Whereas light sources are visible by their own emitted light, objects and materials appear to the eye according to how they modify incident light. The sensation of the color of an object is evoked by the physical stimulation of light-sensitive receptors in the human retina. The stimulation consists of electromagnetic radiation in the visible spectrum comprising wavelengths between about 380 and 780 nanometers.
Perceived color of the object is the result of a combination of factors, such as: (1) the spectral power distribution of an illuminant emitted by a light source that is incident upon the object, (2) the modification of the spectral power distribution of the illuminant by the spectral reflectance or transmission characteristics of the illuminated object, (3) the excitation of light sensitive receptors in the eye by the modified light from the object, and (4) the perception and interpretation by the brain of signals produced by the light sensitive receptors.
The perception of color is attributed to the differing spectral sensitivities of the light sensitive receptors. The trichromacy of color sensation implies that many different spectral distributions can produce the same perceived color. Such equivalent stimuli, which produce the same perception even though they are physically different spectral distributions, are called metamers, and the phenomena metamerism. For example, it is known that the perceived color of an object can change quite markedly when the object is moved from incident daylight into incident artificial light. The spectrum of the illuminating light source is also known to have an effect on the perceived colors of a printed image in spite of the considerable physiological compensation that the eye makes for differences in illumination. Light sources of differing relative spectral power distributions are therefore known to have different color rendering properties; for example, light sources which emit very narrow band, or almost monochromatic, light are considered to render colors very poorly.
According to the concept of metamerism, the respective colors of two objects may appear to be identical even though typically the spectral power distributions produced from the objects are different. Such power distributions, or stimuli, which are spectrally different but visually identical, are considered as metameric pairs. Because we measure light using only three cone types, the differences in these power distributions are indistinguishable. Two objects with different spectral reflectance functions may be perceived to match in color under one illuminant and not match under a different illuminant.
Certain aspects of perceived color have been employed to disguise images by printing an image in one color and then overprinting the first image with a pattern in a different color having approximately the same apparent brightness. Adjacent zones of equal brightness appear to be visually blended, even though they are of differing colors, thereby confusing the perception of the original image.
It is known to print patterns in different colors such that the patterns may be viewed through one or more filters having certain correlated colors, such that the patterns will change, depending upon the colors involved. It is also known to print characters in different colors in an overlapping relationship such that the overlapped characters, when viewed through one colored filter, will give the appearance of only certain ones of the superimposed characters, and when viewed through a second and differing colored filter, will reveal certain other ones of the superimposed characters. Such approaches are known for encoding (or encrypting) information to prevent recognition of the information content of the pattern until the pattern is decoded and made comprehensible. These approaches have been applied to promotional gaming technology and in document security and document verification applications.
Techniques are known for rendering flat, two-dimensional images that can stimulate an illusion of depth perception, that is, of a three-dimensional object or scene. Three-dimensional imaging can be classified into two major groups according to the quantity of information required to record the images: (1) binocular stereoscopic imaging, and (2) autostereoscopy, or three-dimensional spatial imaging. See Takanori Okoshi, Three-dimensional Imaging Techniques, Academic Press Inc., New York, (1976). Devices for performing binocular stereoscopic imaging include binocular viewers, parallax stereograms, lenticular-sheet binocular stereoscopic pictures, and binocular displays using Polaroid glasses or color filters. Devices for performing autostereoscopy include parallax panoramagrams, lenticular-sheet three-dimensional imaging, projection type three-dimensional displays, and integral photography.
In stereoscopy, a three-dimensional image is created by a series of two-dimensional images of an object captured from different perspectives, and therefore the three-dimensional image so produced contains multiple-angle information about the object. Physically displaced views of the same image are presented simultaneously to the eyes of an observer to convey the illusion of depth. These techniques typically employ a multiplexed pair of images, wherein the images are nearly identical and differ only so as to simulate parallax. The multiplexing is performed according to color, polarization, temporal, or position differences between the constituent images. For example, anaglyphic stereoscopy is a well-known process, in which left and right nearly-identical images are color-encoded by use of respective complementary color filters (e.g. cyan and red) for subsequent viewing through correspondingly colored lenses to separate the images as necessary for a simulated three-dimensional effect. When viewed through colored spectacles, the images merge to produce a stereoscopic sensation. The encoded image pair is known as an anaglyph, as it is typically rendered as two images of the same object taken from slightly different angles in two complementary colors.
This stereoscopic viewing of the multiplexed image pair typically requires the use of optical devices to channel each of the paired (left and right) images solely to the appropriate eye of the observer. A few autostereoscopic display techniques are known for providing subjectively three-dimensional viewing of a fixed image plane, without resort to eyewear and the like, by use of alternative devices based upon direction-multiplexed image displays. These devices typically employ optical diffraction, lenticular imaging, or holographic phenomena.