We plan to enrol 120 research participants in total. Research participants will be recruited from two centres: St Vincent's University Hospital, Elm Park, Dublin 4 and Adelaide and Meath Hospital, Tallaght, Dublin 24. These hospitals provide outpatient dermatology services. Psoriasis patients attending these centres who have a psoriasis area and severity index (PASI) greater than 7 and who are due to undergo a course of NB-UVB phototherapy will be considered potentially eligible research participants and will be invited to attend for a screening visit.