Negative/positive paper is a type of colour photographic printing paper specifically designed and sensitised to produce a positive colour image when exposed to light through a colour negative and processed through the appropriate colour print chemistry. It is the standard medium used in conventional colour darkroom printing, and the paper from which the vast majority of colour photographic prints - whether produced in a professional darkroom, a minilab, or a large scale photofinishing laboratory - have historically been made.
The negative/positive relationship that gives this paper type its name describes the two stage process by which a final positive colour print is derived from an original colour negative. In the first stage, the camera records the subject on colour negative film, producing a negative image in which tones are reversed and colours are represented in their complementary opposites - the familiar orange masked colour negative. In the second stage, light from an enlarger is passed through this colour negative and projected onto the negative/positive paper, which responds to the reversed tones and complementary colours of the negative and produces a second reversal, restoring the correct tonal values and true colours of the original subject in the finished print.
Negative/positive colour printing paper consists of a paper or resin coated base onto which three separate light sensitive emulsion layers are coated, each sensitive to one of the three primary colours of light - blue, green, and red - and each containing colour forming dye couplers that produce yellow, magenta, and cyan dyes respectively during the colour development process. When the paper is exposed through the colour negative, each emulsion layer receives an amount of exposure corresponding to the density of the complementary dye layer in the negative at each point in the image, and during development the colour couplers in each layer react with the oxidised developer to form the appropriate amount of each subtractive primary dye, building up the full colour positive image through the combined effect of all three dye layers.
Negative/positive papers are processed through the RA-4 chemistry - or its predecessor the EP-2 process - a relatively simple two bath process comprising a colour developer and a combined bleach-fix solution, followed by washing and drying. The RA-4 process is designed for rapid processing at elevated temperatures, making it suitable for both manual darkroom printing and automated minilab and photofinishing equipment. The simplicity and speed of the RA-4 process compared to the more complex processing required for colour reversal printing papers contributed to the widespread adoption of colour negative film and negative/positive paper printing as the dominant system for consumer and professional colour photography throughout the latter half of the twentieth century.