Companies concerned with producing fabrics and textiles in the exact colour specified by a customer may use spectrophotometers and colorimeters to track quality, but they can easily overlook the need to do proper visual evaluation of colours and colour differences as part of quality control.
At some point in the supply chain, qualitycontrol personnel need to evaluate how the colours of apparel or softgoods look when they are placed under the same illumination as where they are sold or used – illumination that is often specified by the customer. And there isn’t any substitute for the human eye when it comes to making those judgments.
A good programme of visual evaluation can pay for itself quickly. Experts estimate that millions of euros are lost each year in time and resources when brand owners reject test swatches that don’t meet their visual colour standards or requirements. Some textile mills and dyehouses say that they reject about 20% of their first lab dips internally during visual inspection, but specifiers report that they reject up to 80% of the first-time lab dips that they receive from their suppliers. X-Rite Inc, a worldwide leader in colour-data-management solutions for textile companies, estimates that it costs more than €1,400 in time and resources for each sample a dyehouse sends to a specifier for evaluation.
The good news is that manufacturers can take inexpensive and practical actions to assess the visual colour quality of products. Best practices for controlling the visual quality of colour fall into three main areas: quality and intensity of light sources, the colour acuity of those who are responsible for checking products, and clear procedures for preparing samples for quality checks and maintaining records.
The Eye of the Beholder
Some manufacturers mistakenly believe that evaluating colours visually is entirely subjective, based only on the opinions of observers. But over centuries of study, scientists and engineers have developed theories and methods that make visual evaluation of colours reliable and reproducible enough to be considered objective measurements.
Best practices in visual colour evaluation start with proper illumination and knowing what your customer is using to evaluate the colour of products. Manufacturers of colourmeasurement equipment have developed light booths that simulate a number of illumination conditions, such as natural sunlight at different times of the day, the average fluorescent illumination that a shopper may encounter in a store, and the lighting a consumer may experience at home with incandescent lights.
Macbeth Lighting, a business unit of X-Rite Inc that manufacturers light booths, uses filtered tungsten halogen lamps to accurately simulate how daylight looks in the morning, at noon or in the evening, because the perception of colours changes under natural sunlight at different times of the day.
Manufacturers may also need accurate simulations of the average fluorescent illumination that one may find in a retail outlet. Most shoppers today view products under fluorescent lamps with spikes of energy levels among certain green wavelengths that alter how colours are perceived.
Often light booths are equipped with incandescent bulbs to stimulate what customers may see in their homes and ultraviolet lamps to replicate how textiles treated with optical brightening agents (OBAs) would appear under sunlight. Natural daylight contains ultraviolet rays that are invisible to the human eye, but the rays are converted into visible light when they strike certain chemicals used to brighten colours.
CIE Publication 51 sets international standards of controlling ultraviolet light that can cause metamerism, an optical phenomenon where a pair of samples might match in colour under one light source, but appear different under another light source. Lab personnel can also adjust the amount of ultraviolet light in the test stations such as the X-Rite SpectraLight QC light booth, depending on how the sample should be tested.
The Right Intensity
In addition to the quality of light, test samples need to be exposed to a controlled intensity of light.
International standards committees such as the ASTM, ISO, DIN, and BAM have outlined some best practices for illuminating light-, medium- and dark-coloured coating samples when evaluating their colours. For instance, one standard states that the illumination may be as low as 50 foot-candles or 540 lux for viewing very light materials and as high as 200 foot-candles or 2150 lux for viewing very dark materials.
Both lux and foot-candle are measurements of the intensity of light that is brought to bear on a defined area. One lux is defined as one lumen uniformly distributed over an area of one square metre. A typical office may have illuminations in the 320 to 500 lux range, while direct sunlight on a clear day may be measured at more than 100,000 lux. A foot-candle is defined as one lumen uniformly distributed over an area of one square foot, with one foot-candle equalling the power of approximately 10.8 lux.
Companies can determine the lux or footcandles that are falling on a sample with a light meter or with adjustments in the light booth itself.
After manufacturers determine the right light sources to illuminate test samples, they should check whether their quality-control personnel and lab technicians can see colours well enough to perform their duties properly.
Many people cannot see the entire range of visible colours: experts say nearly one in every 12 males and one out of 255 females do not have normal colour vision. In addition to heredity, colour vision can be affected by a person’s age, health, and prolonged exposure to sunlight.
There are simple and inexpensive tests that indicate how well a person can see colours, such as the Ishihara Color Vision Test and the Farnsworth-Munsell 100-Hue Test. The FM Hue test is specified as the proper test in international standards for visual assessment of textiles, fabrics and other materials, to distinguish between individuals who have poor, normal or exceptional colour vision.
X-Rite offers a free online version of FM 100 Hue Test at this internet link: http://www. xrite.com/custom_page.aspx?PageID=77. The online version isn’t exact enough to be used for testing purposes, but it is an entertaining test that gives an indication of a person’s ability to match hues.
Describing a Colour
The third leg of an effective colour-evaluation programme is to establish standard procedures to get consistent results that have practical limits on what colours are acceptable. Many companies cannot operate under a rule of ‘no perceptible colour differences’ because textiles that meet such a standard would be too costly to manufacture.
To provide quality products at a profitable price, manufacturers need to establish standards on what colour differences are acceptable and explain those standards clearly to their quality control personnel.
For example, companies should document:
• how samples are prepared for evaluation
• how samples should presented for viewing
• how observers should conduct their assessments and report results
Some tips on viewing include:
• removing everything but the sample and standard from the light booth
• determining the angle that samples will be illuminated and viewed, generally either at a 0-degree angle or a 45-degree angle. If a viewing geometry is specified, then that is the arrangement that must be used by all supply-chain partners
• test observers should not wear brightlycoloured clothing while evaluating samples in a light booth, and obviously must not wear coloured glasses or coloured contact lenses.
When they are reporting their results, test observers should try to use a very limited vocabulary. For example, when describing the sample’s colour compared to the standard, the observer should identify if the sample is lighter or darker (lightness difference); yellower, bluer, redder, greener (hue difference); and more or less colourful (chroma difference).
Further, the magnitude of the observed colour difference may be described in terms such as ‘slightly’, ‘moderately’, and ‘extremely’. Best practice in reporting results is to use simple, consistent vocabulary to describe colour differences. For example: “Sample is lighter and redder, needs to be darker and greener.” The most pronounced colour-difference element should be mentioned first.
Part of standard procedures is to have a regular method of keeping records, such as:
• the light booth and lamps used to view the sample
• the condition of the lamps when the sample was evaluated
• whether equipment has been calibrated and certified as working properly
• whether observers have been tested for colour-vision deficiencies.
All of this information and more can be recorded automatically by test stations such as the SpectraLight QC light booth when it is connected to a personal computer that is running SpectraLight QC software. Lab personnel can conduct their tests and record their results on the personal computer. The software automatically maintains all of the important records, which can be sent electronically wherever it is needed or printed out to accompany the physical test sample.
Manufacturers that maintain good records on their visual tests can solve problems much quicker and easier when then occur because quality-control personnel can determine if visual-evaluation tests were performed correctly. For instance, lab technicians using the SpectraLight QC software can create a file for a brand owner or specifier that tells exactly the sequence of lamps that were used in a test, to ensure that the wrong lamps were not accidentally selected to cause a bad result.