Milgram lists seven types of MR displays:
1. Monitor-based (non-immersive) video representations. Show video of the real world over which digital images are superimposed
2. An HMD showing the video.Same as Type 1, but content in an HMD
3. Optical transparent HMD. A transparent screen that allows virtual images to appear superimposed over the real world
4. Video transparent HMD. Same as 3, but shows video of the real world in front of the user with virtual graphics placed on it.
5. Monitor-based AV system. 3D graphics are shown on a monitor with superimposed video.
6. Immersive or partially immersive hunting. Display 3D graphics on an immersive screen with video embedded on it.
7. Partially immersive hunting systems. AV systems that allow October additional real object interactions, such as the interaction of one's own (real) hand.
Microsoft's HoloLens is an optical transparent display that allows virtual information to appear in the real world, and is therefore a type 3 Mr display. Similarly, a large screen showing virtual characters interacting with real people in a camera stream is a type 1 MR display.
As can be seen from this list, almost every screen that combines virtual and real images is real-time, a kind of MR display. But they have different characteristics. For example, types 1,2 and 4 are based on video with graphics enhancements, while types 5 are based on graphics with video enhancements.
Therefore, a classification is needed that can be used to classify MR images according to these characteristics.