Introduction
The term camera was coined on “camera obscura”; a Latin term meaning “dark chamber”. The concept of camera was initiated by the concept of dark chamber where a small hole in the wall acted as a shutter and whole room as a real imaging system. The camera obscura was a device consisting of a darkened chamber or box into which light was permitted through a lens. A focused real image was formed on a screen or wall based on the focus of lens. Camera obscura was first invented and studied by scientist Ibn al Haythem in 1015 AD.
The concept of camera evolved with the time and it became portable. The black and white and colored imaging was invented as soon as in 1850.
Working
The camera has several important components. The lens to capture light and image, a shutter to permit appropriate light into camera and the photographic film to form the real image are foremost components.
Lens and Focus: Lens is a curved surface made of glass or plastic. Lens may be convex or concave, depending on its curved surface. The function of the lens is to collect the light rays and focus them to form an image. Light rays falling on lens are focused on the other side and form a real inverted image. The size and quality of this image depends on focal length and aperture of the lens. The focal length is the distance between the lens and its real image. Different cameras have different lenses for various magnifications. This magnifying power depends on the focal length of the lens. Higher focal length means higher magnification. Two types of cameras are commonly used. A single-lens reflex camera uses a movement of mirror and objective lens to focus the image. While a twin-lens reflex camera uses a focusing and objective lens unit in a parallel body.
As the inverted image is formed with a single lens, the image is not very clear and it undergoes “aberration”, a phenomenon whereby the various colors of light are focused differently on the screen. This produces a blurry and a vague image. To handle this, cameras use a number of lenses. There are several lenses of different materials joined to form a single structure of lens such that its focal length, magnification and focus are well aligned. Modern cameras use several lenses for different situations as well; such as lens to capture image during motion, night, in mountains, snow etc.
Shutter: The job of shutter is to limit the amount and time of passage of light through the lens. This is vital to define the appropriate light into the camera as the image may be blurry in case excess light is permitted.
Photographic film: The most important part is the photographic film inside the camera. The photo film is made of a light-sensitive material. As soon it is exposed to light, the light sensitive particles on the surface are transformed. For this very reason, the film is never exposed to light and it is kept in complete darkness except when taking a photograph, where shutter permits the light as appropriate. When light rays fall on the photo film, the particles undergo a chemical reaction that defines a pattern based on the size and shape of the object. This forms a real image on the film. Colors of the object are captured by using three colored layers of photo film.
Developing the Photo: Once image is captured, the photo film is taken out of the camera in a dark room containing only red light of a wavelength not captured by the film. It is exposed to other chemicals that imprint the real image on another material.
Radiography
Radiography is an imaging technology that uses light rays of specific wavelength called X-Rays. X-Ray is form of electromagnetic radiation that has wavelength from 10 to 0.001 nanometers. Due to its short wavelength, X-Ray has its prime utilization in imaging hard to focus and inside-surfaces objects.
Sources: X-Rays are produced from a radioactive element, X-Ray tube or specific high energy X-Ray equipment. Physicists commonly make use of X-Ray production equipment for the purpose. Natural radioactive and artificial substances that spontaneously produced X-Rays were used to generate X-Rays before invention of computers. Radioactive elements emit X-Rays indefinitely and cannot be controlled. Artificially, X-Rays are produced by X-Ray equipment that makes use of a setup that evolves electrons along with alpha and gamma rays. Cathodes are used to accelerate electrons produced by a high energy filament. These electrons at high velocity collide with the metal plate called anode, and this collision produces X-Rays.
Applications: X-Rays have very short wavelength hence cannot be detected by human eye, but they can make visible those places that cannot be seen by naked eye. The foremost diagnostic utilization of X-Rays is capturing the images of inner body organs and bones. The medical application of radiography is called Radiology. A bone fracture or a presence of damaged tissues is most commonly detected by high intensity X-Rays. Lately the field of radiology has gained immense use in detecting very tiny and hard to locate tumors, cancers and other cavities and crevices in bones. The concept of image capturing inside body is based on reflection or passage of X-Rays by different tissues based on their density. The body is placed between the X-Rays equipment and the capturing plate or screen. The X-Rays are absorbed or reflected by the high density areas such as bones and these places are shown as dark places in the image. For example, to take image of the bones the X-Rays are shot through the body with the photographic film behind. The bones are electron-dense hence they absorb the electrons in X-Rays through photoelectric process. The low density areas such as veins and flesh let the rays pass through they appear as white places. Based on the image, the radiologist can detect any fracture, crevice or damaged tissue. Although immensely used by doctors, physicists and radiologists, the harmful effects of X-Rays have been in debate over the years and doctors are turning to safer and more reliable sources such as Ultrasound and Image Resonance.
Other important use of X-Ray is in industries and security equipments. An airport security passage is most common example. Light intensity X-Rays are used to detect things inside the luggage. In industries, X-Rays are used for inspection of density, weight and any cracks inside the materials. Another important and immense use is in crystallography. The X-Rays are used to inspect the molecular or atomic structure of the substances. X-Rays are projected on a metal and they are reflected or diverted in different angles based on which a crystallographer can recognize and draw an atomic structure of the metal. The crystallography is widely utilized in medicine, nuclear power technology, mineralogy, metallurgy and scientific research. A scientist can estimate atomic size, ratio and structure of the metal.
Detection: The detection of X-Rays is made by photographic plates, films, computers, photographic film, plate, and scintillator or semiconductor diode. High energy radiography equipment in medical field makes use of high speed computers.
Bibliography
D. Falk, D. Brill and D. Stork Seeing the Light.John Wiley, New York, 1986).
Nicholas J. Wade, Stanley Finger (2001), “The eye as an optical instrument: from camera obscura to Helmholtz’s perspective”, Perception 30 (10), p. 1157 – 1177.
Charles Hodgman, Ed. (1961). CRC Handbook of Chemistry and Physics, 44th Ed.. USA: Chemical Rubber Co.. pp. p.2850.
David R. Lide, ed.. CRC Handbook of Chemistry and Physics 75th edition, CRC Press. pp. 10–227.