Doug Kerr
Well-known member
Today, as for many years, essentially all "serious" incident light photographic exposure meters feature a prominent dome-like (hemispherical) translucent "collector" over their actual photosensitive element. Here is a currently-available, but classical, example, a Sekonic L-398A exposure meter:
Douglas A. Kerr: Sekonic L-398A exposure meter
This arrangement was invented by Donald W. Norwood in the late 1930s, and since then has been acclaimed as bringing to incident light exposure meters the ability to consistently give, over a range of lighting situations, a photographic exposure recommendation that would lead to an "appropriate" exposure result.
A while ago I became intrigued by this scheme. Just exactly what might we mean by an "appropriate" exposure result? And how does the use of a hemispherical collector make the meter consistently deliver an exposure recommendation leading to such a result?
My quest for an understanding of these issues was lengthy and at times frustrating. My conclusions are covered in detail in a new technical paper, "Norwood’s dome: a revolution in incident‑light photographic exposure metering", available here:
http://dougkerr.net/Pumpkin/index.htm#NorwoodsDome
Best regards,
Doug