ME 8883 Physical Properties of Paper MeasurementLooking at a “spanish yellow” school bus example (from HunterLab)Color SystemsHunter L, a, b systemBrightness“GE” brightness is directionalTechnibite TB-1c conforms to ISO 2469Brightness ImprovementFluorescenceWhiteness, Yellowness and TintInstrument use detailInstrument keypadsBrightness measurement procedureInstrument detailBrightness measurement con’tColor measurementISO opacity on TB-1CFluorescence component1ME 8883 Physical Properties of Paper MeasurementLecture 13 Brightness and ColorMaterial taken from Technidyne™ and HunterLab™commercial literature and other web resources2Color instruments in 351 lab: ‘directional’ and ‘ISO’34Color measurements are dependent on the illuminant – spectrum of incident light. A source is not an illuminant. Most instruments use and A source filtered to produce a C illuminant e.g. QTHD illuminants are representative of spectral power distributions of various of actual daylight. D50 is used in graphics art industry for viewing color transparencies and prints.F2 is cool white, F7 brad band daylight, F11 is narrow-band56Instrument measurements of color require to correlate to visual perceptions of color. Technidyne diagram illustrating concept1931 CIE established the Standard Observer: monochromatic light is projected onto a white screen, 3 controllable red, green blue sources also projected are adjusted by the observer to match the monochromatic spot, wavelength is varied from 400 to 700 nm7HunterLab depiction of the 1931 CIE Standard Observer experiment8In the retina: Rods respond to intensity only – night visionCone receptors 3 types, responsive to red, green and blueIn 1931 it was thought the cones were distributed around the fovea, later found to be distributed farther so experiments were redone fro 10 degrees observation angle in 19649CIE (Commission INternationale de l’Eclairage) System 1st quantitative tristimulus system – 1931 Defined observer – sample illumination 45°, observations 0°, 76 observers Illumination A – Tungsten light at 2854 5°K B – Direct sunlight C – Average diffuse sunlight, northern hemisphere D – Fluorescent light E – Pure white light Primary Colors Red , R – 700.0 nm ⇒ X Green, G – 546.1 nm ⇒ Y Blue, B – 435.8 nm⇒ Z10()λλλλλλλλλλλλdRyECdRzECZdRyECYdRxECX∫∫∫∫====/100Four criteria • Only positive values need be combined to create any color • y - green primary function chosen to match standard luminosity • assume a standard observer • White light when tristimulus values are all equal X = Y = Z Chromaticity Components x = X/(X + Y + Z) y = Y/(X + Y + Z) z = Z/(X + Y + Z) For an illuminant E, Reflection R, per wavelength thamount of the response requied to match a given shade.111931 CIE tristimulus based on 2 deg viewing. 1964 new functions determined for 10 degree viewingDashed curves are the response curves of the colorimeter12Colorimeter uses RGB color filters for measurement. Integrating sphere Sequential filter wheelGet three light levels to produce tristimuluscolorimetric values X,Y,ZGrating or prism for selecting wavelengthsReflected spectrum is mathematically integrated to get X,Y,Z13The overall response of an instrument is given by the wavelength multiplication of the 4 elements above1) Source is quantified as a table of intensities versus wavelength2) The object is quantified as a Reflectance or transmission curve3) The observer is quantified by the CIE standard observed functions14Looking at a “spanish yellow” school bus example (from HunterLab)15Note that the yellow of the bus is a mix of wavelengths, we want to numerically describe this mix of wavelengths iof varying intensities in terms of how it is perceived by the eye.16Pictorial representation of the chromaticity calculation1718Color Systems3 main systems are discussed:1. CIE dominant wavelength, purity and luminosity (1931)2. Hunter L, a,b (1958).3. CIE, L*, a*, b* aka CIELAB (1976).CIE tristimulus functions X,Y,Z are the basic fundamental measurements – these are transformed into understandable color units. Chromaticity coordinates are calculated using:ZYXYy++=ZYXXx++=19ZYXXx++=ZYXYy++=The outside curve is the plot of the visible spectrum wavelengths in terms of the chromaticity x any y co-ordinates Outer locus are 100% saturated colors (no mix). At the center is the C illuminant “white” light.Out of the plane in tensity is called luminosity denoted by “Y”20This sample is “orange-ish” in color A sample’s color is determined by its x,y values – then a straight line from the illuminant point through the sample point to the outer locus points to the dominant wavelength No dominant wavelengths for the purplish shades – mixes or red and blue, here you get “complementary” dominant wavelengths 2122Hunterlab L, a, b system L measures the lightness, 100 for perfect white, 0 for absolute black+a indicates greenness, -a redness+b indicates yellowness, -b blueness23Delta E is the numeric difference in the color of 2 specimens24Hunter L, a, b system25CIE version of the Hunter L, a, B system is the L*, a*, b*, Conversion equations using the tristimulus equations are :2627A delta E = 1.0 is thought to be acceptable but this is subjective. A delta E > 0.4 will be perceptible generally when dealing with pastel shades of paper.Data based on many observers28BrightnessSpectrophotometric curve for pulp throughout the bleaching cycle, therefore, blue reflectance is most sensitive to bleaching effects29“GE” brightness is directionalThe readings in 45/0 (Brightimeter) Tappi 452 are MD CD dependent also dependent on the material texture, measures both effect of surface pigment and surface finish – good correlation with visual observation30We don’t have this portion in our instrument, we use the Opacimeter for Tappi opacity31Technibite TB-1c conforms to ISO 2469d/0 sphere geometry averages non-uniformities in the sampleNo directionality No correlation between ISO and Tappi brightness, e.g. embossing of towel tissue is found to decrease Tappi brightness but increase ISO brightness32Brightness Improvement1. Removal of non-cellulosic contaminants causing absorption2. Bleaching – oxidation through chemicals3. Addition of high brightness fillers4. Addition of fluorescent dyes to counteract yellowing by ligninA 2 point error in brightness can cost millions per year in a high