Do you really need a high speed ballast? What we learned about 1,000Hz
When shooting at a high frame rate then you may get better results using a high speed (1,000Hz) ballast. Why do we say may? Read on to find out more
How does the 1000Hz mode work?
The arc in an HMI lamp is a fairly delicate creature, and can be disturbed in several ways such as by the turbulence in the gases within the arc chamber. Additionally it may take a slightly different path on the two halves of the AC waveform applied to the lamp terminals.
These effects are known as ‘arc movement’ which, depending on the frequency, may be visible to the naked eye, or may only be visible on playback from a high speed shoot. The use of the latest high efficiency PAR fixtures, and the capture techniques of digital cameras, can combine to increase the probability that arc movement will become a problem.
By increasing the operating frequency of the lamp the degree of arc movement tends to be reduced – but there is an upper limit as the lamp can be pushed into a region of resonance which causes severe instability of the arc.
I’m shooting at 1,000fps so I need a 1,000Hz ballast, right?
You don’t necessarily need a high speed ballast. Some lamps have low arc movement at just 100Hz or 300Hz, and the camera will still see the light as being flicker-free even if the frame rate is higher than the Hz setting on the ballast. (You can see this proven here: How We Test: Behind The Scenes of High Speed Photography). But if there is excessive arc movement present then you may get some benefit from gearing up to 1,000Hz. The downside is that you need to be conscious of the possibility of resonance in the lamp and to tune the ballast frequency away from resonance if necessary.
Don’t overlook the 300Hz mode
Increasing the lamp frequency slightly to 300Hz can give a great improvement in arc movement, and there’s no danger of resonance, so no lamp tuning is required. This is why we make this option available together with the 1,000Hz ‘high speed’ option. This can be a really good choice even if you’re shooting faster than 300fps.
Believe us, it’s easier with Autoscan
Tuning the ballast to avoid lamp resonances can be time consuming, and gets more difficult when there are multiple fixtures in use and it’s difficult to see the output of the one you’re tuning. Our 9kW and 18kW ballasts have a time-saving ‘Autoscan’ feature built-in and this allows the ballast to do the lamp tuning for you. You can read more about Autoscan here
