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Ask the Expert a question about color

If you have a question regarding color or apperance in dentistry, you have access to a panel of experts which include Drs. John Powers, William Johnston, Edward Swift and Stephen Westland. Just click on the link below to submit your question. info@scadent.org

 

What is the difference between light source and illuminant?

A light source is a physical emitter of radiation such as a fluorescent tube, a candle or the sun. An illuminant is a table of spectral power distribution that could form the blueprint for a possible illuminant. The CIE has defined several illuminants such as CIE illuminant A (representing incandescent light sources), CIE illuminant C (representing daylight) and the CIE D illuminants (also representing daylight). CIE illuminant D65 is probably the most commonly used illuminant and represents the spectral properties of a northern hemisphere sky around noon on a cloudy day. CIE illuminant D55 is also often used but has a lower color temperature warmer (redder/yellower) than D65. Interestingly there are no commercial light sources that are a perfect match to illuminant D65 though many are marketed to be so and many are very close to the spectral properties of D65.

 

Which bleaching method is the most effective?

Options for tooth whitening include dentist-prescribed, patient-applied (at-home bleaching), in-office, and over-the-counter methods. Of those, the most predictable and well-proven method is at-home bleaching. Overnight application of 10-15% carbamide peroxide in a custom tray is considered the gold standard. However, many other concentrations of peroxide, in either hydrogen or carbamide form, and application times (e.g., short daytime applications) are available and effective for at-home tooth whitening. In-office methods tend to be less effective and less predictable, and when used, are frequently used in combination with at-home applications. The effectiveness of a light source to “activate” in-office bleaching agents remains controversial. OTC bleaching products vary widely in effectiveness. Some, notably Procter & Gamble’s Crest Whitestrips, have performed very well in numerous clinical trials. Many other products are only marginally effective or not effective at all.

 

What is ΔE*?

ΔE* represents the Euclidean distance between two points in CIELAB color space that represent two color stimuli. The asterisk denotes that the Euclidean distance is specified in CIELAB because there are other color spaces than CIELAB that have similar axes and it is therefore important to avoid confusion. CIELAB has become a very popular and common color space since its introduction as a CIE standard in 1976. The value of ΔE* is often used to represent the perceptual color difference between the stimuli represented by the two points. However, it is important to be aware that CIELAB is only approximately uniform in terms of visual perception. It would be useful if we could define a threshold value of ΔE* below which we would find no color difference between the two color stimuli and above which the color stimuli would not be a color match. However, although some texts suggest that this threshold exists and is equal to unity (ΔE* = 1), the lack of visual uniformity of the CIELAB means that the actual threshold of perceptibility of color difference is very much less than 1 for some colors and very much greater than 1 for other colors. The lack of visual uniformity of the CIELAB color space has led to the development of a number of so-called optimized color difference equations such as CIEDE2000 that are more reliable indicators of visual color difference.

 

How are cameras used for measuring color in dental research?

Traditional color-measurement devices take an average measurement over a small area and either measure spectral properties or – in the case of colorimeters – measure colorimetric properties. Devices that measure spectral properties are often of higher quality and certainly are more flexible, since it is relatively trivial to calculate colorimetric properties (such as CIELAB values or CIE XYZ) values from spectral values (such as spectral reflectance factors or spectral radiance values). These instruments can be either contact-based or non-contact-based. A digital camera is a non-contact imaging device that measures neither spectral nor colorimetric values. Cameras record digital RGB values at each point or pixel in the image. However, careful calibration work can allow the RGB values to be converted into colorimetric values. The advantage of cameras is that it is then possible to obtain colorimetric values at each pixel. However, the disadvantage is that the conversion from RGB to colorimetric values is unlikely to be perfect and, therefore, the colorimetric values may be less accurate. It is possible to convert the camera RGB values into spectral data but this conversion is usually particularly inaccurate. Some color measurement devices that have been developed specifically for dentistry are based upon camera technology and therefore the distinction between cameras and more traditional color-measurement technology is becoming blurred?

 

What is a photometric curve and how is it applied in dentistry?

The photometric curve or luminosity function defines the sensitivity of the human visual system to radiation as a function of wavelength in the visible spectrum. The eye is not equally sensitive to energy at different wavelengths, being much more sensitive to the middle wavelengths (around 500 nm) and far less sensitive to the very short and very long wavelengths. It is important to note that there is more than one photometric curve that has been defined by the CIE. For example, the photopic luminosity function describes the wavelength sensitivity of the visual system under moderate levels of illumination, whereas the scotopic luminosity function describes sensitivity under very dark conditions. The photometric curve is used in photometry and colorimetry. In photometric units (such as luminance), every wavelength is weighted by the sensitivity of the human eye, whereas in radiometric units (such as radiance) this is not the case. The photometric curve and the CIE color-matching functions are used in the calculation of colorimetric values such as CIE XYZ.

 

What hand-held shade matching instruments are currently available? Recommendations?

Available are several instruments and systems intended for dental shade matching. These instruments usually involve hand-held components for intra-oral use, but may instead involve digital photography. Included in these instruments and systems are:

  • SpectroShade Micro (MHT Optic Research AG, Niederhasli, Switzerland)
  • VITA Easyshade Advanca 4.0 (Vident, Brea, CA)

Regarding recommendations, there is much to consider before making any recommendation for shade matching. The current standard of visual shade matching has been highly criticized, yet the independent and conflict-free assessment of validity of each instrument must be established relative to a gold standard of shade specification, as validity is the major concern for the recommendation of any instrument. Currently insufficient information regarding the validity and precision of these instruments is available. Furthermore, ease of use and ease of incorporation into a clinical setting should also be considered before a recommendation can be made.

 

Is there an ideal color space for natural teeth?

The CIE has recently recommended modifications to the CIELAB color space in the form of the L’a’b’ color space and its application through a revised color difference formula. Currently, this L’a’b’ color space and the newly recommended CIE DE2000 color difference formula represent the best specification of a color space and its application in practical clinical dentistry and the research of esthetic dental materials. Further research is needed to assess the ideal constants in the CIE DE2000 color difference formula that should be applied to differences in tooth colors.

 

How do you measure fluorescence in natural teeth?

Fluorescence is most frequently measured by means of a spectrophotofluorometer, which can independently adjust and monitor both the illumination and the observation wavelengths. Since available spectrophotofluorometers commonly involve apertures for specimen placement, edge loss may be present in these measures when the path-length of any fluorescent effect exceeds the distance from the illumination beam to the aperture edge. There are also concerns about calibration, since any adjustment of wavelength involves varying the intensity of the light passing through device which adjusts the wavelength. As always, the calibration of light from curved surfaces can be an issue for natural teeth. Nevertheless, practically useful simplifications can be implemented which provide useful information about the fluorescence of natural teeth.

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