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I have a modified pico video projector that I need to add an autofocus to. The focusing ring already has a fast stepper motor added to it.

If the focus had to be done on the middle of the projection frame I suppose I could just put a laser distance sensor in the optical path of the projection by having a 45 degree IR reflect, visible spectrum pass dichro attached after and maybe before the final focusing and zooming lens. But I want the ability to focus to any point in the frame. This requires the sensor to be able to be rotated, or if it goes through lenses, tilted in two axis. I could do that with a small gantry system with stepper motors and the distance sensor attached to it, or galvonometer mirrors that would reflect the distance sensor beam to a given point/angle, but there may be a better way or a better sensor for this I am not aware of which wouldn't require so many parts.

Requirements are ability to measure distance up to 4 meters, accuracy about at least +-5cm up to 1 meter, +-10cm after that, which I believe is not much to ask from a sensor and acceptable accuracy for a video projection focus. Measuring on shiny or reflective surfaces don't need to be supported as projectors don't project properly on those.

Having a camera and software which focuses until software finds the sharpest result would work if it was fast enough (200 ms). As I understand with this method the lens is focused until the sharpest image is detected which is slow.

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  • $\begingroup$ if using the camera you could just get the focusing distance from the camera and apply it to the projector, once calibrated you shouldn't need the back and forth feedback. $\endgroup$
    – L Selter
    Mar 26, 2018 at 12:07
  • $\begingroup$ its a dynamic focus interactive projection $\endgroup$
    – Mark Legault
    Mar 29, 2018 at 22:17
  • $\begingroup$ What I mean is you could just read the focus distance from the camera, and set your projector to that distance, you shouldn't need a feedback system, this would probably be quicker. $\endgroup$
    – L Selter
    Mar 30, 2018 at 23:14
  • $\begingroup$ "read the focus distance", what does that mean? $\endgroup$
    – Mark Legault
    Mar 30, 2018 at 23:54
  • $\begingroup$ Why not a simpler approach: a laser with a diffraction network that displays a specific pattern (always focused at, arbitrary distance) and a camera. First switch projector off, laser on, focus the camera on the laser mark (using the camera's autofocus). Then freeze the camera focus, switch laser off, projector on, display a test pattern and tune the projector until camera detects the pattern is focused. (you may even use the same autofocus controls as in the camera lens, just redirect control to the projector motor!) You don't measure the distance, you just get focused image. $\endgroup$
    – SF.
    Jul 24, 2018 at 8:39

2 Answers 2

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U can use a mirror for feedback of the projected image and a SLR as in cameras (a piece of glass at an 45` angle to measure the difference between projected and image onthe screen and approp. adjust the focus.

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  • $\begingroup$ Terribly sorry, this would work but I forgot to mention, "Having a camera and software which focuses until software finds the sharpest result would work if it wasn't so slow." Added this to my question. Sorry again. $\endgroup$
    – Mark Legault
    Mar 25, 2018 at 9:59
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How about using a Kinect or similar device ?

It's a 3D camera that outputs a real-time depth measurement of the whole scene... pretty fast and low latency (designed for gaming)... if you have depth info for the whole scene you can use it to control your focus.

Here's an example of a kinect coupled with a projector:

https://www.youtube.com/watch?v=bA4uvkAStPc

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