The eye diagram test for optical modules is a common optical testing method used to measure the performance of optical modules during high-speed data transmission. It is ty pic ally used to test parameters such as transparency, transmission distance, and offline storage. Fundamentally, an eye diagram is a graphical representation of a digital signal's quality, formed. Eye height is the vertical distance between the upper and lower boundaries of the eye diagram. Figure 1 shows two Anritsu instruments that feature the latest in eye pattern analysis for manufacturing and field applications.