LIGHT & MOTION
Most of you have been introduced to lumens if you have purchased an LED flashlight or replacement light bulb for your home. We grew up on watts and seemingly overnight watts disappeared and lumens took their place. So what is a lumen?
A lumen is a measure of light visible to the human eye. There are plenty of wavelengths of light the human eye can’t see so lumens simply measure light we can see. But lumens are much harder to measure than watts. This has led to confusion in the market with lots of outrageous claims. Anyone could measure watts with a $20 meter. Measuring lumens requires a fancy integrating sphere. The image above shows Light & Motion’s sphere testing a Sola light.
Brands looking to get an edge claim high lumen numbers. Perhaps based on theoretical efficiency, they often overstate the lumens by 30% to 50% or more. Without a standard for measuring and reporting lumens the consumer is given false claims.
Frustrated flashlight manufactures, tired of the lumen wars, got together to hash out standards for advertising delivered lumens. The result is the FL-1 standard. The purpose is to allow consumers to make apples-to-apples comparisons of lights based on a standardized test method. Brands that agree to the standard can use the FL-1 icons indicating that the brand is following a rigorous reporting standard. Once the CE or FL-1 symbol is employed on the product, the manufacturer is legally bound to follow the standard.
Like any standard, FL-1 has limitations; it only takes a single lumen reading 30 seconds after the light is turned on. Many lights will dim during use. A light rated 1000 lumens by the FL-1 standard may start at 1000 lumens and drop to half that amount by the middle of the run time. As lumen output is critical across the entire run time for divers, Light & Motion publishes test data in the form of a lumen to time plot.
The FL-1 Standard includes lumens, run time, drop test and ingress protection (IP) testing. The Sola Tech 600 delivers 600 lumens for 3 hours and 20 minutes on high, and is depth tested to 100meters as shown in the icons. These icons are published on the side of the box. The drop test requires 6 drops on concrete from the height published in the icon. Light & Motion selected 1 meter as a reasonably height for all their products.
After completing the drop testing, the light then undergoes the IP testing. Light & Motion’s depth ratings on dive lights range from 100 to 120 meters. After 6 drops on concrete the light is placed in a tank and pressurize to the rated depth for 30 minutes. The Standard requires 100% pass on a set of 3 random samples from stock. Below is a picture of the pressure tank being loaded with production lights that will be pressurized to the FL-1 depth, checked for sealing, and then packaged and shipped to customers. Notice the tank’s heavy wall design required to withstand the pressure seen at 120 meters.
What more should I consider when buying a dive light?
Reliability: Many lights are highly susceptible to flooding because they require opening after every dive to charge or replace batteries. Constant opening and closing of the light wears O-ring seals and invites sand and salt water to contaminate the seal and cause a leak at depth. Proper care of your light, removing the O-rings, carefully cleaning them, and applying silicone grease can reduce this risk. But you can avoid the hassle by purchasing a light that is factory sealed and charges externally.
Beam Angle: Beam angle is an important and widely misreported measure for lights. If you are doing video or photography you typically want as wide of a beam angle as possible while technical divers prefer a narrow beam delivering penetrating punch. We have not measured a light wider than 90 degrees in water without a dome port. Many brands claim far wider beams, but these claims are false and are usually measured in air. A light in water measures about 30% narrower than in air due to the refractive index of water. The standard for beam angle measure is “Full Width Half Maximum” or FWHM. This measures the beam angle from the brightest point or center, to the angle off center when the beam power drops to 50% or half the power of the beam at the center.
Summary: Light power in lumens is a basic measure to consider. Beware of manufacture claims that are not certified to the FL-1 standard. Beyond lumens, a quality dive light is sealed so you don’t need to fuss with O-rings and designed to regulate the power to maintain constant output across the entire run time. This matters for photography and videography and is critical in environments like caves and wrecks where you depend on your light for safety. You don’t want your light fading just when you are getting to the darkest point of your dive.
Be an informed buyer and ask for lights that meet the FL-1 standard. Be careful with beam angle claims. Few manufacturers are reporting accurate beam angles. If the price and the lumens look too good to be true, do not be fooled.