A staple of the doctor's coat pocket, the ophthalmoscope provides a critical diagnostic tool for most fast and routine eye examinations. These scopes represent a hierarchy of lens ranges, the widest being +44D to -45D. An ophthalmoscope will similarly stand out in terms of light source, with the most sophisticated offering xenon gas illumination. Some offer lens filters optimized for specific applications; red free filters aid in the examination of blood vessels, while blue filters help identify corneal scratches.