Look at the specifications on a 400W metal halide lamp. While specifications may vary from bulb to bulb, a review of a popular website that offers Metal Halide lamps reveals that a brand new Metal Halide bulbhas the following specifications: Initial Lumens between 32,000 to 36,000 and Life Hours between 15,000 and 20,000 hours. A recent report by Dark Sky Society revealed the mean lumens for a 400W Metal Halide bulb to be about 20,500 lumens and the rated life to be about 15,000 hours. But for the purpose of this topic, we will stick with the numbers we presented.
Let’s look at this information. While initial lumens seems high, in reality a Metal Halide lamp starts off bright and then drops lumens quickly, settling at a lower lumen output quickly. It is not uncommon to show a lumen loss of 25% more in the first six months of operation. So while 36,000 lumens sounds like a lot of lumens, within six months it can be well below 30,000 lumens. Lumen depreciation in a metal halide bulb is fast. It is noted that at 1/2 life of the bulb (8,000-10,000 hours) lumen depreciation is already at 50% or more.
Comparatively, let’s look at the operating nature of a Metal Halide bulb. It is omni-directional. That means light is distributed in every direction equally. So it produces an equal amount of light upwards into the fixture as it does facing downwards. To make this light usable, you need to collect it, reflect it, and deliver it to where you need the light. In designing light fixtures, the use of reflectors are used with omni-directional bulbs to collect all this light and then deliver it where it is needed. The main problem with the process of using reflectors is how effective is the reflector is in gathering the light that bounces off of it and placing it where it is needed. A lumen that bounces off the wall of a reflector and doesn’t get delivered out of the fixture is considered a loss lumen. Anything that bounces more than one bounce is considered throw-away. As much as thirty percent or greater of light can be lost in this reflective bounce phenomenon. So using these numbers, and using a metal halide bulb in a high bay fixture as an example, assuming initial lumens of the Metal Halide lamp at 36,000 lumens, after six months we would expect the fixture real lumens to be:
36000 lumens – 20% (initial lumen loss) = 28800 lumens
Lumens lost from reflector bounce process: 8640 (or greater)
Total real lumens after six months: 20160
This does not take into account any shields, filters or covers the fixture has. Anything that is added between the bulb and where the light is needed takes away from the total amount of available lumens.
Let’s compare this to a LED light source. If you think of the same type of fixture, like a LED High bay, or using a LED Retrofit Kit to replace the Metal Halide bulb, the light is directional and 100% of it is directed to where it is required. You do not need to use a reflector. There is no light to reflect. When you think about lumen depreciation in a LED lamp, it happens significantly slower than the Metal Halide bulb it is compared to. For example, a good LED retrofit kit will have a L70 lumen depreciation of over 100,000 hours. A Metal Halide loses a significant amount of lumens in the first six months of life, LED tends to maintain its lumens much longer.
Think of the life span of a Metal Halide bulb. It’s advertised between 15,000 and 20,000 total hours. A LED bulb should last 2-5X longer. This means that you will be spending a lot of time and money replacing Metal Halide bulbs instead of savoring the savings of using a LED Light source.
To learn more about how LED compares to Metal Halide, click here.