Techniques for determining characteristics of a stream of jetted material in a three-dimensional (3D) printer are disclosed. An example 3D printer includes an ejector configured to release molten droplets along a jetting path from the ejector to a build platform. The 3D printer also includes a sensor positioned adjacent to the jetting path and an optical mask positioned adjacent to the jetting path. The optical mask includes a plurality of regions comprising light-blocking regions and light-passing regions. The optical mask is configured to modulate a signal generated by the sensor as the molten droplets travel along the jetting path. The 3D printer also includes a controller to control the 3D printer based on the signal.