Background: Some clinicians perform cutaneous laser treatments through transparent membranes to protect themselves and the surroundings from contamination by ejected tissue particles. Transmissibility of these membranes influences the tissue irradiance. Objective: We compared the optical transmissibility of 8 membranes. Methods: In part 1, a Schwartz Electro-Optics Q-switched alexandrite laser (wavelength of 720-800 nm, beam diameter of 3 mm) was directed through 8 membranes. A Molectron JD 2000 Joulemeter Ratiometer obtained measurements of percent loss of laser energy (for calculation of percent transmission) through each. To evaluate other wavelengths used in skin treatment, an SLM Aminco UV-VIS model 2000 spectrophotometer measured variance of transmission over a spectral region from 350 to 900 nm in 4 of the membranes. Results: The samples listed in order of increasing amounts of light energy lost across the membrane as compared with control (direct irradiation) were as follows: (1) Handi Wrap plastic wrap: 7% loss, 93% transmission; (2) glass slide: 8% loss; (3) Bioclusive transparent dressing: 11% loss; (4) Tegaderm transparent dressing: 26% loss; (5) Vigilon dressing without plastic backing: 34% loss; (6) Vigilon dressing with one layer of plastic backing: 37% loss; (7) Acuderm dressing: 41% loss; and (8) OpSite IV: 48% loss. Conclusions: Transmissibility of the interposed membranes must be considered when determining the dosimetry of light energy required by the target tissue. (J Am Acad Dermatol 2001;45:919-23.)