Combining Fresnel and block prisms to measure large angles of strabismic deviation.

Purpose: A method was developed to measure strabismic angles >50 by stacking commercially available Fresnel and block prisms in the same direction ("piggyback prisms").

Methods: With a laser pointer (wavelength of 532 nm) as the light source, the deviation of the laser spot produced by the stacked prisms was measured on a tangent screen placed 100 cm away from the prisms. To the obtained data with combinations of Fresnel prisms (5-40) and block prisms (10-50), a cubic surface function was fitted by polynomial regression.