Real-time simultaneous measurements of ink drop velocity and diameter in thermal ink-jet printhead using laser scattering techniques

Abstract

The velocity and diameter of ink drops are important parameters in ink-jet printing technology. They affect print quality and the control of the gray scale. We report here an exploratory experiment in which the velocity and diameter of drops from thermal ink-jet print heads were measured nonintrusively, simultaneously and in real-time. A laser phase Doppler particle analyzer was used in the experiment. The velocity was deduced from the Doppler signal and the diameter from the spatial phase difference of the scattered intensity. From an analysis using geometrical optics, it was determined that the optimum condition for sizing absorbing spheres such as ink drops was that the signal detected be limited to externally reflected rays. The intensity of rays refracted as a fraction of rays reflected by ink drops could vary significantly over their size range and compounded size interpretation. By considering the scattered intensity, the analysis showed that the best placement of the detectors for the present application was at 90° scattering angle, using a perpendicularly polarized light source. In this configuration, twice refracted rays (no internal reflection) were absent. The detected signal was dominated by externally reflected rays. In order to measure ink drop diameter accurately, the complex refractive index m of several ink-jet inks was measured. A typical value was m = 1.37 - ι 0.0049 at 0.6328 μm wavelength. Monodisperse droplets produced with a piezo-electric squeeze tube drop generator were used to calibrate the instrument. With the optimized configuration the drop velocity and diameter of two commercial thermal ink-jet heads and an experimental prototype were measured.