Optics: measuring and testing – By shade or color – With sequential comparison of sample and standard
Reexamination Certificate
2001-11-09
2004-09-07
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By shade or color
With sequential comparison of sample and standard
C356S402000, C356S405000
Reexamination Certificate
active
06788413
ABSTRACT:
CROSS-REFERENCE TO PRIOR APPLICATION
This application claims the benefit of European Patent Application No. 00125189.1 filed Nov. 20, 2000.
BACKGROUND OF THE INVENTION
The invention relates to a method for characterizing, by means of a measurement, the appearance of an object, more particularly, to a method for characterizing the contribution of the surface to the appearance of an object and for predicting the object's surface appearance.
Currently, many products are manufactured from plastic. These products have a particular appearance that depends on how the combination of the object's color and surface texture is perceived by the human eye. The appearance will in general be different when the sample is rotated relative to illumination and/or observer. This is caused by a number of factors, both internal factors such as the kind of material from which the object is manufactured, manufacturing conditions, the colorants (concentrations and types) used in the material, and surface factors, for example, the surface texture, of the object.
Surface texture is one factor considered when an object is to be manufactured. It may, for instance, be required that the surface's texture has a leather-like character, such as used in interiors of cars. This surface texture also affects the appearance.
There is are known methods for predicting what the color of an object will be (see: Practical Color Measurement, Anni-Berger-Schunn, Joseph W. Goodman, ed. J. Wiley, New York, 1994) based on the kind of material (often plastics such as polymers) and the colorants used for manufacturing the object.
A leading theory, with corresponding equations, which is used to match color is referred to as Kubelka-Munk. Many improved theories have been derived based on this theory. The Kubelka-Munk theory is based on diffuse illumination, which can be understood as light coming from all or at least many angles simultaneously. The experimental technique to create such an illumination uses a so-called integrating sphere and the data measured on such spectrophotometric equipment is treated using the Kubelka-Munk theory. In a spectrophotometer, the detector is usually placed in this integrating sphere at an angle of zero or 8 degrees relative to the perpendicular of the sample to be measured. For the user of such equipment, there is little practical possibility to change angles of viewing or illumination in this equipment. Therefore, the reflectance values measured in this classical way are considered angle-independent. This is essentially true, although in principle, a redesign of these spectrophotometers with a different viewing angle may lead to a slightly different reflectance value.
The kind of plastic in its natural state and color, as well as the colorants mentioned, serve as a basis in Kubelka-Munk theory for the prediction of an object's final appearance. However, this prediction is often poorly related to visual perception. For example, the effect of surface characteristics such as gloss on appearance is often not or poorly predicted, and also the effect of changes in illumination and/or viewing angle relative to the surface is not predicted. The integrating sphere used in the Kubelka-Munk (K-M) theory in general also masks the surface effects, for example, the effects of gloss and texture, to a large extent. Thus, neither the K-M theory, as used today, nor its associated measurement is able to predict surface characteristics such as gloss and texture. As a consequence, use of the known art of color prediction in general predicts a color, which is presumed to give a preselected appearance, but in reality more probably results in an appearance totally different than that seen by the human eye. This is particularly true when attempting to match the appearance of different materials.
For example, when a color standard made in polypropylene is used to match a sample to be formulated in a different material, for example ABS (Acrylonitrile Butadiene Styrene copolymer) a predicted formulation using existing color formulation techniques will almost always lead to a visibly unacceptable match. It happens frequently that the measurement of the standard and formulated sample with such equipment indicates that a color difference is small or negligible whereas visible differences are clearly much larger and often unacceptable. Another related problem is that while a visible match can be obtained under one viewing angle, rotation of the sample and reference material may lead to visible appearance differences at some other angles.
More particularly, this means, for instance in the car industry, that it is not easy to manufacture two or more plastic objects having the same appearance such as matching the appearance of one part of the instrument panel with the appearance of another part of the instrument panel made of another material. Thus, an observer notices differences in appearance. In another example, it is not possible to give, for instance, the automotive instrument panel the same appearance as the leather upholstery of a seat. In this case, too, a user of the car notices appearance differences. As mentioned, there are techniques known per se for measuring the color of an object. This technology can also be used to predict what the effects will be of the choice of a particular plastic and colorants (and other additives) on the color of an object to be manufactured. In this way, with existing techniques, an object such as the automotive instrument panel can be manufactured which, as regards its color, corresponds as much as possible with the color of the reference object, such as the leather-upholstered seat mentioned. It has been found, however, that the user still notices differences in appearance.
SUMMARY OF THE INVENTION
An object of the invention is to provide a solution to the problems outlined above. In the present invention, directional or collimated light (that is, more or less narrow beam of light which has distinct (although possible somewhat spread) viewing and illumination angle(s)) is used. Viewing angle can be chosen by the user and is a variable that may have significant effect on the result. Measured reflectance data are strongly dependent on these angles (that is, are “angle dependent”), and are related to the observed appearance. In contrast, the known systems are angle independent systems or, fixed angle systems, that do not allow to the characterization, measurement or prediction of appearance as observed under a variety of conditions, met in practice.
Accordingly, in a first aspect, the present invention is a method for characterizing the contribution of the surface to the appearance of an object. This method is characterized in that the surface contribution to appearance, or the surface reflection k
1
(w,h,i), is calculated from a plurality of reflectance values R
m
(w,h,i) which are established for at a light frequency w and a plurality of combinations of viewing angle(s) (h), and illumination angle(s) (i). The calculation of k
1
(w,h,i) from R
m
(w,h,i) involves the use of the color determining parameters of the material. Preferably, k
1
(w,h,i) is calculated at a number of wavelengths, w, covering the visible spectrum, and a plurality (at least two) combinations of viewing and illumination angles in order to have a more or less complete description of the surface contribution to the appearance. Once the surface contribution to the appearance has been determined, the appearance of the sample to the reference material can be subsequently matched by, for example, changing the colorants or other additives.
The invention is based inter alia on the insight that the appearance of the object depends not only on colorant loading and base material, but also on the angle at which the object is viewed by the human eye and the condition of illumination of the object. The effect that the viewing and illumination angles have on the surface and hence the appearance as perceived by the viewer is taken into account. The illumination is preferably directional light although illumination
Brands Gerrit J.
Dedeyne Evelien M.
Goethals Eric G.
Torfs Jan C.
Dow Global Technologies Inc.
Font Frank G.
Lauchman Layla
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