In article <48a9gs$hkg at surz03.HRZ.Uni-Marburg.DE>, doerner at papin.HRZ.Uni-Marburg.DE (Doerner Wolfgang) says:
A Klett unit is a spectroscopic measurement commonly used in bacterilogy.
It gives a absorbance reading based on a preselected color filter. The
unit is manufactured by the Klett-Summerson company (or used to be) we now
use OD600 to get the same basic info. 500klett = 1.0 OD