A logarithmic measure of sound loudness closely related to the decibel. The unit decibel is used for objective measurements that mean, they measure the actual pressure of the sound waves as recorded using a microphone. The unit phon is used for subjective measurements, which means, measurements made using the ears of a human listener.
A sound has the loudness 'p' phon if it seems to the listener to be equal in loudness to the sound of a pure tone of the frequency 1 kilohertz and strength 'p' decibel. A measurement in phons will be similar to a measurement in decibels, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness x sones has loudness 10 log2 x + 40 phons.

Photo Peak is short for photoelectric absorption peak, the maximum energy measured in a gamma ray absorption spectrum.

See Photoelectric Effect.

The photoelectric effect describes the following interaction of electromagnetic radiation with a metallic surface: a photon with an energy (frequency) above the binding energy of an electron gets absorbed and the electron is emitted. The positive energy difference is transferred to the electrons kinetic energy. If the photons energy is not high enough for the electron to overcome its binding forces, the photon will be re-emitted. It is not the intensity of a photon beam (amount of photons) which allows the photoelectric effect; it is the energy (frequency) of a single photon which will allow the emission of a single photoelectron.
The discovery and study of the photoelectric effect leads to a new quantized understanding in physics. Albert Einstein was awarded the Noble prize for physics in 1921 'for his services to theoretical physics and especially for his discovery of the law of the photoelectric effect'.
The photoelectric effect is the most important effect in medical radiography. E.g. it is photoelectric absorption that is responsible for most of the absorption in a mammogram which creates the contrast in the image.

See also Photon, Electron.

The electron emitted by the photoelectric effect.

See Photoelectric Effect.