Holography

The use of a lens to image an object is one of the oldest principles in optics and photography. However, in 1948 Gabor introduced a two-step imaging process in which an intermediate record, containing the information necessary to create an image, is formed.

The 2-step imaging process consists of first photographing the interference pattern which exists when a diffracted or object field is allowed to interfere with a reference field or background wave (usually a plane or spherical wave) on or off axis with respect to the diffracted field. The 1-st step is called the formation or recording, The 2-nd step, called the reconstruction, consists of placing the photographic transparency (film) into a coherent beam of light and producing an image of the original object.

The resulting record of the interference pattern obtained in the 1-st step is called a hologram after the Greek word "holos", which means "the whole", because the photographic record contains information concerning both the amplitude and phase of the original object.

The hologram bears little resemblance to the object, and upon visual observation contains a seemingly meaningless combination of fringes or diffraction patterns. The image of the object which is obtained from a hologram is referred to as the reconstructed image, and there are two types. A real image is one that appears on the opposite side of the hologram from the source and has the property that no auxiliary focusing devices are needed to record a focused image. A virtual image is one that appears on the same side of the hologram as the source and has the property that an additional focusing device is needed to detect a focused image.

The entire process is known as the hologram process and the wave-front reconstruction process, and also the science of holography.

background wave – опорная волна

coherent beam of light – когерентный пучок света

diffracted or object field – дифрагированное поле или поле предметной волны

fringes or diffracted pattern – полосы или дифракционная картина

interference pattern – интерферограмма, интерференционная картина

intermediate record – промежуточная запись

photographic transparency – фотографическая пленка

plane or spherical wave – плоская или сферическая волна

reference field – опорное поле

two step imaging process – двухшаговый процесс формирования изображения

wave-front reconstruction process – процесс восстановления волнового фронта

Exercise 1. Retell the text, find out three main ideas.

Exercise 2. Read and translate two definitions of the word “hologram”.

1. Hologram is a pattern produced by the interference between one part of a split beam of coherent light, e.g. from a laser, and the other part of the same beam reflected off an object.

2. Hologram is a photographic reproduction of a pattern that when suitably illuminated produces a three-dimensional picture.

TASK 3

Read the text paying attention to the specifications of the optical instruments and the ways they are used.

Optical Instrument

A . The night vision sight NP-75 is designed for aimed hunting gun firing in the twilight and at night. Intensification of luminous flux from the observed scene is taken place by an electrooptical transducer. Luminous laying mark in the field of view of the sight is use for a target aiming. Smooth adjustment of the mark’s brightness allows aiming on targets with different illumination.

Generation: I

Magnification, times: 2

Field of view, degrees: 12

Range of target detection, m: 400

Dimensions, mm: 256х72х76

Mass, kg: 0.95

B . ON 1х20 night vision goggles is an optronic device, intended for visual observation of objects in dark time of the day, to orient oneself in a given area, or on water surface in conditions of natural night illumination.

The device is provided with refocusing of the objectives to observe both remote and nearby objects.

Generation: I

Magnification, times: 1

Field of view, degrees: 40

I

Caution: While test-operating the

goggles in the day time or under

condition of artificial illumination,

NEVER remove protective cover

from the objective lens!

Dimensions, mm: 180х120х170

Mass, kg: 1