What are color mixtures?

WHAT ARE COLOR MIXTURES?

A DISTINCTION IS MADE BETWEEN TWO BASIC TYPES OF COLOR MIXTURES: ADDITIVE AND SUBTRACTIVE. ADDITIVE COLOR MIXTURES WERE DISCOVERED IN 1860 BY MAXWELL. THE TERM MEANS THAT EVERY COLOR CAN BE PRODUCED BY MIXING TOGETHER IN DIFFERENT QUANTITIES RED, GREEN AND BLUE (R,G,B) COLORED LIGHT. IF THE THREE COLORED LIGHTS, - RED, GREEN AND BLUE, - ARE MIXED IN EQUAL PROPORTIONS, WHITE LIGHT IS OBTAINED. ADDITIVE COLOR MIXTURES ARE USED IN ELECTRONIC IMAGE PROCESSING AND IN TELEVISION TECHNOLOGY, WHERE PHOSPHOR DOTS THAT EMIT RED, GREEN AND BLUE LIGHT ARE EMBEDDED IN THE SHADOW MASK OF THE SCREEN. THE DOTS OF COLORED LIGHT CANNOT BE SEEN INDIVIDUALLY BECAUSE THEY ARE SO TINY, SO AN ADDITIVE COLOR MIXTURE IS FORMED AND THIS COLOR IS WHAT IS PERCEIVED.

HOWEVER, PIGMENTS AND DYES RATHER THAN LIGHTS ARE USED TO COLOR MATERIALS. IF THE THREE PRIMARY COLORANTS CYAN, MAGENTA AND YELLOW (C, M,Y) ARE MIXED IN EQUAL QUANTITIES, BLACK IS OBTAINED. THIS IS CALLED A SUBTRACTIVE COLOR MIXING. THIS IS BECAUSE COLORANTS SUCH AS DYES AND PIGMENTS ABSORB SOME OF THE INCIDENT LIGHT AND SUBTRACT IT FROM THE REFLECTED LIGHT. ADDING COLORANTS MAKE THE MATERIAL DARKER. SUBTRACTIVE COLOR MIXING OCCURS IN PHOTOGRAPHIC COLOR REPRODUCTION, PRINTING AND COLORING TEXTILES, PLASTICS, PAPER AND GLASS.

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.

What are tolerances?

WHAT ARE TOLERANCES?

NO PAIR OF SAMPLES HAS 100% IDENTICAL COLOR VALUES, AND YET SUPPLIERS HAVE TO SATISFY THE CUSTOMER'S QUALITY REQUIREMENTS FOR COLOR ACCURACY WHEN SUPPLYING PRODUCTS. WE ALSO KNOW THAT A HUMAN BEING'S PERCEPTION OF COLOR IS VERY SUBJECTIVE.

IT IS THEREFORE IMPORTANT THAT SUPPLIERS AND CUSTOMERS USE COLORIMETRY TO STIPULATE COLOR TOLERANCES, THE MAXIMUM COLOR DIFFERENCES WITHIN WHICH PRODUCTS WILL STILL BE ACCEPTED.

COLOR TOLERANCES CAN BE ESTABLISHED IN ALL COLOR SYSTEMS, INCLUDING THE CIE XYY SYSTEM OR THE CIELAB SYSTEM, WHICH IS THE ONE MOST FREQUENTLY USED. HOWEVER, THE DISADVANTAGE OF THE TRADITIONAL SYSTEMS IS THAT VISUALLY THESE DO NOT HAVE EQUIDISTANT SPACING. WHAT DOES THIS MEAN? IN THE CIELAB SYSTEM, FOR EXAMPLE, YOU COULD ESTABLISH THAT A MAXIMUM COLOR DIFFERENCE OF DE*=1 IS ACCEPTABLE. HOWEVER, WHEN ASSESSING VARIOUS PAIRS OF COLORED SAMPLES WITH A MEASURED COLOR DIFFERENCE OF DE*=1, YOU FIND THAT DE*=1 MAY WELL BE AN ACCEPTABLE COLOR DIFFERENCE WITH BRILLIANT YELLOW OR GREEN TONES, BUT WITH ACHROMATIC, GRAY COLORS DE*=1 REPRESENTS ANOTHER, UNACCEPTABLE COLOR. THE SAME MATHEMATICAL DIFFERENCE OF 1 DOES NOT THEREFORE CORRESPOND TO OUR VISUAL PERCEPTION.

SEVERAL NEW TOLERANCE FORMULAS HAVE BEEN DEVELOPED IN ORDER TO AVOID HAVING TO ESTABLISH DIFFERENT COLOR TOLERANCES IN THE CIELAB SYSTEM FOR EVERY COLOR. THESE FORMULAS CORRECT THE NON-EQUIDISTANT SPACING OF THE CIELAB SYSTEM. TODAY THE MOST COMMON PUBLISHED TOLERANCE FORMULAS ARE THE CMC AND THE CIE-94 FORMULAS. HOWEVER, THESE ARE MORE OFTEN BEING REPLACED BY MORE RECENT TRANSFORMATIONS SUCH AS DIN99 OR CIEDE2000.

What is whiteness?

WHAT IS WHITENESS?

WHITE IS A COLOR TOO. ALTHOUGH IT IS ACHROMATIC, IT IS STILL A COLOR IN THE COLORIMETRIC SENSE. WITH AN IDEAL WHITE, ALL THE INCIDENT LIGHT IN THE 400-700 NM RANGE IS REFLECTED. THIS "IDEAL WHITE" IS, HOWEVER, PURELY A THEORETICAL QUANTITY. BARIUM SULPHATE, WHICH WAS ONCE WIDELY USED AS A WHITE STANDARD, "ONLY" GIVES AN AVERAGE REFLECTION OF 98% ACROSS THE VISIBLE SPECTRUM.

"WHITENESS THAT IS, HOW WHITE A MATERIAL IS IS A YARDSTICK FOR QUALITY FOR MANY TYPICALLY WHITE MATERIALS SUCH AS PAPER OR TEXTILES. THE "WHITENESS" OF A MATERIAL CAN ADMITTEDLY COME VERY CLOSE TO AN IDEAL WHITE IF THE MATERIAL IS BLEACHED, THUS DESTROYING THE COLOR PIGMENTS THAT ABSORB LIGHT. BUT IF A WHITE NEEDS TO BE "BRIGHT" THEN OPTICAL BRIGHTENERS ARE USED TO OBTAIN A "PERFECT" WHITE. THESE OPTICAL BRIGHTENERS, WHICH ARE ALSO FOUND IN EVERYDAY LIFE IN WASHING POWDERS AND TOOTHPASTES, FOR EXAMPLE, HAVE PROPERTIES THAT ENABLE THEM TO ABSORB RADIATION IN THE UV RANGE (<400> 400 NM). MATERIALS THAT HAVE BEEN TREATED WITH AN OPTICAL BRIGHTENER CAN THEREFORE HAVE A REFLECTION VALUE OF >100%. THESE MATERIALS APPEAR TO BE "BRIGHT" WHITE.