So how many lumens of LED do you need to convert from High Pressure Sodium to LED? Do you just replace lumen for lumen? Well, we are here to say NO! If you do that, you will increase the light levels in the area by 5X.
Here's the math.
So when you read the foot candles of a light, you are measuring foot candles with a light meter and that is called a photopic measurement. This has been the standard since light meters were invented. Well, slight problem. First, light meters pick up everything. Including those spectrum's like UV and IR that we can't see. But it collects them and adds them into the total light being measured.
Problem is that is not how we perceive light.
Humans are not light meters, we see light in a different way. And so there are times when we demo a LED light up against a High Pressure Sodium Light and ask "Which is brighter"? and they say the LED. And then we bring out the light meter and it says the HPS is brighter. And now we have a confused customer.
And that us when we explain Scotopic lumens, or the lumens we perceive as people. And it is different than what a light meter reads. And what are we trying to solve. Lighting for people or lighting for meters?
So its good that the smart lighting scientists thought of this and came up with a ratio of how we perceive light by type of lights.
For example, 2100K High Pressure Sodium gets a ratio of .40 and 5000K LED gets a ratio of 1.8. So how does this help?
So lets take a HPS bulb producing 40,000 photopic lumens. Multiply it by the .40 S/P ratio, and the scotopic lumens is 16,000 lumens. Take a LED Light that produces 10,000 lumens, multiply it by 1.8, and it produces 18,000 scotopic lumens.
That's right, we are saying that 5000K 10,000 LED Lumens is brighter than 2100K 40,000 HPS lumens, in terms of how we perceive the light. Put a light meter under each light source and it is going to tell you a different story. Let your eyes judge and you will choose the LED.
For more information on S/P ratios, check out this page for a complete listing of S/P ratios by light source.