Optical and Infrared

"Optical" redirects here. For the musical artist, see Optical (artist).

Optics (appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. Optics explains and is illuminated by optical phenomena.

The field of optics usually describes the behavior of visible, infrared and ultraviolet light; however because light is an electromagnetic wave, analogous phenomena occur in X-rays, microwaves, radio waves, and other forms of electromagnetic radiation. Optics can thus be regarded as a sub-field of electromagnetism. Some optical phenomena depend on the quantum nature of light and as such some areas of optics are also related to quantum mechanics. In practice, the vast majority of optical phenomena can be accounted for using the electromagnetic description of light, as described by Maxwell's Equations.

Optics, however, as a field is often considered largely separate from the physics community. It has its own identity, societies, and conferences. The pure science aspects of the field are often called optical science or optical physics. Applied optical sciences are often called optical engineering. Applications of optical enginering related specifically to illumination systems are called illumination engineering. Each of these disciplines tends to be quite different in its applications, technical skills, focus, and professional affiliations.

Because of the wide application of the science of "light" to real-world applications, the areas of optical science and optical engineering tend to be very cross-disiplinary. Optical science is a part of many related disciplines including electrical engineering, physics, psychology, medicine, and others. Additionally, the most complete description of optical behavior, as known to physics, is unnecessarily complicated for most scenarios so particular simplified theories are used. These limited theories adequately describe subsets of optical phenomena while ignoring behavior irrelevant and/or undetectable to the system of interest.

Classical optics

Before Max Planck suggested that light is quantized, optics consisted mainly of the application of electromagnetism and its high frequency approximations to light. Classical optics divides into two main branches: geometric optics and physical optics.

Geometric optics, or ray optics, describes light propagation in terms of "rays". Rays are bent at the interface between two dissimilar media, and may be curved in a medium in which the refractive index is a function of position. The "ray" in geometric optics is an abstract object which is perpendicular to the wavefronts of the actual optical waves. Geometric optics provides rules for propagating these rays through an optical system, which indicates how the actual wavefront will propagate. Note that this is a significant simplification of optics, and fails to account for many important optical effects such as diffraction and polarization.

Geometric optics is often simplified even further by making the paraxial approximation. The mathematical behavior then becomes linear, allowing optical components and systems to be described by simple matrices. This leads to the techniques of Gaussian optics and paraxial raytracing, which are used to find first-order properties of optical systems, such as approximate image and object positions and magnifications.

Gaussian beam propagation is an expansion of paraxial optics that provides a more accurate model of coherent radiation like laser beams. While still using the paraxial approximation, this technique partially accounts for diffraction, allowing accurate calculations of the rate at which a laser beam expands with distance, and the minimum size to which the beam can be focused. Gaussian beam propagation thus bridges the gap between geometric and physical optics.

Physical optics models the propagation of complex wavefronts through optical systems, including both the amplitude and the phase of the wave. This technique, which is usually applied numerically on a computer, can account for diffraction, interference, and polarization effects, as well as aberrations and other complex effects. Approximations are still generally used, however, so this is not a full electromagnetic wave theory model of the propagation of light. Such a full model would (at present) be too computationally demanding to be useful for most problems, although some small-scale problems can be analyzed using complete wave models.

Topics related to classical optics

• Coherence
• Diffraction
• Dispersion
• Distortion
• Fabrication and testing (optical components)
• Fermat's principle
• Fourier optics
• Gradient index optics
• Optical lens design
• Optical resolution
• Polarization
• Ray (optics)
• Reflection
• Refraction
• Scattering
• Wave

• Geometric optics of:
  • Lenses
  • Mirrors
  • Optical instruments
  • Prisms

Modern optics

Modern optics encompasses the areas of optical science and engineering that became popular in the 20th century. These areas of optical science typically relate to the electromagnetic or quantum properties of light but do include other topics.

Topics related to modern optics

• Circular dichroism
• Crystal optics
• Diffractive optics
• Guided wave optics
• Holography
• Integrated optics
• Jones calculus
• Lasers
• Micro-optics
• Non-imaging optics
• Nonlinear optics
• Optical modeling and simulation methods
• Optical pattern recognition
• Optical processors
• Photometry
• Quantum optics
• Radiometry
• Statistical optics
• Thin-film optics

Other optical fields

• Color science
• Illumination engineering
• Image processing
• Information theory
• Linear optics
• Machine vision
• Materials science - optical properties
• Optical communication
• Optical computers
• Optical data storage
• Optical display system
• Pattern recognition
• Photography (science of)
• Thermal physics - radiative heat transfer
• Visual system

Everyday optics

Optics is part of everyday life. Rainbows and mirages are examples of optical phenomena. Many people benefit from eyeglasses or contact lenses, and optics are used in many consumer goods including cameras.

Wikibooks modules

• Optics (Physics Study Guide)
• Optics

See also

• Important publications in optics
• Transparency (optics)

References

• Hecht, Eugene (2001). Optics (4th ed.), Pearson Education. ISBN 0805385665.
• Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.), Brooks/Cole. ISBN 0534408427.
• Tipler, Paul (2004). Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.), W. H. Freeman. ISBN 0716708108.

External links

• Optics.net - Optical Engineering forum and resource directory
• Optics Book - an online textbook
• Optics & Photonics - SPIE's Optics & Photonics Symposium, held annually, features optics and photonics research, training, and an exhibition.
• Optics2001 - Optics library and community

Image of a small dog taken in mid-infrared ("thermal") light (false color)

Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm.

Different regions in the infrared

IR is often subdivided into:

• near infrared NIR, IR-A DIN, 0.75–1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO₂ glass (silica) medium.
• short wavelength (shortwave) IR SWIR, IR-B DIN, 1.4–3 µm, water absorption increases significantly at 1450 nm
• mid wavelength IR MWIR, IR-C DIN, also intermediate-IR (IIR), 3–8 µm
• long wavelength IR LWIR, IR-C DIN, 8–15 µm)
• far infrared FIR, 15–1000 µm

However, these terms are not precise, and are used differently in various studies i.e. near (0.75–5 µm) / mid (5–30 µm) / long (30–1000 µm). Especially at the telecom-wavelengths the spectrum is further subdivided into individual bands, due to limitations of detectors, amplifiers and sources. Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body).

Plot of atmospheric transmittance in part of the infrared region.

The common nomenclature is justified by the different human response to this radiation (near infrared = the red you just cannot see, far IR = thermal radiation), other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (The common silicon detectors are sensitive to about 1050 nm, while InGaAs sensitivity starts around 950 nm and ends between 1700 and 2200 nm, depending on the specific configuration). Unfortunately the international standards for these specifications are not currently available.

The boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to red light above 700 nm wavelength, but particularly intense light (e.g., from lasers) can be detected up to approximately 780 nm. The onset of infrared is defined (according to different standards) at various values between these two wavelengths, typically at 750 nm.

Telecommunication bands in the infrared

Optical telecommunication in the near infrared is technically often separated to different frequency bands because of availability of light sources, transmitting /absorbing materials (fibers) and detectors.

• O-band 1260–1360 nm
• E-band 1360–1460 nm
• S-band 1460–1530 nm
• C-band 1530–1565 nm
• L-band 1565–1625 nm
• U-band 1625–1675 nm

The Earth as an infrared emitter

The Earth's surface absorbs visible radiation from the sun and re-emits much of the energy as infrared back to the atmosphere. Certain gases in the atmosphere, chiefly water vapor, but also carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons, absorb this infrared, and re-radiate it in all directions including back to Earth. Thus, the greenhouse effect, keeps the atmosphere and surface much warmer than if the infrared absorbers were absent from the atmosphere.

Applications

Night vision

Infrared is used in night-vision equipment, when there is insufficient visible light to see an object. The radiation is detected and turned into an image on a screen, hotter objects showing up brighter, enabling the police and military to acquire thermally significant targets, such as human beings and automobiles. Also see Forward looking infrared.

Smoke is more transparent to infrared than to visible light, so firefighters use infrared imaging equipment when working in smoke-filled areas.

Other imaging

In infrared photography, infrared filters are used to capture only the infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and some camera phones which do not have appropriate filters can "see" infrared, appearing as a bright white colour (try pointing a TV remote at your digital camera). This is especially pronounced when taking pictures of subjects near bright areas (such as near a lamp), where the resulting infrared interference can wash out the image.

Thermography

Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs.

Heating

Infrared radiation is used in Infrared saunas to heat the sauna's occupants and to remove ice from the wings of aircraft (de-icing).

Communications

IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms.

Free space optical communication using infrared lasers can be a relatively inexpensive way to install a Gigabit/s communications link in urban areas, compared to the cost of burying fibre optic cable.

Infrared lasers are used to provide the light for optical fibre communications systems. Infrared light with a wavelength around 1330 nm (best transmission) or 1550 nm (least dispersion) are the best choices for standard silica fibres.

Infrared is the most common way for remote controls to command appliances.

Spectroscopy

Infrared radiation spectroscopy is the study of the composition of (usually) organic compounds, finding out a compound's structure and composition based on the percentage transmittance of IR radiation through a sample. Different frequencies are absorbed by different stretches and bends in the molecular bonds occurring inside the sample. Carbon dioxide, for example, has a strong absorption band at 4.2µm.

History

The discovery of infrared radiation is commonly ascribed to William Herschel, the astronomer, in the early 19th century. Herschel used a prism to refract light from the sun and detected the infrared, beyond the red part of the spectrum, through an increase in the temperature recorded on a thermometer.

Simple infrared sensors were used by British, American and German forces in the Second World War as night vision aids for snipers.

See also

• Night vision
• Infrared astronomy
• Infrared filter
• Infrared photography
• Infrared spectroscopy
• Thermography
• Infrared homing

External links

Journals

• Infrared Physics and Technology (Elsevier) (last access June 2005).

Web sites

• Infrared Spectroscopy NASA Open Spectrum wiki site.
• IrDAOrganization that creates low cost infrared data interconnection standards.