Thermal Emission and Temperature Measurement

Thermal Emission

• Thermal emission is a process where excited electrons release photons as they drop back to the ground state.
• A continuous spectrum of radiation is emitted with a minimum wavelength, and the intensity distribution depends on the temperature.
• Higher temperatures result in a wider range of emitted wavelengths.
• By measuring the intensity of a narrow band of emitted wavelengths with a pyrometer, a material's temperature can be estimated.

Photoconductivity

• Photoconductivity occurs when semiconductors are bombarded by photons with energy equal to or greater than the band gap, creating electron-hole pairs.
• This process generates current, and the current produced is directly related to the incident light intensity.
• Photoconductivity is utilized in photographic light meters, and cadmium sulfide (CdS) is commonly used for detecting visible light.
• Photoconductivity is also the underlying principle of the photovoltaic cell, or solar cell, used for converting solar energy into electricity.

Optical Fibers

• Optical fibers consist of a core, cladding, and coating, with the core transmitting signals and the cladding constraining the light beam to the core.
• The outer coating protects the core and cladding from the external environment, and all reflection losses should be considered.
• Optical fibers have revolutionized the communication industry.

Luminescence

• Luminescence is the process where a material absorbs energy and then immediately emits visible or near-visible radiation.
• It involves electron excitation and then dropping down to lower energy states.
• If the emission of radiation occurs within 10^(-8) seconds after excitation, the luminescence is called fluorescence, and if it takes longer, it is known as phosphorescence.
• Special materials called phosphors have the capability of absorbing high-energy radiation and spontaneously emitting lower-energy radiation.
• The intensity of luminescence is given by the formula: I = I0 * e^(-t/τ), where I0 is the initial intensity of luminescence, I is the fraction of luminescence after time t, and τ is the relaxation time constant for a material.

Types of Optical Fibers

• In step-index optical fibers, there is a sharp change in refractive index between the core and cladding, resulting in pulse broadening.
• Graded-index fibers avoid pulse broadening by adding impurities such as boron oxide (B2O3) or germanium dioxide (GeO2) to the silica glass, varying the index of refraction in a parabolic manner across the cross-section.
• Both step- and graded-index fibers are termed as multi-mode fibers.