When beginning, establish the average depth of the hair being treated. Choose a hair (ideally in anagen phase if possible), grip it at skin level and remove it. The depth from the tweezers to the inner root sheath shows the thickness of the epidermis. In thin epidermal layers the ‘clear’ hair shaft will be approximately one quarter the length of the hair. In thick epidermal layers the ‘clear’ hair shaft will be half the length of the hair. This is only a guide; with experience you will begin to be able to ascertain from the appearance of the hair and to some extent the feel of the probe, what growth stage the hair is at.