Color perception is an important part of human vision and experience. The human eye can see millions of different colors, but we are not equally sensitive to all colors. Research has shown that the human eye has lowest sensitivity to colors at the blue-green end of the visible spectrum.
The Visible Spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. The visible spectrum ranges from wavelengths of approximately 380 nanometers to 740 nanometers. The visible colors we perceive range from violet with the shortest wavelengths to red with the longest wavelengths. The wavelength of light determines its color.
The visible spectrum can be represented as a continuous band of colors:
| Violet | Blue | Green | Yellow | Orange | Red |
Within this band, there are millions of different shades and hues that the human eye can distinguish. But we do not perceive or sense all visible wavelengths equally. The eye has different levels of sensitivity across the spectrum.
Cone Cells and Color Vision
The ability to see color comes from special photoreceptor cells in the retina called cone cells. There are three types of cone cells that detect different wavelengths of light and are responsible for our trichromatic color vision.
- S cones – most sensitive to short wavelengths of light like blue and purple
- M cones – most sensitive to medium wavelengths like green
- L cones – most sensitive to long wavelengths like red and orange
These cone cells contain pigments that absorb light and trigger neural signals to the brain. The combination of signals from the three cone types gives us the perception of all the colors we see.
Spectral Sensitivity
Researchers have measured the light sensitivity across the visible spectrum by testing the minimum radiance needed at each wavelength for subjects to detect light. This generates spectral sensitivity curves for the three cone types.
The spectral sensitivity curves show that each cone type has a peak sensitivity in different regions:
- S cones peak at 420-440 nm (blue light)
- M cones peak at 534-555 nm (green light)
- L cones peak at 564-580 nm (yellow/green light)
The lower ends of visible spectrum (violet, blue) stimulate mainly the S cones. The middle wavelengths (green, yellow) stimulate the M cones strongly. And the longer wavelengths (orange, red) primarily stimulate the L cones.
Lowest Sensitivity to Blue-Green
An analysis of the spectral sensitivity curves shows that humans have the lowest overall sensitivity in the short-wave (blue) to middle-wave (green) range, approximately 450-495 nm.
In this range, the S cone response declines steadily going from blue toward green wavelengths. At the same time, M cone response is relatively weak at the blue end. This results in an area of lowered combined cone response between the peak sensitivities of the two cone types.
Experiments testing color discrimination thresholds also demonstrate that the just noticeable differences between similar hues are larger in the blue-green spectral regions. This indicates poorer perceptual sensitivity.
Reasons for Reduced Blue-Green Sensitivity
There are a few reasons why our eyes are least sensitive in the blue-green wavelengths:
- The S cone density is much lower than M and L cones across the retina.
- The peak transmittance of the eye’s lens declines rapidly below 500 nm.
- Neural processing may compress blue signals relative to longer wavelengths.
So both optical factors like low S cone representation and pre-retinal absorption, as well as neural processing contribute to poorer blue-green sensitivity.
Implications of Weak Blue-Green Sensitivity
Our weak sensitivity in the blue-green range has some interesting implications:
- We are less able to discriminate between shades of blue and green.
- Blue colors may appear darker or less bright compared to other colors.
- Mixing blue and green light results in desaturated perceptions.
- Blurring or haziness is more apparent with blue-green hues.
Our visual systems are essentially set up to be “bluish-green blind”. But despite this weakness, blue-green colors are still visible due to the responses of S cones and M cones. The sensitivity is just significantly lower compared to other parts of the spectrum.
Measuring Sensitivity with Cone Fundamentals
Vision scientists represent the spectral sensitivities of the cones using a standardized color matching function known as the CIE RGB cone fundamentals. These mathematical functions model the average responsiveness across the spectrum for the three cone types based on experimental data.
CIE RGB cone fundamentals:
- r(λ) – long-wave (L cone) sensitivity
- g(λ) – medium-wave (M cone) sensitivity
- b(λ) – short-wave (S cone) sensitivity
Plotted together on a graph, the cone fundamentals illustrate the overlapping spectral sensitivities and the dip in combined response in the blue-green wavelengths:
| Wavelength (nm) | r(λ) | g(λ) | b(λ) |
| 450 | 0.04 | 0.09 | 0.63 |
| 470 | 0.09 | 0.39 | 0.99 |
| 490 | 0.20 | 0.85 | 0.97 |
| 510 | 0.42 | 0.99 | 0.75 |
| 530 | 0.65 | 0.97 | 0.45 |
At 450 nm, sensitivity is dominated by S cones. At 530 nm, sensitivity is dominated by L cones. In between at 490 nm is the region of minimal overall response.
Practical Applications
Understanding the areas of lowered visual sensitivity has useful applications:
Display and Imaging Technology
Display and camera color engineering take advantage of weaker blue sensitivity by emphasizing blue chromaticity. This results in perceived brightness and widened color gamut.
Data Visualization
Visualizations can use blue-green colors for labels, annotations, or codes with minimal visual interference since they are hard for us to see.
Vision Testing
Blue-green color vision deficiencies are assessed by testing sensitivity to wavelengths from 480-510 nm. Normal trichromats perform poorly in this range.
Vision Research
The blue-green window provides a zone to stimulate S cones or M cones separately from L cones. This enables isolation of cone pathways.
Conclusion
In summary, humans have the lowest visual sensitivity in the blue-green spectral region spanning approximately 450-495 nm. This arises due to the combined properties of S cones and M cones peaking at different wavelengths. Understanding the zones of weakness in color perception provides insight into the intricacies of visual processing and enables better application of color in various technologies.