Barco Prometheus I, Safety & Instruction Manual

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5. Lenses & lens holder
5.6 Back Focal Length adjustment
When to adjust the Back Focal Length?
If a lens is used with a throw ratio suited for the application, (lens selection depends on projection distance and screen size) typically
one would NEVER need to adjust the Back Focal Length of the projector.
A Back Focal Length adjustment is only required in case the Focus range of the installed lens does not capture the projection
screen either for FLAT and/or for SCOPE. In other words, when it is impossible to focus the image on the screen for FLAT and/or
for SCOPE. Note that the lenses for the Prometheus are varifocal. So, switching between FLAT and SCOPE (zoom action) requires
a readjustment of the focus.
What is Back Focal Length adjustment?
Back Focal Length adjustment means moving the lens plane (Lp), thus the Lens Holder front plate, closer to or further from the DMD
plane. The closer the lens plane to the DMD plane the further the focus range (Fr) of the lens will be.
SCREEN
DMD
Lp1
Lp2 Fr1
Fr2
Image 5-26
Do not abuse the Back Focal Length adjustment of the Lens Holder. Neglecting this will result in loss of image
quality because of the lens design. Cases requiring Back Focal Length adjustment normally indicate incorrect
lens choice (throw ratio).
Necessary tools
• Nut driver 10mm.
• Allen wrench 3mm.
• Nut driver 13mm.
How to check the Back Focal Length
1. Ensure that the throw ratio of the installed lens matches with the requirements of the application (projection distance and screen
size).
2. Ensure that the correct lens parameters are activated. (See user guide of the Communicator chapter Installation > Advanced >
Lens parameters )
Caution: Not using the correct lens parameters could result in lens damage.
3. Perform a lens HOME & RETURN operation. (See user guide of the Communicator chapter Installation > Advanced > Lens
parameters )
4. Project the green focus test pattern. (screen fi le “no masking” or “no “crop”)
Image 5-27
5. Zoom the lens for maximum image on the screen ( WIDE ).
6. Is it possible to focus the center of the projected image?
If yes, the Back Focal Length is OK.
If no, the Back Focal Length needs realignment. Proceed with the next procedure.
78 R5905963 PROMETHEUS SERIES 09/03/2015