Thanks to dropping prices as well as the boom in internet and digital photography use, ink-jet printers can now be found next to computers in almost every household. With just one push of a button, documents and images can be quickly and easily put to paper. As users, we can easily forget what a major technical achievement lies behind this now everyday printing activity: the controlled and highly precise spraying of ink onto paper via millions of tiny ink drops that combine to form the print image.
Whether with bubble jet or piezoelectric technology, printer and print head manufacturers face the challenge of perfectly controlling the size, speed and trajectory of the droplets to produce clean, sharp, photo-quality images.

Testing ink jets with cameras
ImageXpert is an American company specializing in imaging solutions for quality assurance and printing technology. To support companies using print heads from manufacturers such as Dimatix, Xaar, Kyocera, Konica-Minolta, Epson and Ricoh in their development and quality control, ImageXpert developed a system to precisely analyze a print head's ink jet with the aid of cameras and proprietary LED strobing technology and analytical software.
The  system is available in a one-camera or two-camera configuration using digital cameras from Allied Vision Technologies. In the one-camera option, the print head is positioned at a 90° angle to the camera axis in front of the camera so that the jetted droplets pass between the camera lens and an LED strobe unit. ImageXpert developed a special strobe controller to synchronize the illumination of the droplets such that the light strobes at a specified time delay after the firing pulse. The LED is positioned behind the ink droplet so the image of the droplet is in silhouette, which allows for analysis of both transparent and opaque fluids. The strobe pulsewidth is, at 250 ns, particularly short. "Once the drops are ejected, they move at a speed of approximately 10 meters per second", explains Yair Kipman, President of ImageXpert. "The very short strobe width allows us to capture images with minimal motion blur." JetXpert uses single strobe events, one strobe per image frame, to capture images of individual droplets in flight. This method of image capture is made possible by the proprietary high-powered LED, and minimizes blur caused by droplet aggregation that is common to systems that use multiple strobes per frame.

Extremely short illumination time
The camera is a Stingray F-080b model by Allied Vision Technologies. This monochromatic camera is equipped with an XGA (0.8 megapixel) image sensor and a high-speed FireWire IEEE1394b interface. "The Stingray F-080 delivers 30 frames per second," according to Kipman. "What was really most valuable for us, however, was the ability to freely set the shutter speed between 48 µs and 67 s. By being able to change the shutter speed, we can work with very fast ejection systems such as ink jet technology and much slower dispensing systems such as those used in the fabrication of bioarrays."
The camera is equipped with a Moritex telecentric zoom lens, assuring that small variations in the depth of the ink jet in the image does not lead to a change in scale. The ink jet's trajectory is measured relative to the angle of the print head, but analysis of the full trajectory of drops in flight is only possible with images captured with the two-camera version of the system.
The overall system footprint is quite small for inclusion in R&D and production environments, but applications exist where an even smaller configuration is desired.
"We are extremely satisfied with the performance of the AVT Stingray cameras", says Yair Kipman. "This is why we chose the Guppy, another AVT camera with a very small form factor, to design a more compact version of JetXpert for applications where little space is available".

Ink volumes measured, drop for drop
The camera's images are transferred to the system computer via the Stingray's FireWire interface. There, software developed by ImageXpert analyzes the images to measure important qualities of the droplets such as: drop volume, trajectory, velocity, ligature length, break off length, etc.
The drop volume (in picoliters) is calculated based on the measured drop radius. Velocity is most often calculated from the distance and time interval between two drops with a known delay time between them. The system measures the ink jet's trajectory relative to the print head in either two or three dimensions.

The JetXpert System is already in use by many leading printer and ink manufacturers, and is relevant in markets and research where liquid material is jetted or dispensed, such as printable electronics, adhesives or biotechnology.