Understanding Color Perception

by | Dec 26, 2019 | Dental Treatments | 0 comments

Shade Selection

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:
(a) Red
(b) Blue
(c) Green

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
For Example:  
(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:
(a) Color
(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).

Incandescent Light
2. Fluorescent  Light- This emits  high  concentration of  bluewaves. It is not suitable for shade matching. It has CRI of 50-80.

Fluorescent Light
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.

Natural Daylight

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.

Color Rendering Index or CRI

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 Temperatures

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.


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