Saturday, August 25, 2007

Modify the Sony 717 for IR -- Exchange Hot Mirror

(click on Thumbnail Images for Full Size view)

Hot mirror stage. In this picture you can see the hot mirror on its small stage. It is moved by the small motor and some (half) cog wheels. Check how the cog wheels interact with the stage and the motor. This is very important for correct reassembly of hot mirror stage/cage after modification. In standard mode the hot mirror (cyan color) is in the optical path as in the left half of the picture. In Nightshot mode the hot mirror is moved to the left, out of the optical path as in right half of picture. Just in case you wondered: in Nightshot mode the camera firmware makes a focus correction for the missing window.

Unlock hot mirror. The hot mirror is the small piece of blueish glass inside the stage. It is held in place with a small angled metal clip that has to be removed first. The red arrow shows the position of the metal clip.

Remove hot mirror. View on the hot mirror stage with metal clip removed and hot mirror glass sliding out. Note the position of the motor cog wheel T, and slit P for the pin that moves the stage. Make sure you do not scratch the hot mirror and keep it in a safe place, just in case you want to re-install it again at a later time.

Replace hot mirror. This is what it's all about. Replacing the internal hot mirror that blocks infrared light. IMPORTANT: dust, scratches, fingerprints etc. on the new optical window are NOT allowed! The original hot mirror (left) is a small piece of optical glass, size 11 x 13 x 1.2 mm. I replaced it with a special optical window that transmits light from +/- 300 to 2000 nm, instead of just 400-700 nm for the standard Sony hot mirror. After modification the F717 will record images with light from +/- 350 - 1100 nm. The sensor/glass combination will not work outside of this range. This range includes most of the near-infrared spectrum.
It is VERY IMPORTANT to replace the hot mirror with another optical window with the same dimensions, and refraction index (thickness tolerance about 0.02 mm), otherwise the camera will no longer be able to focus, especially at infinity and wide angle. If you do not replace the original hot mirror with anything, the camera will only focus in Nightshot mode. This is because the optical window is part of the camera's optical system. You can try to make your own window from some piece of optical glass ground to specifications, but it will require a lot of trial and error because the window must be perfectly flat. If it is too thick, it will probably not fit in the stage (maximum thickness is about 1.3 mm). Note that microscope slides have no AR coating, using uncoated glass will cause extra reflections and hotspot problems. A suitable optical window with the exact required specifications and AR coating (and a choice of IR filters) is available from (EU customers only). Theoretically one could replace the hot mirror with a suitable piece of IR filter glass to create an infrared-only 7x7 camera, like it is usually done with DSLR conversions. However, as far as I know there is no IR glass available with the right dimensions. If you are not interested in visible light photography you could add polyester or gelatin IR filterfoil to the stage (e.g. 87/87c filter, thickness usually 0.1 mm). This is cheaper than using an external IR glass filter, but you can no longer experiment with different IR filters (one of the many charms of this conversion).

Focus adjustment. If you use replacement glass with the wrong specifications it may not fit in the hot mirror stage, and you will probably have to adjust the focus. The focus adjustment screw (requires special tool) is below the blue tape. I recommend not to tamper with this, because there is no way you can adjust the focus while the camera is disassembled. You need to power on the camera to check focus, and the camera cannot power on when partly disassembled. The only option in this case is to assemble the camera and if the focus is wrong repeat the whole procedure and adjust the focus adjustment screw.

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1 comment:

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