A light source is configured to be mounted to a vision measuring instrument that includes a primary image capture unit capturing an image of an object to be measured, and an auxiliary image capture unit providing a means to aim the primary image capture unit at a determined position. The light source includes a main body defining a through hole for receiving the primary image capture unit, and a mounting hole for readily mounting an auxiliary image capture unit. A luminescent surface is formed on an inner wall bounding the through hole of the main body. A number of light-emitting diodes (LEDs) is disposed on the luminescent surface.