When red and green light are mixed together, the resulting color that is perceived depends on the relative intensities of the red and green light. Red and green are additive primary colors, meaning that when mixed in equal proportions they produce the secondary color yellow. By varying the intensities of the red and green light, different shades ranging from yellow to orange can be produced. Understanding the science behind mixing red and green light provides insight into the nature of color and vision.
How Light and Color Work
In order to understand what happens when red and green light are mixed, it is helpful to first review some basics about light and color. Sunlight and most artificial light sources emit light across a continuous spectrum of wavelengths in the visible range. The visible light spectrum that humans can see ranges from about 380 nanometers (violet) to 740 nanometers (red). The wavelengths corresponding to the primary colors are:
| Color | Wavelength (nm) |
| Red | ~700 |
| Green | ~550 |
| Blue | ~470 |
When a full spectrum of visible light enters the eye, the light-sensitive cone cells in the retina respond preferentially to red, green or blue wavelengths. The relative stimulation of the red, green and blue cones allows the brain to perceive all the colors of the visible spectrum.
The primary colors of light (red, green, blue) are different from the primary colors of pigments and dyes (cyan, magenta, yellow). When dealing with light and displays, the primary colors are those that when added together can produce the other colors.
Mixing Red and Green Light
When lights of two primary colors overlap in space, they are additive. With additive color mixing, increasing the intensity of either color increases the total amount of stimulation of the cones in the eye, changing the perceived hue.
Specifically, when red light (around 700 nm wavelength) and green light (around 550 nm) mix, the red stimulates the red cones, the green stimulates the green cones, and the intermediate wavelengths stimulate both cones to varying degrees.
If the red and green light are of equal intensity, the red and green cones are stimulated about equally, and the brain perceives the additive mix of red and green light as yellow. Yellow is seen when the red and green cones are stimulated similarly.
By increasing the intensity of either the red or green light in the mixture, the hue shifts towards that color. The brain perceives the additive mix of bright red plus dim green as orange. Conversely, increasing the green intensity relative to the red shifts the hue towards greenish-yellow.
Chromaticity Diagrams
The visual effects of mixing red and green light can be mapped on a chromaticity diagram. The chromaticity (color quality) of light is determined only by its spectral power distribution, not its total power or luminance.
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On this CIE 1931 chromaticity diagram, saturated monochromatic red, green and blue wavelengths are located on the outer curved boundary. Mixing red and green light produces colors along the straight line between them.
Equal intensity red + green light mixes to produce yellow at the center point. Increasing red shifts towards orange, while increasing green shifts towards greenish-yellow. |
Chromaticity diagrams provide a useful visualization of how the hue rendered by additive color mixing changes depending on the relative intensities of the components. The path traced by red-green mixtures falls along a straight line between the red and green primary wavelengths on the chart.
Light Sources and Filters
There are a couple straightforward ways to mix red and green light together in adjustable proportions:
– Use red and green light sources. Red and green lasers or LEDs can be overlapped in variable brightness to produce a range of hues. Mixing lights this way produces the full spectrum of possible colors.
– Start with white light. A white light source like an incandescent or LED bulb provides a full spectrum. Passing it through a red filter removes other wavelengths, while a green filter transmits green wavelengths. Overlapping the filtered beams mixes reds and greens additively.
– Use a RGB lamp or pixel. Many color-changing LED or neon lamps have separate red, green and blue elements that can be adjusted to produce various colors through additive mixing. LED and LCD displays work similarly, mixing red, green and blue light at the pixel level.
Pigment and Dye Color Mixing
It is important to distinguish additive light mixing, as discussed here, from the subtractive color mixing that occurs with pigments and dyes. When mixing paints, inks or other colorants, the pigments selectively absorb and subtract certain wavelengths of light. The color perceived is the wavelengths that are not absorbed and are reflected.
For example, red pigment absorbs green and blue light while reflecting red. Green pigment absorbs red and blue, reflecting green. Mixing green and red pigments produces a dark brown color because each pigment absorbs some of the complementary light that the other reflects.
So while mixing red and green light additively produces yellow, mixing red and green paints subtractively produces brown. The mixing principles for lights and pigments are very different.
Human Color Perception
The sensation of color is all ultimately about human perception – how the brain interprets signals from the eye’s cone cells stimulated by different wavelengths of light.
Part of the complexity of color perception is that it is influenced by visual context, surrounding colors, and other physiological factors. The same mix of wavelengths can be perceived differently depending on the conditions. These include:
– Lightness/brightness – Lighter and brighter colors are perceived as paler or washed out. The same red-green ratio will appear darker or more saturated if the overall luminance is lowered.
– Background – A color may appear more red or greenish depending on surrounding hues. The effect of simultaneous color contrast means perceived color is always relative.
– Adaptation – The eye adapts to ambient lighting conditions, which shifts color perception. Green-red balance may look different after adapting to reddish or greenish light.
– Individual differences – Genes, age and other factors mean people’s eyes and visual processing differ, which affects color perception. Some forms of color blindness change how red-green mixtures are seen.
So while the physics and wavelengths can be measured objectively, perception introduces complexity. The mix of red and green light ultimately produces brain sensations of yellow, orange and related hues.
Applications and Uses
Understanding red-green color mixing has many practical applications, both for art and science:
– Digital displays – All computer, phone and TV screens mix variable red, green and blue light to produce color images. Their filters and backlights determine the range of colors possible.
– Stage lighting – Theater lighting designers use red, green and blue sources to create dramatic colored lighting. Carefully mixing the lights evokes moods.
– Lasers and lights – Red and green lasers can be overlapped to create laser light shows projecting patterns in orange and yellow hues.
– Pigments – While mixing red and green pigments makes brown, adjacent fine red and green patterns can optically mix to seen as orange and yellow when viewed from a distance. This is used in printing.
– Biology – Fluorescence microscopy uses filters to visualize specific red- and green-fluorescing molecular tags in cell biology. Combinations indicate colocalization.
– Astronomy – Analyzing the red-green balance in starlight helps determine properties like temperature, composition and velocity due to redshift.
– Photography – Adjusting red and green channels allows manipulating color balance and saturation when developing and editing photos.
– Color theory – Understanding primary color mixing helps artists use adjacent strokes of pure red and green paint to render a realistic range of hues including orange and yellow.
So from basic principles of photons and vision to high-tech applications, mixing red and green light produces a fascinating interplay of color and perception.
Conclusion
In summary, combining red and green light results in a range of colors including yellow, orange and chartreuse green depending on the relative intensities. This additive mixing of light can be visualized on a chromaticity diagram and implemented with color sources and filters. But color perception also depends on neural processing so the visual effect has psychological complexity. Understanding primary color mixing provides the foundation for working with color across science, technology and the arts.