Photodynamic Therapy Incubation Times for Actinic Keratosis

Inclusion Criteria: 1. Subjects who have been diagnosed with actinic keratosis. 2. Subjects who by the investigator deem to be suitable for photodynamic therapy of the face or scalp. 3. Males or females ≥ 18 years old 4. Fitzpatrick skin types I-VI 5. Subjects are in good health as judged by the investigator. 6. Subjects who are willing and have the ability to understand and provide informed consent for participation in the study and are able to communicate with the investigator. Exclusion criteria: 1. Subjects who by the investigator are not suitable for photodynamic therapy. 2. Lesions suspicious or proven for skin cancer 3. History of 2 or more ALA- PDT treatments within the past 6 months. 4. Use of keratolytics within 2 days of initiation of treatment (e.g., \>5% urea, glycolic acid, lactic acid, salicyclic acid) 5. Cryotherapy within the past 2 weeks. 6. Topical retinoids within the past 4 weeks 7. Procedures, such as microdermabrasion, ablative lasers, ALA-PDT, chemical peels, 5-fluorouraci, diclofenac, imiquimod within the past 8 weeks 8. Systemic retinoids within the past 6 months 9. Pregnant or breast feeding 10. Uncooperative subjects or subjects with neurological disorders who are incapable of following directions or who are predictably unwilling to return for follow-up examinations. 11. Subjects who are unable to understand the protocol or give informed consent. 12. Subjects who in the opinion of the investigator are not suitable for the study.