What is the Lighthouse method equation to predict ADD based on current near VA, goal near VA, and working distance?

What is the Lighthouse method equation to predict ADD based on current near VA, goal near VA, and working distance?

• A. ADD = (Current near VA / Goal near VA) × Working Distance
• B. ADD = (Goal near VA / Current near VA) × Working Distance
• C. ADD = (Current near VA × Goal near VA) ÷ Working Distance
• D. ADD = Current near VA ÷ (Goal near VA × Working Distance)

Answer: (A)

The Lighthouse method estimates the needed near add by linking how much you must improve your current near vision to the distance at which you’ll work. The key idea is that the amount of magnification (the ADD) scales with how far your current near vision is from the goal and with the working distance you’re using. Using the ratio of current near VA to goal near VA captures how much improvement is required: if the current acuity is worse than the goal, this ratio is greater than one, indicating more add is needed. Multiplying that ratio by the working distance converts that required improvement into a dioptric value you can use as ADD. The other forms either invert this relationship or combine the VA terms in a way that doesn’t reflect how changes in acuity and distance translate into the required lens power, so they don’t produce the plausible ADD estimate the Lighthouse method provides. For example, if the current near VA is worse than the goal and you’re working at 0.4 m, the ADD comes out as 0.4 times the acuity ratio, giving a reasonable reading-add value.

The Lighthouse method estimates the needed near add by linking how much you must improve your current near vision to the distance at which you’ll work. The key idea is that the amount of magnification (the ADD) scales with how far your current near vision is from the goal and with the working distance you’re using. Using the ratio of current near VA to goal near VA captures how much improvement is required: if the current acuity is worse than the goal, this ratio is greater than one, indicating more add is needed. Multiplying that ratio by the working distance converts that required improvement into a dioptric value you can use as ADD. The other forms either invert this relationship or combine the VA terms in a way that doesn’t reflect how changes in acuity and distance translate into the required lens power, so they don’t produce the plausible ADD estimate the Lighthouse method provides. For example, if the current near VA is worse than the goal and you’re working at 0.4 m, the ADD comes out as 0.4 times the acuity ratio, giving a reasonable reading-add value.