14 August 2015
For work, play or just relaxing about the house, jeans are a multi-functional, practical piece of fashion that are versatile enough to work in most walks of life. Such is the notoriety of any denim product that a great deal of care goes into creating and choosing the right colour, shades and hues for a certain product. Nowadays there are many shades of blues, purples, greens and so on made out of the denim material. I daresay you can find a pair of jeans in every colour imaginable.
The one thing all denim products have in common is normally the manner in which the finished product is presented to a prospective customer; the majority are hanging next to different sizes of the same product in clothes shops or departments. It is for this reason that monitoring colour consistency and quality is extremely vital. In order to capture a customer’s eye the product has to look appealing; they are not going to consider the quality of the material itself unless they are attracted to its appearance.
This observation calls for a method of colour and appearance monitoring that can ensure consistent shades and hues from batch to batch, product to product. For this type of analysis, a colorimetric spectrophotometer could be used to provide spectral data relative to the fabric that can then be calculated into any colour scale or index required, taking into account differences in types of light sources. For example, a dyer might look at CIE L*a*b* values to compare them to other samples of the same product. When compared to a standard, delta values (dL*, da*, db* and dE*) can tell the operator how different the sample is from a standard. Alternatively, the dECMC tolerance could be used. This is an elliptical tolerance that gives the visual difference to an observer in terms of a single numerical value.
For the measurement of fabrics, consistent sample preparation is essential. The surface for measurement should be as smooth and as flat as possible with any striations facing in the same direction if only taking a singular measurement. Keeping the direction of the fabric the same will ensure any shadowing effect caused by furrows in the fabric will be the same on all samples and will not affect the reliability of the results.
Another aspect of sample preparation to consider is the thickness of the sample. Will it be a single layer, multiple layers or will the sample be folded at all? With a single layer, there may be a chance, depending on the thickness of the material, that some of the light may go straight through and not be reflected back to the detector to be included in analysis. If using multiple layers, it must be carefully checked to make sure that the same amount of layers are used of denim sheets with the same thickness to, again, ensure consistency. Similarly, if folding the sample it must be of the same thickness and folded the same number of times as other samples to ensure consistent sample preparation practices.
An example of a reliable method would be as follows:
Sample size – large enough to cover the measurement port size without gaps (approximately 1 inch in diameter.)
Sample preparation – Using a skein holder, attach a sample of denim fabric so the surface is taut and flat. Have any striations facing in the same direction. Being backed with a skein holder normally ensures an opaque surface for all light to be reflected back to the instrument for measurement.
Quantity of measurements – As a general rule, the more measurements taken, the more accurate the data is at depicting the appearance of the product. A singular measurement may suffice, particularly when time and quantity of samples to be tested is an issue, but at least 2 measurements are recommended. These 2 measurements would allow for a measurement of the samples with the striations going in different directions; these results could then be averaged.
Colour scale – For denim, the CIE L*a*b* colour scale would be the best way to start as this can give data that can be compared easily and provide clear difference values. The dE CMC index would also be useful for matching shades of the product to keep them consistent, using interchangeable tolerances that can be personalised to the maximum acceptable and minimum perceptible differences within their product.
Instrument – A colorimetric spectrophotometer would be best, particularly one that uses directional geometry. A ColorFlex EZ is a compact benchtop spectrophotometer that uses directional geometry to take measurements of colour in relation to how the human eye sees it. This instrument can be used in conjunction with EasyMatch QC software to allow for detailed analysis, comparisons and calculations of results.
Keeping quality and appearance consistent during the production process needn’t be a headache and, with the right tools, there’s no reason why it should be.