A common lighting technology in the commercial space is metal halide, at least in the past it was. At an ever-increasing rate, businesses are actively searching for a more efficient lighting system that will allow them to cut energy consumption and in turn; reduce overhead expenses
When companies or facility managers start their search, they often search for the wrong answer. Here are some common questions that we are asked:
“How many LED watts does it take to replace a 1000 watt metal halide bulb?”
“How can I replace my metal halide light?”
“Is it even possible to replace my metal halide fixture with another technology?”
This two part post specifically will be covering the first question, as for the other two: be sure to subscribe to our email list to know when new posts go live, we are always writing informational content to answer your questions just like these!
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How many LED watts does it take to replace a 1000 watt metal halide?
It’s simply a simple question that requires a complex answer. The reason for this is simple: you can’t base your decision entirely off of watts. Instead, you need to base your decision based on the amount of lumens you need. Once you get the lumens dialed in, the fixture that produces those lumens determines the watts required. For purpose of this discussion we will not differentiate between a new LED Fixture or a LED Retrofit Kit designed to convert your existing fixture over to LED.
What is a lumen?
In simple terms, it’s a unit or measurement of light that basically defines how much light a certain fixture gives off.
Why does this matter? A lot more than you may think, because it’s all that matters when it comes to replacing old lighting systems with new and improved lighting technology.
When replacing an existing fixture (or entire system), you need to know your lumen output per fixture. Let’s say your metal halide fixtures, at least when they were freshly installed, output at 100,000 (initial) lumens per 1000 watt bulb. So, instead of thinking “I need 1000 Watt LED to replace 1000 Watt Metal Halide,” you need to be thinking “I need a LED that can replace 100,000 lumens.” This way, you can forget about how many watts a fixture has and focus solely on the amount of lumens the replacement can output and ensure it matches your lumen output needs.
With this example of 100,000 lumens for a 1000 Watt metal halide, that means it is outputting 100 lumens per watt (initially). In the world of lighting in today’s technology world, this isn’t too bad. However, you need to be aware of some other factors going on here
Understanding Initial Lumens and L70
100,000 lumens may seem bright. It is. However, the bulbs were designed for high initial output because metal halide’s suffer from pretty fast lumen depreciation. It is not uncommon for a metal halide bulb to have lost 50% of it’s lumens at half-life.
L70 is a term for the number of hours before a bulb is performing at 70% of initial lumen output.
Which means it will be at 70% of its lumen output at approximately 5000 hours. So, while the specs proclaim 100,000 lumens, it won’t be 100,000 lumens for long. LED, on the other hand, maintains its lumens extremely well.
You will notice L70 times for LED in terms of 50,000 and 100,000 hours, not 5,000 hours like it is for metal halide bulbs.
Loss of Light due to Reflection
LED light is directional, conventional light sources are omni-directional and require reflectors to gather the light and focus it to where it is needed. Any reflection that is over one bounce effectively loses the effect of the lumen. It has been proven that you can lose up to 30% of the effective lumens in the reflective process.
So, if a metal halide bulb has 100,000 initial lumens, the loss accounted to reflected lumens drops the lumen output to around 70,000 lumens.
Quality of Lumens
This is related to Color Rendering Index, or CRI, and the best explanation is seeing how much better you can see when comparing LED to other light sources. It’s a measure on the quality of light. It is not uncommon to have customers tell us that 20,000 lumens of LED appear brighter than 60,000 lumens of other light sources, like HPS.
The basic truth is you need less quantity when you have higher quality.
Photopic vs Scotopic Lumens
This has to do with how a camera perceives lumens versus how humans perceive lumens.
Photopic lumens are lumens that are detected by a device that is similar to a camera. Scotopic lumens are lumens detected by the human eye.
LED produces light within these spectrums, which means the light perceived by LED is light that we use. It is rare to see a LED light produce IR or UV spectrums. These wavelengths are invisible to people, so they have no value to us from a vision perspective.
The science of scotopic lumens is associated with a factor that allows us to adjust the photopic value to give a true representation of how useful the lumens are. The factor, developed by scientists, is an attempt to level the playing field between different light sources. The factor is used to adjust the effective value of the lumen, either up or down from the advertised photopic lumen listed on the package.
Some lights, like HPS, have a factor that reduces the lumen amount. LED, on the other hand, typically has a factor of 1.7 or greater, which means the lumens it is producing are far more effective to us.
Therefore, light sources with higher scotopic factors need lumens for us to perceive and detect the light.
Read Part 2 of this Post