What is metamerism?

WHAT IS METAMERISM?

EVERYONE IS FAMILIAR WITH THIS EFFECT: A COLORED OBJECT APPEARS TO BE ONE COLOR UNDER A GIVEN ILLUMINANT, (E.G. DAYLIGHT), BUT APPEARS DIFFERENTLY UNDER ANOTHER ILLUMINANT SUCH AS THE LIGHT FROM AN INCANDESCENT LIGHT BULB. THIS CHANGE IN COLOR, WHICH OCCURS FREQUENTLY WITH ALMOST ALL COLORED OBJECTS, HAS BEEN ERRONEOUSLY CALLED METAMERISM. BUT WHAT IS THE TRUE DEFINITION OF METAMERISM?

WE USE THE TERM METAMERISM WHEN THE COLORS OF TWO OBJECTS UNDER THE SAME ILLUMINANT, E.G. DAYLIGHT, (D65) ARE PERCEIVED TO HAVE THE SAME COLOR UNDER ONE ILLUMINANT, SUCH AS DAYLIGHT, BUT HAVE DIFFERENT COLORS UNDER A DIFFERENT ILLUMINANT, SUCH AS AN INCANDESCENT LIGHT BULB. THIS IS USUALLY AN UNWANTED EFFECT. JUST IMAGINE YOU HAVE A BLUE JACKET AND THE MATERIALS USED FOR THE SLEEVES AND BACK EXHIBIT METAMERISM THE JACKET WOULD LOOK PERFECT IN DAYLIGHT, BUT IN THE EVENING IN THE LIGHT FROM AN ELECTRIC BULB THE SLEEVES WOULD APPEAR TO BE A COMPLETELY DIFFERENT COLOR.

THIS EFFECT CAN BE EXPLAINED CLEARLY USING THE STANDARD COLOR VALUES XYZ, WHICH REPRESENT A COLOR PERCEPTION. TWO SAMPLES ARE IDENTICAL IN A CERTAIN LIGHT IF THEIR XYZ VALUES FOR THIS TYPE OF LIGHTING ARE IDENTICAL. THIS IS THE CASE WHEN WE HAVE SAMPLES WITH IDENTICAL SPECTRAL REFLECTANCE CURVES. HOWEVER, METAMERIC SAMPLES HAVE DIFFERENT SPECTRAL REFLECTANCE CURVES. THE TWO CURVES WILL HAVE THE SAME X, Y AND Z VALUES AND HAVE THE SAME COLOR UNDER ONE ILLUMINANT, BUT DIFFERENT X, Y AND Z VALUES AND DIFFERENT COLORS UNDER A DIFFERENT ILLUMINANT.