ABSTRACT
Backlit against a dark background, the naked eye can be used to observe drops as small as an estimated 10 microns in diameter. Optimally, the light source should minimize the heating of the microdrops and their environment. Examples of light sources that provide high brightness with minimal heating are lasers, high brightness light emitting diodes (LED), and fiber-bundle-coupled halogen lamps. Hot light sources, such as collimated incandescent sources, are often not usable because they heat the fluid in the drop ejector, changing its rheological properties, and generate strong, local convection currents in the air that will deflect the drop’s trajectories. The drops are most easily viewed from a position where the illumination beam is shining towards the observer as much as is possible without being in the observer’s eyes. The forward, scattered light from the small drops is significantly brighter than the reflections from the rear or the side of the drops.
