Thursday, September 29, 2011
More printing coolness
If you read this blog, then you know we love the continuing advent of personal manufacturing, which allows the prototyping of mobile, micro and nano devices with computer vision. We are always on the lookout for cool links. Thanks to AJ for pointing out this website, where you can design and order your own custom, printed parts.
Saturday, September 10, 2011
Animal eyes
I recently read a great book by Michael Land and Dan-Eric Nilsson on biological vision sensors. The read was fairly light, since the book is targeted at a range of readers, from undergrads to eye doctors. A colleague at work let me borrow this book, and to be honest I thought I would skim it first to understand how nature and evolution has solved micro computer vision. But instead I read it cover to cover, since it was intensely compelling.
I thought I'd set out some of the interesting facts about animal eyes that I did not know:
1) Animals in the ocean do not use a cornea at all: its just a flap to protect the eye. In addition, sea animals have spherical lenses with variable refractive indices. In contrast, land animals have non-spherical lenses and almost half of our vision comes from the cornea: its our second lens. Amphibians and other creatures that cross the air-water boundary have to somehow "switch" between these two, by either having "goggles" (transparent eyelids that change the eye optics) or by having four eyes.
2) Some sea animals have two or more retinas in the same eye. These look in different directions. The light is shifted around by having mirrors inside the eye!
3) Light scatters so much in the deep sea that certain wavelengths (like infra red) are absent. However, there are some deep sea fish whose eyes still maintain infra-red sensitivity. They also can generate infra-red beacons: so they use infra-red light as a secret communication channel (since there is no other source of this light at that depth).
4) Many nocturnal animals reduce their iris substantially to protect the eye in daytime. Some animals without an iris actually have the retinal cells "migrate" or shift around during the day. There are other animals whose contracted eye resembles templates: for example the contracted chameleon iris is a row of four pinholes: that's a light-field camera!
5) Some birds and reptiles have oil droplets on their retina that allow them to detect color. Its also well-known that many insects and birds are sensitive to polarization and UV light.
6) Birds of prey (raptors) have "pits" in their fovea that allow for an telephoto like effect and gives them extra acuity. This explains their good eyesight from high above.
7) Jumping spiders have a camera array for eyes. Spookfish use mirrors.
8) The infrastructure of retinal tissue (like nerves and blood vessels and so on) is fairly transparent. For example, in land animals (like humans) the nerves that connect to rod and cone cells attach to them in front of the retina. This means that they get in the way, between the scene and the photodetectors. Octopus and squid have the nerves that connect to the retina in the back. A comparison is here.
9) Scallops have mirrors under the retina of their many, many eyes (pic). The retina is transparent and basically the mirror focuses light back into the transparent mush of the retina, and then the image gets detected. Without the mirror, scallops cannot see a focused image. However, many other animals simply have mirrors under the retina, that "resend" light back out from the direction it came from. Since the light retraces its path, it intersects the retina again, allowing the eye a second chance to image the scene point. This enables night vision and is called tapetum. Mirrors in animals are not metallic, but instead use a quarter-wavelength thick film (a thin film) that reflects light through constructive interference of two reflected rays at the medium boundaries.
10) The cell distribution in animal retina is not uniform (for example, fovea in humans). But for some animals, the retinal cells are so non-uniform that they show up as bands in the retina. These bands are associated with regions of importance: for example a "horizon" band for insects that fly over water and need to locate the horizon to navigate. Some animals have "expensive" cells like color cells in a band, and so they need to constantly move the eye around to scan the scene and get a full color image (eg: mantis shrimp).
11) Compound eyes are amazing, and they consist of many ommatidia or eyelets. There are two types of compound eyes which look almost identical from the outside. Apposition eyes are those where the eyelet FOV do not overlap, and each eyelet creates an inverted image of the scene. Superposition eyes have heavy overlap of the same scene point from multiple eyelets and all the eyelets together create one erect image of the scene. Apposition eyes are common in insects whereas superposition eyes are found in lobsters and similar creatures. Many apposition eyes have templates in front of each eyelet, and this creates a moire pattern type effect called a dark spot, which makes it feel as if the insect eye is following you around. Superposition eyelets have a compound lens system with either two lenses or a lens and mirror pair.
I thought I'd set out some of the interesting facts about animal eyes that I did not know:
1) Animals in the ocean do not use a cornea at all: its just a flap to protect the eye. In addition, sea animals have spherical lenses with variable refractive indices. In contrast, land animals have non-spherical lenses and almost half of our vision comes from the cornea: its our second lens. Amphibians and other creatures that cross the air-water boundary have to somehow "switch" between these two, by either having "goggles" (transparent eyelids that change the eye optics) or by having four eyes.
2) Some sea animals have two or more retinas in the same eye. These look in different directions. The light is shifted around by having mirrors inside the eye!
3) Light scatters so much in the deep sea that certain wavelengths (like infra red) are absent. However, there are some deep sea fish whose eyes still maintain infra-red sensitivity. They also can generate infra-red beacons: so they use infra-red light as a secret communication channel (since there is no other source of this light at that depth).
4) Many nocturnal animals reduce their iris substantially to protect the eye in daytime. Some animals without an iris actually have the retinal cells "migrate" or shift around during the day. There are other animals whose contracted eye resembles templates: for example the contracted chameleon iris is a row of four pinholes: that's a light-field camera!
5) Some birds and reptiles have oil droplets on their retina that allow them to detect color. Its also well-known that many insects and birds are sensitive to polarization and UV light.
6) Birds of prey (raptors) have "pits" in their fovea that allow for an telephoto like effect and gives them extra acuity. This explains their good eyesight from high above.
7) Jumping spiders have a camera array for eyes. Spookfish use mirrors.
8) The infrastructure of retinal tissue (like nerves and blood vessels and so on) is fairly transparent. For example, in land animals (like humans) the nerves that connect to rod and cone cells attach to them in front of the retina. This means that they get in the way, between the scene and the photodetectors. Octopus and squid have the nerves that connect to the retina in the back. A comparison is here.
9) Scallops have mirrors under the retina of their many, many eyes (pic). The retina is transparent and basically the mirror focuses light back into the transparent mush of the retina, and then the image gets detected. Without the mirror, scallops cannot see a focused image. However, many other animals simply have mirrors under the retina, that "resend" light back out from the direction it came from. Since the light retraces its path, it intersects the retina again, allowing the eye a second chance to image the scene point. This enables night vision and is called tapetum. Mirrors in animals are not metallic, but instead use a quarter-wavelength thick film (a thin film) that reflects light through constructive interference of two reflected rays at the medium boundaries.
10) The cell distribution in animal retina is not uniform (for example, fovea in humans). But for some animals, the retinal cells are so non-uniform that they show up as bands in the retina. These bands are associated with regions of importance: for example a "horizon" band for insects that fly over water and need to locate the horizon to navigate. Some animals have "expensive" cells like color cells in a band, and so they need to constantly move the eye around to scan the scene and get a full color image (eg: mantis shrimp).
11) Compound eyes are amazing, and they consist of many ommatidia or eyelets. There are two types of compound eyes which look almost identical from the outside. Apposition eyes are those where the eyelet FOV do not overlap, and each eyelet creates an inverted image of the scene. Superposition eyes have heavy overlap of the same scene point from multiple eyelets and all the eyelets together create one erect image of the scene. Apposition eyes are common in insects whereas superposition eyes are found in lobsters and similar creatures. Many apposition eyes have templates in front of each eyelet, and this creates a moire pattern type effect called a dark spot, which makes it feel as if the insect eye is following you around. Superposition eyelets have a compound lens system with either two lenses or a lens and mirror pair.
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