Chalcogenide glasses are widely used in optical systems of thermal imagers for its high transmission and broad spectral band width. However, internal defects brought by composition’s disproportion of glasses during the melting and solidification can affect the imaging performance. A detection system of internal macroscopic defects is introduced by using the principle of perspective imaging based on resolution estimate. The optical power of system is adjustable, which can detect the thickness and diameter of glass samples within several millimeters to dozens of millimeters. It’s resolution can reach 0.19 mm at the distance of 300 mm, and it can make a full image of an object at 63.4 mm. The distinct stripes inside the glasses are obtained by glass edge extration of internal stripe through Sobel operator.