The Ainger Hall Photometer

copyright 2008 R. A. Suomala
In the process of researching the previous articles dealing with the SEI Photometer I came across reference to an earlier "Spot" meter known as the Ainger Hall Photometer.

British Patent 508122 was issued for this invention on June 27, 1939 having been first applied for on March 25, 1938. The inventors are listed as John Ainger Hall, Francis Harold Schofield and William George Haughton Turl (Note 1).

The patent document describes the invention as "A photographic exposure meter comprising in combination an optical system consisting of an objective lens for forming an image of the field to be photographed and an eyepiece for viewing same; means whereby a small part of the image plane viewable through the eyepiece is illuminated by an independent light source, means for adjusting to equality the brightness of the image of a selected object in the field and said illuminated part, and means for indicating said brightness".

The years 1938 and 1939 (remember WWII?) were probably the worst time to try and market a photographic exposure photometer that was not particularly suited for use by military photographers. This instrument was originally manufactured by the Bowen Instrument Company in England. One source (Ref. 1) indicates that some of these instruments were still being hand made on special order in the 1950's by one of the inventors, W. G. H. Turl.



The Ainger Hall exposure photometer (Schematic shown in Figure 1) has an equivalent acceptance angle of half a degree and its range is just over 1,000,000 to 1. The spot intensity is effected by withdrawing the lamp, the lower part of the body pulling out and turning at the same time under the control of a spiral guide slot (Indicated by arrow in Fig 2). No spot color control or self-calibrating feature is included, but a single adjustable wedge is incorporated for resetting the calibration against a candle flame (Note 2) in a darkened room.




During the October 1945 meeting of the Scientific and Technical Group of the Royal Photographic Society J. F. Dunn and G. S. Plant presented a paper (Ref. 2) describing an improved exposure meter based on the same commonly known principle used in the Ainger Hall instrument. This new instrument was the SEI Exposure Photometer. Whereas the Ainger Hall instrument relied on calibration with a candle flame in a darkened room and depended on stable battery voltage between calibrations, the SEI provided a photocell and rheostat that allowed for calibration at any time. During the discussion that followed Mr. Ainger Hall described the possible problems that the SEI might encounter by the introduction of a rheostat, photocell and ammeter. Some of Ainger Hall's other remarks were recorded as follows: " He thought when he designed his instrument that the twenty-five percent accuracy, which meant about seven percent voltage on the lamp would be a great trouble, but to his surprise using an ordinary unit cell, he found he had not yet used a cell so long that needed any change. He usually took the cell out of the instrument for safety's sake when it was in store and he had always lost the battery before it changed from the first calibration". My experience with the SEI photometer verifies Ainger Hall's comments regarding battery life.


References:
1. J. F. Dunn, Exposure meters and Practical Exposure Control, The Fountain press, London, 1952.
2. The Photographic Journal, Vol. 85B, 1945, pages 114-119.

Notes:
1. John Ainger Hall, 10 Kitson Road, Barnes, Surrey.
Franicis Harold Schofield, of 8 Seymour Road, Hampton Hill, Middlesex.
William George Haughton Turl, 15 Cambridge Road, Hampton, Middlesex.
2. I tried this with an SEI photometer using a cheap store bought candle and it works but the flickering of the flame makes it a chore. Initially the standard of luminance was that of a special spermaceti candle weighing six to the pound and burning at the rate of 120 grains per hour or 2 grains per minute (The sperm whale was named after the milky-white waxy substance, spermaceti, found in its head and originally mistaken for sperm. This substance was used in making candles of a standard photometric value). As the requirement for a more precise standard became necessary the term "candela" was adopted to differentiate it from the term "standard candle". Initially, the candela was defined as the luminous emission of a Planckian radiator, a type of blackbody, at the temperature of freezing platinum (2045 K). This correlated roughly to the light emitted by a typical candle, making it a more precise measure. In the late 1970s, it was determined that the experimental difficulties in creating a Planckian radiator at such high temperatures made the existing definition of candela less than desirable. Breakthroughs in radiometry allowed scientists to have a more specific definition, and so the current hertz/watts definition of candela was adopted. It is technically defined as the intensity in a given direction of a source that emits monochromatic radiation of a frequency of 540 x 10^12 hertz and which has a radiant intensity in the same direction of 1/683 watts per steradian. Personally I rather like the idea of using a real candle since this seems to produce results accurate enough for practical photographic purposes.