A device that is connectable to a phototherapy apparatus for applying targeted phototherapy to an area of skin to place a skin condition into remission and a method of determining a maximum tolerable dose of phototherapy applied to a treatment area to determine an optimum therapeutic dose to quickly place a skin condition into remission. The dosimetry device can include a housing and an optical matrix arranged within the housing. The optical matrix includes a plurality of at least one of absorptive, reflective and/or partially transmissive regions that each permits a different percentage of light to be delivered to an individual's skin. An assessment can then be made as to the maximum tolerable dose of phototherapy that can be applied to the individual's skin in order to place a skin condition into remission.