Our previous works presented experimental investigations of the brittle material removal fraction (BRF) on the machined surface of optical glass. It is related to the area fraction of brittle material removed on the machined surface and is used to assess the surface roughness of optical glass during precision grinding. In general, subsurface damage (SSD) is considered to be one of the most important machining qualities in grinding of optical glass. This work presents an experimental investigation of the relationship between BRF and SSD depth of optical glass during precision grinding to understand the significance of BRF on sensing the SSD of optical glass. A series of precision grinding experiments were conducted on a precision grinder machine to investigate the influence of grinding parameters on SSD depth. Based on an image processing algorithm developed in previous work, the BRF was calculated by using a microtopography image of a machined surface of K9 glass during precision grinding. The experimental results show that the BRF was determined for different machining parameters (cutting depth, wheel speed, and feed rate) and was dependent on SSD depth in precision grinding, which implies that the BRF has potential for application as a rapid and affordable online vision measurement approach to monitor surface and subsurface qualities during the grinding of brittle materials.