Metamer Mismatching and Its Implications

​

The color signal of a reflective object is subject to change if the illuminant is changed. Objects with different spectral reflectance functions are called metamers if they induce the same color signals under one light. But their color signals may no longer match under a different light. This phenomenon is referred to as metamer mismatching. Given a surface reflectance illuminated by a given light, there can be many other surface reflectances for which the eye provides an identical cone-triple response or an identical RGB response in the case of a camera. The tristimulus color values of these metamer reflectances can disperse into many different tristimulus values under a different illuminant. The set of all such possible tristimulus values defines a convex hull known as a Metamer Mismatch Body (MMB). A sample MMB is plotted in Figure 1. Metamer mismatching poses several challenges in color-based machine vision.

​

In this work, we address some of the current challenges in the field of Color Science by measuring the extent of metamer mismatching. We begin by investigating the performance of existing color prediction methods in predicting the results of an asymmetric color matching experiment. Because of the possibility of metamer mismatching, it is a mistake to interpret one answer as the ‘correct’ answer. However, we demonstrate that all the studied computational methods are not capturing some important aspects of the observers’ asymmetric matching strategy. By modeling the MMB by its equivalent (in terms of its inertial moments) ellipsoid, a new metric for evaluating the colorimetric accuracy of digital color cameras is proposed. The advantage of the new metric is that it is based on a theoretical principle rather than simply computing the average error over a chosen set of representative test reflectances. Furthermore, a hypothesis that metamer mismatching is the underlying cause of the variations in color discrimination ellipsoids is proposed. Statistical analysis over the existing datasets provides evidence that metamer mismatching can possibly explain color discrimination threshold variation and paves the way to predict the Just Noticeable Differences (JNDs) in color.

​

Emitis Roshan is a doctoral candidate at Simon Fraser University.

​