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## Optical sensors and actuators

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The human eye, like that of other vertebrates, is a marvelous, complex sensor allowing us to perceive the world around us in minute detail and true colors. In fact, the eye is akin to a video camera. It consists of a system of lenses (the cornea and crystalline lens), an aperture (iris and pupil), an image plane (retina), and a lens cover (eye lids). In humans and animals of prey, the eyes point forward to create binocular vision with excellent depth perception. Many prey animals have side -facing eyes to increase their fi eld of view, but the vision is monocular and lacks perception of depth. The eyelids, in addition to protecting the eye, also keep it clean and moist by distributing tears as well as lubricants (the conjunctiva) and protect it from dust and foreign objects in conjunction with the eyelashes. The front dome of the eye is made of the cornea, a clear, fi xed lens. This is a unique organ, as it has no blood vessels and is nourished by tears and the fl uid inside the eye sphere. Behind it is the iris, which controls the amount of light that enters the eye. On the periphery of the iris, there is a series of slits that allow fluid to pass out from the eye sphere. This passes nutrients to the front of the eye and relieves the pressure in the eye (when this is not perfectly regulated one has glaucoma, a condition that can affect the retina and eventually can cause blindness). Behind is the crystalline lens, an adjustable lens, controlled by the ciliary muscle that allows the eye to focus on objects as close as about 10 cm and as far as infinity. When the ciliary muscle loses some function, the ability of the lens to focus is impaired, leading to the need for corrective action (glasses or surgery). The lens itself can cloud over time (cataracts), a condition that requires replacement of the lens. At the back of the eye lies the optical sensor proper -the retina. It is made of two types of cells: cone cells that perceive color and rod or cylindrical cells that are responsible for low -light (night) vision. The cone cells are divided into three types, sensitive to red, green, and blue light, with a total of about 6 million cells, most of them in the center of the retina (the macula). Rod cells are distributed mostly on the peripheral parts of the retina and are responsible for low -light vision. They do not perceive color but are as much as 500 times more sensitive than cone cells. There are also many more rod cells than cone cells -as many as 120 million of them. The retina is connected to the visual cortex in the brain through the optical nerve. Although the lens of the eye is adjustable, the size of the optical ball also plays a role in vision. Individuals with larger eyeballs are nearsighted, those with smaller eyeballs are farsighted.

Chapter Contents:

• 4.1 Introduction
• 4.2 Optical units
• 4.3 Materials
• 4.4 Effects of optical radiation
• 4.4.1 Thermal effects
• 4.4.2 Quantum effects
• 4.4.2.1 The photoelectric effect
• 4.4.2.2 Quantum effects: the photoconducting effect
• 4.4.2.3 Spectral sensitivity
• 4.4.2.4 Tunneling effect
• 4.5 Quantum-based optical sensors
• 4.5.1 Photoconducting sensors
• 4.5.2 Photodiodes
• 4.5.3 Photovoltaic diodes
• 4.5.4 Phototransistors
• 4.6 Photoelectric sensors
• 4.6.1 The photoelectric sensor
• 4.6.2 Photomultipliers
• 4.7 Charge coupled (CCD) sensors and detectors
• 4.8 Thermal-based optical sensors
• 4.8.1 Passive IR sensors
• 4.8.1.1 Thermopile PIR
• 4.8.1.2 Pyroelectric sensors
• 4.8.1.3 Bolometers
• 4.9 Active far infrared (AFIR) sensors
• 4.10 Optical actuators
• 4.11 Problems

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