Louis Harold Gray FRS (10 November 1905 – 9 July 1965) was an English physicist who worked mainly on the effects of radiation on biological systems.

He was one of the earliest contributors of the field of radiobiology.

[6] Amongst many other achievements, he defined a unit of radiation dosage (absorbed dose) which was later named after him as an SI unit, the gray.

[7][8] Gray was born as an only child on 10 November 1905 to parents Harry and Amy Gray.

His father worked at a post office.