1 LANGFORD'S ADVANCED PHOTOGRAPHY Figure 1.4 Colourisperceivedwhenwavelengthsfromalightsourcearereflectedbythesurfaceofanobject.Thehumanvisual systeminterpretsthewavelengthcompositionsasthecolouroftheobject. Methods of colour reproduction Additive method James Clerk Maxwell proved the trichromatic theory in the 1850s by demonstrating with light from blue-, green- and red-filtered lanterns overlapped on one screen. Dimming or brightening individual lanterns recreated all the colours of the spectrum, while an equal mixture of the three primary colours formed a patch of white. Combination of two primary colours results in a secondary colour. This is shown in Figure 1.5(a). This type of colour mixing is called additive. Note that during the 1870s and 1880s the technique of pointillism was used by French Impressionist painters such as Monet, Pissaro and Seurat. Pointillism is the technique of juxtapositioning tiny brush strokes in paints of strong luminous colours and subtle tones. It is a significant fact that the very first colour photography materials to go on sale were manufactured in France (Lumière's `Autochrome', 1907) and worked on what is known as the additive principle. With the additive method, you can create any colour by varying the proportions and intensities of red, green and blue points in an image. When viewing an image from a distance, the eye sees a uniform colour and not individual colour points. One of the applications of this method is in the design of television and computer displays (see page 183). The colours are formed by very small red, green and blue pixels, which you can discriminate only by very close inspection of the screen. The additive method has also been used for instant picture materials and for the design of digital imaging sensors. T wo methods of colour reproduction exist: the additive method and the subtractive method. Subtractive method Another method, the subtractive method, has its origins in work by the French scientist Luis Ducos du Hauron in the late 1800s. He published a book, Les Couleurs en Photographie, in 1868, forecasting several of the colour reproduction systems we now use. However, it was not until the 1880s that Dr Hermann Vogel's research in Germany into sensitizing additives made possible orthochromatic and panchromatic emulsions to add to the blue-sensitive materials used exclusively up until this time. Panchromatic response to all colours, and ability to control colour sensitivity, unlocked the door to practical colour photography. 4