Where Does Black Fall On The Color Spectrum? A Color Scientist Explains

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People love the rainbow of ROYGBIV colors: red, orange, yellow, green, blue, indigo and violet. Human eyes perceive visible light as this array of colors.

You may notice that some colors you can perceive aren't part of the classic rainbow, though. Where is black, for example?

I'm an associate professor of color science , a field that combines physics and perception. Color scientists are interested in learning more about human vision and applying that knowledge to make color systems – such as in cameras, screens or lighting systems – work better.

To understand where black falls on the color spectrum, first consider what light actually is.

Light is energy called electromagnetic radiation. It's made up of a stream of energy particles called photons.

Each photon has its own energy level. There are two characteristics you can use to describe a photon. Its frequency is how fast it vibrates back and forth – or oscillates – as it travels. And its wavelength is the distance between those oscillations in space.

As photons with wavelengths within a range of about 400–700 nanometers stream into your eyes, your brain perceives them as light. Scientists call these photons visible radiation. You perceive photons with different wavelengths as different colors.

Photons outside that range of wavelengths are invisible to human eyes. Shorter wavelength energy includes ultraviolet, X-ray and gamma radiation, while longer wavelength energy includes infrared and radio waves.

Color perception is also affected by the quantity of photons – what physicists call the power – at different visible wavelengths. More photons means more powerful light, which looks brighter. A very vivid color consists mostly of photons of similar wavelength. For example, a pure red may consist of photons that all share the same wavelength near 620 nanometers.

A stream of photons with a wider range of wavelengths will appear as a paler, less saturated color. White light, such as natural daylight, consists of photons with wavelengths spread fairly evenly across a wide range of the visible spectrum. LEDs and other electric light sources are not quite as uniform across the spectrum, but they still appear white or achromatic, meaning without color.

Mixtures of wavelengths combine and appear as new colors. The human visual system interprets pure red light and pure green light combined as yellow. Add in pure blue, and this mix of radiation appears white. Scientists and engineers take advantage of this quality in display devices, which are able to create a huge range of perceived colors by mixing the primary colors red, green and blue.

While there's no black in a rainbow, photons anywhere in the electromagnetic spectrum can be seen as black. Or in some cases, they can't be seen at all!

Radiation within the visible spectrum can appear black if it is low in power – more specifically, lower in power than its surroundings.

Additionally, radiation outside the visible range of wavelengths appears black to our eyes. For example, infrared radiation appears black because it is invisible to humans.

Our eyes detect the wavelength and power of the light, but our brains interpret it. So color perception always depends on the context .

People are good at adapting to a wide range of light levels, from sunlight to starlight. So our perception of color and brightness depends on what's around and what we've been looking at recently. If you step from outdoor daylight into a dark theater, at first you probably perceive the whole environment as black, and you may even have trouble finding your way.

However, your visual system immediately begins to adapt to the low light level. Soon, visual details begin to emerge. What appeared black now has different levels of lightness and color.

Consider the optical illusion that consists of a light rectangle next to a dark rectangle. Each rectangle contains a circle. The circles appear to be different shades but in fact are identical. Against the light background, the circle is dark enough to appear black. Surrounded by the black background, it becomes clear that the circle is merely dark gray. Even when you know the circles are the same, it's hard to believe because the effect of the surrounding background is so strong.

You might be asking yourself, how dark must a color be to appear black? Another way to ask the question is, how low in power must the physical light be in order to look black?

For a visual answer, look at a gradient from dark gray to black. Where in the gradient is the boundary, or threshold, at which you call it black? What if you dim your display or view the screen in a much brighter or much darker environment? Probably the best answer for how dark it must be is,“It depends.”

Color perception is a fascinating topic, and we color scientists are continuing to uncover details of how the human visual system works while also applying our knowledge to many other useful things, including dyes, cameras, printers, LED lighting systems and AR/VR displays.

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