Optical Components, Techniques, and Systems in Engineering

Rajpal S. Sirohi, Mahendra P. Kothiyal

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351426886
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Optics & Light

Meeting the needs of experienced professionals and newcomers to the field, this volume presents major optical measurement procedures-including new work from the authors' laboratory-and offers critical background on optical components and systems, giving access to essential information on modern optics gathered from a range of literature sources. This resource fully describes the capabilities and applications of semiconductor laser diodes used in fiber optics communications and sensors... refracting, diffracting, reflecting, thin-film, and polarization elements... optical metrology... Fourier transform processing, image subtraction, and other operations... coherent and incoher­ent fiber optic sensors... and more. This self-study aid also supplies explanatory illustrations, display equations, simplified discussions, and briefly reviews optical surface sensing, surface evaluation, ellipsometry, and laser anemometry. This guide works as a vital reference for optical, laser, photo-optical, electrical, mechani­cal, and industrial engineers; optical physicists; photonic scientists; metrologists; and holographers; and serves as a useful text for graduate students in applied optics training programs or courses.

CHAPTER 1

Light Sources, Detectors, and Recording Media

1.1 LIGHT SOURCES

Light is that portion of electromagnetic spectrum that is responsible for the sense of seeing. Light can be produced by suitable conversion of other forms of energy. Nuclear reactions at the core of the sun and the other stars produce an almost inexhaustible supply of light energy. Artificially, light is produced by heating, electrical discharge, electrical current injection, etc. Since our visual experiences are due to light, and since also many physical processes can be understood by the light generated, the measurement or quantification of light is essential [1, 2, 3].

It may be mentioned that radiation sources as standards of radiance and of wavelength are used for calibration. Some applications require pointlike sources of high irradiance, while others may require high irradiance sources regardless of size. We shall discuss only a few different types of sources.

1.1.1 Black Body Sources

The energy density ρ(ν)per unit frequency range from a blackbody is given by

$\text{ρ(v) =}\frac{{\text{8πv}}^{\text{2}}}{{\text{c}}^{\text{3}}}\frac{\text{hv}}{{\text{e}}^{\text{hv/kT}}-\text{1}}$

(1.1)

The energy density does not depend on the size or material of the body but only on its temperature. The total energy over the whole frequency range is

$\text{W =}{\displaystyle {\int }_{\text{0}}^{\text{x}}\text{ρ}\left(\text{v}\right)}{\text{dv = σ T}}^{\text{4}}\frac{\text{W}}{{\text{m}}^{\text{2}}}$

(1.2)

where σ is the Stefan’s constant.

Blackbody radiators are only idealizations. They can be approximated by cavity radiators. A cavity radiator consisting of a graphite rod with a coaxial cylindrical hole having a large length-to-diameter ratio and walls having an isothermal temperature profile is a suitable radiation source up to temperatures of 3000K. The graphite wall is resistance-heated and temperature uniformity is achieved by a variable heat source distribution obtained by varying the cavity wall thickness. The rod is radially surrounded by radiation shields. Insulation by radiation shield is very effective. The graphite radiator is to be operated either in a vacuum or in an inert gas atmosphere. In the temperature range of 1200 to 2200 K, it can be heated in vacuum. For higher temperatures (>2200K), the graphite is heated in argon gas atmosphere.

Calibration of radiation detectors, particularly radiation pyrometers, is done with blackbody radiation. Calibration of halogen lamps and of ultraviolet UV lamps used for cosmetic surgery and pharmaceutical research is also carried out with blackbody radiation. Moreover, blackbody radiation can be used as a standard for the transmittance and reflectance measurements of optical elements such as windows, filters, prisms, mirrors, etc.

1.1.2 Filament Lamps

Filament lamps come in a wide variety of shapes, sizes, and power ratings. They are both vacuum and gas filled and are used in a large number of applications. The lamps used as intensity standards are carefully manufactured; the filament plane is well defined, and the base is accordingly fitted. Further, the filament is aged. Other applications may also require planar filaments, but the requirements are not so stringent. For example, the projector lamps usually have flat filaments; normally, halogen lamps are used. Halogen lamps can be operated at higher color temperatures, because the halogen inside the envelope prevents oxidation of tungsten filament.

1.1.3 Arc Lamps

The earliest arc lamp was the carbon arc, an open arc; it was and is widely used for its high radiation and color temperatures ranging from 3600 K to 6500 K. It contains two electrodes in which the arc is maintained. The electrodes are moved toward each other to compensate for the rate of consumption of the material. The rate at which material is consumed (5 to 30 cm/hr) depends on the intensity of the arc. The anode forms a crater of decomposing material that provides a center of very high luminosity. Some electrodes are hollowed out and filled with a softer carbon material to help keep the arc fixed in the anode. The carbon arc is used in three forms: the low intensity arc, the flame arc, and the high intensity arc. The low and high intensity arcs are generally operated on DC; the flame type adapts to either DC or AC. In all cases a ballast must be used.

1.1.4 Enclosed Arcs

The carbon arc is not very efficient and has a short life; and moreover undesirable combustion products during operation are produced. Therefore enclosed arcs, particularly mercury arcs, are preferred. A coiled tungsten cathode is usually coated with thorium, and an auxiliary electrode is used for starting. A high resistance limits the starting current. Once the arc is started, the operating current is limited by the ballast supplied by the high reactance of the power transformer.

Multivapor arcs are also used; they contain other materials besides argon and mercury. Also available are compact arc lamps. Extreme electrical loading of the arc gap results in very high luminance. The lamps have internal pressures of a few atmospheres, and bulb temperatures go as high as 900°C.

1.1.5 Concentrated Arc Lamps

Zirconium arc lamps use cathodes made of a hollow refractory material containing zirconium oxide. The anode, a disk of metal with an aperture, resides directly above the cathode with the normal to the aperture coincident with the longitudinal axis of the cathode. Argon gas fills the tube. The arc discharge causes the zirconium to heat to about 3000K and produce an intense, very small source of light.

The tungsten arc lamp is also used as an intense and small source of light. It contains a ring electrode and a pellet electrode, both made of tungsten. The arc forms between these electrodes and causes the heating of the pellet to incandescence. The ring also incandesces but to a lesser extent.

1.1.6 Discharge Lamps

⁸⁶Kr lamp: One discharge lamp that remained as a standard of wavelength until 1983 is the ⁸⁶Kr lamp. It consists of a capillary with an internal diameter of 2 to 4 mm and a wall thickness of about 1 mm; it is filled with⁸⁶Kr and run at the triple point of nitrogen. The recommended current density is 0.3± 0.1 A/cm². The wavelength of the radiation arises from the transitions between the 2p₁₀ and 5d₆ levels of the isotope of krypton. The vacuum wavelength is given as λ_vac = 6057.80210 Å. The meter is then defined as 1,650,763.73 wavelengths.

Low-pressure lamps containing cadmium, mercury, thallium, sodium, etc. are used as spectral lamps in many spectroscopic and measuring instruments.

There are other kinds of sources that are used for certain applications. For example, UV lamps are used for cosmetic surgery and photodynamic or photodissociative work in many industries. Similarly, infrared lamps may be used for other curative applications.

1.2 LASER SOURCES

The laser [4,5,6,7] is a source of coherent radiation. It consists of an active medium in a cavity. The active medium may be solid, liquid or gaseous. The cavity is an open resonator bounded by two mirrors, one of which is partially transmitting. When the active medium is excited, the atoms or molecules are raised to higher energy levels. They relax to the ground state by spontaneous emission. It was shown in 1917 by Einstein that there is another process of relaxation called the stimulated emission. If the population of higher energy level is raised to the extent that it is more than that of the lower level, a population inversion is created between these levels. A spontaneously emitted photon will cause the excited atoms to relax and release photons in phase. These photons bounce back and forth in the resonator, and the wave grows; a part of the wave ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• About the Series
• Preface
• 1. Light Sources, Detectors, and Recording Media
• 2. Optical Components
• 3. Basic Optical Systems
• 4. Length Measurement Techniques
• 5. Alignment and Angle Measurement Techniques
• 6. Heterodyne and Phase Shifting Interferometry
• 7. Hologram Interferometry and Speckle Metrology
• 8. Optical Data Processing
• 9. Photoelasticity
• 10. Fiber-Optic Sensors
• 11. Miscellaneous Techniques
• Index