Measuring and Classifying Tile Shades
The shade of a tile is the combination of a number of visual characteristics of the tile surface, including its color and the distribution pattern of color or decoration over the tile surface. The tile manufacturing process has many variables that can affect tile shade, including material colors, quantity and mixing; a wide range of printing and other decoration effects; and firing conditions. Manufacturers can only achieve the ideal of having a single shade for each product if they can control all of these process variables extremely well (or if the customers’ expectations for shade are very low—i.e., it is a low-quality, low-cost product). In most cases, however, such control is not possible, and manufacturers must instead deal with the reality of classifying and sorting tiles by shade to maximize their profitability.
Unfortunately, the classifying and sorting process is not easy, and many problems can occur, including:
- Quality/inspection managers often find it hard to establish and control shade groups.
- Manual inspectors do not make consistent, accurate shade decisions against the established shade groups.
- Tiles within the same shade group have too large a visible difference (due either to incorrect inspection or to the defined shades being too broad), leading to customer returns and a damaged reputation for quality and therefore sales prospects.
- Many different shades exist, leading to high costs for storage, distribution and sales.
- During inspection, many tiles do not appear to fit within the established shades, and new shades have to be established or tiles have to be classified into another quality category.
Principles of Shade MeasurementAppearance measurement instruments and systems take objective measurements that correspond with human perception. They illuminate the tile surface with one or more light sources of controlled intensity and color, collect the light reflected from the tile surface, convert it to digital signals and analyze these signals using a digital processor.
To be useful for tile shade classification, an individual shade measurement should:
- measure an appearance variation that is clearly visible to the human eye in a way that correlates with human perception;
- give reliable and repeatable measurement values;
- measure shade variations that occur or could occur during the production of the tile (there is no need to measure something that cannot change);
- be linear with human perception (a one-unit difference at one part of the measurement scale should appear to a person to be just as large as a one-unit difference at another part of the measurement scale [at least within the range of measurement values that occur]).
- be able to detect and measure all of the shade variations that occur or could occur (there must be enough measurements);
- be orthogonal to each other (a change in the appearance characteristic measured by one measurement should not affect any of the other measurements);
- be isotropic (a one-unit difference of one of the measurements should be perceived as being just as large as one-unit difference of any of the other measurements).
The Science of Color MeasurementA tile shade classification system that meets the above requirements is based on the science of color measurement—understanding how people see and describe colors. Three apects can be used to define a color:
- Hue—whether the color is red, yellow, green, blue or some other color
- Saturation—how strong or vivid the color is
- Lightness—how light or bright the color is
This system is widely used across many industries to specify colors and the acceptable variations of those colors. For uniform, plain-colored tiles, such a system can provide an effective method of shade control (though even on these tiles other shade characteristics such as gloss may need to be considered and measured). However, in general, tiles are not uniform and plain. They have a wide range of decoration types, including silk-screen printing, rotary printing, spray and mixed materials to give speckles or blobs. While the average L*, a* and b* values for the tile surface are often useful shade measurements, they are rarely sufficient to fully define the shade of a tile, and a range of other measurements are needed.
Other Shade MeasurementsThe geometric distribution of colors is critical for decorated tiles. A wide range of measurements can be used to define the position, contrast, size and shape of this decoration.
The reflectivity of the tile’s surface—its gloss—is another key appearance characteristic. Variations in the shininess of the surface are likely to be noticed by customers. If one or more tiles in a batch have a much lower gloss than the others and all are laid on a floor, these tiles will look clearly different and may cause returns. In some cases, variations in the texture of the surface are also perceivable and should therefore be measured.
While typical shade measurements on plain tiles are L*, a*, b* (average gloss and surface texture), possible measurements on printed tiles include the L*, a*, b* of base color (weight/contrast of print), the sharpness of the edges of the print (the x and y registration of print), and sometimes even the colors of the print.
On tiles with a random speckle pattern, a typical shade measurement is the L*, a*, b* of the base material, the average size of the speckles and the number of speckles per unit area.
An effective tile inspection system needs to be able to take all of these measurements, as well as determine the tile’s shade, defined by the values of its shade measurements. Unlike simple color measurement, typically more than three shade measurements, and sometimes up to about 10, are needed to fully define a tile’s shade. We can, however, extend the concepts of color measurement to cover these additional measurements. All of these shade values can still be considered to be shade coordinates that define the position of the tile’s shade in n-dimensional shade space (where n is the number of shade measurements). If all of the measurements are linear, isotropic and orthogonal, the shade difference between two tile shades is still equal to magnitude of the distance between the two shade points in this shade space, and this is still equal to the square root of the sum of the squares of the differences of the individual measurements.
As with color measurement, a single tile shade group can be defined by the coordinates of the central or target shade and the maximum difference from it. The shade is therefore a hyper-sphere in the n-dimensional space. We can guarantee that no two shade points within this shade group are more than a given distance apart, equal to the diameter of the sphere.
If we believe that our customers will be satisfied with a shade if any two tiles within the shade group always have a shade difference of less than d, then the largest volume of space that satisfies this requirement is a hyper-sphere of diameter d.
These concepts can be used to establish a strategy for shade classification that is optimized to ensure the maximum shade difference within a shade group and also to minimize the number of shade groups required. Such a strategy lowers manufacturing costs while maintaining customer satisfaction.
Manual vs. Automatic InspectionWhen shade groups are established purely by eye it is impossible to create a shade scheme that even approaches the one described.
Figure 3 shows a scheme of 12 fixed shades that has been developed by one manufacturer, with great care and attention, over a period of several years. Each shade group is indicated by an ellipse. It is noticeable that:
- each of the shades is a different size and shape;
- the shades overlap; and
- there are gaps between the shade groups.
Automatic tile inspection systems use electronic cameras and digital image processors to acquire and process digital images of tiles. They are able to calculate shade measurements and take these measurements in a way that is reliable and repeatable.
Most current systems, however, concentrate on learning a shade classification scheme that has been developed by human inspectors. They are able to learn the sizes and positions of shade groups by passing sample tiles through the system and then to apply this scheme to achieve automatic inspection. At best, this only serves to mimic the manual scheme in a more repeatable, automatic way and it therefore has most of the shortcomings of a manual shade classification.
Far greater benefits can be achieved if the automatic inspection system is programmed to guide the manufacturer in setting up an optimal shade classification scheme. This is only possible, however, if the system supplier has a clear understanding of the requirements and theory of tile shade classification, and experience of what shade measurements are required for different tile types.
The system should guide the manufacturing personnel through a sequence of steps to establish the shade scheme. The interface of the system needs to be clear and simple, using non-technical terms to describe shade differences and clear graphics to show progress in the sequence.
By using such a system, manufacturers can carry out shade classification with a good balance between their key objectives—achieving customer satisfaction, minimized manufacturing costs and maximized profits through a clear, practical shade scheme.