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.

There was a time when buying light bulbs used to be a much easier task than it is today. Then along came LED, CFL, and light bulbs bent on reducing energy. With this, the good old incandescent started losing their place on our bulb holders. Gone are the days when finding a bulb for your home just meant looking at the wattage on the box. If a bulb was rated 100 watts, you knew it would be 25% brighter than a 75-watt one.

An entirely new rating system is now used and this has been giving shoppers grief for quite a while. At the center of this confusion is the word ‘lumens,’ which appears along its older and more familiar cousin ‘watts.’ Learning about lumens is the price you pay to help keep our planet safe. Don’t worry, though. We will tell you everything you need to know about the new lights rating system.

Lumens are units used to measure how much light a bulb puts out. Watts measure how much electrical power the bulb uses. With incandescent bulbs, the expected brightness pretty much depended on the wattage of the bulb. One lumen is equal to the amount of light given out by a birthday candle.

The idea with energy saver bulbs is to use fewer watts and produce more power. Incandescent bulbs produced by different companies are pretty uniform. On the other hand, energy saver bulbs are so varied, you don’t know if two 50 watt bulbs will be equally bright. This is why CFL and LED bulbs have lumen figures printed on them while incandescent varieties only have wattage.

When you hear “lumen per watt”, think miles per gallon. This is a measure of efficiency. It measures how many lumens of electricity a bulb can produce when using up one watt of electricity. This is the factor that should interest you the most when you are buying light bulbs. The first step is to figure out the brightness you want in lumens. If you want a brighter room, go for a bulb with more lumens. To dim it down, consider a bulb with a smaller lumen rating. Also, find the lumens per watt rating to determine which bulb has the greatest efficiency. Usually, the lumens per watt rating is an average, as bulbs reduce in efficiency with time. The lumen per watt rating is calculated by dividing the brightness in lumens a bulb offers by the number of watts it takes to produce.

Watts can be converted into lumens by multiplying the lumens per watt rating by the brightness.

The Federal Trade Commission now requires a product information summary, called the Lighting Facts label, to be attached to every lighting product. Another label that you should be on the lookout for is the Energy Star mark. The system was created by the Environmental Protection Authority in 1992 to govern the energy consumption of various household and industrial appliances. When it comes to lighting, this mark is put on bulbs that have met a certain lumen per watts threshold. This mark effectively spells impending doom for incandescent bulbs because they do not come anywhere near fulfilling these new efficiency ratings. If you are not into reading the fine prints and making calculations, the Energy Star logo will make your life easier. Look for the label on light bulbs to ensure you are getting a good deal.

The chart below can help to understand the Energy Star system.

This chart is an excellent indicator of how incandescents measure up to energy savers.

Aside from the fact that you have one more bullet in your trivia arsenal, your hunt for light bulbs should now return to the simple task it should be.