Dimroth and Reichardt ET

Dimroth and Reichardt determined the wave length of the absorption maximum for a betain dye as a measure of the ionizing power of solvents.
The wave length is converted to ^kcal/_mol (=2.86x10⁴λ^-1) to give E_T(30). The dye is popular because the changes in wave length are in the visible spectrum (see below). However, it is poorly soluble at the ends of the E_T scale and may be protonated.
E_T is mainly a measure of hydrogen bond donation (HBD) and is strongly correlated with →Kamlet and Taft's α, →Gutmann's acceptor number (AN), →Swain's acity (A), →Fawcett's acidity (Ap) and →Catalán's SA.
See → α vs E_T and → AN vs E_T et c.
Normalizing to a scale from TMS (0.00) to water (1.00) gives E_T^N which is a range more in line with other solvent polarity scales.

Note: Colours shown here are very approximate - in particular at the low and high ends of the scale. Click →here to sort by E_T/colour.

See e.g. →IUPAC Gold Book, →Reichardt's dye, →"Solvatochromic Dyes as Solvent Polarity Indicators" (article), →"Lösungsmittel-Effekte in der organischen Chemie" (book), →"Solvents and solvent effects in organic chemistry" (book)