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HMI"S


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#1 Quinten Densak

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Posted 12 July 2008 - 04:18 AM

Hi


Whats the difference between a hmi and a hmi par? can anyone link me to documents that will explain this. What different scenes could both these light s be used in?


Thanks

Q D
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#2 Chris Keth

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Posted 12 July 2008 - 04:23 AM

Most HMIs are PAR fixtures. Anything that is a PAR fixture just has a particular type of reflector and interchangeable lenses to change and control the spread of light. PAR stands for Parabolic Aluminized Reflector.

As for use, that's as limited as your imagination.;)
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#3 Quinten Densak

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Posted 12 July 2008 - 09:04 AM

Thanks for your quick reply. Im still not sure, but what i've understood that the normal hmi uses a fresenel lens and had a wider spread. The hmi uses a par bulb but has external lenses that control the beam of light ( like you said ). So lets take a 4K normal and a 4k hmi par. The differences- lamphead different, voltage different, lamp type different.
Yes most fixtures used are par like you said. But you would go with the normal hmi if you needed less punch? Your right about the imagination bit. Hope you will add more to this.

Thanks

QD
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#4 Chris Keth

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Posted 12 July 2008 - 05:55 PM

Thanks for your quick reply. Im still not sure, but what i've understood that the normal hmi uses a fresenel lens and had a wider spread. The hmi uses a par bulb but has external lenses that control the beam of light ( like you said ). So lets take a 4K normal and a 4k hmi par. The differences- lamphead different, voltage different, lamp type different.
Yes most fixtures used are par like you said. But you would go with the normal hmi if you needed less punch? Your right about the imagination bit. Hope you will add more to this.

Thanks

QD


There really isn't a normal HMI. HMI is just the type of light producer used in the fixture. It's like asking the difference between tungsten lights and tungsten fresnels.

In your example of two 4k fixtures, the lamphead would be different, the voltage and power requirements would be the same, and the lamp type would very likely be the same. It would probably take the same feeder cable and ballast, even. The only difference between a fresnel fixture and a PAR fixture is that the fresnel fixture would have a fresnel and a spherical reflector and the PAR would have a parabolic reflector and interchangeable lenses, often including a fresnel.

A PAR usually has more punch but the beam is less even and so is less desirable for hard lighting people. The light from a bare PAR also just looks harsher than fresnel light.
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#5 Quinten Densak

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Posted 13 July 2008 - 04:08 AM

Chris

I think i'm getting it now. Your last comments about the eveness that par doesnt provide were cool. Any more observations there? Do Par's perform better on windows rather than pretty faces?Great info!!

Going to stop saying normal hmi......just hmi and hmi par?


QD
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#6 David Mullen ASC

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Posted 13 July 2008 - 11:21 AM

"HMI" tells you that it is a daylight-balanced unit and the physical type of design would be a PAR or Fresnel (or if an HMI were in an ellipsoidal like a Source-4, that would be another type.) Some PAR's come with a Fresnel lens you can drop in front, though they are not as good as a true fresnel unit.

Fresnels produce a cleaner shadow pattern and are a more even source edge to edge, but if you aren't using the lights hard & direct, generally PAR's work fine for backlights, softlighting practices, etc. Even fine for sharp patterns if they are far enough away. PAR's can be used for hard keys, just they are a bit hit or miss in terms of the shadow pattern you get when you try to flag them, sometimes it is fringey or you get a double-edge. They can be helped by slightly diffusing the PAR, like with Opal, to keep the semi-hard light effect but blend any messy shadow edges.
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