Eye

Types and Usage

Eyes are organs of the visual system. They provide organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision.

There are different eye layouts—indeed every technological method of capturing an optical image commonly used by human beings, with the exceptions of zoom and Fresnel lenses, occur in nature.

Non-compound eyes:

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa

1. Pit eyes: Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light.

2. Spherical lensed eye: The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable.

3. Multiple lenses: Some marine organisms bear more than one lens; for instance the copepod Pontella has three. 

4. Refractive cornea: In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. In general, the lens is not spherical.

5. Reflector eyes: An alternative to a lens is to line the inside of the eye with “mirrors”, and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.

Compound eyes:

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. Compound eyes are common in arthropods, and are also present in annelids and some bivalve molluscs.

1. Apposition eyes: Apposition eyes are the most common form of eyes, and are presumably the ancestral form of compound eyes. They are found in all arthropod groups, although they may have evolved more than once within this phylum.

2. Superposition eyes: The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. 

3. Parabolic superposition: This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.