that control the attenuation of light provided by the liquid crystal attenuator a feedback system corrects light intensity for losses due to ageing changes in temperature or slight misalignment a photodiode e g 10 placed in the light path monitors the intensity of the incident light beam the feedback signal is digitized by the a d converter and compared to the instructed value any difference prompts the processing unit to introduce corrective action by adjusting the magnitude of the currents delivered to the leds fig1 represents a first embodiment of a novel led variable light source for visual stimulation of the invention leds are semiconductor devices in which an electrical current is passed through a diode junction to produce light emission in active layers of semiconductor material at the junction leds that operate a relatively high powers and having a relatively broad spectral width are called superluminescent diodes sld while slds are known for low modal noise their spectral width typically decreases with increased coherence as the power driving them increases by using state of the art ultra bright leds for example off the shelf leds and slds manufactured by lamina ceramics inc westhampton n j the novel led variable light source of the invention generates u201c monochromatic u201d light beams of at least three different colors red green and blue alternatively an led core provides a range of uv to infra red wave length in the device in one embodiment the three different colored light beams may be activated individually or activated simultaneously in varied intensity to realize a mixed or combined source of mixed wavelength light essentially allowing the user to create any of a palette of colors from the r g and b primary colors color filters are not required.