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mixing HMI light with CTB gelled tungsten


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#1 Tim O'Connor

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Posted 14 December 2007 - 11:02 AM

Okay, light from an HMI has ultraviolet light while CTB gelled tungsten does
not, right? Also, CTB gels are often described as providing "nominal" daylight
although I have suspected that this has more to do with their having Kelvin
ratings lower than HMIs rather than their lack of ultraviolet light.

I was just thinking that it's usually an either/or situation for me. I do remember
once quite a while ago when due to budget I had two 1K Fresnels with full CTB on the
same set with three 1200W HMI pars and (shooting video) there was a noticeable
difference between the gelled lights and the HMIs (although the HMIs were consistent with
each other.)


I haven't otherwise mixed them up much in the same scenes/shots. Have you?
What have you seen? Thanks.
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#2 Michael Nash

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Posted 14 December 2007 - 02:20 PM

There are a lot of little variables that affect the color of tungsten light, HMI light, and gels. Lights and gels are not always true to their specs.

Tungsten bulbs get warmer as they age, and so can the glass in a fresnel (and it gets dirty also). Voltage also affects color temperature. HMI's often burn "hotter" than 5600K, and can have varying amounts of red or green in their spectrum between units. Gels age, and different manufacturers' colors aren't always the same. Add all these things up and you're bound to see differences.

It's helpful to measure the color temp of your HMI's during prep so that you know what variances each units has. If you want to correct for those varaiances with gels on set is up to you, but it's not uncommon to add subtle gel correction to different units depending on the setup.
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#3 Tim O'Connor

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Posted 14 December 2007 - 05:48 PM

There are a lot of little variables that affect the color of tungsten light, HMI light, and gels. Lights and gels are not always true to their specs.

Tungsten bulbs get warmer as they age, and so can the glass in a fresnel (and it gets dirty also). Voltage also affects color temperature. HMI's often burn "hotter" than 5600K, and can have varying amounts of red or green in their spectrum between units. Gels age, and different manufacturers' colors aren't always the same. Add all these things up and you're bound to see differences.

It's helpful to measure the color temp of your HMI's during prep so that you know what variances each units has. If you want to correct for those varaiances with gels on set is up to you, but it's not uncommon to add subtle gel correction to different units depending on the setup.



Thanks. I generally do meter HMIs or somebody else does and correct if necessary. I'm on this tack
because I've been thinking lately about things that I've taken for granted for a while.

For example, I know that a color temperature is calculated based on the idea that black bodies above 700
Kelvin emit visible light that gets cooler as the measurement of degrees Kelvin rises and that some lights
are better suited for this type of measurement.

For example, an incandescent light is considered a good representation of a
black body radiator but a fluorescent light is not, so for fluorescents we have
correlated color temperatures or the handy CRI numbers on the packages.

Why is it that full CTB often changes a 3200K light to what looks like daylight but not the daylight of an
HMI? Rather, a CTB gelled tungsten light falls under 5000K. Is that because a gel that would get a higher color
correction would simply have ridiculously low transmission?

I guess this is a thought experiment* but if you had an HMI corrected/gelled to 4100K and a tungsten light
corrected/gelled to 4100K, would the light look the same on film/video (I'm guessing human eyes would
adjust) or would the presence/absence of ultraviolet light in the two lamps respectively make a difference?


*Unless anybody has handy HMI and tungsten lights and gels and feels like doing this. I would but I don't.
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#4 David Mullen ASC

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Posted 14 December 2007 - 06:02 PM

After getting tired of the color variations of HMI's for a moonlit scene, I once tried gelling a row of 1K tungstens to blue instead -- guess what? I found out how much gels can vary. Had to open-up a new roll and cut new gel for each light to get rid of that variation... and still found some minor variations in the lights after all of that.

But in terms of gelling an HMI to match a tungsten or a tungsten to match an HMI, it rarely seems to work so I avoid doing it if they have to be a perfect match, like a series of key lights that the actor moves through. Variations in the color of the backlight or fill can be easier to get away with.
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#5 Michael Nash

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Posted 14 December 2007 - 06:17 PM

But in terms of gelling an HMI to match a tungsten or a tungsten to match an HMI, it rarely seems to work so I avoid doing it if they have to be a perfect match, like a series of key lights that the actor moves through. Variations in the color of the backlight or fill can be easier to get away with.


It's funny, it seems it doesn't matter whether I gel an HMI with CTO or tungsten with CTB they both go green compared to light I'm trying to match. You'd think one would shift red. Maybe it's the gel...

I'm sure someone else here can explain gel color better than I can (there was a thread about color temp and gel Mired recently), but I'm more of a pragmatist and tend to go for what works...
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#6 Tim O'Connor

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Posted 14 December 2007 - 07:11 PM

Boy, variations among the variations. Yikes, that's funny.

I too definitely go with what works but I enjoy pondering these things off set. One thing that I've found that helps a
lot is that an added strong color reference can help the scene look the way you want it to even if your lights aren't quite
what you want, simply because they look more a certain way in contrast to something else that you've added.

Sometimes the pale blue moonlight can have a little more, or seem to have a little more, snap in the blue if
I can put a white reference in the shot. It doesn't have to be a white light but say a white mailbox that is getting hit
with a white light. Yes, why is there a white light hitting it may be a valid question but if you can justify that
to yourself or the director, I think often the audience simply accepts it and in comparison the moonlight is a bit
blue and ...moonlightier?


Also, you may have to justify why the blue moonlight is not hitting the white mailbox or, if you're in the woods,
then you need all your rhetorical skills to justify white light on white snow in contrast to the blue moonlight
elsewhere but "there's a logical explanation, trust me."
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#7 Jonathan Bowerbank

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Posted 14 December 2007 - 07:15 PM

A cinematography instructor I once had mentioned that in another class he teaches, he shows a certain type of diffusion that (when used with tungsten lighting at least) actually manages to cool the color temperature just slightly, without using CC gels.

I never asked him what specific diffusion he was talking about, but I can ask him.
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#8 timHealy

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Posted 15 December 2007 - 12:45 AM

But in terms of gelling an HMI to match a tungsten or a tungsten to match an HMI, it rarely seems to work so I avoid doing it if they have to be a perfect match, like a series of key lights that the actor moves through. Variations in the color of the backlight or fill can be easier to get away with.


And not all tungsten lights are the same either. Arri's tend to be a bit cooler than Mole for example.

Best

Tim
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#9 Hal Smith

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Posted 15 December 2007 - 12:54 AM

...........lot is that an added strong color reference can help the scene look the way you want it to even if your lights aren't quite what you want, simply because they look more a certain way in contrast to something else that you've added................

You're starting to deal with the tendency of the eye to adjust to a large area of color and judge it as neutral. Another way of putting it is that God gifted us with automatic white balance. :)
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#10 Chris Keth

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Posted 15 December 2007 - 01:22 AM

I've started to just accept that variation does exist. I was checking out some gels for an upcoming project and I compared Lee and Rosco CTO. Odd point number one is that Lee and Rosco even differ in what they say corrects daylight to tungsten. Lee calls that gel full CTO while Rosco calls it 3/4 CTO. Furthermore, they look a bit different, with Lee's version looking a bit more red and Rosco's having more of a yellow look to it. Realistically, you would never see the difference.
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#11 Tim O'Connor

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Posted 15 December 2007 - 03:32 AM

A cinematography instructor I once had mentioned that in another class he teaches, he shows a certain type of diffusion that (when used with tungsten lighting at least) actually manages to cool the color temperature just slightly, without using CC gels.

I never asked him what specific diffusion he was talking about, but I can ask him.



Definitely, that would be cool. I know that there are some blue CC gels that also diffuse (Have seen them but
can't think of brand/number) but the other way around would be good to know. If cooling is kind of a characteristic
of that diffusion, that might come in handy because perhaps there'd be a higher transmission of light than gels
designed to cool and diffuse.
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#12 Tim O'Connor

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Posted 15 December 2007 - 03:44 AM

It's funny. There are a lot of cool details here and it's good to know them and it's fun to discover the differences
between brands of lights or gels but I'm also thinking to remember that since usually any shot is going to have
an actor, actors or something going on, that's what people are going to be noticing primarily. They'll forgive or
adjust variations of color, to an extent anyway, probably not even consciously, as long as the lighting is supporting
the story.


When will they learn that lighting is the story? When will they learn?

Actor: Am I in the light?

DP: Yes, now just don't say anything and the scene will be great.

Actor: But I have lines.


DP: Go over there when you say them.


Actor: But that's out of the shot.

DP: Yes, but the set looks great. Don't you want people to see that? Isn't that better?
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