Doug Kerr
Well-known member
In another thread in this forum I showed a picture of a Sekonic L-398A exposure meter, the latest (and possibly last) model in a series that is described as the "Norwood director" series (after the name of several early members of the family). Here we see it:
Douglas A. Kerr: Sekonic L-398A exposure meter
The photo clearly shows what is the hallmark of the Norwood Director series, a translucent essentially-hemispherical dome "light receptor". In fact today many exposure meters, in their "incident light" configuration, feature a similar dome. (The Norwood Director meters are "incident light" meters in their basic configuration.) It is interesting to reflect on what this ingredient does, and why we want that.
It is widely (but incorrectly) believed that this "dome" receptor is used to give the meter the ability to accurately determine the illuminance on a certain surface, or a certain plane, regardless of how the illumination may be composed of components arriving from different angles. Technically, to do that requires that the instrument have a "cosine" directivity.
But the dome does not give the instrument a cosine response. In fact, the Norwood Director meters have an alternate "receptor", almost flat, that in fact gives the meter a cosine directivity, so it can truly measure the illuminance upon a certain surface (or a certain plane). Some Sekonic meter models have a dual-mode receptor, a dome in its basic configuration, that can be physically changed to yield a cosine directivity, for actual measurement of illuminance.
So what directivity pattern does a hemispherical receptor give, and why do we want that?
Well theoretically, it will give an close approximation to a cardiod pattern (a pattern found, in the electro-acoustic domain, in many microphones).
And it has been determined, empirically, that this directivity pattern, in the context of the key-fill lighting technique, for a human subject (with special attention to the face), over a range of keylight locations (in terms of angle from the camera axis), seems to make the meter give exposure recommendations that produce "good" exposure results.
Now of course we have to deal with the fact that, especially in the case of substantial keylight angles, just what constitutes a "good" exposure result is very subjective. Do we want the side of the face nearest the keylight to be brighter in the image than we would want the whole face to be with "head-on" lighting? Maybe yes, maybe no.
Now, is there some theoretical relationship that wouild show us why this is so? No. And in fact the seminal paper on this matter, published by Don Norwood, who introduced the hemispherical receptor meter, if it were free from error, would show that the "optional" result, in the sense I mentioned above, would come from a slightly different directivity pattern. However Norwood, in that paper, made two errors such that is seems as if the pattern to be expected from a hemispherical receptor would be ideal - exactly ideal (fancy that).
Now, the discrepancy caused by Norwood's errors is not substantial, and we are talking here about a highly subjective assessment of the result anyway, so we might reasonably conclude that a hemispherical receptor is very suitable to pursue Norwood's concept.
Now, I have said that the objective of Norwood's concept is to make the meter provide good exposure recommendations for a human face (head, actually) over a range of keylight positions in the overall context of key-fill lighting. What about in other lighting situations?
Well, to take an extreme alternative, consider the situation of multiple-source lighting on a flat subject (perhaps a painting). Then, we are more likely to get a "desirable" exposure recommendation from a meter with a "cosine" directivity (which measures the actual, uniform illuminance on the subject. So with a Norwood Director type meter, we put on the alternate receptor. For certain other Sekonic meters, we switch the multi-mode receptor to the "cosine pattern" mode.
What about, for example, using incident light metering on a human subject in a typical outdoor setting? Will the "cardiod" directivity pattern (with the "hemispherical" receptor) or the cosine pattern (with the alternate receptor) give the "best" exposure recommendation? It is hard to tell. One's mileage may vary.
Those interested in further details of this matter may wish to red the technical article, "Norwood’s dome: a revolution in incident‑light photographic exposure metering", available here:
http://dougkerr.net/Pumpkin/index.htm#NorwoodsDome
Best regards,
Doug
Douglas A. Kerr: Sekonic L-398A exposure meter
The photo clearly shows what is the hallmark of the Norwood Director series, a translucent essentially-hemispherical dome "light receptor". In fact today many exposure meters, in their "incident light" configuration, feature a similar dome. (The Norwood Director meters are "incident light" meters in their basic configuration.) It is interesting to reflect on what this ingredient does, and why we want that.
It is widely (but incorrectly) believed that this "dome" receptor is used to give the meter the ability to accurately determine the illuminance on a certain surface, or a certain plane, regardless of how the illumination may be composed of components arriving from different angles. Technically, to do that requires that the instrument have a "cosine" directivity.
But the dome does not give the instrument a cosine response. In fact, the Norwood Director meters have an alternate "receptor", almost flat, that in fact gives the meter a cosine directivity, so it can truly measure the illuminance upon a certain surface (or a certain plane). Some Sekonic meter models have a dual-mode receptor, a dome in its basic configuration, that can be physically changed to yield a cosine directivity, for actual measurement of illuminance.
So what directivity pattern does a hemispherical receptor give, and why do we want that?
Well theoretically, it will give an close approximation to a cardiod pattern (a pattern found, in the electro-acoustic domain, in many microphones).
And it has been determined, empirically, that this directivity pattern, in the context of the key-fill lighting technique, for a human subject (with special attention to the face), over a range of keylight locations (in terms of angle from the camera axis), seems to make the meter give exposure recommendations that produce "good" exposure results.
Now of course we have to deal with the fact that, especially in the case of substantial keylight angles, just what constitutes a "good" exposure result is very subjective. Do we want the side of the face nearest the keylight to be brighter in the image than we would want the whole face to be with "head-on" lighting? Maybe yes, maybe no.
Now, is there some theoretical relationship that wouild show us why this is so? No. And in fact the seminal paper on this matter, published by Don Norwood, who introduced the hemispherical receptor meter, if it were free from error, would show that the "optional" result, in the sense I mentioned above, would come from a slightly different directivity pattern. However Norwood, in that paper, made two errors such that is seems as if the pattern to be expected from a hemispherical receptor would be ideal - exactly ideal (fancy that).
Now, the discrepancy caused by Norwood's errors is not substantial, and we are talking here about a highly subjective assessment of the result anyway, so we might reasonably conclude that a hemispherical receptor is very suitable to pursue Norwood's concept.
Now, I have said that the objective of Norwood's concept is to make the meter provide good exposure recommendations for a human face (head, actually) over a range of keylight positions in the overall context of key-fill lighting. What about in other lighting situations?
Well, to take an extreme alternative, consider the situation of multiple-source lighting on a flat subject (perhaps a painting). Then, we are more likely to get a "desirable" exposure recommendation from a meter with a "cosine" directivity (which measures the actual, uniform illuminance on the subject. So with a Norwood Director type meter, we put on the alternate receptor. For certain other Sekonic meters, we switch the multi-mode receptor to the "cosine pattern" mode.
What about, for example, using incident light metering on a human subject in a typical outdoor setting? Will the "cardiod" directivity pattern (with the "hemispherical" receptor) or the cosine pattern (with the alternate receptor) give the "best" exposure recommendation? It is hard to tell. One's mileage may vary.
Those interested in further details of this matter may wish to red the technical article, "Norwood’s dome: a revolution in incident‑light photographic exposure metering", available here:
http://dougkerr.net/Pumpkin/index.htm#NorwoodsDome
Best regards,
Doug