Food Physics - Colour Measurement.pdf

Food Physics Professor Enda Cummins Email: [email protected] Extn:7476 4. COLOUR MEASUREMENT • Appearance – 2 attributes Chromatic – related to colour e.g hue, saturation Geometric – shape, distribution of light, gloss, haze Change in one = perceived change in colour • Define colour in physical...

Food Physics Professor Enda Cummins Email: [email protected] Extn:7476 4. COLOUR MEASUREMENT • Appearance – 2 attributes Chromatic – related to colour e.g hue, saturation Geometric – shape, distribution of light, gloss, haze Change in one = perceived change in colour • Define colour in physical sense in terms of physical attributes of food • Has serious limitations when we use colour measurement as a quality control mechanism for food manufacturing operations • Better approach is to define colour as objectively as possible and interpret output in terms of how human eye sees colour • Munsell system which contains 1225 chips for visual comparison • Results are very subjective; they vary with assessors and conditions and there are limitations in the no. of colours that maybe used • Hence colour spaces have been developed (e.g. CIE, Yxy, L*C* Ho, XYZ, Hunter L * a * b *) • Hunter Lab and CIE L* a* b* most popular used today • Both mathematically derived from x,y, z values • x, y, z standard observer curves are a plot of human response against colour wavelength Lab = L is lightness or darkness, +a is redness, -a is greeness, +b is yellowness & -b is blueness THINGS REQUIRED TO SEE COLOUR A light source An object An observer In order to make an instrument that will quantify human colour perception each item in the visual observation situation must be represented by numbers LIGHT SOURCE A light source appears to be white When dispersed by a prism it is seen to be made up of visible wavelenghts LIGHT SOURCE Small part of electromagnetic spectrum Visible light - Electromagnetic spectrum, 380-750 nanometers LIGHT SOURCE Light source – physical source of light Illuminant – a plot of table of relative energy versus wavelength which represent spectral characteristics of different light sources COMMON ILLUMINANTS A incandescent C average daylight D65 Noon daylight F2 White Fluorescent • The D series are usually known by the first two digits of the Kelvin colour temperature scale D75, D65, etc. Illuminant Colour temp (K) Physical equivalent A 2856 Tungsten filament lamp B 4900 Sunlight C 6800 Average Daylight D50 5000 Daylight, UV corrected D55 5500 Daylight, UV corrected D65 6500 Daylight, UV corrected D75 7500 Daylight, UV corrected By representing light source as a function we can quantify and characterise the source THINGS REQUIRED TO SEE COLOUR A light sources An object An observer OBJECTS Modify light Pigments/dyes adsorb some wavelengths while reflecting or transmitting others Diffuse reflection Absorbtion Specular reflection OBJECTS The amount of reflected or transmitted light at each wavelength can be quantified. This is called a spectral curve (spectrophotometric curve) % reflectance 100% 0% THINGS REQUIRED TO SEE COLOUR A light sources % reflectance 100% An object 0% An observer 4.1 HUMAN EYE • receptor cells, the rods and the cones • Rods sensitive to light and darkness; cones sensitive to colour • 3 types of cone: 1 sensitive to red light, 1 to green and 1 to blue OBSERVER Luminosity is the relative sensitivity of the human eye to various wavelengths of light Light enters lens, focuses image on retina, Rods record light (black or white) Cones provide colour vision Trichromatic theory - 3 different cones sensitive to different bands RGB cones Electrical impulses set up by rods and cones sent to brain via optic nerve which interprets and assembles PSYCHOPHYSICAL QUANTIFICATION – PERCEPTION OF COLOUR Hue- distinguishes between colour Saturation (chroma) – vividness of colour Lightness – bright/dark, their lightness is compared 4.2 MEASUREMENT OF COLOUR • Early methods were based on reflection spectrophotometry • This is achieved by shining a red, green and blue light beam on to a half circle and controlling amount of light from each beam so that the colour produced can be matched to a standard (figure 10.1 in handout) • Thus colour can be defined in terms of red, green and blue (RGB) • Thus we can set up a triangle with RGB stimuli at each corner • Early researchers chose to use XYZ reference stimuli • This cannot be reproduced in the laboratory as they are a purely mathematical concept, however crudely they can be applied to the RGB scale respectively • If data for red, green and blue for the spectral colours is taken and transformed to X, Y and Z co-ordinates, and plot the human response of cones against wavelength, the response of the human eye to colour can be determined • These curves were standardised in 1932 and are called the CIE (Commission Internationale d’Eclairage) x, y, z standard observer curves (fig. 10.4) • The equation to calculate XYZ from spectral data contains the spectrum of the source of light • It is essential that the source of light for visual colour examinations be standardised so as to get reproducible results • In 1931 the CIE approved three “standard” illuminants • Illuminant A, with a colour temp of 2854 K, represents light from an incandescent lamp • Illuminant B, colour temp = 4800 K, represents direct sunlight Illuminant C, colour temp = 6500 K, represents average daylight from the entire sky • In 1966 the CIE proposed a 4th series, the D illuminants • It represented daylight more completely and defined the energy spectra down to 300nm as opposed to 400nm (cut off for A,B,C series) Determination of standard colorimitric observation R G B 2o X X • The experimentally derived X, Y and Z functions were know as the CIE 20 standard observation curves • Experiments redone in 1964 with new knowledge about eye function - known as CIE 100 standard observation curves • They quantify the average response of the human eye to red, green and blue •What is blue? Where does it start?? Relative Human Response Relative Response CIE Tristimulus Colour Matching Curves •100 most often used for commercial applications 3 elements of visual observation can now be modelled A ligh sources Illuminant 100% % reflectance An object Reflectance/transmi ssion curve An observer Standard observer curves Relative Response 0% COLOUR MEASUREMENT Light Source A ligh source Illuminant 100% R & T curves % reflectance An object Specimen 0% SOC An observer Relative Response Spectrometer Psychophysical quantification methods Methods of assigning colours to their appropriate locations in a colour space a) Spectrophotometer b) Tristimulus colorimeter c) Visual colorimetry • Hence the CIE tristimulus colour values X, Y and Z for any sample can be obtained by combining data values for the illuminant, reflectance/transmittance of the sample and the standard observation curves Illuminant X % reflectance 100% Reflectance = 0% Visual Stimulus Relative Response X CIE standard observer curves Y = 41 = = = X = 41 Z = 41 4.3 Tristimulus Colorimetry • need a light source, 3 glass filters with transmittance spectra that duplicate X, Y and Z curves, and a photocell (fig 10.7 handout) • Reading can be obtained that represents colour of the sample • Actual readings depend on reference colour solid • A no. of systems can be used. 2 most popular are CIEXYZ system and the second is the Judd-Hunter L a b solid • Latter represents a colour solid in which L is lightness or darkness, +a is redness, -a is greeness, +b is yellowness & -b is blueness TRISTIMULUS COLORIMETER •Light source to light sample •Reflected light passes through RBG filters to simulate Standard observer functions •Photodetector beyond each filter detects amount of light passing through •These then displayed as X, Y, Z values Sample Light source Filters R X = 41 G Y = 20 B Z=8 Photodectectos Display TRISTIMULUS COLORIMETER SPECTROPHOTOMETER • A light source lights specimen • Reflected light passes through a grating which breaks it into a spectrum • Spectrum falls on a diode which measures amount of light at each wavelength • Spectral data combined with illuminant data and CIE curves to obtain X, Y and Z values. Sample Diode array Data Processor Light source Diffraction grating X = 41 Y = 20 Z=8 Display SPECTROPHOTOMETER COLOUR SCALES • Visual Organisation of colour • Colour has a degree of lightness or darkness • Hue is the colour from the spectrum • Saturation is a degree of colour • XYZ values not intuitive • Other colour scales devised • Relate better to colour perception • Simplify understanding • Improves communication HUNTER LAB COLOUR SCALE • Based on opponent colours theory • 3 dimensional rectangular colour space • L (lightness) – 0 black, 100 white • a (red-green) – positive red, negative green, zero neutral • b (blue-yellow) - positive yellow, negative blue, zero neutral OPPONENT COLOURS THEORY • Red, green and blue cone responses are remixed with opponent coders as they move up the optic nerve to brain Sample • Proof? Red/Green G Black/White B Blue/Yellow BRAIN R COLOUR CHANGE ACCEPTABILITY • Colour differences can be calculated by comparing Sample-Standard readings • Hue differences are more objectionable • Chroma less objectionable • Light differences are least objectionable 4.6.2.1 Malting of Barley •In general there is a trade off between enzyme activity and flavour/colour development 4.6.4 COLOUR MEASUREMENT OF CEREAL BASED BEVERAGES 4.6.4.1 Beer • tintometer (a visual method) • Adapted by the American society of brewing chemists (ASBC) for colour determination of beer and worts 4.6.4.2 Whiskey • Although whiskey is obtained from coloured wort, it is completely colourless after distillation • interaction between the distillate and the cask it matures in • Final colour trimming is achieved by the use of caramel colouring Lovibond discs or instrumentally using an internationally recognised method by the Association of Analytical Chemist (1993) • Method involves measuring absorbance of a sample in a 1cm cuvette, with respect to distilled water, using light at 525 nm • Spectrometer used must have a bandwith less than 10 nm END 4.7.3 CIDER • Minolta colorimeter is used for colour determination • Laboratory sets parameters in terms of a tristimulus reading and a sample from each batch is tested • Minolta colorimeter offers more flexibility in range of beverages than a Lovibond colorimeter