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Spectrophotometry
A spectrophotometer (Fig. 5) is a sophisticated analytical
optical instrument consisting of a stable and precise
light source and light detector (photo detector) capable of
measuring discrete colors or wavelengths with a 1 nm resolution
measured between the ultraviolet and infrared wavelengths
within the visible light spectrum. True colors, shades
and optical properties can be reproducibly quantified by a
spectrophotometer, which engineers and scientists use to
characterize optical ceramic and dental ceramic materials
by measuring the transmission, reflection and absorption of
light as a function of wavelength. The latest digital dental
shade-taking systems use a simplified spectrophotometer to
quantify tooth shades.
Determining Shade
Ceramic crown & bridge restorations are fabricated to the
desired shade and esthetic detail based typically on a standardized
shade guide system, such as the VITA Classical
Shade Guide or VITA 3D-Master Shade Guide. The VITA
Classical Shade Guide was introduced in 1956 and is considered
the gold standard for dental tooth shade quantification.
This guide classifies dental shades into 16 different shade
guide tabs. The VITA 3D-Master Shade Guide introduced
in the 1990s is based on the earlier Munsell color space
and the CIELAB colorimetry system. The VITA 3D-Master
Shade Guide tabs are categorized by value, hue and chroma
colorimetry parameters. The dentist and dental laboratory
technician use the shade guide tabs to determine the appropriate
tooth shade by visually comparing the shade guide
tabs against the patient’s tooth and against the ceramic
dental restoration — in ambient background lighting. This
method, although common, is inherently subjective, dependent
on myriad variations arising from ambient
light interactions and color sensitivity of the
human eye.
Figure 5: PerkinElmer Lambda 35 UV/VIS
Spectrophotometer with Labsphere RSA-PE-20
Reflectance Spectroscopy Accessory
Color and the Human Eye
The eye detects three primary colors: red, green and blue.
The optical focal plane is the retina, which contains two
kinds of light detection sites: rods and cones. Rods make
up the majority of light sensors in the retina. They are extremely
sensitive to light intensity variation, but are not
color-sensitive. Cones are sensitive to specific colors — or
rather the electromagnetic wavelengths that are responsible
for color vision. Cones consist of three primary lengths
attuned to specific colors or wavelengths: long (L-cones),
medium (M-cones) and short (S-cones). L-cones detect red
colors at a peak wavelength of 565 nm, while M-cones
detect green colors at a peak of 540 nm and S-cones
detect blue colors at a peak of 440 nm. The majority of
these retinal cones are of the long and medium variety.
The superpositioning of the three different color signals
results in a peak sensitivity to light wavelength at 550 nm.
The human brain can perceive the continuum of visible
colors between blue (the shortest wavelength the retina
is sensitive to at 400 nm) and red (the longest light wavelength
the retina is sensitive to at 700 nm). Except for red,
green and blue, color is a perception, a result of the brain
processing the additive intensities of the three primary
colors detected by the cones.
Visible Light and the
Electromagnetic Spectrum
The color-related esthetics of natural dentition and dental
ceramic materials result from the interaction of light
with the material in the visible part of the electromagnetic
spectrum (400–700 nm). This visible range, of course,
represents only a small portion of the larger electromagnetic
spectrum, which includes ultraviolet rays, X-rays,
and gamma rays smaller than 400 nm in wavelength; and
near-infrared, infrared, microwaves, radio waves, and longwaves
larger than 700 nm in wavelength (Fig. 4). Wavelengths
that fall within the visible electromagnetic spectrum
are expressed in nanometer units, where 1 nm equals
one billionth of a meter (1 nm=1x10 -9 m). Expressed in
colors, the visible electromagnetic spectrum ranges from
violet (400 nm) to red (700 nm). As described earlier, the
human eye is most sensitive to the yellow-orange color
at a wavelength of 550 nm.
Dielectrics
Dental ceramic materials such as veneering porcelains
and alumina/zirconia crown & bridge frameworks are
electrically insulating materials commonly known
as dielectrics. Dielectric materials are generally
comprised of inorganic oxides, such as silicon
dioxide (SiO 2
), zirconium dioxide (ZrO 2
), titanium
dioxide (TiO 2
) and many others. Additionally,
the calcium phosphate of natural dentition and
polymer materials such as composites are typically
dielectrics consisting also of inorganic nitrides and
some carbides.
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