Photocoagulation was performed ex vivo between tissue slabs by delivering continuous‐wave laser energy from an optical fiber either directly, or by depositing the energy into a 2.4 mm diameter steel sphere at the fiber tip. The dependence of photocoagulation lesions on the following variables was assessed: (1)
energy source:Nd:YAG‐532 nm, 1,064 nm ± steel sphere, (2) tissue type: porcine muscle (light), bovine muscle (dark), (3) delivered power: P = 1.5–3.0 W (porcine), 1.0–2.5 W (bovine), (4) exposure duration: T = 300–1500 s. The resulting cross‐sectional photocoagulation lesions are summarized as follows: 532 nm: elongated; central charring in all cases; 1,064 nm: circular; central charring only in bovine for P ⩾ 2.0 W, T ⩾ 500 s; sphere: circular; central charring in bovine for P ⩾ 1.5 W and porcine for P ⩾ 2.0 W. These experiments suggest photocoagulation lesion size decreases as optical penetration increases. The results indicate that interstitial laser photocoagulation lesions >10 mm diameter can be made without charring in both lightly and heavily pigmented tissues ex vivo by delivering 1,064 nm laser energy at sufficiently low power for at least 1,000 s from well‐polished optical fibers. © 1994 Wiley‐Liss, Inc.