The ProximitySensor Generates events when viewpoint:
{"Image DocumentName": "The ProximitySensor Generates events when viewpoint:", "Image ImageDescription": "Enters Region: SFTime enterTime. Leaves Region: SFTime exitTime. Moves within Region: SFVec3f position_changed. SFRotation orientation_changed. ProximitySensor { center size } Defines an axis-aligned region. Zero size disables the sensor. All relevant Proximity sensors generate events. VRML 2.0.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}