Optics: measuring and testing – By shade or color
Reexamination Certificate
1998-03-12
2002-05-07
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
C356S406000, C356S407000, C356S419000, C433S029000
Reexamination Certificate
active
06384917
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of color measurement and in particular to equipment for measurement of tooth color.
BACKGROUND OF THE INVENTION
Recent advances in the practice of dentistry, especially in the field of implants, have given a more natural look to the use of artificial teeth. Advanced methods are used to temporarily or permanently implant artificial teeth in patients. One of the most crucial decisions a dentist must make in preparing the artificial tooth or teeth is to choose a color that will properly match the color of the natural teeth remaining in the mouth. Normally, a dentist uses a standard Master Teeth Set which consists of a set of artificial tooth models in varying shades of color. The dentist compares these sample artificial teeth visually with the teeth remaining in the patient's mouth and tries to establish which one of the teeth from the Master Teeth Set matches the patient's mouth environment optimally. This procedure is very subjective and can be influenced, for example, by the illumination conditions and the skill of the dentist.
There are known color measurement tools, referred to as calorimeters such as described by U.S. Pat. No. 4,449,821 to Fletcher, U.S. Pat. No. 4,767,703 to Ericsson et al., and U.S. Pat. No. 3,069,644 to Hartmann et al. None of these tools meet the requirements for measuring tooth color within a patient's mouth. Measurement must be made of a very small area with proper calibration.
Thus, it would be desirable to provide an exact and inexpensive device for tooth color measurement which would enable a dentist to properly match artificial teeth to the patient's mouth environment.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to overcome the disadvantages associated with the method of manually matching prosthetic teeth and provide a method of reliably and accurately matching tooth color.
In accordance with a preferred embodiment of the present invention, there is provided a method for measurement of tooth color comprising the steps of:
illuminating a selected tooth zone in a patient's mouth with light;
filtering light reflected from said tooth zone into color components;
detecting each of said reflected light color components and producing a color component signal in accordance therewith;
converting said color component signals to digital data;
processing said digital data and mapping said color component signals into color values of hue, saturation and intensity (H,S,I) for said selected tooth zone;
normalizing said HSI values into a set of normalized color coordinates defining a color vector;
comparing said color vector with a predetermined set of color vectors;
determining a classification of said color vector closest to one of said predetermined set of color vectors; and
providing said classification for matching an artificial tooth.
In a preferred embodiment of the invention, a light source is provided with an illumination lens and a relay lens. A light aperture at the end of a probe directs the light from the light source to illuminate a measurement point in a chosen zone of a tooth selected in the patient's mouth. Light reflected from the tooth is passed through a third lens which focuses the reflected light simultaneously on three detectors. Each detector is provided with a color filter positioned in front of it to allow for only one of the basic red, green and blue (R,G,B) colors to reach the detector.
A method of auto-calibration is provided by using a shutter which in its vertical position shunts a portion of the light produced by the light source to a reference white surface. The processing steps described below are performed, as for a tooth zone.
The three detectors produce an output signal representing the R,G, B color coordinate related to the measurement point. These signals are sent to the signal processing unit (SPU) where they are filtered to improve the signal-to-noise ratio. The SPU converts the analog signal values to digital data suitable to be further processed by a data processing and control unit (DPCU). In addition, the SPU performs a two step auto-calibration of the signal to provide precision and stable color measurement results.
The SPU interfaces with the DPCU to allow processing of the data. The R,G,B values are mapped into values of Hue, Saturation and Intensity (H, S, I) which are normalized using the NF. The three normalized values of (H, S, I) together represent a point and a radius vector. The distance from the point in (H, S, I) space to the closest point of reference value of the Master Teeth Set is determined to affiliate the point with a tooth color vector. The process is repeated for a total of three zones on the tooth. This allows the tooth color to be classified as corresponding to one of the colors in the Master Teeth Set.
REFERENCES:
patent: 4654794 (1987-03-01), O'Brien
patent: 5690486 (1997-11-01), Zigelbaum
patent: 5759030 (1998-06-01), Jung et al.
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