The RAF Rule (Royal Air Force Rule) provides a binocular gauge to measure Objective and Subjective Convergence as well as accommodation in 1 mm increments. The RAF Rule consists of a 50 cm long rule with a slider holding a rotating four-sided cube. Each side has a different target: the first has a vertical line with a central dot for convergence fixation and the other provides a limited number of lines of near-reading examples.

A rest is provided for the cheek to ensure consistency and proper height for the patient. Some studies have shown greater consistency of the NPC when measured with the RAF Rule compared to using a pencil or finger.

This instrument is used for determining the objective and subjective convergence points, examining the accommodation, and determining the master eye. The RAF Rule is useful for both diagnosis and treatment.

This instrument has four sides that have different measurements or functions:

• One side features diopter measurements from 20 to 2
• One side features a ruler from 5 to 50 centimeters
• One side that tests ages 8 through 48
• One side that tests for convergence, normal, reduced, and defective