Luminys Systems Corp. Editorial for Crash Testing Technology magazine
There is much talk about the availability of faster and faster full definition camera speeds. No longer is 1,000 frames per second the upper limit. It seems inevitable that the ever increasing demands of testing will in time dictate faster and faster camera speeds. Faster camera speeds mean more light is needed. But higher light levels alone is not sufficient to satisfy the new demands for faster speeds. Flicker becomes more and more of an issue with higher speeds and faster shutter times. But what exactly is flicker? A broad definition of flicker is when there is a noticeable difference in light “quality” from one frame or image to the next. The classic way this difference occurs is when the shutter opens during a period of momentary lower light level coming from the light source. The light emitting plasma of an HMI light operates with AC current, which means that the light actually turns off and then back on again as its energy is absorbed from each AC cycle. With a square wave “Flicker free” power supply the AC cycle can be anywhere from 50 hertz to several thousand hertz. Ideally the off time for the lamp (the rise and fall time) is very short. But now shutter times can be very short also. If the time when the shutter is open coincides with the time the light is off (during the rise and fall time of the AC cycle), then there will be significantly less light or even no light on that particular image frame. Thus as the image moves at normal speed flicker occurs as the dark frame comes into view. The second phenomena referred to as “arc wander” occurs with AC lamps when the light emitting plasma inside the lamp is jumping back and forth between the tips of the two cathodes but from time to time does not land quite right on the tip of the cathode, but rather lands to one side of the cathode tip. This causes a distinct shift in the optics of the light fixture which then moves the shadow and highlight areas on the subject. Suddenly there is a brief moment when the object appears substantially different due to some parts being hidden in shadow and other parts being accidentally highlighted. This is a random occurrence that varies from lamp to lamp and can suddenly occur or disappear with little warning.
Contrarily, a DC lamp has none of the above issues of flicker or arc wander. A DC lamp employs light emitting plasma that is on all of the time with no interruptions. There is no arc wander because the plasma is continuously streaming from the cathode to the anode.
Not all lamps are capable of running in a DC mode. HMI lamps must run AC. Luminys ESL lamps run only DC. Is your future insured against obsolescence with lamps that will still be compatible with the higher camera speeds to come? Will you be plagued with flicker and arc wander? Cogent questions in this age of fast moving technology.