SI base unit units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the "metre" (US English: "meter") has the symbol m, but the "kelvin" has symbol K, because it is named after Lord Kelvin and the "ampere" with symbol A is named after André-Marie Ampère. Several other units, such as the litre (US English: "liter"), are formally not part of the SI, but are accepted for use with SI. New definitions of the base units were approved on 16 November 2018, to take effect 20 May 2019. The
Einstein (unit) Einstein (unit) The einstein is a unit defined as one mole () of photons. As such, photosynthetically active radiation (PAR) was formerly often reported in microeinsteins per second per square meter (μE m s). This usage is also not part of the International System of Units and when used this way it 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