Film-Tech Forum ARCHIVE: Reverse Scan Lens

This is topic Reverse Scan Lens in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul Trimboli (Member # 1509) on 11-05-2005, 09:15 AM:

What sort of lens is used in a reverse scan system? Does it have a slit in it still? If not can someone explain just how a reverse scan system works?

Posted by Sam D. Chavez (Member # 1841) on 11-06-2005, 02:48 PM:

The lens for reverse scan systems is a spherical lens. Depending on the brand and model they run from 12 to 18mm. They do not have a slit as that function is take care of by the reverse scan solar cell which has it built in. The main difference conceptually between front and reverse scan is that in front direct scan, the slit in the lens lights a defined area of the film with a slit of light, The size of this slit defines the ultimate frequency response of the sound system. The image defined by the slit is passed onto the solar cell. The limitation is that some of the signal intended for the left cell bleeds over onto the right and vise versa. This is because the septum or guard band between the left and right soundtrack is very small.

With reverse scan the general area of interest of the soundtrack is lighted by an LED or other light source. The entire image of this area is projected with some appropriate magnification (about 3.5) onto the solar cell with slit. Only the part that falls in the area of the slit is amplified and processed. Since the image is magnified is much easier to mask off the unwanted area and the slit linearity can be much more carefully controlled. There is almost no crosstalk or bleed between channels and the ultimate separation is superior with this method.