Color is the visual perception of a portion of the light spectrum. This perception is created when light strikes the eye and is interpreted by the brain.
Discover FizziQ
How to measure it in class
FizziQ’s colorimeter uses the smartphone camera to measure the three red, green and blue components of the incoming light, as well as its hue and brightness.
Steps:
- Open FizziQ and select the Colorimeter instrument. Aim at a sheet of white paper lit by daylight and note the R, G and B values: they should be close to one another.
- Aim in turn at red, green and blue cardboard sheets from a constant distance. Record the three components for each sheet and identify which one dominates.
- Repeat the same measurements under another light source (fluorescent tube, white LED, incandescent lamp) without changing anything else. Compare: the measured color of the same object changes with the source that illuminates it.
- Film or photograph a computer screen displaying white through a magnifying glass or a diffraction grating, then measure again: the white of the screen is actually a juxtaposition of red, green and blue.
- Compare the values between two different smartphones aiming at the same target: the absolute differences show that the sensor provides reliable comparisons, not reference values.
Scientific activities on this topic
The study of colors opens the door to many science topics such as waves, light absorption or the physiology of vision. These experiments can be carried out with a smartphone or a tablet.
- Colorimetry of autumn leaves - measure how the RGB components of leaves evolve over the season and relate the color to the pigments present (middle school).
- RGB additive color mixing - recompose colors from three red, green and blue sources and verify the resulting secondary colors (middle school).
Learn more
Color, in its essence, is a human interpretation of the visible portion of the electromagnetic spectrum. This spectrum, to which visible light belongs, encompasses a range of wavelengths from approximately 380 to 780 nanometers. Each wavelength corresponds to a specific color perceived by the human eye, starting with violet at the shortest end of the spectrum, passing through blue, green, yellow, and ending with red at the longest end.
This phenomenon, which everyone can perceive, was studied scientifically relatively late.
One of the most remarkable contributions to the understanding of colors comes from Isaac Newton. In the 17th century, he demonstrated that white light was actually composed of a spectrum of colors. Newton used a prism to decompose white light into colors and showed that by recombining these colors, white light could be reformed.
Johann Wolfgang von Goethe, better known for his literary work, also wrote an influential treatise on colors, “Zur Farbenlehre” (Theory of Colors), published in 1810. He challenged Newton’s ideas and presented a more phenomenological approach to color, focusing on how we perceive it.
In the 19th century, James Clerk Maxwell made fundamental discoveries in the fields of optics and color. He is best known for creating the first color photograph in 1861 using three primary color filters: red, green and blue. Hermann von Helmholtz also contributed significantly to the theory of color perception. He developed the trichromatic theory of color vision, which assumes that the human eye perceives colors through three types of receptors sensitive to red, green and blue. Finally, Thomas Young had proposed an earlier version of the trichromatic theory of color vision. This theory became the basis of our current understanding of color perception.
One of the pillars of color analysis is the phenomenon of dispersion: white light is the complex mixture of all these colors, as can be demonstrated with a prism or as seen in a rainbow. Color can also be influenced by the way light interacts with different materials. For example, an object appears red because it absorbs all wavelengths of light except red, which is reflected toward our eyes. Likewise, an object can appear in different colors under different light sources because of the variation in the spectrum of light emitted by these sources.
Another fascinating aspect of color is its absence in darkness. In darkness, no light means no perceived color, emphasizing that color is a creation of our perception in response to light. The study of color therefore leads us to explore not only the physical properties of light, but also to understand how our eyes and our brain work together to interpret these light signals.
The three factors of perceived color
Color is not a property that the object possesses on its own. It results from the combination of three elements: the spectrum of the source illuminating the object, the absorption and scattering spectrum of the object, and the response of the three types of cones in the retina. Changing any one of the three changes the perceived color. This is why a garment bought under store lighting can appear in a different shade in daylight: the object has not changed, the source has.
Metamerism
The eye does not measure a spectrum; it extracts only three numbers from it, one per type of cone. Two lights with physically different spectra can therefore produce exactly the same triplet of responses and appear identical: these are metamers. This is what makes screens possible: with only three primaries, the sensation produced by very varied continuous spectra can be reproduced. It is also what explains why two fabrics that match under one lamp can clash under another.
Common mistakes
A color is not always a wavelength. The colors of the spectrum, called spectral colors, do correspond to a single wavelength, but white, pink, brown or magenta exist nowhere in the spectrum: they are mixtures. Another common confusion: the color of an illuminated object does not depend only on what it absorbs: under a lamp that emits no red, a red object appears black since it has nothing to reflect.
Orders of magnitude
The visible spectrum extends from approximately 380 nm (violet) to 780 nm (red). The sensitivity maxima of the three types of cones are located around 420 nm (S, blue), 534 nm (M, green) and 564 nm (L, red). The light-adapted eye is most sensitive around 555 nm, in the yellow-green. A human eye distinguishes on the order of a million shades. A digital image with 8 bits per channel encodes 256 levels per component, that is approximately 16.7 million RGB triplets.
Formula
Monochromatic light is characterized by its wavelength, related to its frequency by:
λ = c / ν
where:
- λ: wavelength in vacuum (m)
- c: speed of light in vacuum, 3.00 × 10⁸ m/s
- ν: frequency of the wave (Hz)
A color displayed by a screen is described by its three components, each encoded between 0 and 255:
Color = (R, G, B)
where:
- R: red component (dimensionless)
- G: green component (dimensionless)
- B: blue component (dimensionless)
The perceived relative luminance is calculated from the three components normalized between 0 and 1:
Y = 0.2126 R + 0.7152 G + 0.0722 B
where:
- Y: relative luminance, between 0 (black) and 1 (white)
The coefficients are unequal because the eye is far more sensitive to green than to blue.
Application examples
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A green laser pointer emits at 532 nm, that is a frequency of 5.6 × 10¹⁴ Hz
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The infrared LED of a remote control emits around 940 nm, beyond visible red: the front camera of a smartphone can see it, the eye cannot
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The white of a smartphone screen corresponds to the triplet (255, 255, 255), obtained by turning the three red, green and blue subpixels fully on
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Under the sodium lighting of old streetlamps, which emits only around 589 nm, a red car and a green car both appear gray
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A green leaf mostly reflects between 500 and 570 nm because chlorophyll absorbs blue and red
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The sky is blue because Rayleigh scattering by air molecules is stronger the shorter the wavelength
FAQ
Q: Are the RGB values given by the smartphone colorimeter absolute measurements? A: No. The camera sensor does not have the same spectral response as the eye, and the software applies white balance and gamma correction. Relative comparisons, between two objects measured under the same conditions, are reliable; absolute values are not, and are not comparable from one phone model to another.
Q: Do two objects with identical colors to the eye necessarily have the same spectrum? A: No, this is metamerism. Two different spectra can stimulate the three types of cones in the same way and appear identical. They will often stop looking alike if the light source is changed.
Q: Why does magenta not appear in the rainbow? A: Magenta is not a spectral color: it corresponds to no wavelength. It is the result of the simultaneous stimulation of the cones sensitive to red and blue, without green. It therefore exists only as a mixture.
Q: Does a colorblind person see in black and white? A: Almost never. The most common form of color blindness is an anomaly of one of the three types of cones, most often the red or green one, which makes certain pairs of colors difficult to distinguish. The total absence of color vision, achromatopsia, is very rare.
Q: Why does the same paint not look the same in the store and at home? A: Because color depends on the spectrum of the source. Store fluorescent tubes, household LEDs and daylight have very different spectra, which changes the light reflected by the paint and therefore the perceived color.
Related concepts
Visible Spectrum - Wavelength - RGB Additive Synthesis - Colorimeter - Luminance - Illuminance