Physical world and mathematics

Photoionization

Photoionization is the process in which an atom or molecule absorbs a photon energetic enough to eject an electron, and in astronomy also describes gas kept ionized by ultraviolet light from hot stars.

Photoluminescence

Photoluminescence is luminescence arising from photoexcitation, where a material absorbs photons and re-emits light as fluorescence or longer-lasting phosphorescence; it is widely used to characterize semiconductors.

Photoluminescence blinking

Photoluminescence blinking, also called photoluminescence intermittency, is the random switching of a single light-emitting particle between bright and dark states, first observed in colloidal quantum dots in 1996.

Photoluminescence detection

Photoluminescence detection is a contactless spectroscopy technique that measures the light a material emits after absorbing photons, used to characterize semiconductor defects, impurities, and band gaps.

Photoluminescence imaging

Photoluminescence imaging is a contactless optical technique that maps luminescence from photoexcited semiconductors to assess defects and carrier lifetime, widely used on silicon solar cells.

Photoluminescence spectroscopy

Photoluminescence spectroscopy measures the light a material emits after absorbing photons, using that emission to characterize electronic structure, defects, and optical quality, with time-resolved variants reporting carrier recombination.

Photolysis

Photolysis is the cleavage of covalent bonds in a molecule caused by light absorption, a branch of photochemistry with major roles in stratospheric ozone formation.

Photometer

A photometer is an instrument that measures light in units weighted to the sensitivity of the human eye, such as lux and candela, making it the reference instrument for evaluating light sources and displays.

Photometric analysis

Photometric analysis is the measurement of the brightness of astronomical objects, such as stars, supernovae, and galaxies, from images, together with how that brightness changes over time.

Photometric calibration

Photometric calibration is the procedure that converts raw instrumental brightness measurements into magnitudes on a standard photometric system, correcting for the instrument's response, atmospheric extinction, and standard reference stars.

Photometric parallax

Photometric parallax is a method of estimating a star's distance from its colors and apparent magnitude rather than geometric measurement, valuable for faint, distant stars.

Photometric redshift

A photometric redshift is an estimate of a galaxy's redshift and distance from its brightness in several broadband filters, cheaper to obtain than spectroscopy but less precise.

Photometric system (astronomy)

A photometric system in astronomy is a defined set of wavelength bands, standard stars, and zero points for measuring celestial brightness on a common magnitude scale.

Photometry (astronomy)

Photometry in astronomy is the measurement of light energy received per unit time from a celestial source, expressed on a logarithmic magnitude scale attributed to Hipparchus.

Photomultiplier tube

The photomultiplier tube (PMT) is a vacuum electron device that detects light down to a single photon, invented in 1934 at RCA and widely used in medical imaging and physics.

Photon

A photon is the elementary particle of light and the electromagnetic field, massless and traveling at the speed of light, carrying energy proportional to its frequency.

Photon antibunching

Photon antibunching is a nonclassical property of light where photons arrive more equally spaced than in a laser field, first observed by Kimble, Dagenais, and Mandel in 1977.

Photon blockade

Photon blockade is a quantum-optical effect in which one photon in a cavity blocks a second from entering, converting light into an antibunched stream usable for single-photon sources.

Photon Bose-Einstein condensate

A photon Bose-Einstein condensate (photon BEC) is a gas of trapped light that condenses into a single cavity mode, first observed at Bonn in 2010.

Photon counting

Photon counting is a measurement technique that registers individual photons as discrete pulses, outperforming analog measurement at very low light levels, with detectors including photomultipliers, SPADs, and SNSPDs.

Photon cross correlation spectroscopy

Photon cross correlation spectroscopy (PCCS) is an optical particle-sizing method that cross-correlates light scattered from two laser beams, filtering out multiple scattering in turbid suspensions.

Photon diffusion in biological tissue

Photon diffusion in biological tissue describes near-infrared light propagating through highly scattering tissue as a random-walk process, enabling non-invasive measurements of hemoglobin concentration, oxygen saturation, and blood flow.

Photon echo

A photon echo is a delayed, coherent pulse of light emitted by an ensemble of atoms or ions after optical pulses rephase their dipoles, first observed in ruby in 1964.

Photon energy

Photon energy is the energy of a single photon, given by E = hf, where h is the Planck constant and f is the photon's frequency.

Photon Factory

The Photon Factory is a synchrotron radiation facility at KEK in Tsukuba, Japan, producing intense X-ray and vacuum ultraviolet light for research; completed in 1982, it was Japan's first dedicated X-ray light source.

Photon gas

A photon gas is electromagnetic radiation confined in a cavity and treated as an ideal gas of noninteracting photons, whose thermodynamics yields the Planck distribution of black-body radiation.

Photon polarization

Photon polarization is the quantum-mechanical two-state, or qubit, description of a single photon, the analog of the classical polarization of a light wave's electric field.

Photon recycling in halide perovskites

Photon recycling in halide perovskites is the reabsorption and re-emission of emitted photons within the material, which raises open-circuit voltage in solar cells and boosts light emission in LEDs.

Photon recycling in lead halide perovskites

Photon recycling in lead halide perovskites is the repeated reabsorption and re-emission of emitted photons within a film, converting them back to electron-hole pairs and lengthening effective carrier lifetimes.