Crystal Locking by Power Gems – A new tool for High Speed Shooting
Learn the ins and outs of using HMI lighting for high speed filming with Power Gems. Check out the new feature – the Crystal Locking mode – available on the next generation of high speed ballasts.
In Using HMI lighting in high speed filming applications, we looked at the advantages but saw that under certain circumstances the artefacts in the light can interact with the camera.
We shared tips on how to get the best results by following a few simple rules. However, there will still be occasions when, despite following these rules, some residual effects can be seen in the playback image. In order to solve these lingering problems Power Gems has added a new feature – Crystal Locking mode – to their ballasts.
Here, Dr Phil Ellams of Powers Gems helps us understand the cause of the problem, and explains how Crystal Locking gives you the best possible results when shooting at high speed.
What problem is Crystal Locking solving?
Using the 1000Hz setting on the ballast can help with high speed shooting by reducing rhythmic variations in the arc, and allowing multiple cycles of the HMI lamp current to occur during the frame capture time (shutter open time). This dilutes the effect of the brief dip in the light output which occurs when the lamp current changes direction.
However, in cases when the shutter speed is very high, the number of current cycles per frame becomes quite low, so the dilution effect is reduced. Or in other cases, the ballast frequency can be close to, but not exactly the same as the camera frame rate, and in these cases a ‘beating effect’ can be observed. This is the same as the tremolo effect which occurs when you hear two musical notes which are very slightly different in frequency. In both cases, the end result can be banding or breathing in the playback image.
Crystal-locked frequency
The solution to prevent these types of beating effects is to use a ballast frequency which is exactly the same as the frame rate, or an integer multiple of the frame rate.
In order to achieve this, Power Gems has modified the 1000Hz operation so that when this mode is selected the ballast switches in the first instance to an accurate 1000.0Hz. This frequency is crystal locked with accuracy better than 50ppm.
In most cases when shooting at common high speed frame rates of 250, 500 or 1000 frames per second this will eliminate effects caused by rhythmic variation in the light output, ensuring the best quality images.
Button for mode select
The 1000.0Hz accurate frequency is automatically engaged when the Crystal Locking mode button is pressed to select ‘1000Hz’ on the front panel. However it is possible that the particular lamp being used is not stable at this frequency, and in these circumstances it is best to use the Autoscan system to let the ballast choose a better operating frequency by scanning through the range 750Hz to 1250Hz.
Note that Autoscanning removes the advantages offered by Crystal Locking operation, but if you decide to return to 1000.0Hz this is easy– just press and hold the blue ‘Full’ button for three seconds. If the ballast is powered off it will always return to 1000.0Hz on restarting.
Speed read
- Use the maximum shutter angle possible.
- If shooting at a sub-multiple of 1000fps try to use Crystal Locking mode.
- Check that the lamp is stable at 1000.0Hz (by directly observing the light output and checking for instabilities, or listen for resonance).
- If the lamp is not stable at 1000.0Hz then use Autoscan.
Does Crystal Locking mode replace Autoscan?
Autoscan, developed by Power Gems, is a tremendously useful feature. The new Crystal Locking mode is designed to be used alongside Autoscan and is a further tool to help get the best results with high speed filming.
If you are shooting at 100fps, 250fps, 500fps, or 1000fps and the lamp is stable at 1000.0Hz then using Crystal Locking mode will give the best possible results at high speed.
If you are shooting at a frame rate which is not a sub-multiple of 1000fps, or if your lamp is not stable at 1000.0Hz, then using Autoscan is the next best thing for creating favourable shooting conditions.
Don’t forget 100Hz and 300Hz modes
Due to the tremendous output of HMI fixtures they remain the light source of choice for high speed filming applications. High speed shooting can present some issues as the frame rate approaches the frequency of the light source, but Power Gems ballasts come supplied with features at no extra cost which allow you get the best possible results under all circumstances.
The twin pillars of Autoscan and Crystal Locking mode are powerful tools when shooting at high speed. But we shouldn’t forget the standard 100Hz and 300Hz modes. Under many conditions, particularly modestly high frame rates these frequency modes allow great results to be achieved, with a simple set up.
High speed filming – solve problems with Power Gems
Power Gems ballasts boast the greatest selection of frequency options and come with Autoscan and Crystal Locking mode built in at no extra cost. As specialists in high speed ballasts we know a thing or two about high speed filming. Come back for more advice from Phil or get in touch for help choosing the right ballast for your next project.
