Film-Tech Forum ARCHIVE: 7k bulb worth it?

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Posted by Jack Ondracek (Member # 1466) on 03-22-2013, 09:53 AM:

We recently started our first season with digital. My 86-foot screen is running a DP2K32B with a 6,500 watt bulb. I understand there's a 7kW bulb available. Based on my experience with our film gear, I'm not sure the results would be worth cutting the warranty in half, or if the increase would be noticed much.

Nobody seems to have a good answer to this, and I'm told I should try it out for myself.

Any thoughts?

Posted by Brad Miller (Member # 2) on 03-22-2013, 11:43 AM:

I don't think you will find that much of a difference, especially when you consider the operating cost.

Posted by John Roddy (Member # 7348) on 03-22-2013, 01:19 PM:

One of the other theaters in my company uses 7k bulbs on occasion, but only to push just a little more light through the projector for 3D shows on their wall–wall, ceiling–floor screens. In that particular case, they come in handy. In any other case, I'd chalk it up as a total waste. Unless 6.5k is just barely giving you enough brightness, it ain't worth it.

Posted by Jack Ondracek (Member # 1466) on 03-24-2013, 12:23 PM:

Kinda thought so.

Thanks

Posted by Steve Guttag (Member # 268) on 03-24-2013, 02:10 PM:

The 6.5KW was "optimized" for the DP2K-32B...I don't believe the 7K was.

A problem arises as one increases lamp size...the arc ceases to behave as a point source so a principle known as Etendue creeps up. The short of is...the smaller the aperture, the smaller the point of light needs to be in order to efficiently get through the aperture.

This is why, with film, 70mm could make use of the larger lamps much better than 35mm. It wasn't just the heat, as many would suspect, you can actually get MORE light from the larger arc on the larger aperture.

The same holds true for digital (the physics has not changed). The smaller apertures can not make as much use of larger lamps because the arcs are getting larger too. The is one of the several reasons why an S2K projector will not be as bright as a .98" projector running the same lamp (and will top out at about 2-2.2KW)...likewise, the 1.2" projectors can make better use of the larger lamps than the .98" ones and so forth.

It is also one of the reasons that lasers are the promise of the larger screens (e.g. Drive-Ins) in the future...one can get a lot of light in a small point. One can also get a narrow bandwidth color without having to split white up with a prism, direct the light to three DLP chips and then recombine the same light you split up (and remember, the wavelengths of the lights are very different so the index of refraction is different for each color...so it is very tricky to do all of this).

I know with film systems, I could get a BRIGHTER picture with a 6KW lamp than with a 7KW lamp using the same manufacturer's lamp on 35mm film.

Posted by Marco Giustini (Member # 4544) on 03-24-2013, 05:42 PM:

Also, the two lamps could have different behaviour, particularly if they are from different manufacturer. It may drop the output quicker than the 6.5kW for example. At that point you may end up with a little more light for 100hrs and then less light than the 6.5kW.
Rather than investing in more lamps, it may be worth replacing - every now and then - the reflector. I have not direct experience with it, but manufacturers do recommend replacing it after a while. I think Christie recommends every 8 or 10 lamp changes? Particularly with a big lamp (more heath and more light for the mirror to handle) it makes sense to me.

Posted by Monte L Fullmer (Member # 2797) on 03-24-2013, 10:21 PM:

Let along you only get 300 hrs warranty on a 7k bulb whereas 500 hrs warranty on a 6.5k bulb.