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Old July 21st, 2014, 11:04 AM
Doug Kerr Doug Kerr is online now
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
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Default Incident light metering - Norwood's dome, and noodle, revisted

I have written about this topic several times (including recently) but in fact was never really comfortable with my understanding about what was really going on in this matter. I think that I was "overthinking" it. Now I believe I have a better grasp of what is really going on.

************

Incident light exposure metering

In incident light exposure metering, the meter measures the light incident on the subject, and from that measurement (combined with the sensitivity of the film or digital sensor, develops a recommended photographic exposure (combination of shutter speed and aperture).

Simplistically, the objective is to give each area of the scene an "exposure result" that is proportional to the reflectance of that area. In this way, a scene that comprises a white cat on a snowdrift will lead to an image that looks like a white cat on a snowdrift. Under basic reflected light metering, that image might look like a gray cat on an ash heap.

Two meter configurations

Most serious incident light exposure meters offer two configurations. In one, the receptor is covered by a roughly-hemispherical translucent dome. In the other, the receptor (nominally flat) is bare, or in some models, is covered by a filter-like disk put on in place of the dome. In any case, although in this configuration the receiving organ may not be microscopically flat, the intent is that it will respond as would a classical flat receptor (Bart describes it as "simulated flat").

Thus, in the "flat" configuration, we would expect the meter to measure for us the luminance of the incident light upon the plane of the receptor. In the "dome" configuration, the meter measures something else. Just what that something else is, and why we are interested in the meter knowing it, is the mystery that is the subject of this note.

The uses of the two configurations

Typically, the flat configuration (often, and aptly, called the "luminance measurement" configuration) is recommended for such uses as:

• Measuring the luminance of the illumination on a desktop.

• In photography, measuring the impact of the individual lighting sources when setting up a planned overall lighting setup.

The domed configuration is said to be used for "incident light exposure metering".

The realities of incident light metering

If all the surfaces of the important subjects in a scene face in the same direction (and of course they would all have to be flat for that)—not necessarily toward the camera—then measurement of the luminance of the incident light, with reference to the plane with which all the surfaces are parallel, will tell the meter what it needs to know. That illuminance, by the way, will be the same on all our surfaces (we assume none are "shadowed").

Note that it does not matter wither the ambient illumination all comes from one direction, or one source, or otherwise. The behavior of the flat receptor (what we speak of as a "cosine response") will inherently take account of that.
"Cosine response" means that the response of the meter to a "shaft" of light of a certain potency would vary with the cosine of the angle of arrival of the "shaft" (measured with respect to a line perpendicular to the plane of the receptor).
But of course we rarely have that situation. Suppose our subject is a human face. The forehead has essentially one orientation, the cheeks essentially two others.

Now, depending on the overall nature of the light sources, those three surface areas may receive different illuminance. That of course will disrupt any chance that they all will get exposure results proportional to their reflectance.

And of course we often "play" that to our advantage in getting a desired artistic result. We may intentionally arrange for a potent light source on the right side of the face and a far less potent one on the left, so the left side will appear "shadowed".

Now the matter of incident light exposure metering is not nearly so straightforward. No single measurement can simply lead to the "proper" photographic exposure. After all, the meter has no idea that we want the subject's left cheek to be "darker" than its reflectance would dictate.

Nevertheless, photographers, especially in less extreme situations, are anxious to get "the answer" from their incident light meter.

"The answer"

One approach can be unscientifically described as to try and measure the "average illuminance" on the variously-oriented surfaces of the subject.

One way to do that would be to have a meter whose receptor did not have a cosine response but was rather omnidirectional. That is, its reaction to a shaft of light of a certain potency was independent of the angle at which it arrived on the receptor. We could make a meter that did that by giving it a spherical receptor.

The work of Don Norwood

Famed cinematographer Don Norwood, the developer of the famed series of incident light exposure meters that bear his name, was one of the first to study this concept.

It seems to me as if he reasoned thus:
There is no need to measure the average illuminance over surfaces of all orientations. After all, those oriented at more than 90° from "facing the camera" would not even be seeable by the camera. So perhaps we should ignore light coming from angles more than 90° from the camera.
OK.

In other words, perhaps we wanted to have the meter have an omnidirectional pattern out to 90°, but be "dead" beyond that. And we would always face it toward the camera.

Now to continue my interpretation of Norwood's presumed thoughts:
So instead of giving the receptor a spherical receptor, we would give it a hemispherical receptor. This wouild be in effect a proxy for the subject's head—that is, the part that was visible to the camera. So it would capture the light that we wanted to have noted by the meter.
But if we do that, then the response of the instrument turns out to follow very nearly the mathematical curve called the cardioid—not a uniform response out to 90° and none beyond.

Now does this follow the concept of "measuring the average luminance upon all surfaces not facing away from the camera"? No.

But in fact there is no theoretical basis for concluding that such a measurement would be the "ideal" one to guide photographic exposure, with a single measurement, over a wide range of different situations. It just "sounded reasonable".

Evidently, Norwood found that metering with a hemispherical receptor (to use my words) "gave a good result in many cases".

And so, the "cardioid" receptor response of an incident light exposure meter became enshrined in the armory of the industry. The international standard for the properties of freestanding exposure meters, ISO 12232, even prescribes a cardioid response for the "exposure measurement" mode of incident light exposure meters.

And of course meters based on Norwood's patents (by his firm and various successors-in-interest) became widely used, especially in the cinema industry.

***********

And to think of the hours I spent trying to develop a theoretical model for why the measurement taken with a meter with a cardioid response would be a good one to use for general.

So "domes up", soldiers.

I close with some images. Here is what is arguably the most beautiful exposure meter ever made, the Norwood Director Model B, as made by American Bolex (ca. 1948):



Norwood Director Model B
Photo © James Ollinger

Here we see the noted Turkish cinematographer Erkan Umut making an incident light measurement for Sibel Can, the famous Turkish singer (1996). The meter is a Minolta Autometer IIIF.



Oh! Erkan bey!

Best regards,

Doug
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