Visible Signaling Appliances the Science of Light
Ralph Coco Potter Electric Signal
In this part of our series, we will be looking at the science of light, but before we begin, a little history on how ADA revolutionized our industry. Back in 1990, The American Disabilities Act (ADA) was formed as a civil rights law that prohibits discrimination against individuals with disabilities in all areas of public life, including jobs, schools, transportation, and all public and private places that are open to the general public. The purpose of the law is to make sure that people with disabilities have the same rights and opportunities as everyone else. Back then, buildings in the United States had no way of notifying the hearing impaired of a fire emergency. ADA came to the rescue developing a strategy to implement strobes into buildings. These changes revolutionized the fire alarm industry as every building had to make provisions of installing strobe lights into their facilities. To accelerate the process, major buildings in the United States were served with lawsuits if they failed to comply. The Sears Tower in Chicago was one, and there were other prominent buildings, too numerous to mention. NFPA 72 modified their standards to accommodate ADA and eventually everyone complied. Canada was slow to follow, but eventually our standards were progressively updated to harmonize with ADA in the United States. This included, light intensity in candelas, sizing the number of strobes per room, and synchronization of the flash rate to protect people with epilepsy.
Measuring Light
There are three major ways we can measure light. Namely, Peak Candlepower, Candela and Foot Candles or Lumens. We will now review all three measurements.
Peak Candlepower measures the peak intensity of the light where the flash duration only lasts a few microseconds. As a detriment, Peak Candlepower doesn’t take into consideration total light energy, but Candela ratings do. Candela takes into consideration the total light area under the light intensity curve and includes all of its energy, but it also has a detriment because it doesn’t include distance. Similar to sound, light is affected by distance.
Therefore, we have an inverse square law for light. When the distance is doubled away from the light source, light will decrease to one quarter of its intensity. This means that if we measure a light source at 10 feet and determine it is 75 Candela, then at 20 feet we should measure 18.75 Candela or one quarter of its intensity. If we double the distance again at 40 feet, we should be at 4.69 Candela. Conversely, light will quadruple its intensity when the distance is halved. This means that a 75 Candela strobe at 5 feet (half the distance) will increase by four times to 300 Candela and so forth. Therefore, you can see that distance is an important factor and foot candles (Lumens) takes this into consideration.
From the three different methods of measuring light, illumination will be the best choice because it includes the intensity of the light source and the distance. The calculation formula for Lumens or Foot Candles is as follows.
As an example, we are going to calculate the illumination of a 75 Candela strobe in a 50 by 50-foot room. The formula directs us to divide the candela rating (75) by the distance squared which in this case is (50 X 50 = 2500 sq. ft) gives us (.03 lumens) or foot candles.
The magic number that ADA came up with was .03 Lumens or foot candles. A fact that most people are unaware of is that ADA used this exact formula to arrive at a minimum foot candles for every room size. In other words, the illumination in any room size cannot be less than .03 Lumens. In Canada we calculate room sizes in metres instead of feet, but the same rule was used. In Canada, we didn’t want to reinvent the wheel, therefore we used a lot of the science that was compiled by NFPA 72 and ADA. The table below shows room sizes in metres and none of them fall below .03 Lumens. Canada used a more conservative approach and thus illumination is well above ADA’s minimum.
Polar Light Distribution
Testing the polar distribution of light will ensure that the strobe is visible from all viewing angles. Below are charts that cover Vertical, Horizontal and Vertical Ceiling dispersion of light. The diagrams show a percentage of the intensity that should be met using a 75 Candela Strobe. The same percentages can be used for any candela rated strobe. Surprisingly, very few companies in our industry measure light in the same way they measure sound.
The addition of strobe lights in buildings created another concern. Flashing strobe lights can cause epileptic seizures. ADAAG (American with Disabilities Act Accessibility Guidelines) Technical Bulletin #2, states that 5 flashes per second or more is the trigger point for photosensitive epileptic seizures. This means if 4 strobes in a room are flashing 2 times per second, the combined flash rate could exceed the 5 flashes per second; potentially causing a seizure. ADA recommends that a composite flash rate of 5 Hz be avoided for multiple strobes. The strobe flash rate must be a minimum of 1 Hz across the listed voltage range. (Increasing or lowering the operating voltage will affect the flash rate). In Canada we followed the same guidelines and stated “more than two strobe lights in corridors or rooms in the same field of view shall flash in synchronization regardless of whether the strobe light is served from one or more control units / transponders.”
The crucial factor here is to eliminate the possibility of an epileptic reaction. Moreover, the same synch module will synchronize the temporal pattern code 3. This ensures a distinct code 3 cadence throughout the building. (Please note that panel and notification manufacturers may offer their own synchronization method. The synch feature can be designed in the panel or offered by an optional synch module)
Share this article!