To understand lamp flicker, you need to know a little about how mercury arc lamps work. First of all, there is no filament (a thin, typically tungsten wire) inside of an arc lamp. Instead, there are two tungsten electrodes spaced a small distance apart (called the arc gap) in a mercury vapor. When the lamp strikes, this gap is bridged by a plasma arc. This plasma arc, running at over 7000 degrees Celsius, is the light source of the projection system. Over time, the plasma arc slowly burns away the electrodes, causing the gap to get larger and the projector to get dimmer (the smaller the arc the better the lamp reflector collects the light). This burning also causes the electrodes to become smooth and this smoothness leads to a tendency for the arc to move around, jumping from spot to spot as it tries to find its least energy point. Plasma arcs are “lazy” and always hunt for the smallest distance between the electrodes to conserve energy. As the electrodes wear back, there can exist situations where there are two such places on the electrodes. The jumping back and forth between these two places is known as arc-jump, and is the predominant form of lamp flicker that will be seen in our products.
Lamp flicker takes the form of a brightening and darkening of the image at irregular intervals. This issue can be confused with some copy protection methods on DVDs and video tapes which cause a brightening and darkening at regular intervals. If you think you are seeing lamp flicker, the best way to make sure is to confirm that it still occurs when watching the projector’s internally generated blank screen (push the blank button on the remote), and that way you know it is not in your source. Also, if you occasionally see very quick flashes of light in the image similar to a strobe effect, then this is probably not lamp flicker.
All currently available ScreenPlay projectors have circuitry inside to reduce or eliminate lamp flicker. This circuitry varies the current to the lamp in such a way that the smoothing of the electrodes is interrupted, so that lamp flicker is almost totally eliminated. However, even though the probability for lamp flicker is reduced, it still can occur in rare situations.
For the ScreenPlay 4805, if you observe lamp flicker, there are a number of things you can do.
1. Wait. Once the electrodes burn back a little more the lamp flicker should go away.
2. Go to high power mode. Changing the power setting of the projector will usually cause the arc to stabilize because the arc will take a slightly different shape due to the mercury convection changes around the arc. If you leave the projector at this power setting for a number of hours, the electrodes will be reshaped slightly and the lowest energy point will move. Once it moves, then you can return to low power and the flicker will be gone.
3. Magnetic fields and gravity also affect the arc, so moving the projector, turning it upside down, or even putting a magnet against it can stabilize the arc in a different location. Again, leaving it in this way for a while will allow you to use it while the electrodes reshape themselves.
4. If the lamp is within warranty, InFocus will replace it."

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