An invisible-stylus-based coordinate measurement system via scaled orthographic projection

We present on a simple yet effective method for creating an invisible stylus from which a non-contact 3-D coordinated measuring system (the PiCMS) is realized. This invisible stylus dubbed a Pixel Probe is created through the orthographic projection of a spherical mounted reflector (SMR) through a trifocal camera system. Through this, a single point in space that is linked to a laser tracker world frame is mapped to a unique set of pixel coordinates in the trifocal camera creating the Pixel Probe. The system is constructed through the union of a Pixel Probe, a laser tracker, and calibrated $\text{XYZ}$ stage, and does not require contact to obtain a measurement. In the current configuration, system resolution and accuracy better than $20\mathrm{micro}\text{m}$ is demonstrated on objects in the meso/micro scale that are well below the range of a laser tracker alone. A simple single-point coincidence condition allows the user to specify a measurement coordinate by pointing-and-clicking in the images captured by the Pixel Probe. We describe this system using multi-view geometry vision theory and present proof of concept measurement examples of 2-D and 3-D objects.