Let’s face it, one of the most difficult part in making any restoration beautiful and truly exceptional is when we get the right shade that looks exactly the same as the natural teeth of our patient. Shade selection is truly challenging especially when it is a single tooth restoration in the anterior teeth. This defines who’s dentist has it and who don’t. So, in understanding what truly affects our shade selection we will be able to have an edge in the probability that we will choose the right shade every time it is necessary for us to be really accurate.
There are 2 Main Factors that Affect Our Shade Selection:
I. Our Eyes
The observer’s eyes defines what we interpret as the color of the teeth. Visual shade assessment is based on human vision physiology, whereby reflected light from a tooth enters the retina to activate the cones, from where electrical impulses are passed on to the optical center in the brain, where an interpretation of color is made. This process is subjective because different individuals can have different interpretations for the same stimulus at different times, and also because the ability to perceive a color and then to distinguish it from other colors (color discrimination) varies greatly from one person to the other, within the same person as well as from patients to dentists. This is called Color Perception. The initial process occurs in the retina of the eye. The retina contains millions of cells called Photoreceptors that are sensitive to light. These photoreceptors process light into nerve impulses and pass them along to the cortex of the brain via the optic nerve. The Color Perception is affected by two parts of our eyes that acts as photoreceptors , the rods and the cones.
1. Rods- this functions on Scoptic Vision wherein it works under low light conditions. This interpret brightness but not the color of the objects. Rods increase in concentration and predominate toward the periphery of the retina. There are 120 million RODS in the outer edges of the retina help eyes adjust when one enters a dark room. They are good for detecting motion and for seeing in low light-levels. At low light levels, the rods of the human eye are more dominant than the cones and color
perception is lost.
2. Cones- this function on color vision wherein it becomes active under higher lighting conditions or commonly called Photopic Vision. When brightness becomes more intense, color appears to change and this is called the Bezold-Brucke Effect. There are 6 million CONES in each eyeball which are sensitive to color and concentratedly located at central field of vision. Therefore, there is more color perception on this area of the eye. There are three types of cone cells, each sensitive to the long, medium or short wavelength of light that perceives color and they are sensitive to the following lights of colors, respectively:
Color is also influenced by its surrounding particularly complementary ones. When blue and yellow are placed side by side, their chroma may appear to be increased. The color of teeth can also look different depending on the color of its surroundings such as adjacent teeth, gingiva, lips, face, skin, clothing of the patient and operatory walls.
In addition, anterior teeth usually have slightly different colors according to their position within the arch
(a) Maxillary Central Incisors are the lightest teeth whereas the Canines are relatively redder, yellower and more saturated with color.
(b) Maxillary Anterior Teeth are slightly yellower as compared to Mandibular Anterior teeth.
These fine differences in color of adjacent teeth can make selecting and finalizing tooth shade a testing procedure even for the experienced professionals.
Teeth are also known to possess a color gradation from the cervical area to the incisal area. Cervical color is modified by scattered light from the gingiva whereas the incisal color is most often translucent and affected by its background.
Fabricating an anterior restoration from a single shade is in itself going to result in shade mismatch because tooth color tends to change from cervical area to the incisal edge.
Gums and Lips form the soft tissues immediately adjacent to the teeth and can influence the apparent color through the phenomenon of contrast. As has been reported, the subjects’ perception of tooth whiteness, health and attractiveness is greatly influenced by the color of the adjacent lips and gums.
Besides the color of the walls of the clinic, contrast during shade selection is also affected by the skin color and clothing worn by patient. Skin color can serve as a useful guide for the selection of tooth shade in removable and complete denture prostheses as well as in full-mouth rehabilitations to achieve a pleasing natural appearance, especially in the elderly.
The perception of color is dependent upon human vision capabilities which can alter the shade captured by the eyes or its image made in the brain. Our vision is affected by the following factors:
(a) Age- color of images are perceived as more yellowish and brownish as eyes age.
(b) Gender- it is commonly physiologically believed that women see color more accurately than men. Color blindness is the inability to perceive differences between some or all colors that other people can distinguish and studies show that up to 8% males and less than 1% females are known to be affected by color blindness. However, it is important to take note that there are also many studies that show no gender difference.
(c) Experience- the issue of whether the accuracy of shade matching is innate or an acquired (learned) skill is unresolved. Many studies, interestingly, contradict the notion that experience is influential in shade matching. However, do take note that artistic qualities of an individual can modify the way a shade is observed. The need is not only to have proper training and setup for the routinely used visual method but also to properly understand the various factors involved and their influence on shade selections. Only in this way one can approach this process not as a mere guess work but as a scientifically and technically selected tooth shade. This can ensure reliable, repeatable and accurate shade matches on a day-to-day basis.
(d) Eye Color- it is believed that brown eyes, with more melanin, absorb wavelengths better, possibly influencing the sight EXCLUSIVE of color perception.
(e) Eye Conditions- Having Astigmatism, Hypermetropia and Myopia conditions may affect the way we perceived color. However correcting these refractive errors with eyeglasses or contact lenses make their ability to influence shade-matching becomes insignificant.
II. Light Source and Conditions
The condition and the source of the light greatly affects how are eyes perceived the actual shade of your patient’s teeth. Dental Practitioners are required to understand the following:
(b) Light and Related Characteristics of Porcelain and Resins.
(c) The ability to communicate instruction with their Laboratory Technicians.
The Quality of Light is very critical during shade selection. Energy distribution of a light has definite effects on the type of color being perceived. We should try and use a source of light that contains full spectrum of rays without the dominance of any wavelength; because when an object is viewed under lights dominating in particular wavelengths (color bands), that specific color becomes dominant to the observer.
Using the proper light source during shade selection is very critical. There are three types of light sources:
1. Incandescent Light- This emits high concentration of yellow waves. It is not suitable for shade matching. It has low Color Rendering Index (CRI).
3. Natural Daylight- Northern daylight is considered the best because it is closest to emitting the full spectrum of white light. It is used as the standard by which to judge other light sources. It has CRI close to 100.
Unfortunately, most dental offices are fitted with incandescent and fluorescent lights. Thus, we need most of the time to find another source of light during our shade selection. In order to understand what light is needed, we need to know where the ideal light quality and source are based upon. The quality and type of light source for dental use are based on 2 things:
(a) Color Rendering Index or CRI- It is Northern Daylight, which can be close to full-spectrum white light and often, is used as the “normal” standard for judging light from other sources. It has a color rendering index (CRI) close to 100. The color rendering index, on a scale of 1 to 100, indicates how well a particular light source renders color as compared to a specific standard source.
The nature and quality of light is dependent upon the light source being used, whether it is sunlight or artificial light. We have 2 source of lights:
i. Sunlight - this remains a major source of energy on earth. However, its distribution and intensity depends on different factors such as time of the day, relative humidity, environmental pollution, weather conditions and season of the year. For instance, light during morning or evening hours is rich in yellow and orange but lacking in blue and green, with its distribution changing under cloud cover. It also changes the light reflected from the object, in which case the actual color perceived by the eye is different. Therefore, Sunlight has been standardized for shade selection purposes in the form of “Northern Daylight”. This is the sunlight around noon hour on a bright day with slight overcast. It has a CRI of close to 100 and is also used as a normal standard for judging light from other sources.
ii. Artificial Light - this is almost universally used in dental surgeries. It can be Incandescent (which emits higher concentration of yellow light) or Fluorescent (which emits higher concentrations of blue light waves). Neither of these is pure white light. The only artificial light suitable for reproducing the standardized northern daylight is a D65 light source. Unfortunately, the D65 lamps are very expensive and not readily manufactured on a commercial level. This leaves room for using fluorescent lighting in dental surgeries and laboratories. The most commonly used is the Cool White Fluorescent Tube Lights which they have a CRI of between 50 and 80, however, they are NOT recommended for shade selection. The need is for a “Color-Corrected” fluorescent lighting with a CRI of 90 or above, in line with the recommendations of the American Dental Association. Fortunately, this standard can be acceptably reached by using the newer cool white daylight energy saver lamps, which are cheaper and easily available. There are also LED lights that are available and used in dental chair lights and gadgets that can achieve the “Color-Corrected” quality of light.
(b) Color Temperature- This is related to the color of a standard black body when heated. Color temperature is reported in degrees Kelvin (K), or absolute (0° K = -273°C). Northern daylight has an average color temperature of around 6500°K, but this varies with the time of day, cloud cover, humidity, and pollution. Although daylight is often used as the standard against which other light sources are compared, never use direct sunlight to take tooth shade. The distribution of light waves from the sun depends on the time of day and on humidity and pollution. Morning and evening incident light has shortened blue and green waves scattered and only the longer waves penetrate the atmosphere. Therefore, daylight at dawn and dusk is rich in yellow and orange but is lacking in blues and greens.
Color and Light plays into our color perception, it is a condition called Metamerism. Two objects may appear to be identical colors under a certain kind of light, yet under another kind of light they may appear totally different. The problem with metamerism can be avoided by selecting a shade and confirming it under different lighting conditions.
This may all sound so technical and boring for you and may deemed to be unnecessary to learn and understand. However, in understanding how we perceived colors and know the ideal environment and conditions to take shade selection, we will be able to gauge the accuracy of our shade selection and correct our ways during the procedure. It will also allows us to have an in-depth understanding as to why there are strict instructions that we need to follow when we take and select the shade of the tooth during our procedures, so, that we will be able to do it properly and achieve the most accurate shade selection to our case. This is the service we need to provide to our patients, so, that we can honestly say what I believe should be the right meaning of “Service” in our profession. It is not making our charges cheaper but doing our job the proper way with correct knowledge as to why we need to do it.