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  • Shade Control In Indigo Dyeing | Part 3

    This is a technical article by Harry Mercer on the process of Shade control in Indigo dyeing. This is the  third in the series. The first  and second part of this article were published before .

    Standard Illuminants

    standard light illuminantsAs stated earlier in this series, colour starts with a light source or illuminant. Illuminants have been standardized since 1931 by the Commission Internationale d’Eclairage (CIE). Illuminants are specified by a reference colour temperature such as 6500 degrees Kelvin., which is commonly referred to as D6500, (D identifying as a “daylight source). D6500 is a standard daylight illuminant in the United States and Northern Europe, originally to replicate light on the north side of a building in Chicago at noon. This specification of a light source by a colour temperature was originally established with a device known as a blackbody radiator. This is a small metal sphere with an opening which emits this specific illuminant when heated to a temperature of 6500 degrees Kelvin. Another example is Illuminant A, basically the colour of a tungsten filament lamp, which has a heavy red influence and a correlated colour temperature of 2856 degrees Kelvin.

    Specular Component

    “Specular” refers to the mirror-like quality of a fabric surface-the manner and directions in which light is scattered, which is a separate consideration from the colour itself. Appearance of a fabric is a combination of color and surface effects. The specular reflection of a sample differs depending on fiber characteristics, yarn count, yarn twist, fabric construction and finishing. These surface effects are neutralised during visual colour evaluation by using specific viewing geometries which relate the angle of the light source to the fabric and the angle between the human observer and the sample. For example, the 45˚/0˚viewing geometry, which neutralises the specular component, allowing a clear view of the colour only, means that the angle between the light source and the sample is 45˚ and the angle between the observer and sample is 0˚ or perpendicular to the sample. Colour measuring instruments offer this and other special viewing geometries to eliminate the specular or surface factors in measurement. If it is desired to include the specular component, then the option known as Specular Excluded (SEX) can be selected. The other option is include the specular component known as Specular Included (SIN). The Specular Excluded is superior for focusing on dyeing variation and prediction of washed colours, while Specular Included measurements are useful for rating the combined colour and physical differences of denim.

    Tolerances

    A tolerance, in regard to colour measurement, is a numerical difference between the standard color approved by the customer and the colour measurement of the fabric that was provided to the customer. In order to be indicative of a true visually noticeable colour difference, the tolerances should be established based on what a human being would recognize as a visual difference between 2 colours. As a general rule, if there is a total colour difference of ΔE equal to 0.2 between 2 samples, these samples can be considered visually different. With Indigo-dyed denim, this is also true if the colour difference between 2 samples has a value of ΔL = 0.2 in light-dark difference or Δb = 0.2 in yellow-blue difference. However, shades of Indigo are more sensitive in the Δa, red-green difference. In the U.S., it was standard to use a tolerance of only 0.08 for Δ a, meaning that if 2 samples differed by that much, they were considered different shades. This is not only because of the greater visual contrast in red-green differences, but also because of Indigo colour after washing. As Indigo develops a redder or less green shade, it loses colour in washing more quickly. For that reason, the Δa is a good general indicator of how easily Indigo will fade. Δh (hue difference) provides the same indication and is a more precise predictor.

    Many denim operations, as well as other textile operations use larger tolerances to specify shade differences from standard. These can be as high as 1.0ΔE to distinguish a slight shade difference, but this tolerance size allows up to 5 different shades to be shipped to the customer as the same colour.

    By the numbers

    Only by intelligent use of available colour data can the extensive problems with managing shade variation in Indigo dyed denims be systematically brought under the control needed to conform to the customer’s expectations. Colour measuring instrument programs provide massive amounts of direct information that is objective, but the data must be studied and analyzed in order to effectively manage colour, which is the most critical factor in denim success. Factors such as Standard Deviation are the most useful in determining progress, or the lack of it, in reducing Indigo shade variation. The Standard Deviation will demonstrate the relative degree of variation in Indigo dyeing between companies, products, machines, procedures, workers etc. and is the best tool for setting objectives for managers. In far too many denim companies, little concern is demonstrated within the production areas for identifying the sources of colour variation. The standard approach is to resort to shade-sorting or tapering programs that only establish the degree of failure in colour management.

    Recommendations

    1) ILLUMINANT- Illuminant A has long been recognized as a better light source for distinguishing true colour differences in denim, because of the greater sensitivity of Indigo to red-green colour difference. This will increase the number of to allow the laundry to sort the fabric more reliably in terms of wash-fastness.

    2) SPECULAR COMPONENT-While the Specular Included is normally used for fabric shipments, the Specular Excluded is more useful for evaluating the effects of dyeing directly, avoiding the influences of weaving and finishing on denim which is significant.

    3) TOLERANCES-Review the colour tolerance used in shade sorting or tapering to determine if it is realistic and allows conformance to the customer’s expectation.

    4) COLOUR ORDER SYSTEM- While the L*a*b* colour arrangement is in common use, the L*c*h*(chroma and hue) arrangement allows better communication between denim producers and customers. While the L*a*b* arrangement orders colour in a way that approximates how garment buyers see colour, the L*c*h* arrangement relates more directly to how colour managers, as in dyeing perceive colour.

    5) EDUCATION-In order to raise the level of professionalism of those responsible for colour management, detailed, scientifically-based training is badly needed in the denim business in every sector in order to minimize dissatisfaction on the part of customers. There are training resources available from colour instrument vendors on line or colour experts for in-factory training. Selection of staff for colour nmanagement is critical since less than 5% of people are able to grasp the concepts involved, so objective evaluation of colour staff is necessary to maintain an image of professionalism.

     


    image    This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies.He is an expert colorist for measurement and color matching as well as textile testing. He can be contacted here

     

     

     

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  • Shade Control In Indigo Dyeing | Part 2

    This is a technical article by Harry Mercer on the process of Shade control in Indigo dyeing. The first part of this article can be found here

    While it cannot be disputed that, ultimately, how the individual consumer values color with their human subjectivity is the true test of coloration, in large-scale manufacturing of fabric and garments, the resort to evaluation of color visually is extremely unreliable. Color vision is denim eyedifferent among human observers and varies with the same observer depending on factors like fatigue, age, emotional state and even race Color blindness related to difficulty in distinguishing red-green color differences is most common among Caucasians, affecting 8% of males, but only 0.6% of females, 5% of Asian males and 0.25% of females and 4% of African males, 0.16% of females. The Ishihara test is used to quickly identify colorblindness problems. Many fashion houses require colorists to pass the Farnsworth-Munsell 100 Hue test for employment. This test provides a rating of ability to distinguish colors and demonstrates how different individuals are in this regard. When I had the test, it showed that I have excellent color vision in terms of distinguishing violet and Indigo, but hopeless with shades of red. These differences in individual color perception result in endless disputes regarding conformance-to-standard expectations. For final determination of whether a sample and a standard are identical, the eye is still the best instrument, it fails in describing the exact quantity and quality of the differences in a submitted sample to the approved standard. Color measuring instruments, on the other hand, will objectively and precisely measure the exact differences in multiple dimensions, how the standard and sample are related to each other and will determine if the sample is acceptable within a given tolerance through thousands of tests with extreme accuracy and repeatability which nearly impossible with visual measurements.

    How Color Measuring Instruments Work

    Without engaging in the complex physics and mathematics of color science and measurement, the basic principles of determining color involve a standard light source, an object to bounce the light off of and an observer (human or color measuring instrument). Color starts with light. In a color white light color measurement in denim measuring instrument, a standard white light source illuminates the sample to be measured. This white light can be thought of as containing equal amounts of all visible light colors at a high level-100 units of red, orange, yellow, greens, blues, Indigo, violets etc. Dyes and pigments absorb these colors of light in different amounts and allow the rest to escape, which the instrument measures. With a light shade of Indigo, of 100 units of each light color illuminating the sample, perhaps reflected are 50 of violet, 70 of Indigo, 60 of blue, 50 of blue-green, 30 of green, 20 of yellow-green, 10 of yellow and 25 of red. The instrument collects these quantities and converts that into various numerical values. A curve is produced from this data that serves as a “fingerprint” of a color that distinguishes it from all other colors. The data collected by measuring the escaping light colors is mathematically converted into basically 3 numbers that are coordinates in a 3-dimensional color space. This color space is roughly like a sphere in which all colors thatcolor measurement in denim hunter scale can be perceived by the human mind are contained. The distance and direction in 3-dimensions precisely relate each color to all other colors. From these mathematically precise locations the differences are calculated. These values are then mathematically transformed into a color coordinate system, most often L*a*b*.

    Color Descriptors

    The starting point of these color spaces is the L-scale which is a vertical axis around which all colors are organized. The L-scale establishes the differences in lightness between samples-how dark or light a sample is. The L-scale starts at the top of the sphere with a value of 100, for a perfect white and ends at the bottom with 0 for a perfect black. In between are degrees of grayness. This axis is neutral with regard to color, having no color or hue, meaning without red, green, blue etc. A dark shade of Indigo may have an L-value of 30 while a light shade may have an L-value of 60. If a color difference were calculated between the dark and light values it would be a minus 30, reported as a DL -30 (delta L, or ΔL, Δ being the scientific symbol for “difference”). Starting from this neutral central axis of this sphere where are colors are mathematically located, moving outward hue is added, hue being the scientific name for color.

    From the central, neutral axis, color is added gradually so that when the outer edge of the sphere is reached a saturation limit is reached at which the human eye perceives that the color is too dark to be at the same level of lightness. For example, at a lightness (L*) level of 85, which is very white, as you move outwards at first there is a small amount of red added and you have a color that would be identified as pink. If you add the maximum amount of red, then the color becomes a dark, but bright red. When the maximum amount of red has been added, more than the human eye can recognize at the L* level of 85, the lightness level will drop to 80 because the color had increased to a darker level.

    If this color sphere is bisected into a series of planes that go from top to bottom of the sphere, these planes are initially divided into 4 quadrants. In the most commonly used color space (L*a*b*), there is one line labeled a* that represents red-green differences and a line labeled b* that represents yellow-blue differences. Red and green are considered to be opposing colors as are yellow and blue, just as east and west, north and south are opposites.

    On the a* line a positive number like +3.5 indicates a red color and a negative number like -3.5 indicates a green. If a standard has an a* value of +3.5 and a sample has a value of +0.5, then the difference between standard and sample is expressed as Δ a*-3.0, simple subtraction. However if the standard has a value of +2.0 and the sample has a value of -1.0, then the total red-green difference is Δa*-3.0 because these are Cartesian coordinates. The total color difference is the same, but the overall visualcolor measurement in indigo denim color chart difference is much greater because crossing the boundary between the red and green sides of color space results in much greater contrast. Red-green differences are the most significant with Indigo, not only because of the visual contrast, but also red-green differences are good predictors of wash-fastness of Indigo and even small differences in the a*values means higher variation in the laundry.

    The b* line is similar for yellow-blue differences. The –b* values should be monitored as a way to evaluate changes in the original dyed color of Indigo on yarn.

    The third part of the article Shade control in Indigo Dyeing follows shortly..


    image    This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies.He is an expert colorist for measurement and color matching as well as textile testing.