Apparatus and Method for Measuring Optical Characteristics of an Object This application is a continuation of application Ser. No. 11/118,903, filed Apr. 29, 2005, now U.S. Pat. No. 7,119,892, which is a continuation of application Ser. No. 10/302,215, filed Nov. 22, 2002, now U.S. Pat. No. 6,888,634, which is a continuation of application Ser. No. 09/462,020, filed Dec. 29, 1999, now U.S. Pat. No. 6,501,542, which is the National Stage of International Application No. PCT/US98/13764.
The present invention relates to devices and methods for measuring optical characteristics such as color spectrums, translucence, gloss, and other characteristics of objects, and more particularly to devices and methods for measuring the color and other optical characteristics of teeth, fabric or other objects or surfaces with a hand-held probe that presents minimal problems with height or angular dependencies.
Various color/optical measuring devices such as spectrophotometers and calorimeters are known in the art. To understand the limitations of such conventional devices, it is helpful to understand certain principles relating to color. Without being bound by theory, Applicants provide the following discussion.
The color of an object determines the manner in which light is reflected from the object. When light is incident upon an object, the reflected light will vary in intensity and wavelength dependent upon the color of the object. Thus, a red object will reflect red light with a greater intensity than a blue or a green object, and correspondingly a green object will reflect green light with a greater intensity than a red or blue object.
The optical properties of an object are also affected by the manner in which light is reflected from the surface. Glossy objects, those that reflect light specularly such as mirrors or other highly polished surfaces, reflect light differently than diffuse objects or those that reflect light in all directions, such as the reflection from a rough or otherwise non-polished surface. Although both objects may have the same color and exhibit the same reflectance or absorption optical spectral responses, their appearances differ because of the manner in which they reflect light.
Additionally, many objects may be translucent or have semi-translucent surfaces or thin layers covering their surfaces. For example, some materials have a complicated structure consisting of an outer layer and an inner layer. The outer layer is semitranslucent. The inner layers are also translucent to a greater or lesser degree. Such materials and objects also appear different from objects that are opaque, even though they may be the same color because of the manner in which they can propagate light in the translucent layer and emit the light ray displaced from its point of entry.
One method of quantifying the color of an object is to illuminate it with broad band spectrum or “white” light, and measure the spectral properties of the reflected light over the entire visible spectrum and compare the reflected spectrum with the incident light spectrum. Such instruments typically require a broad band spectrophotometer, which generally are expensive, bulky and relatively cumbersome to operate, thereby limiting the practical application of such instruments.
For certain applications, the broad band data provided by a spectrophotometer is unnecessary. For such applications, devices have been produced or proposed that quantify color in terms of a numerical value or relatively small set of values representative of the color of the object.
It is known that the color of an object can be represented by three values. For example, the color of an object can be represented by red, green and blue values, an intensity value and color difference values, by a CIE value, or by what are known as “tristimulus values” or numerous other orthogonal combinations. For most tristimulus systems, the three values are orthogonal; i.e., any combination of two elements in the set cannot be included in the third element.
