Sir Kenneth Corfield's contribution to photographic history should not be underestimated. Among other things, the Corfield Periflex was a brilliant solution to the shortage of Leica bodies in the UK after World War Two; the Corfield 66 was no mean SLR; the WA67 was a superb special-application camera; Corfield lenses were the first to use computer optimization; K.G. Corfield distributed the Shirley Wellard cassette; Sir Kenneth was one of the people who saved Gandolfi (along with Brian Gould); and then there was the Corfield Lumimeter, an enlarging exposure meter. The original model was introduced in 1949 and was rather more complicated in use (though not in basic design) than the 1951 Lumimeter 2 illustrated above. These turn up from time to time at camera shows, and most people don't even know what they are.
The Lumimeter is a comparison photometer, where the brightness of a chosen area of the negative (projected by the enlarger) is compared with a spot of variable brightness lit by a special mains-powered standard bulb. The brightness of the spot is controlled by twisting a knob, and when it matches the area of the negative, you have the density. Actually, there are further refinements in the shape of a paper speed scale (a chart was provided with the Lumimeter) and a paper contrast scale, divided into Extra Soft, Soft, Normal, Vigorous and Hard. By reading the full tonal range of the negative (shadows and highlights), the appropriate paper grade could be chosen. Then, by reading the densest shadow area you can read off the appropriate exposure from 1 to 240 seconds
It's a very handsome piece of design, effectively late Art Deco, and it cost £4:10:0d on its introduction (£3:2:9d plus £1:7:3d purchase tax), which equated to an inflation-adjusted £120 or so in 2010. Today, there are many easier ways of achieving the same result, but 60 years ago this was a pretty clever piece of technology, and it's an interesting piece of photographic history.
Better still, for those who really want to know how they work, here are the instructions:
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© 2010 Roger W. Hicks