I would like to know if quantitative ferrography (DRF) only measures ferrous particles.

yes,
quantitative ferrography(Direct reading ferrography) in strument was basically desgned measure ferrous particles only. as the principle depends on light obscuration measurement by using optical sensors some times it can detect(measure) non ferrous & contaminants if these particles present in the sample are quantitatively more. dr ferrography segregates larger size particles(above 5 microns=DL) & smaller size larticles(below 5 microns=DS) an precipitates separately in a glass tube called precipitator tube subjected to two light sources in a designed channel some of the light will be obstructed due to particle deposition and some of the light will be transimitted on the other side tube two light sensors were mounted to catch the transimitted light and it will be converted to a nemerical reading which is an emperical value. both these readings DL & DS put together gives WPC(WEAR PARTICLE CONCENTRATION) of that particlular machine. even though magnetism was employed to quantify there are two more factors involved in measuring one is light source and other one is slope for precipitator tube to control velocity of fluid. thats why some times they can detect non ferous & contaminants.