Fundamentals of Optics and Radiometry for Color Reproduction

International audienceColoring a surface to form an image is a very ancient activity, which is based on a simple principle: the deposition of coloring layers on a reflecting support. In painting or analog photography, these coloring layers are continuous. They cover the entire surface and more or less absorb light depending on their thickness and dye concentration. The obtained colors are called “continuous tone” or “contone.” In printing, the coloring layers are discontinuous. The inks have fixed thickness and colorant concentration; the different tones are obtained by varying their surface coverage ratio, yielding the “halftones” colors. Historically, the color rendering of images was controlled by the painter, the photographer, or the printer, that is, by a specialist who somehow acquired his expertise in the selection of the materials and the control of the coloration process. The new printing technologies have increased the image reproduction quality and the ubiquity of color images in everyday’s life. They have also provided he possibility for nonexpert consumers to print themselves at home, thanks to fully automated printing processes. In the absence of a color expert, the printer needs to be calibrated by the constructor. This requires to relate color rendering to technical parameters (dye concentration, ink thickness, etc.) according to a scientific approach relying on the physical measurements. The task for the scientists in this domain is double: understanding the physical phenomena being at the origin of the color rendering and predicting the color rendering for given printing specifications. Before entering into the physical characterization of the printed colors, it should be recalled that color is not a physical quantity but a physiological sensation. This sensation is the response of our visual system to a light signal striking the retina. During the twentieth century, scientists managed to elaborate a mathematical description of the color sensation and to connect it with the spectrum of the light received by the retin . However, the study of color and the study of light are two scientific domains based on very different concepts called colorimetry and optics, respectively. Light can be characterized by its energy, speed, wavelength, direction, and polarization, but it would be erroneous to say that it “has a color.” Similarly, it is simplistic to say that a surface has a color. We should rather say that it has the aptitude to reflect a fraction of the ambient light depending on its spectrum attenuation capacities, which creates a luminous signal that human brain perceives as a color. A complete description of the print color rendering should, therefore, rely on physical and perceptual analyses. However, we generally assume that these two analyses can be treated separately, that is, the luminous signal issued from the surface is fully characterized by the physics and the interpretation of this luminous signal in terms of color is described by colorimetry. In this chapter, we focus on the physical analysis. Optics is the sci ntific study of light and its interaction with matter. It covers a wide range of phenomena and applications. We focus here on the basic notions necessary to understand surface color prediction models. We, first, recall briefly what is light and what kind of light is considered in color reproduction. Then, we introduce radiometry, the branch of optics that deals with light measurement, and models for absorption, reflection, and refraction by a surface, scattering, and special effects such as fluorescence