View part of your screen in grayscale or filtered to simulate color vision deficiency.

Install with pip install monolens and then run monolens in a terminal or do it in one command with pipx run monolens.

Monolens was featured on Twitter by Matthew Feickert and Matplotlib, and picked up in this episode of Python Bytes. Thanks, guys!


  • Drag the lens around by holding a Mouse button down inside the window
  • Press Escape, Q, or double click on the lens to quit
  • Press up, down, left, right to resize the lens
  • Press Tab to switch between greyscale and different forms of simulated colorblindness
  • Press I to switch view label on/off
  • Press M to move the lens to another screen

On OSX, you need to give Monolens permission to make screenshots, which is safe.

Known limitations

  • The app is tested on OSX and Linux. It may flicker when you move the lens (less so on OSX). If you know how to fix this, please help. :)
  • Pulling the lens to another screen is currently not supported. See usage on how to switch screens instead.
  • The lens actually uses a static screenshot which is updated as you move the lens around. This trick is necessary, because an app cannot read the pixels under its own window. Because of this, the pixels under the app are only updated when you move the lens away first and then back. This is also the reason why the lens only works for static images.
  • On OSX, an ordinary app is not allowed to read pixels outside of its window for security reasons, which is why this app needs special permissions. Giving permissions is safe; Monolens contains no networking code and will neither store nor send your pixels anywhere.

Future plans

  • Support gestures to rescale the lens (pinch etc)