Object-Colour Manifold
Colorimetry can predict which lights will look alike. Such lights are called metameric. Two lights are metameric if they have the same tri-stimulus values. Using the tri-stimulus values as Cartesian coordinates one can represent light colours as points in a 3D space (referred to as the colorimetric space). All the light colours make a tri-dimensional manifold which can be represented as a circular cone in the colorimetric space. Furthermore, colorimetry also claims that reflecting objects illuminated by the same light will look alike as soon as they reflect metameric lights. All the object colours are then represented as a closed solid inscribed in the light colour cone provided the illumination is fixed. However, as argued in this article, the reflected light metamerism does not guarantee that the reflecting objects will look identical (referred to as colour equivalence), especially when there are multiple illuminants. Moreover, colour equivalence cannot be derived from metamerism. The colour of a reflecting object under various illuminations is shown to be specified by six numbers (referred to as its six-stimulus values) that can be established by experiment. Using the six-stimulus values one can represent the colours of all the reflecting objects illuminated by various illuminants as a cone (without a vertex) through a 5D ball.
colour colour equivalence colour space colour theory