A direct reading phoria measurement technique using vertical prism displacement. Designed by Dr. Edwin Howell BSc Optom PHD and based on the priciple attributed to Prentice and Thorington. A horizontal row of numbers is doubled vertically by the prism. An arrow from the top row points down to a number on the bottom row indicating the instantaneous phoria value. Horizontal prism “neutralisation” is not required. Moment by moment changes in the phoria can be measured and can be used in conjunction with a trial frame or phoropter. 

“Free space” phoria determination with optimisation of Proximal Accommmodation and Convergence factors, particularly when used with large aperture trial lenses, is possible. 

The gradient AC/A ratio for positive and negative lens challenge can be determined using +1/-1D and +2/-2D flippers. 

Conceptually simple – most 4 year olds can cope with the task if they know their numbers. Eso/Exo discrimination by both colour coding and odd/even numbering possible. 

This set consists of two cards. The small card is calibrated for measurements at 33 cm; the larger card is calibrated for 3 m distance. The small card contains a X-cyl target and a variety of letter and word targets useful for near acuity, accommodation amplitude and facility, near retinoscopy and vergeance challenge testing.