Light Units Used on Weston Meters
Weston light meters, including the luminance as well as exposure meters, are not consistent in the units that appear at the end of the needle pointer when it moves in response to the light. Once you attempt to compare one meter model with another it gets confusing. Indeed, even within one model - such as the S85 - the units can differ. So what do there different units spread across the scale amount to?
I have made some attempt here to understand and explain them (with the help of Wikipedia). I hope I've got it right. If any reader of this page can contribute to or correct the information, I would appreciate it. - So here goes:
Although not actually mentioned on the majority of Weston exposure meters, the unit in most common use is candles per square foot, or Candles/Ftsq. This is NOT the same as foot-candles although the simple relationship between the two terms means that a Weston meter can be used to convert between them. Early Weston light meters call this 'Light Values'. On the Master series, just the term 'Light' is most commonly used.
The luminance meters (not designed specifically for photography) however, do NOT use candles per square foot. In the main they use Foot-candles. But they also use Lux x 100 and Lumens per square foot. (Oh dear! Oh dear!)
Let's start with foot-candles. This is a measure of the QUANTITY of light being emitted by a point source. It starts with a standard candle (presumably one purchased from a hardware store). Imagine it being enclosed by a globe of one foot radius (or 2 foot diameter). A foot candle represents the quantity of light falling on the entire inside surface of that globe. It can also be thought of the amount of light falling onto a surface one foot away from the candle. AND it can be thought of as one foot candle being equal to one lumen of light across one square foot area. Foot-candles is a unit most suited to INCIDENT light measurement - that is light falling directly onto a cell's surface.
Two Luminance meter scales marked up in Foot-Candles. Model 703 (left) and one variant of the S85 (right).
Candles per square foot, by contrast (excuse the pun), is a means of interpreting the light REFLECTED off a subject upon which it is falling. As the name suggests, it is a unit of INTENSITY rather than quantity. Weston Electrical Instrumentation Corporation's own early documentation indicates that it is measured off a sheet of white paper in the plane of the subject. The Weston exposure meter can then be used to convert between the two units: From Candles per square foot to foot-candles. Simply put, it's a matter of multiplying the candles-per-square foot reading off the meter, by a factor of x4. So a reading of 10 candles per square foot would represent 40 foot-candles.
This can't be a perfect way of arriving at an accurate foot-candle reading using an exposure meter in reflectance mode, because the whiteness of a piece of paper can vary depending on its bleach, finish and brightness. But Weston believed in it. The evidence is in the features present on every Master Universal (not the cine models) dial they made: Either side of the main pointer are 'A' and 'C' secondary pointers. Lining up the A pointer on the measured light reading will allow the foot candles value to be read off the C pointer.
The A and C pointers were retained on all of the Master Universal family. This is their mark up on Master IV.
The 19 unit Relative Brightness logarithmic scale used on the early model 819 Cine Exposure Meter.
So all dial-based Weston Exposure Meters read candles per square foot. But they don't all use that scale marked directly on the meter face. The cine meters especially use a much simpler scale called 'Relative Brightness'. This is a simple linear numerical scale which translates the candles per square foot logarithmically. The 819 Cine meter, for example, uses a relative brightness scale from 0 to 19. The later Cine Model 720 goes up to 26. Some later Weston Universal meters - The Weston V and Euro-Master for example, use a Relative Brightness scale. Some Direct Reading meters also use this scale.
To make the Relative Brightness scale clear, each number increment represents a doubling of the candles per square foot value. So, if the Relative Brightness value of 13 represents 8 candles per square foot, then 14 would represent 16 candles per square foot, and so-on.
The pegging between the two scales I have shown here is not arbitrary. It is taken from the ONLY Weston that includes both scales - and that is the Ranger 9. And this is a CdS meter - not a Selenium celled one. Extrapolating back down the scale will show that a relative brightness value of 1 corresponds to a candles per square foot value of 0.002. This is far below the minimum light level that a Selenium cell would respond to. It can only be concluded that Weston revalued the relative values scale for that particular meter - the only CdS meter they made in America.
This then begs the question: Is the earlier relationship between Relative Light Values and Candles per Square Foot as used across the Master family consistent?
And the answer is YES. - Yes because it can easily be tested. Taking 2 Masters - one marked up in candles per square foot - such as the Master IV and one marked up in Relative Light Values - such as an Euro-Master, I set each one to the same film speed - 100 ASA (or 80 Weston for a Master II or similar). Then I set the pointer to the top value listed on the meter - 1600 for candles per Square Foot and 16 for Relative. Having done this I got a reference shutter speed for f16 of 1/600th sec on both meters - proving that both types are scaled identically.
The Candles per Square Meter scale on a Weston Master IV (left) compared to the Relative Values scale on the Euro-Master II (right). Point for point they are an exact match.
That may be great for the Master family but it doesn't help us with the other cine meters or Direct Reading ones that have scales all over the place. I do not know how they compare. I might try and find out sometime. Please find a reference table below where I have listed all the exposure meters and given their scale types and range:
Note: The above list excludes meters that read exposures - such as f/numbers directly: Leica meters, cine loupe meters and some models of Direct Reading meter.
So that covers the Weston Exposure Meters, but not the Luminance Meters designed mainly for industrial purposes. The Foot-Candles unit has already been explained and this is used on the Models 614, 703, and one variant of the S85. My Wollensak 757 Fastax meter also uses Foot-Candles x1000, being optimised for use in very bright light. This is a variant of the 756 which uses a standard Foot-Candles scale.
The S5.11 Photometer however, uses Lux x100 and two other variants of the S85 use Lux x100 and Lumens per square foot.
Starting with Lumens: This is an international (rather than imperial) unit that represents a total quantity of light radiating from a point source. Now, if that amount of light is spread over one square metre, that represents its value in Lux. So 1 Lux = 1 Lumen per square metre. That explains the Scale of Lux x 100 - just add two zeroes to the scale reading.
Remembering that Lux is based on a square metre, it then relates to Lumens per square foot by a factor of 10.764. That is the difference between the area of one square foot and one square metre. The same amount of luminance falling on one square foot is more intense than that on one square metre (at any point) by that factor. So Lumens per square foot are equal to Lux (or lumens per square metre) x 10.764. For most practical purposes this factor can be simplified to x10.
I hope that helps to explain it. If you have read through this and have now lost the will to live, then I apologise.