A diagnostic technique has been developed for rapid characterization of optical and scan properties in optical printers. The technique provides simultaneous real-time measures of beam profiles, including spot size, centroid and energy, at all or multiple positions along the print scan line for dynamically moving beams in laser printers or for static optical beams in LED printers. As such, it facilitates adjustments of f- lenses in laser printheads with air bearing spindles without the need to stop the polygonal scanning mirror. Also, since an entire raster is measured at once, measurements of the scan line bow and linearity characteristics are obtained in real-time. Polygonal scanner jitter characteristics can be measured in seconds. For LED printheads, it provides for optimal adjustment of lens/array assemblies, and for measurement of the many thousands of LEDs in seconds, yielding linearity, MTF, and power compensation values. When compared to conventional slit-scanning diagnostic methods, the 3σ measurement accuracy for spot size is slightly less, but the 3σ centroid accuracy is improved, primarily because there are no moving parts. For instruments using this diagnostic technique, real-time performance yields up to a thousandfold increase in measurement speed, with corresponding reductions in test time, allowing for vast improvement in fabrication tools and in quality control and assurance in printhead manufacturing.