The Patented Welch Allyn Coaxial Vision System optics produce a shadow-free spot, easier entry into undilated pupils, and a larger field of view. Aperture : The aperture selection dial allows the practitioner to choose six different apertures. Micro-Spot: The micro-spot aperture allows quick entry in small, undilated pupils. Small: The small aperture provides an excellent view of the fundus through an undilated pupil. Large: The large aperture is the standard aperture for dilated pupils and general examination. Fixation: The fixation aperture has graduated cross-hairs for measuring eccentric fixation or locating lesions. Cobalt: The cobalt filter is used with fluorescent dye to view small lesions, abrasions, and foreign objects in the cornea. Silt: The slit is used to determine levels of lesions and tumors. Filters : Filters give the ophthalmoscope added diagnostic capability. The red free light filter excludes red rays from the examination field, and the cross linear polarizing filter reduces corneal glare by 99%. 3.5V Power Source : The traditional ophthalmoscope works with all Welch Allyn 3.5V rechargeable power handles and wall transformers. Brow Rest : The rubber brow rest allows the practitioner who wears glasses to use the ophthalmoscope properly. Focus Distance : 450mm Angle of View : 45 Degree + 5 Degree Illumination Source : LED Rechargeable Ophthalmoscope (Welch Allyn)