Articles
The hair is clipped short (less than 1mm) and the unit is placed directly on the scalp. An assessment is made from a standard of 10mm2 field. Multiple measurements taken from different part of the scalp are often helpful, particularly if there is significant variability from one location to another.
Densitometry helps analyzes the scalp under high-power magnification to give information on hair density, Hair density is measured by the present number of hairs per unit area, 
Follicular unit density is determined by the present number of follicular units per unit area, Hairs per follicular unit is determined in both the range (observed) and the average (calculated), Presence of miniaturization can also be observed closely
Reference:
Bernstein RM , Rassman WR: The logic of follicular unit transplantation. Dermatologic Clinics 1999; 17 (2): 277-95..
Bouhanna P. The advantage of phototrichogram in hair surgery. International Advanced Hair Replacement Symposium. Birringham, AL, February, 1982.