color and optical effects in dentistry

Color and Optical effectsin dentistry

By

Mohamed Mahmoud Abdul-MonemAssistant lecturer

Dental Biomaterials DepartmentAlexandria University

EgyptMohamed_mahmoud.biomaterials@yahoo.com

Nature of light and the role of human vision

• LIGHT is electromagnetic radiation that can be detected by human eye.

• The eye is sensitive to wavelengths from approximately 400nm(violet) to 700 nm(dark red).

• For an object to be visible ,it must reflect or transmit light incident on it from an external source.

• The incident light is usually polychromatic; a mixture of various wavelengths (white light).

• Light from an object that is incident on the eye is focused in the retina (cone shaped cells which are sensitive to red, blue and green )and converted to nerve impulses that are transmitted to the brain.

• Constant stimulation by a single color may result in color fatigue and decrease in eye’s response.

Nature of object under view

• The nature of the restorative material under view, determines how that material will appear.

• Light interacts with an object through : reflection ,absorption ,refraction or transmission.

• These phenomena determine the opacity, translucency or transparency of an object.

• Opacity is related to the amount of light a material can absorb and/or scatter.

• The opposite of opacity is translucency.• Transparent materials absorb no light and

transmit 100% of light that passes through them.

• Light reflected from rough surface scatters in many directions because it is reflected at many angles by the un even surface.

• This leads to an appearance that ranges from mirror like for a perfectly smooth surface (specular reflectance) to flat,dull appearance(diffuse reflectance) as chalk.

3D of color• Color perception is described by three

objective variables : hue ,value and chroma .These variables constitute the 3D of color space.

• Hue : The dominant color of an object. ( red, green, blue).

• Value (gray scale) : value increases towards the high end (lighter) and decreases towards the low end (darker).

• Chroma: degree of saturation of a particular hue.

L*a*b* color space

• Color chart based on Comission Internationale de l’eclairage.

• L* represents value• a* measurement along

red-green axis• b* measurement along

yellow-blue axis

Color matchingShade guide

Hue A B C D A red-brownB red-yellowC grayD red-gray

Value1 2 3 4

1 lightest4 darkest

• Arrangement from lightest to darkest • B1,A1,B2,D2• A2,C1,C2,D4• A3,D3,B3,A3.5• B4,C3,A4,C4

Vita 3D Master shade guide

• To ensure maximum esthetic results :• Shade guide• Drawings • Photographs• Subjective preference of patient

Effect of observer on color matching

• Factors interfering with true perception of color :

• Low or high light levels• Fatigue of color receptors• Sex• Age • Memory• Cultural background.

• At low light levels,the rods in the retina are more dominant than cones and color perception is lost.

• As brightness becomes more intense,color appears to change(Bezold-Brucke effect).

• If the patient is observed under an intense-colored background,the dentist may select a tooth shade with hue shifted towards the complementary color of the background.

• For example a blue background shifts color selection towards yellow.

The effect of light source on color matching

• The appearance of an object is dependant on the nature of the light in which it is viewed.

• Daylight, incandescent light and fluorescent lamps are common light sources in the clinic or lab.

• Metamerism: Objects that appear to be color matched under one type of light may appear different under another type.

• Thus color matching should be done under two or more different light sources,one of which should be daylight

Fluorescence

• Natural tooth absorbs light at wavelengths too short to be visible to the eye.

• Wavelengths between 300-400 nm (Near UV radiation).

• The energy that the tooth absorbs is converted into light with longer wavelengths and the tooth itself becomes a light source.

• The emitted light is a blue-white color (400-450 nm)

• Fluorescence makes a definite contribution to the brightness and vital appearance of human teeth.

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