Optics: measuring and testing – By shade or color – Tristimulus examination
Reexamination Certificate
2002-09-06
2004-06-15
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By shade or color
Tristimulus examination
C600S127000, C600S160000
Reexamination Certificate
active
06750971
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to instruments for measuring optical characteristics—for example color, translucency, and/or gloss—of objects and, more particularly, to such instruments for use in dental applications.
The determination of shade or color of an object is a process that is frequently performed in the field of dentistry. To perform dental restorations of a damaged tooth, a dentist visually compares the color of the tooth to be replaced with an assortment of shade tabs. These shade tabs are physical tabs representing the color of commercially available restorative material, such as ceramics. The tabs include the exact specification of materials needed to produce a restorative tooth with the shade of the original tooth as determined by the dentist's visual comparison. Once the dentist finds a shade tab that matches the color of the tooth, or remaining adjacent teeth in some cases, he is in a position to manufacture the required restoration. This process, however, is very time consuming and quite subjective, and frequently results in poorly shaded restorations.
In the field of dentistry, intraoral cameras are frequently used to acquire images of teeth and determine treatment plans for cavities and other mechanical reconstruction. These cameras are designed to be versatile and able to collect measurements in tight places often found in the mouth; however, they do not preserve the color fidelity—that is, they do not collect the true color—of the object measured.
Some dentists attempt to use intraoral cameras to assist in the shade determination process. Unfortunately, conventional intraoral cameras suffer two problems: distance sensitivity due to illumination geometry and color discrimination error due to sensor limitations.
With regard to the first problem, intraoral cameras typically use fiberoptic illumination to reduce the size of the handpiece. Such a device is disclosed in U.S. Pat. No. Re. 36,434 to Hamlin et al, reissued Dec. 7, 1999. The goal of Hamlin, and most intraoral cameras, is to provide a small measuring tip on a handpiece that can be used to probe hard to reach areas in a mouth. Although fiber-optic illumination is useful for providing high levels of illumination and is compatible with small measuring probe tips, a drawback of any small illumination source that illuminates a larger area is that the projected beam must be divergent light. The intensity of a divergent beam is governed by the inverse square law given below:
I
=
D
2
(
D
+
Δ
D
)
2
(
1
)
where I is intensity, D is the distance from the illumination source, and &Dgr;D is an increase in distance D from the light source. The concept of Equation 1 is illustrated in
FIG. 1
, where fiber optic source
115
projects illumination flux
112
to distances D and D plus &Dgr;D. There, the intensity of flux
112
at distance D, according to Equation 1, is greater at distance D from light source
115
than at a distance D plus &Dgr;D.
It is known that when the distance change to the illumination source is significant with respect to the distance to the source, the illumination output varies significantly, creating what is called non-uniform illumination. Particularly with objects positioned close to the fiber optics, certain regions of the object are non-uniformly illuminated because the light from the illumination source rapidly diffuses as it travels away from the source. Moreover, when multiple sources of light are used to illuminate an object, the object may be non-uniformly illuminated in different regions.
An example of non-uniform illumination of the surface of an object is understood with further reference to FIG.
1
. As depicted there, a curved surface of a tooth T, slightly exaggerated for purposes of discussion, is illuminated within flux
112
projected from light source
115
. Region of the tooth
113
, lies distance D from light source
115
, and region
114
, lies distance D plus &Dgr;D from the light source
115
. As explained above, the intensity of light is greater at distance D than at D plus &Dgr;D. Accordingly, regions
113
and
114
are not illuminated with the same intensity of light, that is, illumination is non-uniform. Sensors sensing light reflected from tooth T will collect inconsistent color information from these regions.
An example of non-uniform illumination of regions of an object with a multiple fiber optic light sources is illustrated in FIG.
2
. Exemplary fiber optic light sources
120
and
122
project light fluxes
130
and
140
to illuminate the tooth T. These light fluxes reflect from the tooth and are collected by an image sensor not shown for the sake of simplicity. As can be seen, tooth region
122
is illuminated primarily by light flux
140
, but region
124
is illuminated by a combination of light fluxes
130
and
140
. Of course, this illumination is three-dimensional even though it is depicted here in only two dimensions. Further, if more fiber optic light sources are added, the tooth is subdivided into even more regions of different illumination overlap. Given this non-uniform illumination, a color sensor, sensing the light reflected from the tooth, will invariably collect inconsistent color information from region to region. For example, what is sensed as “lighter shade” in region
122
may be sensed as “darker shade” in region
124
due to the non-uniform illumination.
With non-uniform illumination, conventional intraoral cameras critically rely on illumination source positioning which can not be maintained in practical use. This results in significant errors affecting tooth shade determination.
Other devices, specifically designed for tooth shade determination, have been proposed that use bi-directional fiber optic illumination. Such a method is described in U.S. Pat. No. 6,038,024 to Berner, issued Mar. 14, 2000. A limitation of this method of illumination is that the illuminant intensity is maximized at the intersection of the two projected beams. Often, significant portions of the measured area are not illuminated by both beams and hence have a lower and unpredictable illumination value.
Berner's non-uniform illumination is depicted in
FIG. 3. A
fiber optic bundle
150
is supplied with light at one end. Prior to arriving at the probe tip, the bundle is bifurcated, or divided into two bundles
152
and
154
. The bundles are mechanically aimed at the target tooth T in some fixed angularity. Collimating lenses
156
,
158
are often added in the path of illumination between the fiber optic bundle and the target T to lower distance sensitivity of illumination output. Each bundle generates a light flux
162
and
164
projected onto tooth T from two directions with collimating lenses
156
,
158
. As can be seen, fluxes
162
and
164
intersect on tooth T resulting in the intensity in region
169
being greater than the intensities in regions
167
and
171
because those regions
167
and
171
, and other peripheral regions, are each illuminated by light fluxes
164
and
162
individually. The fluxes reflected from the tooth T are not shown for simplicity.
Given this non-uniform illumination, a color sensor, sensing the light reflected from the tooth, will invariably collect inconsistent color value information from region to region. For example, what is sensed as “lighter shade” in region
167
may be sensed as “darker shade,” in region
169
due to the non-uniform illumination. Moreover, with multiple light source paths, gloss artifact potential is increased. Where glare artifacts exist, the color of the target is washed out by the image of the light source itself rather than the desired tooth subject.
In addition to non-uniform illumination, today's intraoral cameras utilize color filter array (CFA) image sensors that frequently contribute to inaccurate color measurement because the filter array is applied to the image. Many intraoral cameras include color filter arrays such as red, green and blue (RGB) arrays, and cyan, magenta, yellow and green (CMYG) arrays, to name a f
Overbeck James L.
Van Andel Richard J.
Smith Zandra V.
Warner & Norcross & Judd LLP
X-Rite Incorporated
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