Isaac Newton had demonstrated that all colours can be produced by an appropriate combination of red, green and blue light. Once black and white photography had been perfected to the point when film was sensitive to all colours (panchromatic) then the race for a viable system of colour photography was on. Early attempts took separate photos through red green and blue filters and then used various means to combine these to produce prints or images on a screen. The easiest method was to make black and white transparencies from the original negatives and then use 3 projectors to project the transparencies through red, green and blue filters to give a full colour image on a screen. This was obviously not a practical system for widespread use.

A Frenchman called Louis Dufay invented a process whereby the back of a photographic plate was covered with a fine grid of red, blue and green before being coated with sensitive emulsion. The plate was exposed through the back and then processed to give a positive transparency. When viewed against a bright light source then a full colour image resulted. (It is this process of viewing combinations of red, blue and green dots or stripes that gives us our PC colour monitors today. Look at the screen of your monitor through a magnifier to see what I mean)

Dufay's plate of 1908 was later taken up by Dufay-Spicer in England and applied to roll film and sheet film, which was marketed by Ilford. The same principle of red, blue and green filters was used with a 'fast' black and white panchromatic emulsion. Spicers were a precision printing company and it was their technology that enabled the extremely fine grid or 'reseau' of lines to be printed on the film. For roll and 35mm film the reseau was repeated at 23 groups per millimetre.The film was first marketed in England in 1935, and was even available later as 16mm movie film.

I was given an old book about the process in the 1960s and then during the early 70s amazingly I found an advert in a photo magazine for rolls of 35mm film. By then it was well out of date, but still usable. I bought a roll of about 50 feet and proceeded to try it out. The first problem was the film speed. The book gave exposure details for various conditions and I used these as a starting point. I used the formulæ for processing in the book and got little or no results. So I gave up for a time. Later in the decade I had been processing my own black & ciné film with success, so I though I would try the process on the Dufay colour film. The slides shown below are a few of the resulting slides. The originals have low contrast (old film) and are rather pink (too much red filter), so I have corrected this on my Mac computer. They give an idea of the best that might be achievable from the process. With the arrival of subtractive dye image processes such as Agfacolor and Kodachrome, Dufaycolour was doomed and ceased to exist in the 1950s. It's been great fun to take an old process like this and get some results. I still have about 10 feet of unexposed film, but by now it must be considered near useless.

	Magnified detail of the grid or reseau on the back of the film. The lines were set at 45° to the film edge and were repeated at 23 line sets per millimetre. Note that the red line is half as wide as the blue and green squares in order to give an even colour balance. In my sample the red lines must have been too wide as the slides had an overall red cast.
	This is the centre of Cambridge, England. Exposure was based on about 2 ASA. The low speed of the film is caused by a slow emulsion to start with, which has lost speed though age, and the light loss though the filter. Less than 1/3rd of the available light can reach the emulsion.
	A cottage in the country just outside Cambridge. This was chosen for the red roof contrasting with the blue sky. All pictures were taken using a Praktica LTL 35mm camera with Meyer Oreston 50mm f1.8 lens.
	My house in Portsmouth, showing my wife's wonderful display of flowers. As can be seen this process produced a full colour range and was probably quite acceptable with new film, particularly in larger sizes.
	County public house in Cambridgeshire. Slides were scanned using a UMAX 3450 scanner and Graphic Converter software onto my iMac 350. Graphic Converter was used to adjust the contrast, brightness and colour balance. I think this picture needs some more work.

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