply (Figure 2).