In 1892 the red window was introduced by the Boston Camera Manufacturing Company as a means to control film advance of paper-backed roll film in the No. 2 Bulls-Eye camera. Later this became the film advance control device of nearly all cameras for the popular roll film formats 127 (4cm) , 120 and 620 (6cm). The window shows an exposure number that is printed on the film's backing paper. The appearance of the next exposure number in the window means that the film is advanced enough for achieving non-overlapping exposures. Some cameras have an exposure counting window of different color, for example the Rollex 20 (green window), but red was by far the most popular, as early black-and-white films were quite insensitive to red light.
|Looking behind a 120 film box camera's door: the |
number visible through the red window is one of the
exposure numbers of the eight possible 6×9cm
exposures. Other number rows are visible through
exp. counting windows of cameras with smaller
frame formats for 12 or 16 exposures..