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6. Quality of Light

When you take a measurement of light with a light meter, it reads lumens. And this measurement of all the lights that contribute to the light at that location creates a measurement called foot candles. But let’s think about that light that is being measured. Metal Halide creates all sorts of light, in all spectrum’s, visible or otherwise. This includes UV and IR spectrum’s, visible to the measuring device but not visible to the human eye. LED, on the other hand, does not produce UV and IR. Therefore, its readings using the same light meter is only picking up visible spectrum’s.

So there is a fun little test you can do. Have 2 light sources, LED and Metal Halide. And first ask, which one is brighter? And if the right LED source was selected, they should say LED. And then use a light meter, and the light meter may say the Metal Halide area is producing more foot candles. And so now you know part of the reason, but wait, there’s 2 other pieces of information you need to know.

One is called Color Rendering Index, or CRI. It is a measurement of Quality of Light. It’s a scale between 0 and 100, 100 is excellent. And LED tends to have a high CRI value. So the other golden rule we say is “You need less quantity when you have more quality”. Now, Metal Halide bulbs can be good, and are certainly much better than their High Pressure Sodium counterparts. But LED tends to be much better, so we perceive the light generated by LED to be brighter. More about this in the next section.

LED Color Rendering Index (CRI)

7. Photopic vs Scotopic Lumens

Many years ago, even the discussion of photopic vs scotopic lumens was perceived as voodoo magic talk. There were those who believed in the difference and as many who discounted it. What this is is a discussion of how we perceive light (scotopic) vs how a camera or light meter perceives light (photopic). It is true people are able to see, and light is a big part of why we are able to see. Walk into a dark bat cave and you will quickly understand how important light is. Our eyes consist  of rods and cones, and how they work allows us to see colors and perceive objects at night. You will notice that it is harder to see colors in the dark. That is how our eyes work. Cameras and light meters work differently, they detect light in a totally different way, but what they do read they register and they interpret what it is ‘reading’.

Then scientists and physicists came along and they tried to make sense of all of this. And they came up with this concept of scotopic lumens. But then they took it a step further and decided to create a series of factors by light source of how lights really look to us scotopically versus how a light is measured photopically. What came out was a series of factors between 0 and 3. Some light sources effective lumens were reduced by these factors, like High Pressure Sodium. Some light sources were increased by these factors, like LED. What was determined was the factor associated with LED was higher than Metal Halide. So in the discussion above about perceived light, it helps to understand why we think LED Lights are brighter than other light sources, even though the light meter tells us differently.

LED Photopic Luemsn

8. Rebates at time of purchase

Utility companies are trying to get their customers to convert to LED. Why? It’s a matter of economics for them. Demand for electricity is growing, so once they reach capacity, they have to choices, build more capacity or reduce demand. Reducing demand is far more cost effective than increasing capacity. So the utility companies offer rebates to purchase LED. They want you to convert from Metal Halide. Every utility company runs their own rebate program but in general, there are some consistent processes and guidelines. For the most part:

  • Utilities offer both prescriptive and custom rebate options. A prescriptive rebate is where they would say a 400W Metal Halide bulb is to be replaced with a LED bulb with these specifications. A custom rebate is for those exceptions not covered by prescriptive conditions
  • Most LED products need to be DesignLights Consortium Qualified. This is a symbol of efficiency and performance. It is sort of the equivalence of Energy Star for consumer products. It tells the customer the lights are tested to a standard and has passed.
LED Rebates

These rebates help reduce the cost of purchase and ultimately affect the quickness in pay back of the investment. With less cost at time of purchase, the time it takes to repay the investment goes down with the addition of rebate dollars.

With Metal Halide, no rebates. Sorry.

9. New versus Retrofit

So maybe at this point you are thinking that converting from Metal Halide to LED is a good idea. But this is where the journey begins. What to choose, what vendor to work with, new or retrofit. Think of the LED Market as the wild, wild west, and with every gun show there are the snake oil salesman trying to get you to buy their lotions. In the world of LED, there are many lotions, misconceptions and marvelous marketing ideals. For example, well built LED Retrofits  will perform as well as a new fixture, and vice versa. It doesn’t always make sense to replace the fixture, nor does it always make sense to retrofit a fixture.


There you have it, 9 highly backed up and legitimate reasons the fight of LED vs MH is no longer a fight. LED is a clear winner in every aspect of a light. We think of it as the perfect HID Replacement.

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Read the first part of this post – reasons 1 to 5

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