Frequently Asked Questions

How is beam angle different in air vs water?

Water will narrow the beam angle of a light by about 30%. This is due to the angle of refraction of water vs air and glass. The light is bent as it travels from the LED through the glass and then into the air or water. Many manufacturers report a beam angle but never tell you what medium the beam angle refers to. A 100 degree beam angle in air will measure approximately 70 degrees in water. If you are evaluating underwater lights, make sure the manufacturer is reporting “in water” beam angle. 

What is the round plot you show on your web site under the lumen graphs?

The round plot is called a polar plot. It is a nice and fast way to see the results of the beam angle measurement. The center of the plot is the light source. The light is pointed directly to the right at the “0” point on the circle. Imagine a lux meter located at this “0” point on the outside ring. This is the max power of the beam. The outside ring is the 100% ring - or the max lux value of the light. Since we are not reporting power in this test, the plot just shows the max power as 100%. Now imagine the lux meter staying at the ‘0’ point and the light slowly rotating in place toward the top of the page. As the light is rotated away from the “0” point, the power of the beam drops (measured at the lux meter still sitting at the “0” point). The line representing the light’s power falls toward the center of the plot. When the power crosses the second ring - the 50% ring - we capture the angle which the light has rotated away from the “0” point. This is the 1/2 angle. Add the other side and you get the full angle of the beam in air. If this is plotted for a dive light, we subtract 30% to report the approximate beam angle based on FWHM in water. 

What is a lumen and what happened to watts?

A lumen is simply a measure of light visible to the human eye. We don’t care about light in the spectrum we can’t see so lumens ignores all non-visible light. Watts are a measure of power and have no direct correlation to lumens. A 100 watts conventional light bulb delivers about 1600 lumens. A 20 watt LED source can also deliver 1600 lumens. Watts no longer are a good reference for light as different styles of lights can use vastly different amounts of power. 

What is ANSI and how does it relate to the FL-1 standard?

ANSI stands for American National Standards Institute. The Mission of ANSI is:


“To enhance both the global competitiveness of U.S. business and the U.S. quality of life by promoting and facilitating voluntary consensus standards and conformity assessment systems, and safeguarding their integrity.”

Learn more at

The FL-1 standard is a registered standard with ANSI and is available to purchase by any manufacturer. The FL-1 standard was created by a broad group of US and some international companies to address the gross discrepancies in performance reporting of lights. 

How does wetestlights get its lights for testing?

WeTestLights purchases all the lights that are tested directly from mainstream retail stores. 

How does wetestlights determine what lights to test?

The site gets requests from the market. We screen the requests for recognized brands and we generally limit our testing to rechargeable lights, although in some cases we test lights powered directly from wall power. 

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