This blog post is our second post to one of our most popular posts, “How Many Watts does it take to replace 1000W Metal Halide”. While many of the concepts are the same, there are some important differences between 400W and 1000W Metal Halide.One of the most common questions we get asked is:
“How many LED watts does it take to replace a 400 watt metal halide bulb?” “I just talked to another LED company and they said to use their 150W LED Fixture. How much is your 150W LED Fixture?” “I’m looking to convert my fixtures over to LED High Bay Lights and what wattage should I get?”
Replace 400W HID High Bay Fixtures
Replace 400W HID Shoebox Fixtures
Replace 400W HID Flood Lights
Replace 400W HID Pole Top Lights
Probably the best advice we can give anyone is never buy a LED product based on watts, buy it based on lumens and then figure out how little watts you can use to make those lumens.
What is a lumen?
In simple terms, it’s a measurement of light that basically defines how much light a certain fixture or bulb produces. It is essentially the light you are replacing. Fixtures produce lumens, consume watts, and all the fixtures lumens combined provide a foot candle measurement. When we do a photometric calculation, we take the scientific information associated with that fixture or bulb, insert it into photometric software, and calculate the foot candles for the area. In real terms, a light meter will give you a foot candle reading at the place you are standing. Move close to a light source, the foot candles will rise. Move away, it will fall. The foot candle reading in between 2 fixtures is the collective lumen output of the 2 fixtures distributed to that point where you are standing.
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 initially produce 32,000 lumens per 400 watt bulb. So instead of thinking, “I need 400 Watt LED to replace 400 Watt Metal Halide,” you need to be thinking
“I need an LED that can replace 32,000 lumens.”
With this example of 32,000 lumens for a 400 Watt metal halide, that means it is outputting 80 lumens per watt (initially). In the world of lighting in today’s technology world, this isn’t that efficient, with LEDs approaching 200 lumens per watt. However, there are some other factors going on here that you need to be aware.
Understanding Initial lumens and L70
32,000 lumens seems bright. It is. However, bulbs were designed for high initial output because Metal Halide suffers from
pretty fast lumen depreciation. It is not uncommon for a metal halide bulb to lost 50% of it’s lumens at half life. Full life for a metal halide bulb is 15,000 hours. That means at 7500 hours of use, it has lost half (or 16,000) of its lumens.
L70 is a term the number of hours before a bulb is performing at 70% of initial lumen output. For metal halide, that calculates to 5,000 hours of use.
You will notice L70 times for LED is spoken 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, that means all its light is delivered where it is pointed. It does not need a reflector to collect it and deliver it to where its needed. Conventional light sources are omni-directional and require reflectors to gather the light and focus it to where it is
needed. Any reflection of a lumen that is over 1 bounce in the reflector is effectively a loss of that lumen. It has been proven that you can lose up to 30% of the effective lumens in this reflective process.
So if a Metal Halide bulb has 32,000 initial lumens, the loss accounted to reflected lumens drops the lumen output to around 20,000 lumens.
Quality of Lumens – CRI
Color Rendering Index, or CRI, is the best explanation in comparing the quality of light source. Simply said, the higher the quality of light, the less quantity you need. It is not uncommon to have customers tell you 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 – How we perceive light versus how a light meter perceive light.
Photopic lumens are lumens that are detected by a device, like a camera or a light meter. It picks up all sources of light that we cannot see, like UV and IR. Scotopic lumens are lumens detected by the human eye. Its how we as people perceive light.
LED produces light within these spectrum’s, which means the light perceived by LED is light that we use. It is rare to see a LED light produce IR or UV spectrum’s. These wavelengths are invisible to people, so they have no value to us from a vision perspective.
It is not uncommon to stand under 2 light sources, one LED and one not, and first read the fc measurement with the light meter of both light sources. And then ask individuals standing near by what light source they think looks brighter. And have the one that looks brighter contradict what the light meter tells us.
We perceive light sources and brightness different, in some cases, than light meters.
Read Part 2 of this Post