RESEARCH ARTICLE Coloured shadows—Why they can be photographed Cecilia Sik-Lanyi 1 | Veronika Szücs 1 | Robert Hirschler 2 1 Department of Electrical Engineering and Information Systems, University of Pannonia, Veszprém, Hungary 2 Colour Consultant, Budapest, Hungary Correspondence Robert Hirschler, Attila utca 93-95, H-1039 Budapest, Hungary. Email: robert.hirschler@yahoo.com Abstract Coloured shadows have been observed, studied, and described for centuries, raising heated debates on whether they were objective or subjective, real or imaginary, psy- chological or physiological. We can demonstrate this phenomenon by projecting coloured light from one and white light from the other projector on a screen, where an object casts two shadows: one (illuminated by coloured light) expectedly coloured and the other (illuminated by white light) unexpectedly coloured. One of the seemingly most solid arguments for the objectivity of the unexpected colour was the fact that it could be photographed. The aim of this work was to photograph the shadows both with conventional (colour slide film) and digital cameras and explain why the unex- pected colour of the shadows appears in these photographs. With spectroradiometric measurements of the shadows themselves and also those of the photographs, it may be proven that this is simply a spectacular case of adaptation and simultaneous contrast. Digital rendering of the coloured shadow situation also supports this conclusion. Coloured shadows are excellent tools for the demonstration of adaptation and simulta- neous contrast. KEYWORDS adaptation, coloured shadows, complementary colours, simultaneous contrast 1 | INTRODUCTION Coloured shadows are a curious phenomenon, caused by two light sources of different colours illuminating an opaque object from two different directions, thus casting two shadows. Figure 1 shows a modern experiment using two slide projectors, demonstrating coloured shadows. Magenta light from projector 1 is blocked by the object, so it casts a shadow illuminated by white light (therefore, we can consider it a half-shadow). In the same way, white light from projector 2 is blocked by the object, so it casts a shadow illuminated by magenta light. Where the object is not blocking the light from either projector, we have a background formed by the mixture of magenta and white light (a kind of light magenta) (see Figure 1). What is surprising is that the half-shadow illuminated by white light is not gray but appears definitely green (or, depending on the coloured light from projector 1, always its complementary colour). To distinguish between the two “coloured shadows” (which, as we shall see, are most certainly not formed the same way), Starrett 1 called the one illuminated by coloured light sim- ply coloured shadow and the other one illuminated by white light unexpectedly coloured shadow. In German literature, these are often called gefärbten Schatten resp. farbige Schatten. Sorensen 2 goes so far as to declare that coloured shadows are really “filtows,” bodies of filtered light, and shadows are not bodies of light—but this seems to be just a philosopher's exaggeration. We suggest calling the first coloured shadow because it is really coloured by the magenta light source. The shadow illuminated by the white light source is, in the physical reality, as we shall see, neutral (gray)—it only appears to be green; we suggest call- ing it apparently coloured. The phenomenon of coloured shadows has been observed, studied, and described for centuries. Leonardo da Vinci 3 already observed that shadows are not necessarily black. He Received: 16 April 2019 Revised: 10 July 2019 Accepted: 11 July 2019 DOI: 10.1002/col.22420 Color Res Appl. 2019;1–16. wileyonlinelibrary.com/journal/col © 2019 Wiley Periodicals, Inc. 1