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Can you measure the wavelength of a color?

The wavelength of visible light determines what color our eyes perceive. Light wavelengths range from 380 to 750 nanometers (nm) within the electromagnetic spectrum. The longest wavelengths appear red while the shortest appear violet. In between, the colors of the rainbow emerge. But can we assign exact wavelengths to each distinguishable color?

The Visible Spectrum

The visible spectrum encompasses the wavelengths of light that human eyes can detect. This range falls between infrared light and ultraviolet light on the electromagnetic spectrum. Infrared has longer wavelengths (750 nm to 1 mm) while ultraviolet has shorter wavelengths (10 to 380 nm).

Color Wavelength Range (nm)
Violet 380-450
Blue 450-495
Green 495-570
Yellow 570-590
Orange 590-620
Red 620-750

As shown in the table, violet light has the shortest wavelength visible to humans while red has the longest. The other colors fall in sequence between them. But the ranges for each color overlap due to the continuous nature of the spectrum. Our eyes cannot perceive exact boundaries between them.

The Trichromatic Theory of Color Vision

According to the trichromatic theory, human color vision relies on three types of cone photoreceptor cells in the retina. Each type is sensitive to a different range of wavelengths:

  • S cones – short wavelengths (blue)
  • M cones – medium wavelengths (green)
  • L cones – long wavelengths (red)

The combination and relative stimulation of the three cone types allows us to perceive the entire spectrum of visible colors. However, there is no single cone type devoted to each color. The wavelengths that stimulate a cone depend on its pigment. Red light, for example, strongly activates L cones but barely activates S cones.

The Subjective Nature of Color Perception

Color perception is subjective – what we see depends on the observer. Here are some of the factors that influence it:

  • Cone ratios – Everyone has S, M and L cones but in different quantities. This affects color sensitivity.
  • Color blindness – About 8% of males lack an M or L cone type which alters color perception.
  • Age – The lens yellows over time, filtering out short wavelength (blue) light.
  • Experience – Memory colors form based on what we’ve learned to associate with objects (e.g. bananas are yellow).

Since color is constructed in our visual system and brain, assigning an exact wavelength is impossible. We may all perceive a subtly different shade for the same wavelength.

Spectral Power Distribution

While we cannot specify a single wavelength for each color, we can characterize the light emitted by a source. The spectral power distribution (SPD) measures radiant power at each wavelength across the visible spectrum. It reveals the predominant wavelengths and relative intensities.

For example, a red laser will have a tall, narrow spike around 650 nm while an incandescent bulb will have a broader peak in the red/yellow region. SPDs allow us to determine why two light sources may look different despite having similar color designations.

Dominant vs. Complementary Wavelength

When describing colored light, we often specify the dominant or complementary wavelength instead of a range:

  • Dominant wavelength – Wavelength where the SPD curve peaks in intensity.
  • Complementary wavelength – Appears as the “opposite” color when mixed with the dominant wavelength.

For example, a neon red sign may have a dominant wavelength of 610 nm (in the red range) and a complementary wavelength of 494 nm (blue-green). This provides enough information to characterize the light’s color by approximating where it lies in the spectrum.

Chromaticity Diagrams

Chromaticity diagrams, like the CIE 1931 chart below, map colors by their x and y chromaticity coordinates:

CIE chromaticity diagram

The x and y coordinates are calculated from the SPD curve. Any color that humans can perceive has a unique set of chromaticity values. These charts improve on representing color compared to just specifying a dominant wavelength.

Color Temperature

Color temperature describes the “warmth” or “coolness” of white light sources. It is measured in kelvins (K) based on the temperature to which a blackbody radiator needs to be heated to produce a comparable color:

  • Low color temps (
  • High color temps (>5000K) look more blue (“cool”)

We cannot assign a single wavelength to white light. But measuring the color temperature gives insight about its SPD and chromaticity.

Measuring Wavelength

To empirically measure a light source’s wavelength, you need a spectrometer. This instrument splits light into its component wavelengths using a prism or diffraction grating. Photodetectors then measure the intensity at each wavelength.

Spectrometers yield an SPD curve for the source. The peaks indicate the dominant wavelengths and their relative strengths. Importing the data into software allows you to calculate additional metrics like chromaticity coordinates and color temperature.

Conclusion

While we may refer to light as a certain color, the notion of an exact wavelength is problematic given the nature of human vision. Colors form as our brain interprets the signals received from photoreceptors. Describing a color quantitatively requires metrics like dominant/complementary wavelength, chromaticity coordinates and color temperature.

Instruments like spectrometers measure a source’s spectrum in detail. But ultimately, color depends on the observer’s visual system and neural processing as much as the light’s physical properties.