DIN99c and DIN99d are the latest iterations over the "official" DIN99b color space.
It promises best accuracy for simple euclidian color distance calculation.

Goals:

• Implement a higher color distance accuracy without the CIE2000 complexity.
• Provide a color model for efficient mass color distance calculations (e.g. compare amounts of colors in a database).

Unfortunatly you can't just convert CIELab to DIN99c/d because it needs a different CIELab calculation first: For better accuracy in the blue regions they modify X from XYZ. The chain would be:
Anything -> XYZ -> modify X to X' -> CIELab' -> DIN99c/d.

The best formula source I could find so far is in German language, alas:
https://tuprints.ulb.tu-darmstadt.de/5870/13/D17%20Einfluss%20der%20Normspektralwertfunktionen%20_final_digital.pdf,
pages 29ff.
I could help translating it, if needed.