At first glance, white may seem like a simple, singular color. However, white light is actually composed of all the colors of the visible spectrum. So in technical terms, there are infinite colors that make up white light. The human eye sees white when it perceives all wavelengths of visible light equally. This article will explore the science behind how white light is created and how many colors ultimately comprise white.
The Physics of White Light
In 1666, Isaac Newton demonstrated that white light is made up of all the colors of the rainbow by passing a beam of sunlight through a prism. This separated the light into the visible spectrum with wavelengths ranging from violet to red. When these component colors are combined, the result is white light.
This occurs because white light is composed of a continuous spectrum of wavelengths across the visible range from about 400-700 nanometers. Each wavelength corresponds to a particular color. Shorter wavelengths are violet and blue hues while longer wavelengths appear more orange and red. Our eyes detect these wavelengths as distinct colors.
| Wavelength (nm) | Color |
|---|---|
| 400-450 | Violet |
| 450-495 | Blue |
| 495-570 | Green |
| 570-590 | Yellow |
| 590-620 | Orange |
| 620-700 | Red |
When all these wavelengths strike our eye simultaneously with roughly equal intensity, our visual system perceives the combination as white.
The Color Theory Behind White Light
According to the trichromatic theory of color vision, our eyes contain three types of color receptors called cone cells. There are cones sensitive to red, green and blue light. By combining input from all three types of cones, the eye can perceive the entire spectrum of visible colors.
White is seen when the red, green and blue cones are stimulated about equally. This equates to perceiving roughly equal parts of all visible wavelengths of light.
The reason we see white when all cone types are activated equally is because of color mixture. Red, green and blue are the primary additive colors. Mixing equal amounts of primaries yields white light. This is why combining red, green and blue light creates white illumination.
Computer monitors and televisions take advantage of this fact and produce color images by combining variable intensities of red, green and blue light. When the intensities of all three primaries are set to their maximum value, the screen displays white.
The Number of Discernible Colors in White Light
Physically speaking, white light is made up of infinite wavelengths across the visible spectrum. However, the human visual system has limits to color perception determined by the eye’s biology.
Research indicates the average person can perceive about one million different colors. This is far fewer than the essentially limitless spectral combinations contained in white light.
Color discrimination is limited by the density of cone cells in the retina and the overlap in sensitivity between different cone types. We can’t distinguish extremely subtle gradations between similar wavelengths.
But even though we can’t see all the nuances, white light still comprises the full visible spectrum. The multitude of wavelengths blend together to create the uniform white appearance we perceive.
Primary Colors that Can Create White
There are sets of primary colors besides red, green and blue that can mix together to produce white light. In color theory, any three primary hues that are not too closely related and that combine to make white can form an additive primaries system.
Some examples of primary triads that yield white include:
- Red, green, blue
- Cyan, magenta, yellow
- Red, blue, green
- Red, yellow, blue
Printers and painters rely on the cyan, magenta and yellow primaries to create color images. Mixing pigments, paints or inks of these three colors together deposits equal amounts of the primaries onto the paper or canvas and absorbs all wavelengths of light to produce white.
Computer and TV screens use red, green and blue to generate color because they rely on mixing colored light rather than pigments. But any set of primaries that are balanced and span enough of the spectrum can produce white through additive mixture.
The White Light Spectrum
As we have seen, white light encompasses the complete range of visible wavelengths. The visible spectrum emitted by the sun or an incandescent bulb is continuous with no gaps between colors. This uninterrupted spectrum allows all colors to reach our eyes and be mixed into white.
However, not all sources of white light emit a full spectrum. Some technologies like fluorescent or LED lamps have an emission spectrum consisting of discrete spikes at certain wavelengths rather than a smooth continuum.
But the spikes are designed to stimulate our red, green and blue color receptors in equal proportion. Although discontinuous, these multi-peaked spectra still appear white to us because they produce the same combined cone response as a continuous spectrum.
Perceiving White Light
We perceive white when our visual system receives suitable stimulation of the three cone types. But individuals can sometimes experience white slightly differently.
Color perception is subjective and influenced by factors like age, gender, ambient conditions, and variations in eye anatomy. The optical density and peak sensitivities of photoreceptors also shifts over time.
For example, yellowing of the lens and changes in cone pigments make older adults see whites as slightly yellowish. Differences in cone ratios can also cause people to perceive white at somewhat different color temperatures.
Despite these effects, the basic color mixing principles that underlie white perception remain the same. White always involves a balanced, proportional stimulation of the eye’s red, green and blue photoreceptors.
The Balance of Component Colors
White light comprises a precise balance of component colors. If one wavelength begins to dominate, the light will take on a hue shift.
For instance, warmth from a candle or light bulb comes from extra long wavelengths. This makes the spectrum slightly orange or red tinted. Cool, blueish-white light has more intensity at shorter violet and blue wavelengths.
True white exists right at the point where no single color overwhelms the others. The proportions of wavelengths must be distributed evenly across the spectrum with no major imbalances.
This equilibrium is precisely what allows the eye’s three cone classes to be stimulated uniformly. Even minor deviations can produce visible color casts and prevent the full combining of primaries that generates white.
Additive vs Subtractive Color Mixture
There are two main ways to mix colors: additive and subtractive mixing. Both methods can create white through different primary color combinations.
Additive mixing involves light sources. Combining beams of colored light adds wavelengths together to form new hues. Red, green and blue lights blended together produce white via additive mixture.
Computer screens, TVs and other displays create images through additive color mixing with their RGB pixels. Additive primaries are bright, luminous and transmitted.
Subtractive mixing uses pigments and dyes. Inks, paints and other colorants subtract wavelengths through selective absorption. Cyan, magenta and yellow pigments subtractively mix together to yield white.
Printers, paintings and most physical objects use subtractive color combination. Subtractive primaries are dark, muted and reflected.
But whether additive or subtractive, any primary system that fully stimulates the red, green and blue cones will produce white through the combined response of our visual receptors.
Diffuse White Reflection
When we see white objects, it is because they diffusely reflect and scatter all wavelengths of light equally in all directions. This even, broadband reflectance ensures a balanced stimulation of the eye regardless of lighting conditions.
Whiteness arises from wide spectral absorption and extensive diffuse reflectance. A perfectly white surface absorbs no light and reflects all visible wavelengths in a isotropic, matte manner. This property allows white materials to maintain consistent appearance in different illuminants, whereas colored objects change hue.
Paper is an example of a highly diffuse white reflector due to the voids between fibers that scatter light. Other white materials like snow, salt, sugar, and clouds owe their appearance to similar microscopic scattering structures that make them non-directionally reflective.
Achromatic vs Spectral Whites
There are two categories of white: achromatic and spectral. Both contain the full visible spectrum but in different ways.
Achromatic white results from equal stimulation of the red, green and blue cones. It does not rely on any single wavelength. Achromatic white is the “pure” white we typically think of and has no hue.
Spectral white contains subtle tinting from extremely narrowband primaries. For example, spectral white could combine 445nm, 540nm, and 605nm wavelengths from narrow-linewidth lasers. This white is not completely achromatic but retains a faint residual hue.
Regular white light sources produce achromatic white via their continuous spectrum. Lasers and other spectrally narrowband technologies can create near-white spectral light by combining three very pure primaries.
Whiteness in Nature
White commonly occurs in the natural world for a variety of physical and biological reasons. Some examples include:
- Snow and ice – reflections off crystal structures
- Clouds – scattering of light by tiny water droplets
- Salt flats – high reflectivity of salt crystals
- White sand – diffuse scattering by fine mineral grains
- White flowers – pigments that reflect many wavelengths
- Polar bear fur – translucent optical fibers
- Coconut milk – emulsified lipid droplets
Nature has evolved many microscopic structures that effectively spread out and distribute the visible spectrum to ensure broad, even reflectivity yielding white coloration. These include things like porous scattering layers, reflective crystals, fiber networks, and colloidal suspensions.
Whiteness in Manmade Materials
In addition to natural white substances, many engineered products also exhibit whiteness through careful manipulation of surface structure, additives, coatings and pigmentation. Common white materials and products include:
- Paper – titanium dioxide particles for high reflectance
- Plastics – solid and voided scattering particles
- Paint – titanium dioxide or zinc oxide pigments
- Textiles – bleaching agents to remove color
- Ceramics – non-absorbing compositions
- Cosmetics – titanium dioxide or bismuth oxychloride
- Foods – air bubbles in foams or emulsions
The key to achieving whiteness is usually the incorporation of nano or micro-structures that make the material non-directionally reflective and scattering through Rayleigh or Mie mechanisms. This provides uniform, matte reflectivity.
Measuring White Light
There are a few ways to quantify white light and determine how closely it approaches ideal achromatic whiteness:
Spectrophotometry – Measures a light source’s power at each wavelength. The output should be reasonably continuous and flat across the visible spectrum.
Chromaticity – Plots color on x/y or u’/v’ coordinate diagrams. White has coordinates near (0.33, 0.33). Deviations indicate color cast.
Color rendering index (CRI) – Compares color appearance under test and reference illuminants. Higher CRI indicates better color rendering and white quality.
Correlated color temperature (CCT) – Describes white balance in terms of blackbody temperature. Lower CCT is warmer, higher is cooler. Equal energy at ~5000K is ideal.
By leveraging standards like these, lighting engineers can design and assess white light sources to ensure they deliver bright, uniform, high-quality white illumination.
Applications of White Light
High-quality white light has become indispensable to modern life thanks to applications like:
- Illumination – interior/exterior lighting
- Displays – computer, TV, phone screens
- Imaging – photography, microscopy, telescopes
- Sensors – color accuracy in automation
- Healthcare – endoscopy, diagnostics
- Art – pigments, dyes, color matching
- Publishing – paper, printing, reproduction
- Metrology – measurement standards
From consumer electronics to precision optics, white light enables color-critical performance. Careful attention to balanced spectra makes vivid, accurate color possible throughout technology and media.
Conclusion
In summary, white light comprises the complete span of visible wavelengths from about 400-700nm. It stimulating the eye’s red, green and blue photoreceptors evenly such that no one color dominates. While we can discern only about a million distinct colors, the spectrum incorporates essentially limitless subtle variations that fuse into uniform white. Engineered materials and coatings can mimic natural whiteness through scattering structures that reflect all hues equally. This fascinating interplay of physics, biology and perception comes together to produce the white light fundamental to our vision and technologies.
