There are two common sources of movement in discharge lamps:
A. Arc movement. This is the fast movement you can see around the edges of the beam. You see it better if you look away from the light as the receptors around the edges of your eye are more sensitive to movement. It is the same frequency as the output of the ballast, which can be 50 – 100hz depending on the ballast and its settings. Often people call this “flicker” but it is not the same effect as the “flicker” seen on film, which a “flicker-free” ballast is designed to prevent.
The cause is the movement of the arc itself. Because the supply to the lamp is AC the current flows through the lamp in one direction, stops momentarily as the supply voltage goes through zero and then flows again in the opposite direction. In effect, the arc is collapsing and reforming very quickly each time the current stops.
When the arc reforms, it does not always reform in exactly the same place – the “feet” of the arc move on the surface of the electrodes. In fact, this happens at twice the supply frequency, which is beyond the ability of the eye to perceive. However, the eye is sensitive to the changes that occur between one pulse and the next and it is these changes which are at the supply frequency.
This movement can be influenced by several factors:
1. The age of the lamp. On a new lamp, the surface of the electrodes are clean so the arc can reform anywhere and so moves around. After a few hours, the arc will burn a pit into the electrodes so it is held in one place and is stabilized. When the lamp gets near the end of its life, the surface is oxidized so that the arc jumps from place to place trying to find a good spot to burn and so the movement increases again. Many times a user complains about “flicker” – he says “I tried another lamp and the flicker did not go, so the fault must be in the ballast”. Of course the have used a new lamp and then replaced it with another new one. After some hours of burning the user finds the flicker will be greatly reduced.
2. The magnetic field from the long electrode. In an MSR lamp (single-ended), the long electrode runs close to arc. This magnetic field tries to push the arc away. This does not occur in the double-ended lamps.
3. Thermal currents in the lamp. The hot gas tries to move upwards and the arc is carried up with it.
We recommend putting lamps into the Arrisuns with the long electrode at the bottom. That way the magnetic and thermal effects act in the same direction and the arc is more stable.
Two things have a big influence on how much the arc movement can be noticed:
1. The frequency of the lamp supply. Higher frequencies are less visible. With an electronic ballast, the most visible effect is seen when the ballast is set to “25fps” which gives a 50Hz supply to the lamp. On the “30fps” setting the output is 60Hz and is less noticeable. On “flicker-free”, the output is 75Hz or more, which is faster than most people can recognize.
2. The optical system. The effects are reduced by the mixing of the light, which occurs in the lens. In an open-face “PAR” type lamphead such as an Arrisun, the reflector is projecting a picture of the arc so these effects are very visible. When a lens is added the effects are reduced – more for the wider angle lenses. In a head with a Fresnel lens the effect is much less visible. Some other manufacturers of “PAR” type lampheads put a diffusing texture onto the reflector to reduce this effect. ARRI does not do this because it reduces the performance of the lamphead, increases spill light and has no positive effect when wide angle-angle lenses are used.
B. The second type of movement is much slower and is often described as “flaming”. It is due to the movement of hot gasses inside the lamp. Again, the effect is more visible in the Arrisuns and also, it is more noticeable in the bigger lamps. On the bigger lamps there is an orange halo around the arc, which makes this effect. If you take maybe six or eight layers of ND filter in a filter frame and look at an Arrisun 60 lamp when it is running, you can see this. It is very beautiful, but be careful not to look directly at the light.
In some cases, a very high level of arc movement can be attributed to a lamp that is faulty, at the end of life or incompatible with the ballast. This can be readily identified buy substituting the lamp with a known good one.
Andy Barnett, ARRI GB Lighting Technical Support