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Another caution about "Ev"

Doug Kerr

Well-known member
I have written about the possible confusion in two uses of the symbol "Ev". I have just realized that there is another place where what looks like this same symbol shows up, with a wholly different meaning yet again.

Let be review the earlier-discussed matter first.

The APEX quantity exposure value (Ev) is a logarithmic measure of the joint effect on photographic exposure of a shutter speed and an f/number. (The "v" can be either a subscript upper-case "V" or a plain lower case "V".)

There is also the widely used convention (which I deplore) of using "this same" Ev to denote scene luminance, this way: the "Ev" that is stated is the Ev that would be recommended (for use in photographing the scene) by a properly-calibrated exposure meter, metering the scene of interest, with its exposure index set to ISO 100.

Now the third situation. We commonly use, in photometric work, the symbol "E" to represent illuminance (the measure of the illumination on a surface).

But in fact the actual formal symbol for illuminance is Ev (here it is subscript lower-case "V"). This has absolutely nothing to do with the APEX symbol Ev (in either its real or "extended" meaning).

This notation comes from the parallel drawn in formal standards between radiometric and photometric quantities.

Radiometry is the discipline of measuring (or describing ) the "strength" of electromagnetic radiation, which includes radio waves, infra-red and ultraviolet light, and even visible light. The measures there are defined in terms of objective physical properties, such as "power".

Photometry is the discipline of measuring (or describing ) the "strength" of visible light The measures there are defined in terms of the perception of the human eye; in particular, these measures take into account the differing sensitivity of the human eye to different wavelengths.

For each radiometric quantity there is a parallel photometric quantity. The difference is solely whether or not the definition takes into account the differing sensitivity of the human eye to different wavelengths.

Accordingly, the wonks who formalized the International System of Units decided that each radiometric quantity should have a symbol, and the corresponding photometric quantity would have the same symbol with the subscript "v" (for "visual").

The radiometric quantity that corresponds to the photometric quantity illuminance is irradiance, and its symbol is E. Thus, for the corresponding photometric quantity illuminance (whose really-formal name is "luminous irradiance"), the symbol is Ev.

Again, this has nothing to do with the APEX value Ev; it is for example not logarithmic in its scale, and it relates to illuminance rather than photographic exposure (or even, God forbid, luminance). The similarity of the symbols is just an unfortunate coincidence.

Where might we run into this? Well, we may encounter it in the documentation of certain types of light meters, such as the Minolta Chroma Meter CL-200.

That meter is an incident light "color-aware" meter. It can report the "illumination" on a surface in the "Ev x y" color space. What is that?

An important CIE color space is called "Y x y". There, Y represents luminance, and x and y together represent chromaticity.

But the Y there is not in any given photometric unit. It is "relative luminance", on a scale of 0-1. If we are going to measure real incident light, we need to have it measured in some actual photometric unit (and of course that needs to be a unit of illuminance rather than luminance).

Thus by making the "Y" scale of the Y x y color space work in real photometric illuminance (formal symbol Ev, unit the lux), we can meet that need. This is just what happens in the Minolta CL-200 in one of its output modes. This form of the Y x y color space is known as the Ev x y color space. And the meter reads out the illuminance of the incident light (in lux, usually) as its "Ev".

Aren't you glad you asked!
 
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