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Mirroring Your Desktop For The Projector

Logo of Promethean Boards Bound: A Blog About Educational Technology featuring a cartoon map of the Upper Cumberland region

by MARK WEST 1/19/10


This tutorial was done on Mac OS 10.5.8

Your computer is ready with your ActivInspire lesson. You look at the computer and see this:

But when you turn on the projector aimed at your Promethean board, all you see is something like this (your computer's wallpaper and no programs, not even your dock):

This tutorial will show you how to fix the issue.

Note: If you really like this wallpaper, it's availble on Flickr (or you could grab it from the above image, as that is the wallpaper). The image is called "Bokeh Effekt - Wallpaper", the artist is named "bayfan". The image is listed as having a Creative Commons Attribution-Noncommercial-Share Alike 2.0 Generic license [you can legally share it]. And the link to the image is which is blocked by ENA's web filter, so you may want to get it here.

The problem

If you have your computer hooked up to another display device, such as a TV or a projector, you can show the image on your computer to the other display device (the projector, in this case).

  1. Open System Preferences (Apple logo in upper left corner and go to "System Preferences")
  2. Click on Displays
  3. Set both displays to have the same color depth (such as "Millions") and the same resolution (such as 1280 x 800).
  4. Click on Arrangement*
    *Note: you'll only have arrangement to choose if you have multiple displays (TV, projector, etc.) hooked up to your Mac; there's nothing to arrange with only the built in laptop display.
  5. Select Mirror Displays

That should fix it.

So why did they put that "dumb thing" in Mac OS, anyway?

You can unselect Mirror displays if you want something on the overhead, but not showing something confidential (like your email, or gradebook, etc.). So it's technically not a "dumb thing"; it has its purposes.

So why not just turn off the projector?

  • projector bulbs are expen$ive and many contain mercury, so we want to replace them as infrequently as possible.
  • To prevent bulb loss, most projectors won't allow you turn them back on until they are completely cooled off. That can take about 10-15 minutes.

Whether you undo mirroring or power off is a situation call. If you get a chat request, and you expect it to be a brief exchange, such as "I need you to come see me regarding a purchase order" to which you reply, "OK"... it's probably more effective to turn off mirroring and reply privately that way rather than turning off the projector and being forced to wait 15 minutes for your class to continue.