Cinematography Mailing List - CML

Which Is Brighter?

I am looking to satisfy a curiosity with respect to moonlight.

I am about to start a new project which requires moonlight rigged in condors. The DP has expressed that he likes his moonlight as HMI corrected with 3/4 CTO. I have done moonlight with half CTO and I know it takes away a bit of the intensity of the lamp.

My curiosity is this: Which is brighter? A 20K with a quarter blue correction or an 18K with 3/4 CTO correction? An additional curiosity (and perhaps more important) is how much is the color temperature affected on the HMI with the additional magenta that is in the CTO? Is the magenta noticeable and will I have to correct ground fixtures to compensate, perhaps with an eight magenta?

It is so early in the show that I don't know which film stock we are using but I do know it is 35mm (as opposed to HD). It is a TV movie and will (I assume) be going through the telecine process. Perhaps these concerns about color temperature are unfounded. However, sometimes these "TV movies" are low budget features in disguise and I have been burned on this before...

I have a sense that the corrected 18K will still beat the corrected 20K in terms of output but I would like to know how others have approached this. A discussion of Dino’s and Maxi brutes led to the notion that he prefers the look of a fresnel as opposed to a par.

Any help or insights would be most appreciated. Thanks in advance.

Andrew Gordon
Regina, Saskatchewan

>Which is brighter? A 20K with a quarter blue correction or an 18K with >3/4 CTO correction?

Any HMI, on average, puts out about 3X the light of a comparable tungsten instrument. The 18K wins hands down, gel or no gel. I can't address the magenta question, but it has never presented a problem in the work I've done.

Trevor O. Houghton

Set Lighting Technician
Sherman Oaks, CA

I used to have a little chart from LTM that fit in my pouch, and showed the output of all their HMI lights with the various different configurations. I recall that in the past I've had to make this same choice, and I compared the output of the LTM 18k (off their chart) with the data from the Mole-Richardson catalogue for their 20k, and the 18k was only 2/3 of a stop brighter - the amount of a full CTO. So, to me, the rule of thumb is that a 20k and an 18k with a Full CTO in front of it are essentially equivalent.

What I might do in this case is try to determine specifically what color temperature the DP would like to maintain and then find the most efficient way to maintain that color temperature. It would be interesting to come up with some kind of chart that would show how much the shift is for each gel for a particular starting color temperature - in other words, starting with a 5600k HMI, where does the color temperature land when a 1/8 or 1/4 or ½ or full CTO is placed in front of it.

Ted Hayash
Los Angeles, CA


From an exposure point of view I’ve actually tested this situation, unfortunately the anal part of me has to point out that the output of different manufactured units differs by +/- 1stop, so the first thing to clarify is which units you plan to use. In general you get approximately ½ stop more out of the corrected 18k. Moles, Arri & other manufacturers post photometric data on their units on the internet, which you can use to calculate your exposure. I posted a question on cml 2 years ago which was answered by Jay Holben, which led me to download a demo which had a photometric calculator built in, I’ve used this extensively, & verified the stop the calculator gave me, with a meter on many occasions, even with 2 Dino’s with med spot bulbs @ 600’ away, 110’ on a crane with med. spot bulbs with ¼ ct straw, to find we were 1/10 of a stop out.

Unfortunately Crescit software from whom I downloaded the plotting demo never brought a later version, I had e-mailed them as I wanted to buy their product if it accepted jpeg images but they don't seem to have this product available any more.


James McGuire

Since your DP is looking for a source closer to 3200k,I would go with the 20k no question.

Although HMI's have come a long way regarding reliability, having a 18k up on a lift which will not strike/restrike/goes out/etc... is a gaffers worst night(shoot)mare.

With a 20k (or 2 Dino’s thru Heatshield/opal/1/4 CTB practically nothing can go "South" that you (or your Best Boy) cant fix fast.

Dino’s/Maxis would be my choice. At 100+ feet with opal the shadows are not "Razor sharp", (like a Brute arc) but workable

Josh Spring
Gaffer Washington DC-

Thanks for all of your input.

I'll be at the rental shop shortly and I will do my own tests in order to compare the brands of fixtures as well as the color temp. Incidentally, I have been told that the 20K's available are Moles and the 18K's are Sunrays.

Additionally, the DP has ordered some HMI helium balloons for testing and we will be looking at those while we test the Fresnel. Obviously, these fixtures are not going to have the same throw so I guess I have to get there and figure out just what is going on with these night exteriors!

Like you, Josh, I too have become quite partial to the look of Dino’s at night as well as the reliability of that technology.

Andrew Gordon


I believe your sense about the corrected 18K being brighter than the corrected 20K is correct. I did the following calculations: Using the Mole Richardson catalogue (invaluable for photometrics and fixture specifications), and using an arbitrary distance of 150 feet at full flood as a base of comparison, an uncorrected 20K provides 19 foot candles as compared to the 18K's 30 foot candles. Now for the gel corrections. Rosco 1/4 CTB has a transmission of 74%, which should equal about 1/2 stop. Subtracting 1/2 stop from 19 foot candles leaves you with about 14 foot candles. Comparatively, Rosco 3/4 CTO has a transmission of 58%, equalling about 3/4 stop. Subtracting that from 30 foot candles leaves your 18K with about 18 foot candles. Of course, the real world has a way of adjusting these figures. But more is not necessarily better when it comes to moonlight. Often, all you need is just enough for the eye to register an outline, a slight separation on trees, cars, whatever.

Too much moonlight is a common mistake and the results make a night exterior look terribly over lit.

Having been a gaffer for many years, with all else being relatively equal, I would always opt for tungsten over HMI. Less risk of breakdown, less set-up time, less expense for production. Put four or five Molepars (same correction) in your condor basket to pick off some dark areas of the frame which your 20K doesn't reach and your DP will be very happy.

The big question, though, is what diffusion to use over your moonlight. But that's another discussion. Good luck on your project.


Jeff Levy

Thanks for all the input.

I appreciate everyone's willingness to share their opinions. I will report back on the night exteriors and the tests that I will do at the rental shop.

Andrew Gordon

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