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

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

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^Δ-40^Δ) and block prisms (10^Δ-50^Δ), a cubic surface function was fitted by polynomial regression.

The combined effect of stacked prisms was always greater than the arithmetic sum of the labeled values of two prisms (by up to 66^Δ), increasing exponentially with each prism power and reaching the maximum of 156^Δ for the Fresnel/block combination of 30^Δ/50^Δ. We obtained contour plots to evaluate the optically induced additivity error and constructed look-up tables for quickly determining the combined effect of the prisms based on their labeled values.

Stacking prisms is a practical method to evaluate a large strabismic angle that cannot be measured by any single prism and is especially useful in dealing with severely paralytic strabismus.