Norwood's dome in incident light exposure meters

[Doug Kerr]

Many incident light exposure meters, both electromechanical and electronic, have a light collector in the form of a translucent hemispherical dome, or something similar.

This construct has its roots in a development by Donald W. Norwood in the late 1930s. The story is a fascinating one, with many twists and turns.

The story, best I can reconstruct it, is told in considerable detail in my article on The Pumpkin, "Norwood’s dome: a revolution in
incident‑light photographic exposure metering".

I have just posted to The Pumpkin a revised issue of that article (as Issue 6). There is essentially no new nor substantially-changed information in this issue. It is primarily intended just to improve the presentation.

The article is indexed here on The Pumpkin:

The Pumpkin

Index page of Doug Kerr's The Pumpkin

dougkerr.net

Best regards,

Doug

 

[Doug Kerr]

The basic concept behind the "dome" light collector on incident light photographic exposure meters is this.

A given "potency' of illumination falling on a subject from some direction other than "from the camera position" is "less effective" in creating a certain exposure result than the same "potency of illumination" falling on the subject in the direction from the camera position.

This cannot be predicted with any theoretical model - do not look for ways to prove this that involve cosines or such. This is an empirical reality. This comes about largely because the human visual assessment of when two exposure results are "comparable" depends in an unpredictable way on how a subject's features may be "shaded" by illumination coming from different directions.

But various researchers - including Don Norwood - have by subjective testing determined typically by what degree a certain potency of illumination affects the assessment of the exposure result as a function of its angle of arrival at the subject.

If we make an incident light exposure meter whose directivity (sensitivity as a function of the angle of arrival of the light) follows that function, then a single measurement made with the meter, regardless of the direction from which the illumination comes, can recommend the desirable photographic exposure for the shot.

And that is what the dome collector on an incident light exposure meter does.

Best regards,

Doug