Converting to LED - S/P Ratios

A lot of the time, we are asked what is the equivalent LED is to convert from Metal Halide. Typically, we have a product in mind that works based on previous experience.

But how did we come up with that product? And how do we determine what the proper replacement is? There’s a science to understanding light, color, and quality. Each light had its own characteristics.

For example, it takes less LED to replace High-Pressure Sodium lights than it takes to replace Metal Halide. And we are talking about 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 spectrums that are invisible to us. Now, we humans see things too, and this is called scotopic lumens.

Our eye’s rods and cones read and interpret light. And they do so in a different way than a light meter. So, it is not uncommon when we demonstrate an LED Light against an 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 it all, and fortunately, there are some handy tables called S/P 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.

How to Use the S/P Ratio Calculation

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 by 0.4, the effective scotopic lumens are 16,000.

To get the same number of lumens from LED, take 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, and the 16,000 LED lumens is the scotopic lumens measurement. So, in a nutshell, you need 8,888 lumens of LED to replace 40,000 lumens of HPS from a scotopic point of view.

These results apply to all light fixtures, whether it’s an outdoor fixture like a shoebox, wall pack, or flood light. It also applies to indoor light fixtures like high bays, warehouse lights, and linkable LED shop lights.

S/P Ratio Light Source Chart

• 1700K Low Pressure Sodium (LPS): 0.25
• 2100K High Pressure Sodium (HPS): 0.40
• 2700K Incandescent: 1.36
• 3000K Fluorescent (830): 1.29
• 3000K LED: 1.21
• 3000K Quartz Halogen: 1.50
• 3500K Fluorescent (735): 1.24
• 3500K Fluorescent (835): 1.41
• 3500K LED: 1.41
• 4100K Fluorescent (741): 1.54
• 4100K Fluorescent (841): 1.65
• 4100K LED: 1.65
• 4300K Metal Halide: 1.49
• 5000K Fluorescent (850): 1.96
• 5000K LED: 1.80
• 6000K LED: 2.00
• 6500K Fluorescent (865): 2.20
• 6800K Mercury Vapor: 0.80