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Old October 15th, 2010, 09:26 AM
Doug Kerr Doug Kerr is offline
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Default White balance for the iPhone camera

Drew Strickland's firm, ColorRight Tools, LLC, has introduced a white balance color correction facility for the iPhone camera (and similar iOS based device cameras), the iBal for iPhone system. You can read about it here:

The hardware aspect of the product is the ColorRight iBAL White Balance filter, a small circular disk of translucent plastic that is held in front of the camera lens to make the measurement. It is essentially a white balance measurement diffuser.

The software aspect is an iOS app, available (free) from the Apple App store.

In the basic mode of operation, with the app in place, the app is activated for "measurement", the filter held in front of the camera lens, the camera faced toward the subject, and a shot taken. The software then analyzes the results of the shot and evidently establishes a "white balance color correction vector". The app then evidently causes the application of that vector when subsequent actual shots are taken by the camera, providing in-camera white balance color correction.

I think, from something I saw on the descriptive page, that with some cameras the system also accommodates measurement with the filter-equipped camera placed at the subject.

Test shots published by the manufacturer show images with correction applied that are probably more "pleasing" than ones of the same subject (we are to assume under the same illumination) without it.


Those with an interest in how things work may wonder, as in the case of many of the ColorRight products, how a measurement taken from the shooting position, with the filter-equipped camera facing the subject, can determine what the camera needs in order to do white balance color correction: the chromaticity of the incident illumination on the subject.

The mystery is actually deepened when the manufacturer (as it does from time to time, in connection with some of its products) describes the process as working on the light reflected from the subject. After all, the chromaticity of that light depends both on the incident illumination and the reflective color of the subject. If the subject is large red barn, we do not want the camera to conclude that the incident illumination is way red.

And this outlook is emphasized when the manufacturer (not always in a clear way) touts what turns out to be a fairly-narrow acceptance pattern for some of their measurement diffusers (compared, for example, to the "cosine" pattern traditionally preferred for white balance measurement).
It is not that the manufacturer does something special to get that narrower pattern. Rather, it is inherent in any "simple" diffuser. They just don't apply any of the special features used in many other white balance measurement diffusers to create the cosine pattern.
So far as I can tell, the success of white balance measurement from the camera position in many cases relies on one or both of these:

In some cases, the average reflective color of the entire scene regarded by the diffuser is nearly "neutral"; thus the average chromaticity of the light from the scene nearly mimics the chromaticity of the incident light.

In reality, the diffuser-equipped camera really regards the chromaticity of the incident illumination on the camera. (It's not ideal for that, owing to its non-cosine acceptance pattern, but that is not a biggie in most cases where this is workable anyway.) In many situations (many outdoor settings, a ballroom illuminated by an array of identical chandeliers, etc.) the illumination on the camera is very similar to that on the subject.

So, like the drunk looking for his dropped keys under the streetlight, rather than in the alley, where he dropped them ("It's too hard to see back there"), we measure what is handy, and hope it is about the same as what we need to know.

In any case, Drew is to be congratulated for bringing his approach to white balance measurement to the world of the iPhone camera.

Best regards,

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