Color Perception

Color perception in humans is a complex process that involves the brain interpreting and processing the relayed electrical signals.^16,17 These electrical signals originate in the retina and are transmitted via the optic nerve to the brain's visual center.^16,17 The perception of color begins with the way light interacts with objects and how the eye processes these interactions to generate visual signals.

Human Eye

When light strikes an object, the object absorbs some of the wavelengths of light and reflects the rest of it.¹⁸ The reflected light from an object enters the eye through the cornea.¹⁹ The cornea, along with the lens, directs the light onto the retina.^19,20 The retina contains photoreceptor cells known as rods and cones.^19,21 These cells are sensitive to specific ranges of the light spectrum and are involved in transmitting electrical signals to the brain.^21,22 The center of the retina contains the cone cells whereas its periphery contains the rod cells.^21,22 Rod cells interpret the lightness and darkness (the value scale) of an object.²¹ They help with the vision in low light and at night time.²¹ Cone cells, on the other hand, detect different colors; however, they are only activated in bright light.²¹ Cone cells are classified into three types (red, green, and blue) based on their sensitivity to various wavelengths of light.²¹ This trichromatic system functions collectively to facilitate the perception of a full spectrum of colors through the process of additive color mixing.²³

The eyes are susceptible to color fatigue, wherein constant exposure to one color decreases the eye’s response to that color. However, visual color perception is not only influenced by observer-associated factors (eye fatigue, experience, vision disorders, and age)^7-9 but also environmental factors such as surroundings, lighting conditions, and the metamerism phenomenon.²⁴ While the eye plays a crucial role in detecting light and distinguishing colors, the concept of color itself is fundamentally linked to the interaction of light with objects and the way it is perceived by the human visual system.

Color

Color may be defined as a phenomenon of light or visual perception that enables one to differentiate between objects that are otherwise identical.²⁵ It may also be defined as the quality of an object or substance with respect to light reflected or transmitted by it.²⁶ Color is strongly associated with light. Without light, color does not exist! ^27,28 Objects have no inherent color—they only reflect specific wavelengths of light from the color spectrum.²⁷ For example, A red object reflects red wavelengths of light while absorbing all others. The perceived color of an object may appear different with variations in lighting conditions and the surrounding colors.^24,28 For example, the teeth look brighter when the patient is wearing brightly colored lipstick compared to when they are wearing no make-up. In dentistry, an understanding of color is essential, particularly when assessing variations in tooth shade and the factors that contribute to discoloration.

Tooth Discolorations (Intrinsic and Extrinsic Coloration)

A tooth may appear darker or stained due to the combination of intrinsic and extrinsic colorations.^7,29 Intrinsic coloration (Fig. 1) is associated with the absorption characteristics of enamel, dentin, and pulp, as well as light scattering.^7,29 Extrinsic coloration (Fig. 2) arises from the absorption of various materials such as red wine, tea, iron salts, and chlorhexidine onto the pellicle and the surface of enamel.^7,29

Fig. 1 - Intrinsically discolored maxillary central incisors and canines

Fig. 1 - Intrinsically discolored maxillary central incisors and canines

Fig. 2 - Mandibular teeth affected by extrinsic discoloration

Fig. 2 - Mandibular teeth affected by extrinsic discoloration