photobiology Photobiology is the scientific study of the beneficial and harmful interactions of light (technically, non-ionizing radiation) in living organisms. The field includes the study of photophysics, photochemistry, photosynthesis, photomorphogenesis, ...

, the einstein (symbol E) is defined as one mole of

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...

s.Incoll, L. D., S. P. Long, and M. A. Ashmore. 1981. "SI units in publications in plant science". Commentaries in Plant Science. 2: pp. 83–96. As such,

photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more o ...

(PAR) may be reported in microeinsteins per second per square meter (μE⋅m⁻²⋅s⁻¹). The einstein is not part of the

International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

(SI) and is redundant with the mole. Since the unit does not have a standard definition and is not part of the SI system, it is usually better to avoid its use. The same information about photosynthetically active radiation can be conveyed using the SI convention by stating something such as "The photon flux was 1500 μmol⋅m⁻²⋅s⁻¹". The einstein may have been originally defined as the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

in one mole of photons.Albrecht Folsing, Albert Einstein: a biography. p. 299. New York. 1997. Viking Since the energy of a photon is proportional to its

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

, this definition would make this ill-defined as a unit of energy. This unit was named after physicist

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...

References

Photochemistry Units of amount of substance {{electromagnetism-stub