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A scratch test is performed to ensure that a projector, camera, printer, platter, or other transport system is not scratching film. This test should be performed regularly, and especially if equipment has been unused for long periods of time, or any mechanical adjustments have been made to your projectors.
The two best films for scratch tests are unexposed print stock and clear leader. Unexposed print stock is more useful for evaluating scratches on the bench, and clear leader is more useful for evaluating scratches on screen. An unused trailer, lightstruck, or black leader will also work in a pinch provided it is in perfect condition.
General: It is helpful to do a deep and thorough clean of your projector before a scratch test.
If evaluating for scratches on screen, use a full aperture lens and plate so that the maximum possible image area (including the soundtrack) is visible.
Because film behaves differently when running at full speed, advancing a scratch test by hand is not sufficient. Advancing by hand may be useful in determining if a roller is scratching film, but not so much for determining if your lower loop is hitting the bottom of your intermittent casing, for example.
If your scratch test passes, you can save your film to use again.
Prepare a loop of film to run through the entire length of the projector, including rollers on reel arms. Set up platter rollers to guide the film away from any projector surfaces, or cover surfaces with lens tissue to avoid scratching. Start and run the loop several times (50-100 is ideal), and check for scratching on screen or by shining a flashlight on either side of the film as it leaves or enters the projector.
Because soundhead flywheels will take a few seconds to come to speed, at least a few feet of your loop will inevitably scratch at startup and can be ignored, though if they are visible on screen in one pass it would be cause for concern and further investigation. For 70mm scratch tests with DTS only, this is not a concern because no flywheels are involved.
This method is most useful for evaluating Slap Scratches or any scratches with a repeating pattern.
Prepare 50-100' of test film on a house reel. Project as normal, rewind, and repeat several times (50-100 is ideal, 10-15 may be more practical). Check for scratching on screen and/or on the bench when rewinding. Keep track of base and emulsion side so you can evaluate if anything shows up.
This method is most useful for evaluating vertical scratching cause by rollers and flywheels.
Spot Check Method
Prepare ~50' of test film on a house reel. Run enough film through for the flywheel(s) to come up to speed and let run a few seconds longer. Stop the projector, and with a sharpie, carefully mark each point of contact with the film (sprocket, roller, gate, etc.). Carefully remove the film and inspect for scratches. If a scratch appears, take note of where in the film path it occurred and evaluate. Using this method the flywheel will inevitably scratch the film when the projector is stopped.
Visual Evaluation of Film Path
Because the above tests do not take into consideration running a full reel of film, and it would be impractical to use 2000' of expensive film stock to run a scratch test several dozen times, a careful and thorough check of a known good test reel should conclude the scratch testing process. Run a full reel of film and examine the film path as it plays: Are all rollers turning? Are the arms aligned so that the film is completely clear through the entire reel? Is the film entering the projector evenly? Look for any new scratches or other issues on your test reel.
If a scratch test reveals that a projector is scratching film, the next step is to determine the cause.
A slap scratch or standing scratch occurs when the film slaps against a surface of the projector once per frame. The most common cause is an upper or lower loop being too big or too small, but slap scratches can also occur in the gate from excessive vibration or mechanical misalignment.
Vertical scratching can occur in the gate, on flywheels, on rollers, or in the gate, or simply by brushing against part of the projector.
Some examples: If a straight vertical scratch appears near the center of the frame, the film is likely being scratched in the gate. Check that the lateral guide rollers are turning freely and that the studio guides are not too tight and are not pinching the film inwards.
If vertical scratches on the far edge of the image and soundtrack appear, check that pad rollers are turning freely and are not pinching the film.
If random "dancing" vertical scratches appear across the entire frame, check that the flywheel is spinning freely.