Fig. 4. A. Ablation of an irregular surface with a fluid of inadequate viscosity will not adequately protect the valley in which runoff can occur, and irregularities will persist as ablation progresses. B. Ablation of an irregular surface with a highly viscous fluid will be irregular because of the variable coating of different thicknesses of the fluid. C. The ideal fluid has adequate ultraviolet absorption and moderate viscosity to cause ablation of the exposed peaks while masking the underlying valleys. (Kornmehl EW, Steinert RF, Puliafito CA: A comparison study of masking fluids for excimer laser phototherapeutic keratectomy. Arch Ophthalmol 109:860, 1991)