The Science Behind Color

By Barry Santini, ABOM

Release Date: December 1, 2017

Expiration Date: October 23, 2019

Learning Objectives:

Upon completion of this program, the participant should be able to:

  1. Understand the sensitivity of cones and rods to light's visible wavelengths.
  2. Learn why color is more important than you might have imagined and the opportunity it provides.
  3. Understand the ways that color enhancement can be created using filter lenses.

Faculty/Editorial Board:

Barry Santini Barry Santini, ABOM, graduated from New York Technical College in 1975 with an AAS in Ophthalmic Dispensing. He is a New York State licensed optician with contact lens certification, is ABO certified and was awarded an ABO Master in 1994. As sales manager for Tele Vue Optics from 1987 to 2003, Santini developed his knowledge of precision optics and has been an owner of Long Island Opticians in Seaford, N.Y. from 1996 to present. In addition, Santini is an amatuer astronomer and lecturer and plays bass trombone in the Brooklyn Symphony.

Credit Statement:

This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). General Knowledge Course SWJH006

A century ago, color blindness or more correctly, color vision deficiency was a condition little known or appreciated because the underlying science behind it was still in its infancy. Fast forward to today, where the enhancement, corrective and therapeutic possibilities of tints, filters and coatings are being intensively investigated by a growing number of researchers, clinicians and commercial companies. Yet many eyecare professionals are unconvinced that various optical, neurological, behavioral and sensory deficiencies can be alleviated or corrected by the scientific application of color filters.

Even the newest, most sophisticated products that promise to enhance, intensify or restore a wearer’s perception of color are viewed with skepticism by many ECPs. Yet there is strong evidence supporting the science behind color enhancement. From helping to improve color deficiencies, to providing relief for sufferers of dyslexia, to remedial social interaction help for autistic children, or simply increasing our enjoyment of color, there are many untapped avenues to improve and enhance people’s lives if we open our minds to the possibilities intrinsic in the technology of color enhancement. This course weaves a story of color into an opportunity for the optician and the doctor to deliver filter lenses that can enhance sports, tasks at work or driving, for indoors and outdoor pursuits or work to help those that are color vision deficient to provide a more colorful world.


Given that early humans dressed themselves in the muted tones of animal skins and forest materials for tens of thousands of years, the discovery of natural pigments such as okra and lapis lazuli must have been both quite surprising and exciting. It wasn’t until the Industrial Revolution where the need for ascertaining whether any single individual saw color “like we do” became a priority. With the growth of railroads and textile mills, the need for uniform recognition of color in track signals and between different types of threads began to rise in importance.

The arrival of automobiles and airplanes, which led to the development of traffic lights, the increased use of colored wires in the electrification of cities, telephone and computer networks, and even the rise of leisure pursuits like pleasure boating made it necessary to test individuals for normal color vision. Certainly, in almost any medical discipline today, it’s not hard to imagine how the diagnostic and treatment skills of a health care professional could be severely compromised by the inability to correctly recognize the overall pallor or change in a patient’s skin color. Even the simple act of finding a vein to draw blood can become a traumatic event for the patient if a vein is mostly invisible and for the medical technician, especially if the technician is less experienced or color deficient.


Everyone has his or her favorite color. But color is more than a simple preference. From the hue of the car we drive, to the tone of our cell phone case, to the close attachment we form to a particular favorite, people see color as a personal expression of who they are, how they see the world and most importantly, how they want the world to see them. Color is innately personal, peculiarly cultural and manifestly associated with our moods.

Clearly, the tone of our skin and the color of our eyes are very important personal identifiers and descriptors of who we are. Change your hair color, and you change how the world sees you, because color has become an important and defining aspect of any society. In view of this, it is interesting to note that big business has only recently begun to exploit our personal relationship to color. Less than 20 years ago, Apple introduced their revolutionary new computer, the iMac, in a color called Bondi Blue, which forever freed electronic devices from the prison of muted gray, black and taupe tones. Within a year, Apple increased the iMac’s color selection by five more hues. Soon a buyer could make a personal selection from a range of up to 11 colors. Today, Apple leverages people’s love of color, stroking the Apple Watch buyer’s anticipation with the lure of choosing your favorite from amongst a dozen different colors.


The choice of one’s political, recreational or social affiliation almost always comes with a specific color association that often has cultural or geographic origins. For example, countries have strong national color associations, with Russia represented by red, Israel by blue and white, and the U.S. by red, white and blue. While humans as a species have evolved a clear sensory reaction to the color red, there’s no escaping the fact that there are quite specific psychological and emotional responses to the same color within different cultures. In the West, we use black as the color of mourning, while Ethiopians use white and Iranians use blue for the same context. Yet despite culturally-bred differences, some color associations transcend culture. Those associations occur in the realm of human mood.


Certain colors are commonly associated with certain human moods. Yellow is the color of happy, as in sunshine, while blue often means sadness. Mark Changizi, PhD, a neurocognitive scientist, has formulated a theory that human color vision primarily evolved to help us sense mood and health by becoming exquisitely tuned to recognizing the amount of blood concentration and blood oxygenation under our skin. As skin is essentially a translucent medium, humans learned to recognize how greater blood concentration gave faces a red flush, possibly indicating anger, while less oxygenated blood would cast a blue-purple tone, indicating poor circulation or poor health. It’s no coincidence that depressed or sad people are said to be feeling “blue.” But color can also have multiple personalities. Green can mean readiness or envy. Red can indicate love, anger or deceitfulness. Blue can mean serenity, sadness or strength. Considering color’s clear importance in all types of personal and societal interaction, being able to avoid ambiguity surrounding the reading of moods makes a strong case for the importance of screening children for deficiencies in color vision. Certainly life today is complicated enough without starting out handicapped in sensing people’s moods because of poor color discrimination.


The human eye has two types of receptors, the rods and cones, the responses from which are used to determine color in the visible light spectrum from 400 nm to 700 nm.

Rods—The eye contains approximately 100 million rod cells, the majority of which are located in the peripheral retina, although some are also found within the outer areas of the macula. Rod cells are about 100 times more light responsive than the cones, capable of firing off a signal when stimulated by a single photon of light. In addition, the response of multiple rod cells is collected and aggregated into a single interneuron, further amplifying their low light detection ability. Rods have an asymmetrical response curve, ranging from 400 nm to 600 nm, featuring a peak response sensitivity centered around 498 nm. While the rods are very light sensitive, their reaction time is slower than the cones, taking about 100 milliseconds to fire off a signal in response to a stimulus change.

Cones—The eye’s 6 to 7 million cones are mainly populated in an area called the macula, approximately 6 mm wide, corresponding to a visual field angle of approximately 15 to 18 degrees (30 to 36 full moons wide). But the majority of the cones are actually found in a smaller, exclusively rod-free area called the fovea or fovea centralis. This area (Fig. 1) measures 1 mm to 1.5 mm wide and corresponds to a visual field angle of approximately 3 degrees (6 full moons wide). Within the fovea, there is a region 0.35 mm wide, even more densely packed with cones called the foveola, subtending a 0.5 degree visual field angle (1 full moon wide). Here, where the sharpest human vision resides, the density of the retinal mosaic is said to theoretically allow a potential visual acuity of 20/08. Even though the cones allow us to see color and provide our sharpest vision, they are less sensitive to light than the rods, requiring between 10 times and 100 times more photons of light before they will fire. Yet their response time is four to eight times faster than the rods.


fig1If nature’s goal were to design the ideal eye for determining color, she would have placed a color receptor, with sensitivity limited to 1 nm at each individual wavelength of visible light from 400 nm to 700 nm. Unfortunately, the complexity of wiring 300 individual color receptors is not compliant with the economical dictates of evolution. Instead, nature gave us just three receptors, each of their frequency response is both broad and overlapped enough to span the entire spectrum of visible light. These are called S, M and L receptors, colloquially corresponding to the short, middle and long wavelengths, respectively, of blue, green and red. Depending upon its unique photopigment or opsin, a receptor will fire in response to varying intensity thresholds across its covered range of wavelengths. Within each color receptor’s frequency range, the aggregated threshold intensities that prompt it to fire determine its tristimulus value.

Color signals are then fired off along the optic nerve, where they encounter a sharing substation called lateral geniculate nucleus (LGN). In the LGN, vision signals are coordinated between other sensory processes, including smell and hearing. Beyond the LGN, selected nerve fibers then crisscross within the optic chasm before finally terminating in the occipital lobe of the brain. The actual color we perceive is the net result of complex visual processing calculations comparing the responses between the different receptors.

The sum of these calculations averaged across the visible light spectrum is a wavelength versus intensity contour plot that identifies the eye’s overall tristimulus curve. The inclusive range of colors that can actually be discriminated is called the gamut and is defined by the response of these three receptors. A mathematically-based model of this gamut that correlates the perceptual relationship for a color’s hue, saturation and brightness is referred to as the eye’s color space. Interestingly, despite only having three color receptors, it is theorized that our eye can discern over 7 million different colors.


For us to see colors as accurately as possible, three basic processes must work normally: first, the receptor’s tristimulus response; second, the timing of the receptor signals through the optic nerve and in the LGN; and third, the processing of nerve signals both in the LGN and the occipital lobe of the brain.

When an anomaly in color perception is presented, it is broadly termed a color deficiency. If the deficiency is severe enough, a diagnosis of color blindness may be applied. With threecolor receptors, there are a number of possibilities from which color vision deficiencies can originate.

The most common deficiency is an alteration of the light frequencies covered by an individual photopigment’s response. For example, in Fig. 2, note how closely the L and M, or red and green receptors overlap. This overlap can be impacted by a shift in the frequency range to which a receptor responds and a shift in the threshold intensities of a receptor’s wavelength response, which may include a loss of most or all of a receptor’s functional response. For example, a shift of the M cone sensitivity (Fig. 3) results in a red-green color deficient individual described as “Deutan.” The nature and degree to which this response is impacted has a well-defined vocabulary. These departures from normal response create miscalculations in visual processing in the LGN and the brain, which alters our color perception. Reduced or incorrect color discrimination, aka chromatic contrast, is the result, and an overall diagnosis of color deficiency is applied.


fig2 and 3As early as the 1850s, James Maxwell, in an attempt to correct for color deficiency formulated the first glasses designed to help the color impaired. It was not successful. There have been many attempts since.

I n order to properly address a color deficiency, one must first analyze precisely what is affecting color perception within the visual chain. To do this, tests have been created that identify and quantify where the problems may lie. The most commonly used test for color vision today is the Ishihara color plate test, which is designed to primarily identify the response of L and M cones. Problems here manifest themselves as a red-green color deficiency, the most common form of color vision anomaly. This is the same one that affected my Dad’s vision. Other color vision tests include the D-15, Farnsworth Lantern test and Farnsworth Munsell 100 Hue test. This last test uses the threshold test of “just noticeable differences” to help screen people in jobs requiring critical color discrimination skills, such as film colorists and computer or visual arts specialists.

Once the diagnosis of the deficiency is known, the process begins to find and apply the proper spectral control filter to correct or reduce the deficiency. But discovering the best and most efficacious filter can require extensive and sophisticated tests, including those designed to create a custom map of an individual’s color space. This process is somewhat analogous to hearing tests used in tuning the response of a hearing aid. Although improvements can be found serendipitously through trial-and-error use of broad spectrum tints and overlays, if there are signs that point to a larger disorder, such as autism spectrum disorder, or ASD, only the skills and experience of a specialist will do.


When the response of one or more color receptors departs from normal, the calculations carried out in the LGN and the brain become corrupted, resulting in the incorrect recognition or discrimination of colors. The solution, in the broadest terms, is simple: Apply a filter designed to help bring the tristimulus responses closer to normal. In common red-green color deficiency, for example, where the overlap of the L and M cones response is often reduced, a properly designed notch or band filter, having a suppressive action in the overlapped area, can help better separate the signals passed to the LGN and brain. Any alteration of the tristimulus responses that helps it to more closely mimic normal will result in improved color identification and discrimination. But because color vision deficiencies can differ in origin, interaction and degree, optimal correction may require the expertise of a trained eyecare professional.


This tweaking of the tristimulus curve is found in the various sunglass technologies that promise a color-enhancing experience. It should be obvious now that even for a normal eye, a properly designed and tailored spectral control filter can improve chromatic contrast and enhance the experience of color. Further, enhancement can even be tailored for specific activities in differing spectral environments, such as boating, golf and tennis, or simply to provide an overall enhancement of color, regardless of the intended activity.

Our eyes generally have difficulty with blue light, both for focusing and from its increased tendency to scatter. Color enhancement often begins in this region through some attenuation of the blue-violet region ranging from 400 nm to 500 nm, regardless of actual base lens color. These wavelengths of high energy visible light, or HEVL, are also known to penetrate the macula pigment, and therefore considered prime suspects in the damage seen in age-related macula degeneration or AMD. But light around 460 to 465 nm influences our daily circadian rhythms and therefore should be allowed to reach the eye because its absence is associated with seasonal affected disorder.

Most color enhancement technologies enhance color perception by influencing the tristimulus values of the L and M receptors by altering the yellow region between 570 nm and 580 nm, where their responses overlap. Note that this approach is similar to that used in addressing red-green color deficiency. As the eye is most sensitive to light in this yellow wavelength region, an accompanying improvement in overall light sensitivity is also generally noted. Further, there is another overlap area between the M and S receptors (green and blue), which also responds positively to color enhancement when notch filters are similarly applied in this area. At the far end of the visible red spectrum is the infrared, or heat radiation, and further attenuation here helps increase comfort by reducing tear evaporation.

Some color-enhancement technologies have their origins in research targeting occupational benefits. The company O2Amp co-founded by Dr. Mark Changizi pursued technology that amplified the signal our red-green receptors evolved to detect in the first place, namely oxygenation variations in the blood under the skin. His color enhancing lenses derived from special filters designed to help health care professionals better identify changes in skin color, thereby improving both normal and palliative care. Another company, Enchroma, has developed color corrective and re-timing technologies that originated from designing lenses to protect surgeons using lasers.


Broad spectrum tints, defined as a transmission contour, free of the excessive peaks and troughs characteristic of specialized notch and band filters, remain the tints most broadly used to provide a targeted enhancement of color for different lifestyle activities. For example, in activities where contrast enhancement is desired, such as hunting, golf and tennis, tints like brown, orange/vermillion and rose are used to selectively block the blue end of the spectrum, reducing haze and scatter.


Making a change in the color of sunglasses comes with a little appreciated secret benefit: You brain begins to pay more attention to everything around you. Because all of our senses have evolved to be acutely sensitive to any change in habitual stimulus, changing your sunglass color changes the tristimulus response your brain is used to, and through the LGN puts vision along with other senses on higher alert, ready to pay more attention to everything going on. This “changing up” is part of the reason why different lens companies produce lenses of different lens colors, but all of which promise improved performance for the same targeted activity.


Understanding the way that humans “see” color helps to understand and create an opportunity for color-enhancing sunglasses. And because competition requires that all ECPs differentiate themselves and their practices, adding color to a practice is just good business.

In Part 2, we’ll discuss new color-enhancing products and provide an overview of how they can be used effectively.