The Actinograph is one of the earliest exposure calculators and the first to be based on scientific investigation. The important aspects of the meter are the incorporation of Hurter and Driffield's research into plate speeds and the light curves showing a value for the light at different times of the day and year.

The Actinograph was patented in early 1888 and sold by Hurter and Driffield themselves soon after that date. The chronology here is quite important as this was more than a year before their paper that set out the definition of inertia and plate speeds and nearly four years before the first plates were marked with H & D Speed Numbers.

• 1888, the plate speed scale is marked as 'inertia'. Time scale marked 0.1s to 100s. No Sky scale.¹
• By early 1889 the plate speed scale is marked as 'Speed of Plate' and defined from tonal range values. Time scale marked 0.05 - 60s. Sky scale present.²
• By 1890 the plate speed is defined as 34 / inertia (with no change to the Actinograph scales).³
• 1892 the meter was marketed by Marion. H & D Speed Numbers, defined from inertia, are marked on boxes of their plates.⁴

Prior to the adoption of H & D Speed Numbers by plate manufacturers the user of the Actinograph would have had to find out the plate speed for himself. This could be done using the Actinograph in reverse; the plate speed was estimated and, based on that, the Actinograph was used to provide an exposure time, the photograph was taken and the estimate could be judged to be too high or too low by looking at the negative.

The Actinograph was produced for 13 latitudes.⁵ The price was originally 17/6, by 1892 this was reduced to 10/6. Later models (c. 1897), still called the Actinograph, did not have the light values printed on the revolving drum, a flat sliding table at the rear of the meter was used, the price then was 8/-.⁶

The Actinograph was not the reason for Hurter and Driffield's research, more a by-product of it, but there is a contrast between the accuracy invested in the scales and the photographer's need to determine the speed of the plate they were using. Manufacturers were not especially quick to label their plates with H & D Speed Numbers so it was left to the photographer to hold the test negative to the light and judge. It would have been better had up-to-date speed lists been issued as was done by Watkins.

The BJA 1899 states that J.W. Towers of Widnes were manufacturing H and D Photometers.⁷

Hurter and Driffield

The Actinograph was one of the outcomes of Hurter and Driffield's systematic research into the action of light on sensitive material. This was the first scientific investigation into the effect varying light levels have on the amount of silver deposited on a plate. Their findings led to the familiar characteristic or S curve, analysis of how the subject tonal range is reproduced in the negative and provided definitions for terms such as opacity and density.

During the early period of their work the concept of 'inertia' to express the slowness of a plate was used but not fully worked out. By the time of the introduction of the H & D Speed Number the definition of inertia had been quantified and plate speeds were expressed as 34 / inertia. Their system of speed rating was gradually adopted by plate manufacturers following Marion who used it in 1892.

Hurter's first meter (loosely called the Actinometer) went through a number of modifications, one of which was a self-registering device that was used in their research, it was calibrated in degrees with 100 as the maximum.⁸ The term 'actinometer degrees' was used in the patent for the Actinograph and elsewhere.

Actinograph Scales

The light curves are drawn on a revolving cylinder, next to this is a sliding plate carrying the Lens scale (nearest the cylinder) and the Exposure scale. There is then a short movable slide carrying the Sky scale and the index that points to the Plate Speed scale, which is fixed. The 1888 patent shows a meter without the short slide so the Time (exposure) scale abuts the Plate Speed (Inertia) scale.

Light curves

The light level curves, one of the fundamental parts of the Actinograph, were drawn from testing the light levels in Britain at different times of the day and for each day of the year. From these measurements a formula was derived that could be used to calculate curves for other latitudes and interpolate missing data. In making the measurements, using the earlier actinometer, the intensity of the light was expressed in 'actinometer degrees'. The mean light value, which is used in their calculations, was found to be half of the theoretical maximum.

The formula used to compile the light curves on the Actinograph was ((sin A x 100) + 5) / 2. Where A is the altitude of the sun. The value of 5 compensates for the period of twilight which would otherwise result in a value of 0. The logarithm of the value obtained was plotted as abscissa against the date as ordinate. Curves were drawn for different hours of the day.

Hurter and Driffield, as they acknowledge, were not the first to plot light levels as a function of the sun's altitude.

Inertia or Plate Speed Scale

1888 Patent - This scale as shown in the patent is marked in inertia ranging from 0.01 (on the left of the scale) to 1, faster speeds have smaller inertia. Inertia was originally defined as "By this term we express the time, measured in seconds, which the sensitive plate must be exposed to a direct diffuse daylight, of the intensity 1 'actinometer degree', in order that on development sufficient of the silver salt on the plate may be reduced to silver, so as to become as dense as the sky of a well exposed landscape negative ought to be." Most plates then on the market had inertia values, as measured by H and D, ranging from 0.08 to 0.35 of a second, 'ordinary' had roughly a value of 0.2, 'rapid' 0.14 and 'extra rapid' 0.08. In constructing the inertia scale, the logarithms of the numbers are marked on the scale.

By 1889 the 'Speed of Plate' was marked on the scale, the definition was "the time taken to produce a density ratio of 1.7 - 1 for an ordinary landscape negative. A plate of speed 1 produces this ratio of density between sky and foreground (1.7 - 1) in 1 second with 1 degree of light (actinometer degrees), a plate of speed 100, produces the same result in 1/100th of a second." H and D give details of how in practice the speed can be determined using the calculation: Plate speed = S / I × t, where S = slowness of lens, I = light intensity and t = time.⁹ This definition includes the tonal range but does not use the term inertia, later the plate speed would be defined in terms of inertia and adopt the same numerical values.¹⁰

The speeds marked on the scale range from 0.5 up to 300 (left of scale); 0.5 is about the speed of a wet plate, and 50 is about the speed of a quick dry plate. Roughly the speeds for gelatino bromide plates were grouped as: slow, 2 to 6; ordinary, 6 to 15; rapid, 15 to 25; and extra rapid, 25 to 50.

Lens Scale

One of H and D's earliest calculations was on the light transmission of lenses. Possibly uniquely, for an exposure meter, they included the absorption properties of the lens as a factor. The exposure calculations included the slowness of the lens based on the f number and the number of lens elements. From experiments they found that the required exposure time is about 15% more than indicated by the f number for single lenses, 32% more for double lenses, and 53% more for triple lenses. The lens scale is marked with small figures of 1, 2 and 3 above each f number for the three types of lens.

Exposure Scale

In the 1888 patent this is shown as being marked in seconds from 0.1s to 100s. By 1889 it is scaled from 0.05s to 60s.

Sky Scale

The scale has five indices, marked 'Very bright', 'Bright', 'Mean', 'Dull' and 'Very dull'. This is not present in the 1888 patent.

Table of Factors

The light curves were correct for open landscape scenes, for other subjects the indicated exposure time had to be modified. The lid of the Actinograph contained a table of subjects and the appropriate exposure factors, these ranged from ¼ to 300 times the value indicated by the meter.

Taking a Reading

In use an index is set against the plate speed. The aperture to be used is set against the light value graph on the drum. One of the five indices for the sky conditions is used to give the exposure time.