Howell Phoria Near/Distance Pair

Howell Phoria Near/Distance Pair: Business, Industry & Science. Buy Howell Phoria Near/Distance Pair at UK. Free delivery on eligible orders.. A direct reading phoria measurement technique using vertical prism displacement. Designed by Dr. Edwin Howell BSc Optom PHD and based on the principle 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. 。 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. 。