the SEI Exposure Photometerwas manufactured by Salford Electrical Instruments Limited, Times Mill, Heywood, Lancashire, England. It is a unique and marvelous instrument, which to my knowledge it is no longer in production..
the SEI Exposure PhotometerFrom the Introduction to the manual:
"The S.E.I. Photometer is a precision light measuring instrument for the photographer and illumination engineer. For the photographer it provides exposure values in general photography, reversal processes, copying and enlarging and for the accurate measurement of transmission and reflective densities. for the illumination engineer it is an accurate instrument for the measurement of brightness and illumination levels.
The main feature of the S.E.I. Photometer which makes it so suitable to solving the most difficult problems in both these fields is its ability to measure at a distance, over a great brightness range, the brightness of very small areas. The brightness range which can be measured is a million to one and the angle subtended by the spot in the field of view only 1/2 °.
the instrument contains its own comparison lamp and calibration circuit which enables the user to set this to a predetermined standard brightness before making measurements.
For photographers the S.E.I. Photometer measures with precision from the camera position, the brightness of a small area of the subject and can thus measure the brightness range of the subject. This enables the photographer to determine with certainty the character of his negatives."
The SEI
Photometer does contain a light sensitive photo-electric cell, but
NOT for the purpose of measuring the luminance of the object. It is
used solely to calibrate the brightness of the small internal light
bulb, which will illuminate the "comparison spot".
To negate changes in bulb and battery condition, a rheostat on the bottom is used to set the lamp brightness to a specific value, measured by the photo-electric cell, the output of which is displayed by a microammeter on top of the unit
Once the "standard" brightness has been set for the illuminating bulb, and thus the brightness of the small "comparison spot", any deviation will represent a measurable variation from the known reference value.
The area being evaluated is viewed through a telescopic viewfinder, in the middle of which is the superimposed small oval "comparison spot". The brightness of the spot is varied by rotating the base of the instrument until it disappears (has the same brightness as the surrounding area), at which time the value of the spot can be read from the scales adjacent to the rotating base.
Thus, there are 2 basic steps in the process.
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Although, ASA, f/stop, and shutter speed scales are present, the luminance values are read from a scale of LOG FOOT LAMBERTS, from which FOOT LAMBERTS, CANDLES PER SQUARE FOOT, and EV values can be calculated. 
 
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