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Old August 23rd, 2008, 11:37 AM
Doug Kerr Doug Kerr is offline
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Location: Alamogordo, New Mexico, USA
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Default "Incident" and "reflected" white balance measurements

Two widely-discussed techniques for using a "measurement diffuser" on a camera to make white balance measurements are sometimes distinguished by calling them the "incident" and "reflected" techniques. These terms, borrowed from the world of exposure metering, seem intended to suggest that that in the former technique we measure (the chromaticity of) the incident light upon the subject, while the second implies we are measuring (the chromaticity of) the light reflected from the subject.

Now, putting aside the fact that I cannot imagine why we would want to do the latter, this pairing of terms does not usually reflect what actually happens, and in fact gives some wrong impressions.

Simplistic descriptions of these techniques are:

Technique A ("incident light") - The camera, with the diffuser in place, is placed at the location of the subject. (There is a second-order issue as to in which way the diffuser should be oriented, but we need not deal with that now.)

Technique B ("reflected light") - The camera, with the diffuser in place, is placed at the location it will have for the actual shot (presumably "aimed at" the subject).

Now, let's first assume that we have a measurement diffuser with a fairly "broad" acceptance pattern, perhaps even the "cosine" pattern often thought to be most appropriate for Technique A over a range of common situations.

Now, what happens in Technique B? Well, a fraction of the light "accepted" by the diffuser (and presented to the camera for determination of its chromaticity) is in fact light reflected from the subject. (That will be a very small fraction if the "subject" is of modest size, in terms of the solid angle it subtends from the camera position). A fraction will be light reflected from objects to the side of the subject. A fraction will be light directly received from (if we have an outdoor setting) the sky.

Now let's compare this with the situation with Technique A and this same diffuser. Here, a fraction of the light "accepted" by the diffuser can in fact be light reflected from the the photographer's assistant (standing at the tripod, awaiting the return of the camera). A fraction will be light reflected from objects to the side of the camera location. A fraction will be light directly received from the sky.

So in in technique B, part of the light (often a small part) is reflected from the subject (obviously, the premise for its "designation"), while in technique A, part of the light (often a small part) just as well might be reflected from the photographer's assistant.

With such a diffuser, in Technique A, the light that is measured, overall, is the light incident on the subject; in Technique B, the light that is measured, overall, is the light incident on the camera location for the shot.

Now, is there a situation in which the designation "reflected light" for technique B is apt? Yes, indeed.

Suppose we have a measurement diffuser whose acceptance pattern is "very narrow". This might mean that, for a subject of the size we have in a particular case, a substantial fraction of the light that is measured is in fact the light reflected from the subject. Thus here, "reflected light" as a description of the technique is apt.

Suppose now we use such a diffuser in Technique A. Now, it may well be that a substantial portion of the light that is measured would be light reflected from the photographer's assistant and from objects slightly off to her side (Porta-Potties, perhaps). Now, the moniker "incident light" for Technique A is no longer apt (remember my opening statement about the type of diffuser).

So I urge we not use "incident light" and "reflected light" to distinguish between Technique A and Technique B. It would be much better to describe them by their actual distinguishing characteristics: "at the subject" and "at the camera position".

Now why, in fact, would we want to use Technique B? Because Technique A is hard to use in our situation (photography of a grizzly bear, for example).

Why in fact would someone want to use a narrow-acceptance-angle diffuser in Technique B? Perhaps because he believes that actually concentrating on measuring the light reflected from the subject is desirable in the use of Technique B (rather than measuring the incident light upon the camera position, which is what a "broad angle" diffuser actually does in Technique B).

How does this give us what we need to do proper white balance color correction (it is generally considered that what we need to know is the chromaticiticy of the incident light upon the subject)? Beats me.
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