Shooting TFT-Displays and Back Projections

In any film that plays in the present day, a computer or a TV monitor is almost a standard prop on the set. If we choose a tube device, the film camera must be synchronized with the monitor to avoid the black bars in the picture. A TFT display is a helpful alternative, because it normally doesn’t require synchronizing. It could, however, be the case that the image seen on the TFT display is not visible in the film camera’s video assist. This effect is caused by polarization. Light emitting from a TFT display is linearly polarized. The beam splitter, which distributes the light in the film camera between the optical viewfinder and the video assist, also polarizes. If the different planes of polarization do not match, the image from the filmed display might not reach the video assist.

All beam splitters used today display the following effects:

Unpolarized or circularly polarized light entering the film camera exits the beam splitter’s two beam paths as linearly polarized light.

With already polarized light, the critical factor is the plane in which the light was polarized. In extreme cases, the polarization in the beam splitter could lead to light only being emitted along one beam path, while no light is emitted along the other path – this would result in image loss.

If light enters the beam splitter at another polarization angle, (in the third example rotated by 90°), the polarization plane of the incoming light would correspond to that of the beam splitter. It is now also possible for the light to be emitted in an upward direction. However, the light emitting from the side is now weaker in intensity.

The other optical elements in the beam path – the lens, the aperture, and the ground glass – do not change any of the fundamental characteristics: Unpolarized light which reaches the camera will also reach the optical viewfinder as well as the video camera. If this light comes from a TFT display and the polarization plane is not oriented correctly to the beam splitter, no light will reach the Video Assist.

As a result, the ability to view the image displayed on the TFT in the Video Assist recording depends on the relationship of the light polarization of the TFT display to that of the beam splitter. With some TFT displays – very often with computers – the polarization is diagonal to the display. These displays are normally seen in the Video Assist recording. With TV displays, in particular, the polarization lies horizontally, while the beam splitter in the ARRICAM LT splits vertically. That’s why an ‘erasure effect’ is seen primarily in the ARRICAM LT, leading to the Video Assist either displaying no image, or only a dark representation of the image on the TFT display. Other monitors could theoretically produce the similar effects with other cameras, but this has not yet been observed.

To address this problem, the Depolarizer LT was developed. The linearly polarized light from the ground glass is transformed by the Depolarizer into circularly polarized light, which can pass without interference through the beam splitter.

Similar effects also occur when polarized light from other sources (rear-screen projection, blue screen areas, etc.) reach the camera.

The depolarizer is easily attached between the camera body and the viewfinder of the ARRICAM LT. It is available from April 2007.

Klaus Jacumet