A lot of time we are asked what is the equivalent LED product to convert from Metal Halide, High Pressure Sodium or Mercury Vapor. And a lot of time, we have a product in mind that works based on past experience. But how did we come up with that product, and how did we make the original decision on what was the proper replacement?

There’s a lot of science that goes into light, color, quality, and each light had it’s own characteristics. For example, it takes less LED to replace High Pressure Sodium than it takes to replace Metal Halide. And we are talking a lot more than just lumens. We are talking about color temperature, quality of light and something called photopic / scotopic lumens.

In the world of lighting, we measure foot candles with a light meter, this is called a photopic measurement. The light meter picks up everything, including spectrum’s that are invisible to us. Now, we humans see things to, and this is called scotopic lumens. Our rods and cones read and interpret light. And it does so differently than what a light meter does. So it is not uncommon when we demo a LED Light against a HPS light or Metal Halide, we first ask “Which is brighter” before we show the light meter reading. It leaves a few people scratching their heads. Well, that’s the science of of it all, and fortunately there is some handy tables called SP ratios that allow us to take the lumens of a particular light (photopic lumens) and multiply it by the S/P ratio to get the scotopic lumens equivalent.

For example, the S/P ratio for High Pressure Sodium is 0.40. The S/P ratio for LED at 5000K is 1.8. So if you take 40,000 lumens of HPS and multiply it 0.4, the effective scotopic lumens is 16,000. To get the same amount of lumens from LED, take the 16,000 and divide it by the S/P ratio and you get 8888 lumens. So the 8888 lumens of LED is the photopic measurement, the 16,000 LED lumens is the scotopic lumens measurement. So in an nutshell, you need 8,888 lumens of LED to replace 40,000 lumens of HPS from a scotopic point of view.

Here is a handy chart of common lights and ratios for you to use.

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