Because I've always wanted a Minolta Color Meter, I was just searching around for links.
I found this on the dv.com site: http://www.forums.dv.com/jive3/thread.jspa;jsessionid=SEW312QV2K3YQQSNDBGCKH0CJUMEKJVN?untiRecursion=true&threadID=61000001&untiRecursion=0
You'll need to register to see the original (it's free).
There was a discussion of trying to balance color by reading a color meter. The discussion moved to handling fluorescent sources.
Here's the pertinent text:
Looking at my color meter III it is telling you how to balance between warm (3200) and cooler (5500) color temperatures by adding CTO (amber) or CTB gelles or filters. The second reading is telling you how to balance between the magenta and green. A typical office flo gives 5690K and +137 blue to get to sunlight and 19M to absorb the green.
Mr. Bill
That's for a 3200K balanced film and a specific light tub/fixture/paint/furniture etc. combination. The colour temperature is close enough to daylight that you'd really be fine with daylight film, and just a 20M and be done with it. The problem with heavy filtration like a 137 mired (actually expressed as -137) is that the filters are not pure, and usually are off on the green/magenta axis, and also unless very fresh and tested are likely not to be the stated colour. Gels are commonly used, and even when kept away from light and stored correctly fade noticeably. Best to use a film matched close to the light source, and use lighter filtering. The Blue/Yellow colour temperature filters usually come in the oddly stepped and named Wratten series; the yellows being 81(9), 81A(18), 81B(27), 81C(35), 81D(42), 81EF(52), 85C(81), 85(112), 85B(131). There are also stronger filters which aren't really designed for taking pictures, but for photometry, in an 86 series. 86(242), 86A(111), 86B(67) and 86C(24) comprise that series. In practice, if heavy filtration is called for, the 86 is useful. The CC's, usually the magenta and greens, are available in 5CC increments. I carry 5, 10, 20, 30 and 50 magenta and 5 and 10 green. About CC5 is the subtlest filter necessary, as are 10 mired steps in colour temperature. Some product shots need more accurate filtration, but that is usually handled at the reproduction stage.
The units used for colour temperature filtration are Mireds (the -137 above). 'Mired' stands for MIcro REciprocal DEgrees, and each light source has a mired value associated with it (mired value = 1,000,000/Colour temp. in �K). So, for example to convert from a 5500K degree daylight film to say, a 2450K incandescent bulb (60 to 100 watt, depending on age etc), you divide 5500 into 1,000,000 and get 181.8. Then you divide 2450 into 1,000,000 and get 408.2. So to get from 181.8 (your film) to 408.2 (light source), you need +226.4 (yellowish). This is where the + and - come in. So you'd probably use two Wratten 85's, which are +112 each. This is heavy filtration, but the yellow filters are a little more consistent and pure than the blues, and don't have as great a filter factor. 85's have a filter factor of 2/3 stop, while their reciprocals, the 80B's, have a factor of 1-2/3 stop.
The Minolta Colour Meter is quite good, and will allow you to filter correctly for one light source, but multi-sources are a problem of course that filtration really can't fully handle.
Yesterday I wrote about 'decamireds', and dealt with them like mireds above. That was, of course, wrong. A decamired is 10 mireds; filter with a small warming effect is sometimes marked 1.5; that is in decamireds which would be 15 mireds.
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