Eye tracking system

An optical instrument for forming a viewable image of an object, comprising:

• an objective lens for forming a viewable image of the object at an image plane;
• an eyepiece through which a user can view said viewable image at said image plane;
• an eye sensor for sensing a direction of gaze of a user viewing the viewable image through said eyepiece;
• and
• means for adjusting a controllable function of said optical instrument in dependence upon the sensed direction of gaze;
• characterised in that said eye sensor comprises:
• (i) a light source for outputting light through said eyepiece towards the user's eye;
• (ii) a lens associated with said light source for causing the light from said source to be substantially collimated when it exits said eyepiece towards said user's eye;
• (iii) an imaging transducer for imaging the front of the user's eye through said eyepiece and for generating an electrical image signal of the front of the user's eye;
• and
• (iv) processing means for processing the electrical image signal from said imaging transducer to determine the distance between the location, within the image, of the user's pupil centre and the location, within the image, of a reflection of the light from said light source off the user's eye, and to determine said gaze information using said distance and predetermined calibration data which defines a relationship between said distance and the point on the image plane the user is currently looking at.