Pathé invented the amateur 9.5mm format in 1922, and it remained popular throughout Europe until after the Second World War. However, during the 1950s, the 9.5mm format declined in popularity. It was no longer considered the most cost-effective format for non-professionals. The amateur film market favored 8mm, which benefited from the huge popularity of 8mm Kodachrome, as well as the fact that 9.5mm stock was twice the price of 8mm. To preserve its flagship format and remain competitive, Pathé capitalized on the popularity of the new widescreen formats in commercial cinemas to modernize and extend the uses of 9.5mm. 

Research for a new format began in November 1954. Without discarding the technical characteristics that had defined the 9.5 format – its width, frame pulldown, or central-perforation principle – the engineers began to think about a new perforation system that would enable them to change the image ratio while keeping the same width of film. The single central hole was replaced by a pair of perforations close to the central axis. With this new double-perforated film, known as Duplex, it was possible to shoot in two different framings: vertically, in the standard 9.5 format; or horizontally, in a panoramic format occupying half of each frame. In panoramic mode, after processing the Duplex film was slit in half lengthwise to become a Monoplex film, 4.75mm-wide, with a single, central perforation. This doubled the Monoplex projection time compared with Duplex. Double format, double length – the system offered mulitple economic advantages. 

Although the horizontal Monoplex format did not increase the size and resolution of the projected image – as was the case with theatrical widescreen formats like CinemaScope and VistaVision – it did offer amateur film enthusiasts the opportunity to make films with a panoramic aspect ratio, which was, at this time, becoming the new aesthetic standard for cinematographic art. Similar initiatives followed from industry competitors, with other manufacturers offering “wide” formats based on 16mm and 8mm – such as the 16/8 Panoramique camera from Dimaphot and Emel’s Panascope (Pan-8) in France – but Monoplex was the first amateur widescreen format brought to market. The image area of Monoplex was 60 per cent larger than that of a standard 8mm frame. Pathé’s claim that the 9.5 Duplex was “the cheapest [widescreen] film in the world” was arguably true. The Monoplex widescreen mode was an inventive way of “modernizing” 9.5mm, presenting a more attractive and cost-effective offering to the consumer.

Société Commerciale et Industrielle Pathé (SCIP) signed an agreement with Kodak-Pathé to produce 9.5mm Duplex perforated film and develop the industrial tools required for its manufacture: special perforating machines and slitters. To support Duplex’s launch and hopefully ensure its commercial success, SCIP hired Bureau Technès, a leading industrial design consultancy headed by Roger Tallon, to design the new, state-of-the-art equipment.

The resulting Lido camera, and accompanying Monaco projector, were praised for their innovative design and served as a showcase for the new format. SCIP premiered 9.5mm Duplex film at the inaugural Biennale de Photo-Cinéma, held at the Grand Palais, Paris, in May 1955. Commercial distribution didn’t fully begin until spring 1956, mainly in France and neighbouring European countries, but also in Australia and New Zealand. The invention of the Duplex-Monoplex format was acclaimed by the trade press and raised the hopes of customers loyal to the 9.5mm. However, it was not as successful as hoped. The narrowness of 4.75mm Monoplex film made it difficult to handle, the increasingly competitive amateur film market and the ascendancy of 8mm made the decline of 9.5mm inevitable. Although manufacture of the Lido camera and Monaco projector was discontinued in 1962, Kodak-Pathé continued to manufacture double-perforated Duplex film until about 1968.