Colour Correction Filters
How To Convert Daylight to Tungsten
OK, let’s do a quick review on colour correction so far; in my first blog in this series we’ve talked about the importance of colour temperature, how it’s measured and how that relates in terms of the colour or hue of light. Warm looking light has a low colour temperature and cold looking light has a higher colour temperature. We’ve said that the colour temperature of natural daylight is 5,600K but changes throughout the day and we’ve now talked about how to convert (or correct) tungsten light into daylight using specific blue colour temperature gels know as CTB gels. Now I want to move on and discuss HMI’s (these produce daylight) and how we can convert them into Tungsten light or give the HMI light a warmer look.
So HMI’s are arc lamps that emit light in the 5,600K range (daylight). To convert that arc light into tungsten we use a gel known as Colour Temperature Orange (CTO). Just like the Colour Temperature Blue gels (CTB) we discussed in my last blog, that convert tungsten light into daylight, there are a range of CTO gels that convert daylight into tungsten light. As with CTB gels, CTO gels vary in density.
Here is the Rosco CTO chart:
As you will see the CTO gel fractions 1/8,1/4,1/2,3/4 and Full are the same fractions as for CTB. So to convert daylight, created by the HMI lamp, into tungsten light we place the CTO gel in front of the HMI light source and as if by magic the light is now converted to Tungsten. But it’s not magic, so how does it actually work?..
How Do Colour Correction Gels Work?
Well without getting too nerdy about it, all gels and filters create a certain coloured light by subtracting certain wavelengths of colour from a specific light source. The actual CTO filters are orange in colour, it just so happens that one of the characteristics of orange filters is that they absorb blue light (as we already know daylight is predominantly blue). Absorbing the blue from the spectrum allows orange wavelengths to pass through and coverts the blue daylight colour temperature (5,600K) to orange, tungsten light (3,200K). End of nerdy bit!!!
So in practical terms, why would you want to convert daylight into tungsten light? Well there are many reasons but let’s just focus on two of the most common situations for now, these examples occur often in both drama and documentary filming.
Imagine you’re going to film in a large conference hall or lecture theatre and there are no windows in the hall (and even if there are windows they are very small and there is very little daylight coming in), or you are filming in the evening and there is no natural daylight at all.
Lets say the room is lit with tungsten light which is coming from the actual conference hall lighting. Despite this existing tungsten light, the room is quite dark; remember it’s a large room so small filming lights (the sort you would use for interviews and close up work) won’t have any significant effect on lifting the light levels in the room. You need to bring in the big boys… The HMI’s. HMI’s are very powerful and produce huge amounts of light.
But now hang on… HMI’s are Daylight lamps and the room is predominantly tungsten so we’re back to that odd mixed lighting thing I talked about in my last blog! Don’t panic, by putting CTO gels on the HMI’s, or ‘gelling up’ the HMI’s as we say in the trade, we balance the HMI light to tungsten!! So now we have the perfect solution, powerful lamps that can easily light the conference hall that have a tungsten colour temperature. Problem solved!
You’re filming on location and you want to light an interior scene to emulate late afternoon sunlight streaming through a window (see example image below).
First of all to get this effect you will generally need (but not always) an HMI. We use HMI’s for this because as I mentioned earlier they are extremely powerful in terms of the level of the light they produce. I mean you’re trying to recreate the sun, I think you’ll all agree that’s a pretty powerful light source… right?!
To get this effect you’ll need to put your HMI outside the building near the window. But HMI’s produce daylight at 5,600K cold light, you don’t want that cold light look, you want a warm evening sun effect. So to warm up the HMI just gel it up with CTO gel. Now you have the best of both worlds: a powerful light source and a warm colour temperature…congratulations you have just re-created the setting sun!
Obviously there are many scenarios where you’ll need to know how to correct daylight to tungsten and you’ll thank me when you do. It’s all pretty simple really; not too much to think about.
Tungsten to Daylight – use CTB
Daylight to Tungsten – use CTO
In my next blog we’ll look at the most commonly used gels and filters and explain why we use them.
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Stay creative people and see you next time!
Free Spirit Film & TV
Film, TV & Online Video Production.