Reflection and transmission of laser light from the esophagus: the influence of incident angle

The application of lasers in gastrointestinal endoscopy is rapidly expanding. Because of the tubular configuration of the gastrointestinal tract, endoscopists often deliver laser energy at large angles of incidence. As incident angle affects the fraction of radiation reflected from the tissue surface, we measured the transmittance and reflectance of laser light from in vitro esophagus as a function of incident angle, using integrating sphere and goniometric techniques. At a wavelength of 633 nm and angles of incidence less than 50 degrees, the total transmittance of the esophagus is approximately 25% and the total reflectance is approximately 45%; both are isotropically distributed. At larger angles of incidence, a specularly reflected component becomes evident and the total reflectance increases. The absorbed light per unit area illuminated decreases with increasing angle, because the area illuminated by the laser beam is proportional to the secant of the incident angle. The data suggest that during endoscopic laser procedures the incident laser beam should be directed within 50 degrees of normal for optimal performance and safety.