Principle

The principle of the Helmholtz-Screen is the same as that of the Hess-Screen. Filter goggles in complementary colours are used in the same way to separate the two eyes.

The Helmholtz co-ordinates were first described more than 100 years ago by Hermann von Helmholtz. They correspond to the Hess co-ordinates in the primary and secondary positions. Deviations are present in the tertiary position of gaze. In mathematical terms the Helmholtz co-ordinate system is derived in a similar way to the Hess co-ordinate system. The eye of the observer is situated in the middle of an imaginary sphere and looks vertically onto the screen, which is placed at a distance of r = 0.5 meters (= radius of the sphere) The co-ordinates are hyperbolae, which result from the projection of two of the sphere’s concentric circles. In comparison with the Hess co-ordinates, however, the projected concentric circles have, at the far pole of the imaginary sphere a common tangent, which is either horizontal or vertical. Each circle through the horizontal tangent cuts vertically all the circles, which pass through the vertical tangent, and this occurs at every position on the sphere. Thus, in the eye of the observer, the circles represent the horizontal and vertical line. By projection onto the surface they become hyperbolae, whose curves are less pronounced than the Hess co-ordinates. The Helmholtz co-ordinates thus represent, as a result of the orthogonal nature of their original circles on the surface of the sphere, the mathematically correct basis for the measurement of cyclodeviations in the tertiary position of gaze.

Ocular torsion cannot, however, be measured on the Helmholtz-Screen. In this case this co-ordinate system offers only the practical advantage of needing less space, because the fixation points lie nearer the centre in the tertiary positions of gaze. On the other hand the screen with its 133 cm sides offers the opportunity to take measurements in the secondary directions of gaze downwards and horizontally up to a 50° angle of view.

HELMHOLTZ, H. VON: Handbook of Physiological Optics, Volume I to II, Teaching Visual Perceptions. J.v. Kries (editor), 3. Ed. Voss, Hamburg and Leipzig 1910

G.H. KOLLING, H.J. SIMONSZ, B. VAN DIJK: The Significance of Coordinate Systems for Diagnosis of Motility. In: MUeHLENDYCK, H., RUeSSMANN, W. (Editors) Eye Movements and Visual Perception (The Ophthalmologist‘s Bookshop. Volume 121). Enke, Stuttgart 1990