Basic principles and techniques in colorometry
- Introduction
In dentistry, the main criteria for the aesthetic success of a restoration are the correct choice of shape, color and surface finish. In his daily practice, the dentist is frequently confronted with the problem of determining the color of natural teeth. This subject has always been considered delicate by most practitioners and laboratory technicians. The subjectivity of the visual choice, frequently practiced in an inappropriate lighting environment, and the difficulties of color reproduction in the laboratory lead the profession to take an interest in colorimetric, spectrophotometric measuring devices or other innovative systems that are increasingly numerous on the market.
- Optical properties
2.1 – The four levels involved in determining color
The nature of the light source is the first of these levels. A light source must be suitable, in terms of intensity and type. In dim light, we can distinguish the shape of an object, but we cannot determine its exact hue. Blue or red lighting completely alters the determination of this hue. It would seem ridiculous to us to choose a hue with such lighting. However, the same is true with lighting by incandescent lamps or neon tubes; their nature is very different and insidiously modifies our choice of color compared to a choice made in daylight.
The second level is the object being observed. It can be extremely complex, combining several colors, its transparency, and its surface condition, which will disrupt the choice. The light reflected or transmitted by the object to the sensor, the eye, depends on the nature of the source.
The eye is the third level . It captures a limited portion of the photons through its sensory cells, the rods and cones. This is the visible spectrum. It transmits information to the center of vision, at the level of the occipital cortex.
The fourth and final level corresponds to the occipital cortex where the transmitted information will be collected and analyzed.
Abnormalities can exist, and while they are systematically sought in certain professions, such as audiovisual specialists, they are not at the dentist’s. Partial dyschromatopsia (color blindness) is a disturbance of color vision. The most common is the confusion of green and red hues, less common is the confusion of blue and yellow. Finally, some subjects cannot distinguish any color, only shades of gray: this is achromatopsia.
But beyond these conditions, it is important to know that vision has been trained since childhood and that the brain completes and interprets images by giving them meaning. Each person will therefore have a subjective vision and their own personal interpretation. It is therefore necessary to practice identifying shades.
2. 2 – The three characteristics of color
2.2.1 – Brightness
( syn.: brightness, luminance or value, English term “value”)
Brightness refers to the amount of reflected light. If the entire spectrum of daylight is reflected, the observed object is white. If nothing is reflected, the object is black. Within this range, depending on the amount of light, the object appears more or less gray. The difficulty in choosing brightness is to ignore hue and hue saturation. The cells specialized in brightness are rods.
2 . 2 . 2 – The tint
( syn.: chromatic tone, chromaticity, tone, English term “hue” )
This tint is exclusively linked to the dominant wavelength of the reflected light. It is part of the visible spectrum ( figure 1 ).
Figure 1: Electromagnetic radiation and visible spectrum
The limit of the visible spectrum varies from one individual to another, the extremes could be from 380 nanometers to 800 nanometers. Ultraviolet and infrared are not visible. The 6 or 7 colors usually listed are: violet (and indigo) [380-450nm], blue [450-490nm], green [490-560nm], yellow [560-590nm], orange [590-630nm], red [630-800nm]. The blue-green-indigo limit is difficult to discern. In reality, the variation in hue is continuous and this distinction is completely arbitrary. The eye (cones) is most sensitive in the green-yellow interval and less sensitive from red and blue.
2 . 2 . 3 – Saturation
( syn.: intensity, density of color, in English “chroma” )
This is the amount of tint in the material. To dilute a tint, simply add white to it.
Color is therefore the combination of these three characteristics: brightness, hue, and saturation. It is therefore important to distinguish between color and hue, which are usually used interchangeably in everyday language.
- Color modeling and colorimetric dimensions
Two systems are used to classify these colors. The oldest is the Munsell system. Today, the L*a*b* system is used ( Figure 2 ).
CIE L*a*b system:
This is the chromatic sphere. It adopts the L*a*b color representation system, established by the International Commission on Illumination, responsible for designing a table of
standardized colors based on a mathematical principle, capable of meeting the quest for precision and objectivity. This system is an evolution of the previous system, established in order to satisfy all modern industrial applications and requirements.
L* is the vertical axis quantifying brightness; a* and b* are the rectangular chromaticity coordinates where the (-a*, +a*) axis is the green-red axis, and the (-b*, +b*) axis is the blue-yellow axis.
Natural human teeth occupy a rhomboid-shaped space commonly called
“chromatic banana”
This is an area high in the chromatic sphere, and quite close to the white-black L* axis. This means that natural teeth are very bright and desaturated. In addition, it is located in the quadrant between the +a* (red) axis and the +b* (yellow) axis: the chromatic tone of all natural teeth is yellow-orange.
In addition to this demonstration, this model is of significant interest: it allows us to easily express a difference in color between two objects, noted ΔE, it corresponds to the distance between two color points in the sphere and is calculated by the root of the sum of the squares of the differences between variables:
ΔE= [(L* 1 -L* 2 ) 2 + (a* 1 -a* 2 ) 2 + (b* 1 -b* 2 ) 2 ] ½
CIE L*C*H System:
According to the recommendations of a 2003 conference on color measurement in our profession, this third system developed by the CIE would be the most suitable for our activity, and in particular for research.
The system is identical to the L*a*b color space, except that the position of a color in space is described by its polar coordinates, rather than its angular coordinates.
Similar to L*a*b space, lightness L* is the vertical axis and ranges from zero (black) to one hundred (white), and saturation c* (chroma) is represented by the distance between the color location and the vertical neutral axis.
Hue is measured over an angle ranging from 0° to 360°. Angles ranging from 0° to 90° represent reds, oranges, and yellows. From 90° to 180°, they represent yellows, yellow-greens, and greens. From 180° to 270°, they represent greens, cyans (blue-green), and blues.
Finally from 270° to 360°, they represent blues, purples, magentas, and then return to
red.
- Optical properties of light radiation
When light hits matter, three things can happen: energy can be reflected, transmitted, or absorbed.
- Absorption:
When light falls on a material, some of the radiation is neither reflected nor transmitted, but absorbed. It then transforms into heat. Any absorbed radiation is subtracted from the perceived color. It is the unabsorbed rays that will determine the object’s color as well as its complementary properties. For example, an object is considered red when it absorbs all violet, blue, green, orange, and yellow wavelengths, and reflects only red. A body that absorbs all wavelengths is perceived as black.
- Transmission:
These phenomena depend on the number of particles included in the object and their size. They determine the translucency and, conversely, the opacity of the object. A material that does not allow any transmission is completely opaque.
- Refraction is the change in direction of a light wave as it passes from one medium to another. Thus, incident light striking a translucent material will have some of its rays reflected, and others will penetrate the material by changing direction. A material that does not change the direction of the rays passing through it is said to be transparent.
- Diffraction is the phenomenon by which light waves can pass around obstacles of dimensions approximately equal to their wavelengths.
This particular behavior of light is found throughout natural tooth enamel. It determines its translucency.
- Scattering is the propagation of light rays in a beam in all directions.
- Reflection:
It is the change of direction of a wave on a surface. It does not penetrate the
medium. This occurs when the particles of an object are larger than the wavelength of the incident light.
- Optical characteristics of natural teeth:
The tooth is a very complex object to analyze: its optical behavior is a combination of many parameters, starting with color and its three fundamental dimensions: brightness, saturation and chromatic tone, but we must also consider additional parameters such as transparency and translucency, opalescence, fluorescence, surface brightness, and characterizations. These light effects make the dental structure very difficult to map, but they are what give the tooth its “natural” appearance.
- translucency
The translucency or transparency of a material refers to the extent to which some or all of the incident light can pass through it. The translucency of dentin is 40%, while that of enamel is 70%.
- Opalescence
Opalescence refers to the bluish and orange effects that are often visible on the edges of natural enamel. This is called the “opal effect.” It is observed that when light is reflected, the enamel preferentially reflects short wavelengths, which gives it a bluish appearance.
Whereas in light transmission, the enamel will produce a red-orange appearance because it allows long wavelengths to pass through.
- fluorescence
The physical concept of fluorescence is the ability of a body subjected to non-visible ultraviolet radiation to immediately re-emit this light in a visible spectral band of short wavelength and bluish-white color] (fig. 9 a and b). Dentin is responsible for the fluorescence of natural teeth.
- The surface condition of natural teeth can vary greatly and significantly influences color perception.
- Characterizations are inseparable from the description of the color of a natural tooth. These are specific and localized colored aspects such as opaque white spots of demineralization.
IN SUMMARY
Dentin is responsible for the saturation, color tone and fluorescence of the tooth, while enamel is responsible for the brightness, gradient effects, transparency and opalescence of the incisal edges.
- Determination of colors by spectrophotometry
Spectrophotometers analyze the reflected wavelengths of visible polychromatic incident light . The reflected spectrum is measured at many points, at small intervals, and compared to a database to deduce the color of the tooth. For an optical measurement, several million reference points are analyzed on a tooth. These devices are currently the most accurate and do not pose any problems of aging of the light source.
- Conclusion
The analysis and communication of the color of natural teeth has progressed enormously with the development of shade guides incorporating 3D color analysis and the appearance of high-performance spectrophotometers and colorimeters. It is still necessary to understand what the color of a natural tooth is.
It goes far beyond the definition of a base shade taken from the middle third of the reference tooth. It also goes far beyond the three-dimensional analysis of color classically broken down into brightness, saturation, and chromatic tone. It develops in depth, intimately in the stratification of the tissues of the natural tooth. These are the six other dimensions of color which are opacity and translucency, opalescence, fluorescence, pearlescent effect, texture of
surface and the characterizations that must be taken into account, to arrive at a perfect description of the natural.
Basic principles and techniques in colorometry
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