Doug Kerr
Well-known member
Don Norwood, in his 1950 paper before the SMPTE, reported this finding from a series of visual experiments. The context here is the use of the key-fill lighting context. The description is in my own terms.
• The effectivity, on the visual impression of the resulting exposure, of a light source of a given luminous flux density varies with the angular position of the light source.
To elaborate: if the light source of a certain luminous flux density is at an angle of 45° from at the camera position, it will have 75% the effect on the visual exposure result compared to its being at the camera. If it at an angle of 90° from at the camera position (exactly to the side), it will have 50% the effect on the visual exposure result compared to its being at the camera.
It is implied that if there are two or more discrete light sources, the overall effect on the visual exposure result will be the sum of the respective sources' luminous flux densities, each weighted by the "effectivity" factor for its angular position.
The exposure meter Norton developed reckons that overall effect with a single measurement by having a directivity pattern that follows that function of "effectivity" vs. the angle of the source's location with respect to the camera. Thus, the light from each source is inherently "weighted" by the meter as appropriate to its angular location.
It turns out that this directivity pattern is fairly closely approximated by the directivity pattern of an incident light meter with a hemispherical ("dome") light collector.
******
It has seemingly been determined, by subsequent work, that this concept of the "effectivity" of any light component on the visual impression of exposure is applicable in the general case. This includes the case of an outdoor scene, in which the elements are illuminated by light coming from the entire hemisphere above the earth, but with different luminous flux density from the different directions (this being greatest in the direction from the position of the sun).
Thus today most incident light exposure meters have, in their basic configuration, either a dome collector or a collector of a more complex shape that nevertheless gives essentially the directivity pattern needed to follow the "Norwood system" of exxposure metering. And using the meter in the mode that follows Norwood's system (the meter being at some location where the illuination is the same as willl be received by the subject, with the axis of the dome toward the camera) is recommeded for most photograhic situations.
Best regards,
Doug
• The effectivity, on the visual impression of the resulting exposure, of a light source of a given luminous flux density varies with the angular position of the light source.
To elaborate: if the light source of a certain luminous flux density is at an angle of 45° from at the camera position, it will have 75% the effect on the visual exposure result compared to its being at the camera. If it at an angle of 90° from at the camera position (exactly to the side), it will have 50% the effect on the visual exposure result compared to its being at the camera.
It is implied that if there are two or more discrete light sources, the overall effect on the visual exposure result will be the sum of the respective sources' luminous flux densities, each weighted by the "effectivity" factor for its angular position.
The exposure meter Norton developed reckons that overall effect with a single measurement by having a directivity pattern that follows that function of "effectivity" vs. the angle of the source's location with respect to the camera. Thus, the light from each source is inherently "weighted" by the meter as appropriate to its angular location.
It turns out that this directivity pattern is fairly closely approximated by the directivity pattern of an incident light meter with a hemispherical ("dome") light collector.
******
It has seemingly been determined, by subsequent work, that this concept of the "effectivity" of any light component on the visual impression of exposure is applicable in the general case. This includes the case of an outdoor scene, in which the elements are illuminated by light coming from the entire hemisphere above the earth, but with different luminous flux density from the different directions (this being greatest in the direction from the position of the sun).
Thus today most incident light exposure meters have, in their basic configuration, either a dome collector or a collector of a more complex shape that nevertheless gives essentially the directivity pattern needed to follow the "Norwood system" of exxposure metering. And using the meter in the mode that follows Norwood's system (the meter being at some location where the illuination is the same as willl be received by the subject, with the axis of the dome toward the camera) is recommeded for most photograhic situations.
Best regards,
Doug
Last edited:
