u201c bow tie u201d or trapezoidal diode lasers emitting near single mode radiation with a wavelength of 670 nm are used to produce a high quality beam these diode lasers have a flared highly diverging region that is in the shape of a trapezoid or a u201c bow tie u201d the beam is then coupled to an oligimode optical fiber with suitable optics such as step mirrors so as to preserve beam quality during coupling for delivery of the treatment radiation to the eye the optical fiber is coupled to a slit lamp adaptor the slit lamp adaptor determines the spot sizes and includes attenuation means that effectively attenuate the power received from the laser source thus a monotonic relationship between the spot diameter and the laser power administered is easily achieved preferably the attenuation means is chosen such that the intensity of the beam is independent of the spot size in a preferred embodiment the attenuation means is in the form of an iris whose aperture is varied in accordance with the movement of a selection ring to select a beam spot size in another embodiment the attenuation means is an attenuator disc rotated in accordance with selection ring at a suitable point in the optical path in this manner the laser unit itself can be simplified to only produce a constant power output and if this is desirable only the time of irradiation needs to be varied to achieve different treatment dosages other embodiments may include adaptors to indirect opthalmoscopes or may include a slit lamp that has.