The primary objective of this study is to compare the improvement of psoriatic plaques with and without excimer laser (308-nm UVB) treatment, applied in a randomized and blinded fashion, in subjects on acitretin 25 mg or tazarotene gel 0.1% QD. The primary endpoint will be the comparison between the change in NPF score of plaques treated with excimer laser and those treated with sham treatment.