The precise measurement of the wavelengths of emission or absorption bands on spectrographic plates is a vexing and difficult problem. The usual commercial microphotometer when coupled with a strip chart recorder does not give accurate results because the rate of plate travel is not constant. This error may be traced ultimately to the plate drive mechanism which operates by means of a screw or by means of a friction clutch arrangement.
G. Dana Johnson, "Wavelength Determinations on Ebert Plates," Appl. Spectrosc. 31, 565-566 (1977)