One standard light source for refractometers and polarimeters is the yellow light (D-line) from a sodium vapor lamp. Recently, there is an increasing use of interference filters on halogen lamps and LEDs to transmit the D-line.
The D-line from sodium vapor lamps is often thought of as one bright line, but it is actually emitted as two bright lines, D1 (589.6 nm) and D2 (589.0 nm). A D-line is comprised of both the D1 and D2 bright lines. From a numerical point of view, 589.3 is the midpoint of 589.0 and 589.6. As for the light intensity ratio, D2 has a stronger intensity, and the midpoint of the intensity is between 589.3 and 589.6.
For ICUMSA, the D-line wavelength is expressed as 589 nm or 589.3 nm for refractometers and 589.4400 nm for polarimeters. (The values found in the revision history and surrounding text are assumed to be in vacuum. In the air the D-line wavelength is 589.29 nm.)
According to the Japanese Industrial Standards (JIS), the D-line for refractometer and polarimeter standard light sources is 589.3 nm (for example, K0062 Test Methods for Refractive Index of Chemical Products and K0063 Test Methods for Optical Rotation of Chemical Products).
The Japanese Pharmacopeia's standard light source for both refractometers and polarimeters is D-line, but it does not reference a wavelength.
ATAGO uses 589 nm for the D-line in its catalogs and instruction manuals.