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| Issue date:01/12/2006 |
| ATA Journal for Asia on Textile & Apparel - Dec 2006 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Dr Sanjay Gupta |
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The key element in successfully implementing a textile color matching and quality control system is the accurate and repeatable measurement of the samples being evaluated. Spectrophotometers form a major part of instrumentation for such color using industries, both as a tool for measuring color and subsequent computation of color difference for color approval.
Color data is now the choice as a "standard" for use in specification, in preference to a physical sample, due to the accuracy, stability and mobility. Moreover, with textile production and sourcing becoming a truly global phenomenon, where manufacturing and retailing are literally continents apart; color communication along the textile product supply has acquired great significance. Digital measurement and electronic communication of color information has greatly improved the efficiency of the supply chain in terms of remote operation of processes such as color approval, trim and range color co-ordination, and hence the ability of industry firms to manage color successfully in a global environment.
Choice of right equipment is critical for this application. Today's spectrophotometers not only measure color, but also enable Enterprise Color Management (ECM) on a global scale. Without accurate, reliable color measurement, ECM is moot. Several types of spectrophotometers are available and an understanding of their types and how they work is useful in determining the appropriate equipment. So what are the key considerations for selecting the right instrument for enterprise color management?
Selecting the right instrument
First consideration is geometry that refers to the placement of a sample relative to the light source and measuring lens in a spectrophotometer.
The most common geometries found in modern instruments are diffuse/8 and 45/0. Diffuse/8 spectrophotometers use an integrating sphere and light reflected off its coated interior to strikes the sample from all angles. The reflected light is then measured at an angle of eight degrees relative to a perpendicular line drawn from the sample. This minimizes the influence of surface irregularities, allowing measurement of reflectance due to color rather than surface variations. This is especially helpful in laboratory shade matching and in production dyeing where it is preferable to measure the difference in color of two samples with little consideration for differences due to construction or surface irregularities.
The 45/0 spectrophotometer uses one or more light sources to illuminate the sample at an angle of 45° and measures the amount of light reflected from the sample surface with a lens placed at 0° position. Such instrument will be more sensitive to surface irregularities, measuring "appearance" as well as color, and thereby finding use in quality control applications where differences in surface texture and finish are important.
Either instrument may be used for measuring textile materials but one will have to decide before purchase as to whether the color of the material or its appearance is to be measured. Reflectance data cannot be transferred between two systems unless instruments with the same geometry are used. Even in instruments with the same geometry, there will still be some difference in readings due to manufacturing variance, but the color difference between two samples measured on two instruments will be similar.
Next is the light source. Spectrophotometers typically make use of either a tungsten filament bulb, which is similar to a common projector bulb, or a xenon flash bulb, which is similar to the flash bulb of a camera. Tungsten filament bulbs burn continuously and generate considerable amount of heat. Temperature and light sensitive samples hence cannot be placed at the instrument port until immediately prior to measurement of the sample. Extended exposure of some samples to the heat and light from the tungsten filament bulb will cause a dramatic change in the color of the sample. Xenon flash bulbs do not generate heat but are instead rich in ultraviolet energy, which may excite any fluorescing chemicals or dyes present in the samples leading to inaccurate match predictions. Filters are usually used to minimize the effect of ultraviolet energy for shade matching and for calibrating the amount of UV energy for accurate whiteness calculations.
Difficulties arise when FWAs (Fluorescence Whitening Agents) are applied to textile substrates. These chemicals absorb light in the invisible, or near ultra-violet region of the spectrum and then re-emit this light as fluorescence in the visible region, usually between 420-500nm. The effect is a greater degree of reflectance in the blue region of the spectrum, therefore a "bluer" white. The visual and instrumental evaluation of such materials requires the simulation of the ultra-violet component found in daylight. A pulsed-xenon light source contains a high amount of UV energy, which is easier to filter to D65, and hence can simulate D65 daylight needs to be used.
Further, lamps and light handling elements of a spectrophotometer are susceptible to changes and deterioration over time. The instrument needs to be continuously monitored, results corrected and compensated for long-term stability, which is critical for implementing ECM. Diagnostic tests to check the accuracy of the measurements, drift in read-to-read and long term repeatability must be carried out routinely. Calibration is carried out with a black/white check every eight hours and a color check (using ceramic tiles) every two weeks. An annual service to check the instrument against master standards is an essential part of the process. Manufacturers are offering systems to ensure instrument monitoring and correction/compensation. GretagMacbeth's NetProfiler is a web-based tool that is claimed to reduce any spectrophotometer drift or error by at least 50%. Other manufacturers have local tools, such as DCI's Maestro and X-Rite's SPectroSynch, which do a similar job in bringing results into line with a master reference.
A range of aperture sizes is typically needed to allow measurement of both small and large samples. It is always preferable to use the largest aperture size possible to minimize the influence of uneven dyeing, but smaller ports also needed for measuring even the smallest of samples. Results from different aperture sizes are however not comparable due to the different amounts of reflected light being measured. The same sample measured with different apertures will give different results. Lab and production standards should therefore be prepared with the intention of using the largest area view available on the spectrophotometer. Samples measured with small apertures will require additional reads to ensure minimal measurement error. It is often impossible to measure some customer standards, especially when only threads or small clippings are provided for shade matching.
Materials such as shiny paint chips, plastic panels, and magazine pages appear glossy when viewed from a particular angle. When measuring such materials or when samples are measured behind a glass plate, it is important to exclude the glossy reflectance from the sample measurement. The glossy reflectance - or specular gloss - is automatically removed when using a 45/0 instrument but would need to be removed from a diffuse/8 instrument by use of a specular gloss port. The specular gloss can be safely included when comparing two identical materials but may be excluded when comparing two materials with significantly different gloss levels as long as the same measurement technique is used.
Specifications of spectrophotometer
Although the visible spectrum (the rainbow) is only around 400 nanometers wide, a good spectrophotometer can easily separate this into 10nm or even 5nm divisions. Newer instruments include two built-in data profiles that let you select 5, 10 or 20 nm data compatibility for seamless integration within the instrument network. Such profiling capability and Internet-based applications and services for instrumental color measurement are expected as ECM reaches critical mass.
Lastly, spectrophotometers are available in handheld, compact bench top and bench top sizes. The size, once indicative of precision and additional features, has now become a matter of user preference and application requirements. However if application requires the measurement of transparent/ translucent films, liquids, etc., a transmission measurement capability is required which is usually found on desktop instruments. Considering such changing needs, many systems offer multiple areas of view and special sample holders that allow added flexibility.
With a little up front planning, one can obtain an instrument that already includes these features; otherwise a rule of thumb is desktop in the laboratory and portable on production. Care must however be taken that its software application allows import/export of stored color standards between portable and desk systems. Also in terms of computer's data interface, although USB is the preferred choice, many systems still use RS232. The system should therefore accommodate both interfaces.
A requirement for successful global operation is electronic communications associated with color measurement. Colorimetric information generated by one system is not readily consumed by other systems, as it is in a format known only to the system maker. In order that dissimilar systems can be used effectively in communicating color, a common data standard is required. XML, a meta-mark-up language developed for use with the Internet allows the exchange of data between dissimilar systems. XML provides data and data about the data (meta data) as well, thereby providing a logical data structure to cover all types of colorimetric data and allowing dissimilar systems to understand the contents of a standard XML document.
Datacolor 600 high-precision reference grade spectrophotometer handles fluorescent materials |
ECM relies on consistent and comparable color measurement across the supply chain. This has led to emergence "reference" quality spectrophotometers that are actually quite similar in design and operation, although details and features vary. This means that the results measured under a given set of conditions on one instrument are relatively comparable with results from a similarly set up instrument from another manufacturer. The reference grade instruments, such as a Datacolor S600, Hunterlab UltraScan PRO, Konica Minolta CM-3700, X-RiteColor Premier 8400 and GretagMacbeth Color-Eye 7000A are considered to be the ultimate precision instruments and specified throughout supply chains by many global manufacturers.
Future: measuring by the pixel
Spectrophotometers can only measure samples of a certain type and dimension, limiting their use to relatively large sample areas and single, solid color substrates. Many textile applications however, like prints, color weaves (gingham for example), and yarn marls, fiber blends, as well as appliques, embroideries and trims, etc. have multiple color areas in close proximity. The solution being developed to measure these colors involve the use of digital cameras to capture images of the entire design or product and process the image pixel by pixel to generate spectral data. There are two commercial systems currently available and they do approach the problem in slightly different ways.
Reference grade spectrophotometers |
DigiEye from Leister uses a characterized digital camera in which a reference color chart, measured by a spectrophotometer, is mapped to the camera's RGB output. The result is a 'synthetic' spectral curve. The hardware consists of a digital camera, an illumination cabinet, calibrated high definition CRT monitor and a computer that controls four functions: camera characterization, color measurement, monitor characterization and texture profiling. The camera captures the image of sample, illuminated by two sets of lamps at 45° to the sample, and the software enables the average color to be calculated for any desired area by creation of synthetic spectral curves for solid areas and pixels (see illustration below).
ColorAIXperts of Germany on the other hand uses a high-precision, multi-spectral 16-channel camera (rather than the standard 3 (RGB)) to build a "real" spectral curve across the visible spectrum for every pixel.
Digital image captured by the camera is converted into average color by the software |
Either system can produce spectral data, similar to that from a traditional spectrophotometer, which can even be exported and used by traditional color management systems to create and compare color in production. At present, the extra size, complexity and cost of these systems limits their application to areas where traditional spectrophotometers cannot be used or where shaded or multicolor objects or designs, or multi-component constructs, need to be viewed on-screen and assessed as a whole.
In the longer term it is expected that this technology will become popular and prove to be a useful and essential addition, or even replacement, to the spectrophotometers currently required for color management.
Dr Sanjay Gupta is Professor of Textile Design & Development at National Institute of Fashion Technology, Hauz Khas, New Delhi.
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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