A method for correction of CA/C ratio based on linear model of accommodation and vergence

Measurement of the response CA/C ratio has required that the target used to stimulate the vergence system provides no blur information to the accommodative system. Although several methods have been proposed to open the accommodative feedback loop, it is difficult to ensure that a vergence target produces no stimulus for accommodation. To avoid this problem, we have derived a formula based on a linear model of the accommodative and vergence systems that allows the CA/C ratio to be estimated when the accommodative loop is not opened completely. An experiment was conducted to verify the derived formula. Two targets, Snellen letters and a small point source, were used to provide different blur-inputs to the accommodative system. CA/C ratios were estimated from the formula using measures of accommodation and vergence obtained with these two targets for eight subjects and showed a high correlation. The formula, therefore, was shown to provide a consistent estimate of the CA/C even when the accommodative loop was not opened completely.