1264
- Equipment for making a consistent exposure to a
plate, - Implementation of an agreed development proto-
col, - A means of accurately measuring the ensuing densi-
ties.
The designs of the fi rst sensitometers incorporated
ingenious exposing devices. Leon Warnerke relied
on an energised phosphorescent block of calcium
sulphide for his exposing source. James Spurge (work-
ing with J.D. Mucklow) proposed a system of tubes
that channelled controlled amounts of daylight onto
the test plate. Exposing through “a screen” (a step
wedge) impressed a set of numbers on the negative
and, after development, the fi rst identifi able number
established the rapidity of the plate. For example, in
1890, Marion & Company advertised their Ordinary
dry plates as “Averages 19–20 Warnerke’s Sensitom-
eter” and advised users to “double the exposure of the
Instantaneous,” which had an average rapidity of 24–25
(on Warnerke’s scale).
Before then, however, some workers had encountered
conundrums, which were not resolved until sensitometry
improved. In 1874, William de Wiveleslie Abney noticed
a disparity between time of exposure and intensity of
light whilst examining the opacity of his negatives. By
the time he had solved the riddle, he had developed his
own method for measuring the deposits on negatives
and was confi dently testing photographic materials
objectively.
To measure the transparency (or the opacity) of
a negative, Abney used the shadow photometer that
Benjamin Thompson (Count Rumford), had designed
to compare the strength of different forms of artifi cial
lighting in 1793. Although Abney introduced improve-
ments to the instrument by inserting rotating sectors (i.e.,
a system of variable apertures), which were positioned in
front of one of two light sources, he retained Rumford’s
principle of balancing the intensities of the two shadows
projected on a screen. The amount of adjustment that
was necessary to achieve the match indicated the opac-
ity of the deposit.
A limitation of the method when applied to photo-
graphic work was the need to use relatively large speci-
mens. Warnerke’s sensitometer and Abney’s photometer
for measuring density (that is, a densitometer) were
pragmatic solutions devised to solve specifi c questions
as they occurred. When practitioners appreciated the
value of sensitometry and densitometry, the techniques
improved and evolved as an important branch of pho-
tographic science.
By 1888, Ferdinand Hurter and Vero Charles Driffi eld
had begun to collaborate on experiments in photographic
chemistry. The initial work had been undertaken with a
view to improving the calculation of camera exposures,
but their investigations were later directed towards
understanding the action of light on photographic emul-
sions. To make their studies comprehensive, Hurter and
Driffi eld standardised their tests by exposing plates to
the light of a standard candle, modulated by a rotating
disk designed to control the amounts of light reaching
the plate. They also specifi ed a standard developer (fer-
rous oxalate) and described apparatus they had made for
making measurements.
Hurter and Driffi eld also introduced the word “den-
sity” to describe the individual amounts of silver that
were produced during development, and by plotting
density measurements against the given exposures, they
constructed a curve, which displayed the characteristics
of the sensitised material being examined. Hurter and
Driffi eld formed their conclusions from a range of den-
sity readings, (instead of the single number obtained by
Warnerke), and their technique was consequently more
reliable. From their characteristic curve, they were able
to determine the sensitiveness of plates, and by the end
of the 19th century their method was recognised and
accepted as the H & D Speed System.
A contemporary of Hurter and Driffield, Henry
Chapman Jones, described his densitometer in 1895,
which he claimed as an improvement on other designs
because he had been “desirous of getting a more simple
arrangement.” His achievements were to eliminate the
need for two light sources (the reference beam and the
measuring beam), to reduce the loss of light in the opti-
cal system and to measure in “one simple movement..”
Jones’s densitometer remained popular until improved
designs were introduced in the 20th century.
Ron CallenderSee also: Ponton, Mungo; Warnerke, Leon; Marion
& Son, A.; Abney, William de Wiveleslie; and Hurter,
Ferdinand, and Driffi eld, Vero Charles.Further Reading
Abney, William de Wiveleslie, A Treatise on Photography, 8th
ed., London: Longmans, Green and Co., 1893.
——, “Photometry, The Royal Society of Arts’ 3rd Cantor lec-
ture.” Photographic News, 38, 499, 521, 542, 547, 1894.
——, The Action of Light in Photography: Evening talks at
The Camera Club, London : Sampson Low, Marston and
Company, 1897.
——, Instruction in Photography, (11th edit.), London : Iliffe
and Sons Limited, 1905
——, “On the opacity of the developed photographic image.” The
Philosophical Magazine (September 1874): 1–5.
——, “Chemical Action and exposure; or, a failure in a photo-
graphic law.” Photographic Journal, XIX, (1894), (Compli-
mentary reprint).
Brown, S C, Benjamin Thompson, Count Rumford, Cambridge,
MA: The MIT Press, 1979.