What is an arc source, and why would we want to switch from it to an LED source?
You know what an arc source is; they're those lamps that have been in your video projectors since like forever. And if you're fortunate enough to afford them, they are in your moving light/intelligent fixtures too. You can recognize them when you turn on that projector and for the first seconds no light comes out of the projector, because it first has to "strike" the lamp. And even then, you have to wait for it to come up to full brightness. That's why some projectors exhibit a countdown timer, before turning on their output image.
The same is true for lighting fixtures that use arc lamps, but they may not be set to automatically strike on power up. They typically are struck one of two ways, based on your user settings. One choice is to strike once they see a valid DMX signal. The other is to wait until you send the right combination of settings over the DMX signal. The latter is done in your console and varies based on the console.
Why does this matter?
Well, every time you strike an arc lamp (also called discharge lamp, or High Intensity Discharge HID) you lose several hours of run time from the life of the lamp. And their life has typically been only about 1,500 to 2,000 hours! For our comparison, we'll look at the OSRAM Sirius HRI 440W with a rated life of 1,500 hours, which is used in the Martin MAC Axiom Hybrid.
So, if you strike up your fixtures several times a week (once for rehearsal and once for Sunday morning) then you've lost about 200 hours in one year of operation. That's about 13 percent of the lamp's estimated life in one year! And they cost a few bucks too. For this one, they cost a little more than $300 each. Not too bad if you have only, say, 10 fixtures.
However, if you use your 10 fixtures for two hours of rehearsal and around four hours on Sunday, then you're hitting that 1,500 hour mark in less than three years. Expending $3,300 after three years or so may not be too bad for you, but there are other factors we need to consider.
As that lamp ages, it gets dimmer and "browner." This is because the color temperature of the lamp is dropping. Here's why a change in color temperature matters.
You've noticed in recent years that headlights on cars vary more than they used to. Some are just white, while others are a blue crisper white. And if a car is very old, or has a problem, the headlights could be even brownish. That's color temperature.
This arc lamp in the Axiom has a color temp of 7,000 degrees Kelvin. That's very white. Blueish white, in fact. As the lamp ages, it will shift down in color temperature. At some point, it will be noticeable when compared to a new lamp. No problem if you change all your lamps at the same time. But if you end up changing them at different times, you will notice the difference between them.
This is all sounding pretty bad for the good old arc lamp, so why would we ever use it for lighting? And why not replace it with an LED source fixture?
Only recently have arc lamps had competition from LEDs for use in moving lights. Even still, the arc lamp's ability to produce large amounts of light relative to the power put into them beats LEDs when it comes to large moving lights. But many small- to maybe mid-sized moving light fixtures have come on the market recently that use LEDs for their light source. a comparable LED sourced fixture we'll compare to the Axiom, would be CHAUVET Professional.
However, careful attention must be paid to comparing specs.
Let's take a look and them and see how they compare.
|Arc Source||LED Source||Model||Martin MAC Axiom Hybrid||Chauvet Maverick MK2 Spot||Power In (W)||600||740||Color Temp (K)||7,000||6,808||Output (fc, ~13deg @ 20ft)||1,618||1,130||Lamp Life (hours)||1,500||50,000||Lamp Cost ($)||330||not applicable|
First, I want to point out the amount of power (watts) going in relative to the amount of light (fc) coming out.
Here we get a feel for the lamp and fixtures efficiency. While these two fixtures are close in their performance, we see that the LED actually takes more power to produce less light. But, 1,130-foot candles still produce a decent amount of light.
The big difference, of course, is in the lamp hours. LED sources are essentially forever. Forever in terms of the life of the fixture. Let's put this in perspective. If we power this fixture up and focus it on an object, we could leave it on 24 hours a day for about 5.7 years! That is before the lamp source theoretically fails or becomes no longer useful in terms of its output. In more relevant terms, at double the usage of our earlier example of 10 hours per week we could run this one at 20 hours per week for 40 years. The fixture itself won't last that long. And certainly newer and better technology would make it obsolete before then.
So far, the one big advantage we see for LEDs is lamp life. What other advantages or disadvantages are there?
An LED lamped fixture does not need to be struck when powered up and doused when shutting down. And the LED lamp is immediately ready, once the fixture is done booting up. Therefore, no warmup period after striking.
One possible disadvantage for changing from arc to LED is that you may need more LED-lamped fixtures to get them same coverage as you were from your arc sourced ones. This really depends on your specific application and choice of fixture.
A key spec to look at when comparing LED to arc lamps is their color temperature. The two above are close enough that the average person wouldn't notice the difference after a change was made. If the LED source had been lower, like maybe below 6,500, then I would start to get concerned. This brings me to my final point.
Test, test and test drive some more!
Just to be clear here, we are talking about switching to a whole new fixture that has been designed to use an LED as its light source. As far as I know, there are no viable options for just changing the lamp in an arc source fixture to an LED source. At least none that are worth doing. With that in mind, let's move on.
If you are considering a change from arc to LED, then you are looking at a substantial expense. One that is enough to allow your supplier to bring in fixtures for you to test with. A side by side comparison will allow to judge for yourself if the LED sources will be adequate for your needs. I cannot stress enough the importance of doing a shootout test like this. I have had the opportunity to be part of several shootouts. In one case, the favorite going into the test ended up not being chosen. Test, test and test!
As a closing caveat, we've focused (pun intended) on white source LEDs specifically for replacing fixtures that have arc source lamps. We haven't discussed replacing arc source fixtures with RGBA or RGBAW LED moving head fixtures.
That's a whole other article.
But don't let that stop you from getting some to test with, as long as you're doing a shootout to see what works best for you.
As always, I hope this has helped you in some way and that your ministry benefits from the articles presented here. God Bless!