There is a popular saying, “seeing is believing” meaning how we see things is how we believe them to be. The same could be said of another saying, “perception is reality”, however, different perceptions can show different realities. The way human’s eyes see the world is not equivalent to how another species on Earth may see things.
For example, the eyes of a common bee see a whole different range of wavelengths on the electromagnetic spectrum, including ultraviolet light, allowing them to detect color patterns in flowers that are otherwise imperceptible to humans.1
Color spectrum visible to the human eye (top) compared to bee vision (bottom). (Image Source: West Mt Apiary).
Unlike the single-lensed human eye, bees have compound eyes (like all other insects). Additionally, bees have three small eyes, known as ocelli, that detect the ultraviolet spectrum. Hidden from the sight of humans, some markings of different varieties of flowers are only visible in the ultraviolet range.
These markings typically function as a target to insects, thus drawing bees towards a flower’s center where the nectar sits. Bees feeding on the nectar of a flower will then simultaneously gather pollen from the flower as it sticks to their body and then carry it to other flowers, thus, pollinating various flowers, plants, and crops.
Images of a Mimulus flower in visible light as seen by humans (left), and ultraviolet light as seen by bees, butterflies, and other insects (right), showing a dark strip called a “nectar guide” directing pollinators to their target. (Image Source).
Generally, compound eyes consist of thousands (or in some insects, millions) of minute facets called ‘ommatidia’, each of which has a miniscule individual lens that concentrates on light and pigments for detecting color.
Each hexagon-shaped facet is connected to a cone with eight photoreceptor cells. “In each cone, there are two receptors for each of the colors blue, yellowy-green, and ultraviolet. Information from all the facets is collated by the bee's brain and makes a mosaic picture of its surroundings. It sounds complicated but it's amazingly fast. Bees perceive color three to five times faster than humans. This means they can see individual flowers blowing in the wind even while flying.”2
Thousands of facets in a bee’s compound eye. Image Credit: Radiant Vision Systems
Human Visual Perception
The human eye is an extraordinary instrument that has the capacity to see with a considerable degree of visual acuity, accurate depth perception, and detailed resolution, at both far and near distances. The human eye also has great sensitivity to color across the spectrum of electromagnetic light from around 380 to 740 nm, which is known as the ‘visible’ spectrum since it’s what’s visible to human vision.
Human eyes are categorized as a ‘camera-type’ eye due to the fact the human cornea focuses light entering the eye onto the retina, a light-sensitive membrane, similar to the way a camera lens focuses light onto a receptive layer of film.
The cornea refracts (bends) the rays that enter the pupil as the iris (the colored portion of the eye that surrounds the pupil) opens and closes, controlling the diameter of the pupil. This process regulates the amount of light passing into the eye.
Moreover, as light passes through the cornea it moves through a thin lens, which also contracts and expands to adjust its shape, further bending the rays to focus them directly on to the retina.
Sitting at the back of the eye, the retina is a thin layer of tissue that has millions of tiny light-sensing nerve cells. These nerve cells are called rods and cones on account of their distinct shapes. Cones are condensed in the center of the retina, in the macula - the macula is responsible for all of the central vision - allowing humans to see most colors and fine details as well the ability to recognize faces.
Therefore, cones deliver clear, sharp central vision to identify color and detail in bright light conditions. Containing color pigments, or photo-detecting molecules, there are three types of cone, each sensitive to a different wavelength of light: red, blue, and green.
A diagram of the structures of the human eye including the pupil (where light enters the eye), and the cornea and lens, which both help focus the light onto the retina, where rods and cones are found. (Image adapted from the American Optometric Association).
On the contrary, rods are situated outside the macula and reach all the way to the outer edge of the retina. They are responsible for peripheral or side vision, and enable the eye to see monochrome in dark conditions including at night. Rods and cones transform the light they receive into electrical impulses, which are transmitted through the optic nerve to the brain, and the brain converts the signals sent from both eyes to generate an image which is what humans ‘see’.
Human vision is particularly acute because of the concentration of cones in the macula. Reportedly, the resolution of the human eye is equivalent to 576 megapixels.3 The color perception of the human eye is also extremely refined – on average, a human eye can differentiate between about 1 million various colors, although some people can identify up to 7 million or more.
Groundbreaking research conducted at the University of Rochester4 found that the amount of color sensitive cones present in the retina can vary by as much as 40 times, but the number of cones is not associated with distinct perception of color (e.g. red versus orange). This suggests that a human’s color vision is not so much a result of the eye’s structure but more of a function regarding how the brain interprets and makes sense of the information it receives.
And Now, About Those Birds
Typically, birds possess much greater visual acuity than humans do. Especially necessary for protection when in flight as well as detecting fast moving prey. Unsurprisingly, the eyesight of raptors (eagles and hawks) is among the most acute on the planet, hence the saying, “eyes like a hawk”. The eyes of raptor birds are packed densely with cones for fine-grained, high-resolution images – in fact, the resolution of an eagle’s eye is about 2.5 times greater than a human’s.5 However, they have limited rods, so they tend to be daytime rather than night-time hunters. Whereas, nocturnal hunting birds, including owls, possess a much higher concentration of rods – this enables them to see extremely well in the dark.
Harris Hawk (left), barred owl (center) and the rear-view of a woodcock (right). Image Credit: Radiant Vision Systems
Like most birds, the eyes of a hawk are slightly turned towards the front of its head, allowing them to have some element of binocular vision. Binocular vision offers the added benefit of precision depth perception – ideal for tracking prey that moves rapidly on the ground. Owls have the greatest binocular vision of all birds, with forward-facing eyes and a near entirely overlapping field of view similar to a human. By way of contrasts, the eyes of a woodcock rotate upwards towards the rear of their head, which provides them with binocular vision above and behind – a useful trait for detecting predators overhead when rummaging in the dirt for food.
The visible spectrum for dogs (left), and a comparison of what colors a human sees vs. a cat (right). (Images sources: left and right).
Furthermore, the list below offers some additional and useful facts with regards to vision in the animal kingdom:
- Eyes on horses and zebras face sideways: this gives them extraordinary peripheral vision (for spotting predators like lions or wolves) but creates a blind spot right in front of their noses.6
- Owls are the only bird who can perceive the color blue.6
- Snakes have thermal vision, with infrared-sensors situated in deep grooves on their snouts, allowing them to see warm-blooded prey.6
- Some animals don’t depend on vision at all. For instance, the star mole is blind, but uses its exceptionally sensitive touch organ to detect, catch, and eat food quicker than the human eye can follow: around 300 milliseconds.7
The Natural History Museum in London offers slider photos for visitors to play around with in order to compare human vision with that of various creatures. For example, visitors can see demonstrations of how dogs, snails, geckos, giant clams, and even jumping spiders see the world around them.
Alternatively, there is a free open source software app that can be downloaded to a smart phone that lets allows a user to view photos as they might appear to different animals. The App is called the Quantitative Colour Pattern Analysis (QCPA)8 framework and was developed by scientists in Australia, as outlined in a recent paper published by the British Ecological Society.9
Four images of a nudibranch (sea slug) using the QCPA app. Top left: the image taken with a digital camera, top right: image as perceived by a triggerfish in 5m depth at 10cm viewing distance. Bottom left: color contrast of edges as perceived by a triggerfish. Bottom right: a heatmap of the perception of color saturation by the triggerfish. (Image Credit: Cedric van den Berg et al.8).
Photometry: Matching Human Visual Perception
Radiant, has spent more than 25 years designing and developing systems for the precise and accurate measurement of light and color as observed by the human eye. The foundation of all of Radiant’s technology—photometry and colorimetry—are the sciences of measuring light relative to how humans see brightness (luminance) and color (chromaticity).
Light measurement systems including Radiant’s ProMetric® Imaging Photometers and Colorimeters are developed to measure brightness and color in line with that of human vision. Using CIE-matched (tristimulus) optical filters and scientific-grade imaging sensors, Radiant’s high-resolution camera systems capture significant data that guides human-centric design and the precise evaluation of many of today's lighting and display products.
To find out more information regarding measuring light and color corresponding to the visual perception of the human eye, view Radiant’s recent webinar: Principles of Light & Color Measurement. Throughout, Radiant discuss the basis of photometry and colorimetry and present photometric technologies that influence these principles to properly quantify the human visual response precisely for the purpose of quality assurance in light and display products. Topics covered include:
- Quantifying color based on CIE tristimulus curves
- How the human eye responds to light and color
- Optical metrology systems and benefits of imaging for light measurement
- Technology designed to replicate human visual response
Image Credit: Radiant Vision Systems
References
- "2019: What Just Happened?", New York Times Special Section, December 29, 2019, page 6
- Smith, Barbara, “How Bees Use Their Unique Vision to Search For Food and Find Their Way Home”, Stuff, August 28, 2017
- Hamer, A., “How Many Megapixels Is the Human Eye?”, Curiosity, December 19, 2016.
- “Color Perception is Not In the Eye of the Beholder—It’s In the Brain, Science Daily, October 26, 2005
- Preston, E., “How Animals See the World”, Nautilus, March 20, 2014
- “32 Facts About Animal Eyes”, Discovery Eye, August 5, 2014 (retrieved April 13, 2020)
- “Star-nosed Mole”, Wikipedia https://en.wikipedia.org/wiki/Star-nosed_mole (retrieved April 9, 2020)
- Ferreira, B., “Scientists Created Open Source Tools to See in Animal Vision”, Motherboard, December 3, 2019
- Van den Berg, C., et al., “Quantitative Colour Pattern Analysis (QCPA): A comprehensive framework for the analysis of colour patterns in nature”, Methods in Ecology and Evolution, December 2, 2019, https://doi.org/10.1111/2041-210X.13328
Acknowledgments
Produced from materials originally authored by Anne Corning from Radiant Vision Systems.
This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.
For more information on this source, please visit Radiant Vision Systems.