This interesting device from Harrison in America, was made for use with the Weston Master series of exposure meters and in effect, turns the meter into a colour temperature meter. It comprises of three mechanical components: An adjustable baffle, red and blue filters. Once locked over the exposed (with the baffle out) photo-electric cell, the meter is offered up to the incident light source. The calculator panel is slid out but the filter plunger is pushed fully home, switching the red filter into place.
The toothed adjustment wheel of the baffle is then rotated until the meter needle reads 3.2 (Candles / Sq.Ft.) This reference point refers to a colour temperature of 3200 degrees Kelvin (Tungsten photographic lighting). The plunger is then released to switch in the blue filter. The new meter reading is noted.
From this point the calculation table is referred to. This will give the colour temperature for the noted value. In addition it shows the filter reference needed to provide correct colours for Tungsten, Photoflood and Daylight balanced films. Exposure compensation values are also listed.
For photographers who may not have an invercone available, the unit can also be used to provide incident light readings, the variable blind being set to a specific reference point on the adjustment wheel.
The rear of the Harrison unit showing the locking ring, the variable baffle and serrated adjustment wheel and the blue filter (plunger out) which gives the final colour temperature reference.
The metal slide out calculation panel allowing colour temperature, filter and exposure compensation references to be read off. The colour correction discs (filters) listed on this were also available from Harrison.
The red filter in position (Plunger in). The blinds are adjusted to give an initial ight reading and reference point of 3.2 Cd/SqFt
As well as the Color Correction attachment for the Weston meter, Harrison also marketed sets of correction filters to support the listings given in the calculation panel (above). 22 filters represent the range from B 1/8 to C8. C9 to C14 only apply to tungsten corrected film (and may possibly have been achieved using multiple filters).
The sets were presented in high quality robust hardwood boxes with each gelatine filter sandwiched between optical quality glass. In this example, 40mm filters are contained in felt lined aluminium cans. A table (below) details the set and lists the exposure correcting factors needed.
These sets were available in different configurations. Some included the Harrison color correction attachment itself and space for a Weston meter. Square lantern-slide format sets of filters were made for work with larger primary camera lenses.