Color and Optical Effects in Restorative and Prosthetic Dentistry PDF

Color and Optical Effects in Restorative and Prosthetic Dentistry Prof. Muawia Qudeimat Dr. Maryam Aljutaili Introduction to Color and Optical Effects in Dentistry Increasing Emphasis: Color and optical effects have gained more attention, especially with the rise of bleaching and whitening tech...

Color and Optical Effects in Restorative and Prosthetic Dentistry Prof. Muawia Qudeimat Dr. Maryam Aljutaili Introduction to Color and Optical Effects in Dentistry Increasing Emphasis: Color and optical effects have gained more attention, especially with the rise of bleaching and whitening technologies. Challenges: Developing a general-purpose, tooth-colored, and color-stable restorative material remains a significant challenge in dental materials research. Importance of Knowledge in Esthetic Dentistry Artistic and Scientific Demands: Dentists and technicians need both artistic skills and a strong understanding of the scientific principles of color and optics for successful restorations. Ceramic Restorations: The increasing use of ceramics in dental restorations further emphasizes the need for knowledge of color and optical effects. Nature of Light and Human Vision Electromagnetic Spectrum: The human eye detects light in the 400-700 nm wavelength range, known as the visible spectrum. Visible Light: Objects become visible by reflecting or transmitting light, usually "white light" consisting of multiple wavelengths. Nature of Light and Human Vision Human Eye Mechanism: Light reflected from objects is focused on the retina, converted to nerve impulses, and processed by the brain. Rods: Sensitive to dim light, located around the outer retina. Cones: Responsible for color vision, located in the center of the retina. Nature of Light and Human Vision Color Vision: Processed in the brain, but prolonged exposure to a single color can cause "color fatigue." Color blindness arises from defects in color-sensing receptors. Human Eye as a Colorimeter: The eye compares colors side by side, similar to scientific instruments, but is more adaptable for rough or curved surfaces. Interaction of Light with Restorative Materials Esthetics and Light: The way light interacts with dental materials must mimic the behavior of light with natural teeth to achieve a natural appearance. Optical Properties: Reflection: Light bouncing off surfaces. Absorption: Light taken in by a material. Refraction: Light bending when passing through a material. Transmission: Light passing through a material without change. Interaction of Light with Restorative Materials Surface Finish: Specular Reflectance: Mirror-like reflection from smooth surfaces. Diffuse Reflectance: Scattered reflection from rough surfaces. Interaction of Light with Restorative Materials Opacity and Translucency: The amount of light absorbed/scattered determines opacity. The opposite is translucency. Transparent Materials: Absorb no light, transmit 100%. Enamel: Translucent, with a refractive index of 1.65, and allows reflection, refraction, absorption, and transmission of light. Three Dimensions of Color Limitations of Verbal Descriptions: Written descriptions of colors (e.g., "puce") are subjective and often inconsistent, making them unsuitable for precise communication in dentistry. Three Dimensions of Color Munsell Color System: Uses three independent variables to describe color accurately: Value: Lightness or darkness (z-axis). Higher value indicates lighter color, lower value indicates darker color. Hue: The dominant color (red, green, blue) determined by wavelength. Chroma: Saturation or intensity of a color. Higher chroma means more vivid color, while lower chroma is duller. Three Dimensions of Color Munsell Color System Example: The Munsell system organizes colors in a 3D space, with discs representing various hues, value (lightness/darkness), and chroma (saturation). Quantifying Color: CIE Lab Color Space CIE Lab System*: Used for precise color measurements. L*: Lightness, from black to white. a*: Red-Green axis. b*: Yellow-Blue axis. ΔE (Color Difference): Formula to measure the difference between two colors in the CIE Lab* space. Quantifying Color: CIE Lab Color Space Perceptibility and Acceptability Thresholds: Perceptibility Threshold (PT): The smallest color difference detectable by 50% of observers (ΔE* = 1.0 most commonly reported). Acceptability Threshold (AT): The color difference acceptable to 50% of observers (ΔE* = 3.3 to 3.7 reported as common). ISO Standards: ΔE* = 1.2 for PT and ΔE* = 2.7 for AT according to ISO 28642:2016. Color Matching in Dentistry Use of Shade Guides: Dentists use shade guides to match the color of ceramic veneers, inlays, or crowns to natural teeth. Shade-guide tabs are categorized by: Hue (color tone): A = red-brown B = red-yellow C = gray D = red-gray Value (lightness/darkness): Values range from 1 (lightest) to 4 (darkest). Color Matching in Dentistry Modern Arrangement: Many shade guides are now arranged by value, from lightest to darkest, for easier selection (e.g., B1, A1, B2, etc.). This method simplifies the matching process, making it more reliable. Challenges in Communication While shade guides help match tooth color, communicating exact details to dental labs remains challenging. Dentists often provide additional information such as photographs, written descriptions, and drawings to ensure better accuracy. Direct observation by the technician increases the likelihood of a successful color match. Patient Preferences Even if there is a near-perfect match between the natural teeth and the restoration, patients often prefer a slightly lighter restoration. Example: A slight mismatch in value may still result in patient satisfaction, as long as the restoration appears brighter or more aesthetically pleasing, as demonstrated by two central incisors selected to be higher in value than lateral incisors (Figure). The Effect of the Observer on Color Matching Human Eye Sensitivity: Color perception is influenced by cones in the retina, which are sensitive to red, blue, and green wavelengths. Factors affecting color perception include: Light Levels. Color Receptor Fatigue. Background Influence. Color Blindness. Do you See What I See. Part 4 A video will be played The Effect of the Observer on Color Matching- Light Levels At low light levels, the rods in the retina dominate, leading to a loss of color perception. High light levels can cause color to appear different. The Effect of the Observer on Color Matching- Color Receptor Fatigue Prolonged focus on a particular color can cause a temporary complementary afterimage when looking at a neutral background (e.g., red followed by green). The Effect of the Observer on Color Matching- Background Influence The color of the background can shift the perception of the tooth’s shade. A blue background may cause a shift toward yellow, while an orange background might shift toward blue-green. The Effect of the Observer on Color Matching- Color Blindness About 8% of men and 0.5% of women suffer from color blindness, usually red-green blindness, but this typically does not impair shade selection in dentistry. The Effect of the Observer on Color Matching- Variability in Color Matching The range of hues, chromas, and values found in human teeth is relatively narrow, making accurate color matching difficult even with a small number of available shades. Digital Technologies: Newer digital tools for color matching aim to reduce inconsistencies and enhance precision. The Effect of the Light Source on Color Matching- Metamerism The appearance of a color can change under different lighting conditions (e.g., daylight vs. incandescent or fluorescent lighting). To reduce the risk of mismatches, color matching should be performed under multiple light sources, including daylight, to assess how a restoration will look in different environments. Fluorescence in Dental Materials Natural Tooth Fluorescence: Natural tooth structure absorbs near-ultraviolet (UV) light (300–400 nm) and re-emits it in the visible range (400–450 nm), resulting in a blue-white glow. This fluorescence gives natural teeth a bright, vital appearance. Aesthetic Importance: If dental materials (e.g., ceramic (a) Dentin portion of the restoration was built up with a high-fluorescent composite. crowns) lack a fluorescing agent, they may look unnatural, (b) Nonfluorescent restorations result form a low-fluorescent composite applied over the especially under UV light, like blacklight, making the high-fluorescent layer. restoration appear as though the tooth is missing. Radiopacity in Dental Materials- X-rays and Optical Properties X-rays (a form of electromagnetic radiation) interact with dental materials similarly to light, and radiopacity is considered an optical property. Radiopacity in Dental Materials- Importance in Dentistry Radiopacity helps differentiate dental materials from surrounding tissues on an x- ray, aiding in the detection of issues such as marginal defects, dental caries, or microleakage. Composite Radiopacity: Restorative materials often contain radiopaque fillers like strontium- or barium-containing glass to ensure they are visible on x-rays. Denture Radiopacity: Denture polymers rarely contain additives like barium sulfate to make them radiopaque. However, when used, they help ensure dental appliances are detectable on x-ray in the event of accidental ingestion or trauma. Radiopacity in Dental Materials- Material Absorption X-ray absorption depends on the material’s density, thickness, and atomic number. The higher the atomic number, the greater the material’s absorbance. Polymers and Resins: These materials are typically radiolucent, meaning they are not easily visible on x-rays. Metal Radiopacity: Metals with atomic numbers higher than potassium (atomic number 19) are radiopaque and show up clearly on x-ray images. Radiopacity in Dental Materials- Dental Restoration Radiopacity To comply with the American Dental Association standards, dental resins must have a radiopacity similar to that of aluminum to be considered radiopaque. For optimal detection on a chest x-ray, denture resin fragments must have a radiopacity about three times greater than aluminum, which ensures the fragment is visible and distinguishable from surrounding tissues. Clinical Color Difference Evaluation Perceptibility Threshold (PT) Acceptability Threshold (AT) Clinical Color Difference Evaluation- Perceptibility Threshold PT refers to the smallest color difference detectable by 50% of observers under standard conditions. PT values range from 0.4 to 4.0, with ΔE* = 1.0 being the most frequently reported. Clinical Color Difference Evaluation- Acceptability Threshold AT is the color difference deemed esthetically acceptable by 50% of observers. Reported AT values range from 2.0 to 6.8, with a typical range of ΔE* = 3.3 to 3.7. ISO sets PT at ΔE* = 1.2 and AT at ΔE* = 2.7 based on multicenter research studies.  Optical Properties of Dental Ceramics Optical Properties of Dental Ceramics Achieving successful aesthetics with a restoration is a difficult process due to the complex optical characteristics of tooth colour. The tooth reflects, absorbs, diffuses and transmits light reaching its surface. Thus, for acceptable aesthetic results, favorable shade matching of the all ceramic restorations should be achieved by controlling the reflection, transmission and light absorption of dental ceramic materials. Optical Properties of Dental Ceramics Optical properties of ceramic materials are the main factors responsible for the natural appearance of the ceramic restorations. These properties are affected by several factors such as: composition, crystalline content, porosity, additives, grain size and the angle on incidence of light. Optical Properties of Dental Ceramics The optical properties of ceramic materials include: a. Wavelength dependant optical properties: Colour (hue, chroma and value). Translucency. Opalescence. b. Bulk properties: Refractive index.  Colour Colour in ceramics is one of the characteristic optical properties that occur when light and matter interacts. When light beam strikes a polycrystalline material, this interaction may result in scattering, absorption into its bulk, transmission through its mass, reflection depending on the surface roughness or refraction,figure (1). All these behaviors of light can be used to explain the optical properties of dental ceramics such as: transparency, refractive index and colour matching. Factors affecting the optical properties of dental ceramics: a.Translucency b.Opalescence and counter opalescence c.Fluorescence d.Colour stability e.Shade matching f.Fabrication method  a.Translucency It is the ability of the material to permit the passage of light. It is a very critical property of dental ceramics. As the amount of light passing through the material ↑ the translucency ↑. a.Translucency In all ceramic restorations, the translucency is dependent on 1. The nature of the reinforcing crystalline phase within the core matrix 2. Their chemical nature 3. Size a.Translucency Alumina and conventional zirconia-based systems → opaque. Leucite reinforced are more translucent. The translucency of spinel-based systems is comparable to that of lithium disilicate. Lithium disilicate ceramics are translucent even with the high crystalline content, owing to the low refractive index of lithium-disilicate crystals. Recently, translucent zirconia systems are available.  a.Translucency The matching between the refractive index of the crystalline phase and the glassy matrix is a very important factor affecting the translucency of the porcelain. As the matching in the refractive index increases the translucency of the restoration increases. a.Translucency Regarding the effect of the crystalline phase, increasing the amount of crystalline phase in many high strength ceramics decrease the translucency and increase the opacity. So, these materials are used as a core in all ceramic restorations and have to be veneered with a more translucent ceramic layer. a.Translucency Veneering ceramics (Layering technique): Veneering ceramics are used to veneer metals in ceramo-metallic restorations and relatively opaque core ceramics in all ceramic restorations. In ceramo-metallic restorations, the ceramic is built up in three main layers of different translucencies: core, dentin (body) and enamel (incisal) porcelains. The opaque core has very low translucency allowing it to mask the metal substrate, then a dentin shade and finally an enamel shade. The enamel porcelain has the highest translucency values giving the restoration a natural appearance. a.Translucency In all ceramic restorations, dental ceramics should display both translucency and opacity to mimic the dental structure. Since, there is no dental ceramic product that can offer both translucency and opacity in a single material. Therefore, manufacturers offered three basic types of porcelain powder: § Opaque ceramics for coverage of dentin or discoloured abutment (they have high masking ability) § Translucent glazes (dentin and enamel shades) to be used in layering technique to simulate the different natural tooth structure successfully b.Opalescence and counter opalescence Opalescence is a form of scattering that happens when the crystal size is smaller than or equal to the wavelength of the light. In reflected light, the shorter blue-violet wave lengths are transmitted. While, in transmitted light, the longer red- orange wave lengths are transmitted. Opalescence is a desired property to optically mimic the natural appearance of human enamel. b.Opalescence and counter opalescence Human enamel has a filter that selectively passes the long waves and reflects the short waves at the same time. That’s why the incisal enamel has a bluish white colour. When the transmitted long waves hit the dentin, they are reflected giving the enamel an orange glaze. This effect is known as counter opalescence. In ceramics, the opalescence and counter opalescence effects are obtained through different opalescence glazes which are fine refraction oxides particles. Some have bluish colour and others have orange colour. c.Fluorescence Dental ceramics should exhibit fluorescent characteristics in order to create the effect of luminosity. Fluorescence is achieved in dental ceramics by adding rare earth oxides such as cerium, ytterbium, and europium oxides. When a dental ceramic restoration is not fluorescent, it tends to have a greyish appearance (reduced vitality look) especially in dark light.  d.Colour Stability Many factors can influence the colour stability of dental ceramics, such as; extrinsic dyes type of ceramic material thickness of ceramic material the degree of porosity The lower the degree of porosity, in the ceramic material after processing, the higher the colour stability  e.Shade Matching Shade matching is a very challenging step in replacing natural teeth. The shades of commercial dental porcelain powders are in the yellow to yellow-red range. Since the shades of natural teeth are much greater than the yellow and yellow red range, modifiers were added to porcelain for adjustment. These modifiers are strongly pigmented porcelains supplied in blue, yellow, pink, orange, brown and grey. Another way of modifying or changing the appearance of a ceramic crown is the application of highly pigmented glazes. The main disadvantages of this extrinsic surface staining are; lower durability, increased solubility and the reduction in translucency.  f.Fabrication Method The fabrication methods of dental ceramics include: 1.Sintering. 2.Heat pressing. 3.Slip casting. 4.Machining (CAD/CAM). The manufacturing technique of dental ceramics can affect the optical properties of the final restoration. f.Fabrication Method In sintering, it is a process of firing the compacted ceramic powder at high temperature to ensure optimal densification, pore elimination and viscous flow when the firing temperature is reached. This densification and pore elimination increase the translucency of the final restoration. In heat pressed all ceramic materials, it relies on application of external pressure at high temperature. This technique promotes excellent crystal dispersion, higher crystallinity and smaller crystal size which increase the opacity of the final restoration. So, for aesthetics, two techniques are available for leucite based and lithium disilicate based heat pressed ceramics: a staining technique, or the layering technique (application of veneering ceramic). f.Fabrication Method In slip-cast all ceramic materials, there are three types of ceramics available for this technique: alumina based, spinel based and zirconia toughened alumina. The main disadvantage of this technique is the highly opaque restorations (except for spinel-based restorations). The alumina crystals have high refractive index which accounts for some degree of opacity, while the spinel phase has better sintering, less porosity and thus better translucency. Factors affecting light scattering and translucency of monolithic zirconia In order to make a dental ceramic restoration more aesthetic and translucent, the light scattering from the bulk of the material must be reduced. Internal light scattering may result from several sources such as; pores, different crystalline phases, incomplete sintering, impurities and grain boundaries. Factors affecting the light scattering and translucency in dental ceramic materials include: a.Intrinsic factors: b.Extrinsic factor: 1. Composition. 1. Thickness. 2. Grain size. 2. Cement layer. 3. Sintering. 3. Light source. 4. Porosity. 4. Colour matching. Methods to increase the translucency of zirconia The conventional Yttria-stabilized tetragonal zirconia (Y-TZP) is a widely used ceramic biomaterial owing to its excellent mechanical properties, however, its high opacity lead to the development of a more translucent zirconia for use in fixed dental prosthesis. The translucence of zirconia is influenced by grain size, crystal isotropy, thickness, porosity, alumina content and many other factors.  Composition Y-TZP is partially stabilized in the tetragonal phase at ambient temperature by metal oxides (yttria). Tetragonal crystals have high mechanical properties because of the phase transformation toughening from tetragonal to monoclinic, but they have reduced translucency. Tetragonal crystals are optically anisotropic because of the differences in the refractive index in different crystallographic directions known as (birefringence). This can create discontinuity of the refractive index at the grain boundaries if the crystal orientations of the adjacent grains are not the same inducing reflection, scattering and refraction at the grain boundaries and reducing light transmittance.  Composition Therefore, a new approach to increase the translucency of zirconia is to produce fully stabilized cubic zirconia material (FSZ). This was achieved by increasing the percentage of the yttria to stabilize zirconia. Cubic grains have an isotropic orientation, which reduce light scattering and less interference with light transmission among the grains. Also, cubic grains are larger than tetragonal grains, so, they have reduced number of grain boundaries which are sources of light scattering.  Composition The content and distribution of alumina is an important factor that affect zirconia’s translucency. Alumina in incorporated in zirconia-based ceramics to increase their aging stability. Unfortunately, alumina has a different refractive index than the zirconia which results in light scattering and decreased translucency. Another approach in increasing the translucency of zirconia was increasing the lanthanum oxide content to 0.2% mol.  Grain size Concerning the effect of grain size on the optical properties of monolithic zirconia, two concepts have been introduced. 1. The first suggests that large grains are associated to fewer grain boundaries and increased light transmission. Large grains usually produced through higher temperature sintering, lead to better elimination of porosity and increased density, which makes the material structure more compact, thus increasing translucency.  Grain size 2. The second concept according to the Rayleigh scattering model, when light rays are incident on a birefringent material, the greatest scattering is recorded when grains have similar size with the visible light wavelength (400–700 nm). Conventional tetragonal zirconia grain size lies between 0.2 and 0.8 μm; which is greater than the wavelength range of visible light. Therefore, the use of nanometric tetragonal crystals should minimize the birefringence effect and improve light transmission, decrease scattering and increase translucency. Consequently, the grain size alone cannot define translucency and other parameters such as presence of cubic phase, porosity, and final density are important.  Sintering Sintering parameters such as temperature and holding time have been shown to affect the optical properties of monolithic zirconia. All of the studies investigating higher sintering temperatures reported an increase in translucency, correlated to the associated increase in grain size, pore elimination and consequently increase in density. This decrease scattering and increase translucency. Most monolithic zirconia ceramics should be sintered in a sintering temperature between 1400–1550 ◦C and no higher than that, as at temperatures of 1600 or 1700 ◦C grain boundary cracks can be generated, increasing light scattering and decreasing translucency.  Porosity Pores are the main cause of light scattering, especially when they are of a size similar to that of the wavelength of visible light (400 to 700 nm). They play a significant role in the optical properties and in particular in the translucency of zirconia ceramics. That happens because there is a difference between the refractive indexes of air (n = 1) and zirconia (n = 2.1– 2.2). Pores can be either intra-granular or inter-granular. Inter-granular are the pores among grains with a different orientation, while intra-granular pores are located inside a grain. For transparent polycrystalline materials, an extremely low porosity (˂0.01 vol.%) is required. If the pores are incorporated by growing grains during densification, they become intra-granular and impossible to remove. The inter-granular pores are more favorable to be eliminated during sintering.  Porosity In the last stage of sintering, the pores are closed, and spherical and grain growth is evident. The pores are gradually decreased by diffusion to the grain boundary region. However, this low porosity can be achieved under sintering conditions involving high temperatures and long holding times. Pores larger than 50 nm can cause significant scattering negatively affecting light transmittance. In order to minimize pore size, the starting zirconia powder needs to be in nanometric scale. Using 40-nm instead of 90- nm powder reduces pores, improves the sintered density and reduces scattering 19. However, porosity alone cannot be considered as the main scattering generator, as other defects such as impurities or oxygen vacancies may be present in polycrystalline zirconia materials, acting as light-absorbing or scattering centers.  Thickness Zirconia translucency values vary inversely with its thickness. As the thickness of the restoration decreases, the translucency increases.  Cement layer The cement shade and translucency have an important effect on the final colour of the ceramic restoration. The thickness of ceramic should be at least 2 mm in order to mask the effect of the underlying discoloured tooth on the final colour of the restoration. In many clinical cases, achieving a 2-mm reduction is not possible without affecting the pulp and compromising the strength of the remaining tooth structure. In such cases, using cement with an appropriate thickness and colour might be the only available solution to mask the colour of the substructure and its effect on the final colour of the restoration.  Cement layer The white and yellow cements can change the final colour of ceramic so that it would be clinically acceptable. The white cement not only have a greater effect on the final colour of ceramic but also decrease the effect of the discoloured substructure on the final colour of the restoration. In contrary, the translucent cement will cause no clinical change in the final restoration colour compared to the colour before cementation, that’s why it is preferred in cases of non-discoloured substructure that don’t need masking.  a.Measurement of color Measurement of Optical b.Measurement of the translucency Properties of Dental c.Measurement of opalescence ceramics d.Measurement of Refractive Index e.Shade matching and measuring tools of colour a.Measurement of color The two primary systems used in measuring colour are 1. The Munsell system 2. The CIE system (Commission Internationale de l’Eclairage).  Colour Difference The colour difference (ΔE) between two colours is the distance between the colour points of the two colours. If two points in the L*a*b* colour space are coincident, the colour difference between them will be zero. A small ΔE* ab value indicates that the colours are close to one another. As the distance in colour space between the two points increases, the colour difference between them increases.  Colour perceptibility and Acceptability Thresholds  b.Measurement of the translucency Translucency can be determined from the transmitted light through ceramic material, where the light source and detector are on opposite sides of the sample. Measurement of transmitted light is done by spectrophotometer that records (L*source) with no sample in place. The sample is placed and (L*sample) is recorded. The percentage of total transmission is calculated using the following equation: T%(L*sample/L*source) x 100  b.Measurement of the translucency In addition, two important measurements of translucency of ceramic materials are 1. Contrast Ratio CR is a measure of translucency that is defined as the ratio of reflectance (Y) of a given material measured with a black backing (Yb) to the measure of reflectance of the same material with a white backing (Yw), using any instrument capable of quantitatively measuring visible light intensity. CR values range from 0.0 (transparent material) to 1.0 (opaque material). Luminous reflectance (Y) of the specimens recorded on black (Yb) and white (Yw) backgrounds are used to calculate the contrast ratio (CR) as follows:  b.Measurement of the translucency 2. Translucency Parameter (TP) TP is defined as the colour difference (ΔE) between a sample of uniform thickness measured with a white and black backing. A dental spectrophotometer is used to record the CIE coordinates (L*, a* and b*) of the same ceramic sample placed on a black (B) and a white (W) background to determine the translucency parameter (TP) by calculating the colour difference according to the following equation: The major factors that affect TP are specimen thickness and the reflectance parameters of the black and white back grounds  b.Measurement of the translucency 2. Translucency Parameter (TP) If the TP value is zero, the material is completely opaque. The greater the TP value, the higher the translucency of the ceramic. TP values of human enamel and dentin at 1mm are (18.1) and (16.4). TP values of glass ceramics at 1mm thickness range from (14.9-19.6). TP values of zirconia at 1mm thickness range from (5.5- 13.5). c.Measurement of opalescence Opalescence improves ceramic restoration appearance to look more natural. The values CIE coordinates (a* and b*) of the ceramic samples placed on a black (B) and a white (W) background were used to calculate the opalescence parameter (OP) according to the following equation: OP values of dental structures are between (19.8 and 22.6). OP values of monolithic ceramic restorations are between (2.5–13.3). d.Measurement of Refractive Index Refractive Index Measurement or Refractometry is the method of measuring substances refractive index and assess their composition or purity. Refractometry is a technique that measures how light is refracted when it passes through a given substance and the amount of the refracted light. e.Shade matching and measuring tools of colour 1. Manual (visual) shade matching Most commonly, the colour matching of teeth to ceramic materials is done manually and visually with dental shade guides as it is much cheaper to obtain. However, the colour selection done in this method is subjective and influenced by the ability of the human observer to choose the best matching shade. Many factors affect the accuracy of shade matching such as; observer colour perception, ambient lighting, eye fatigue, age, stress, prior exposure of eyes to light, the effect of the surrounding environment, metamerism and the acceptance threshold of mismatch. To avoid these factors, colorimetric instrumentation can be used to choose standardizing close colour match. e.Shade matching and measuring tools of colour 2. Automatic “Instrumental” shade matching These instruments were introduced to overcome the limitations of the visual tools. They can be classified as 1. spectrophotometers 2. colorimeters 3. digital imaging devices.  1.Spectrophotometer Spectrophotometers measure and record the amount of light energy reflected or transmitted from an object’s surface along the visual light spectrum (380-780 nm). This device has high precision, sensitivity to measure absolute colours and is equipped with spectral distributions of various illuminates.  1.Spectrophotometer Spectrophotometer can be used to: 1. Measure the colour of ceramic materials: - Measure the spectral reflectance of a colour and converts it into a tri-stimulus value (CIE L*a*b*). 2. Measure the translucency of ceramic materials: - Measure percentage of transmittance (T%). - Measure the illuminance (Y) over standard white and black background. 3.Measure the absolute transmittance and absolute reflectance. 4.Convert readings taken into other calculations of optical properties, such as refractive index. 2.Colorimeters Measure quantitatively visible light intensity and difference in colour intensity, which is related to colour absorption. However, they are less accurate than spectrophotometers as colorimeters do not register total visual light spectral reflectance. 3.Digital cameras Are based on the RGB colour model in which the camera obtains red, green and blue data that is used to produce the colour image and generate a broad arrangement of colours. Edge loss effects One issue associated with spectrophotometers and colorimeters is phenomena of edge loss effects. These instruments measure colour with a small window (measuring area) for illumination and measurement, resulting in loss of light at the edges of the illuminated area during reflectance measurements of translucent materials. The edge loss effect occurs in translucent object when light is transmitted and scattered through the edge of an object and not reflected back to the instrument to be measured creating a blurring effect at the edges. This can affect the accuracy of measurements by these instruments.