Refraction Problem Set: Question 11 Dr. John Lloyd University of Toronto
Question 1111. An eye has a spectacle correction of plano −1.00 x 180. Which lines will this eye see as darker on the astigmatic refraction dial?
Answer 1111. An eye has a spectacle correction of plano −1.00 x 180. Which lines will this eye see as darker on the astigmatic refraction dial? 6 o’clock – 12 o’clock line
Answer 11Astigmatic DialThe astigmatic dial is a tool designed to help determine a patient’s axes of astigmatism The dial has lines arranged in a radial, clock-face pattern A patient with astigmatism will see certain lines as darker, depending on their refraction and astigmatism
Answer 11Astigmatic DialThe simplest way to determine which lines are seen as darkest: 1) Convert the CORRECTING refraction into MINUS CYLINDER form 2) Divide the axis by 30This gives the clock hour at which the lines will be seen as darkestEg. Spectacle correction of: Plano −1.00 x 180 (already in minus cylinder form) Axis = 180 degrees / 30 = 6 Therefore, the 6 o’clock, 12 o’clock line is seen as darkest
Answer 11Astigmatic DialTo understand why this is the case, let’s look at how the astigmatic dial worksThe eye is first fogged (given extra plus power) to relax accommodation, which pullsboth focal lines anterior to the retinaAs a result, themore posteriorof the focal lines is closer to the retina andis seen asdarker and sharper on the dialIn this case, we can use the same table as was constructed in question 10 in orderto create a power cross for the eye error
Answer 11Starting with the spectacle correction given in the question: Refractive Error: Eye Error: Spherocylindrical Power Cross Spherocylindrical Power Cross −1.00 +1.00 Plano −1.00 x 180 +1.00 −1.00 x 090 plano plano
Answer 11Eye Error: From the power cross, it can be seen that there is excess plus power in the vertical meridian Power Cross Greater converging power in the vertical meridian creates a +1.00 HORIZONTAL focal line ANTERIOR to the vertical line When fogged, the line that is more posterior is seen as plano darker, therefore the VERTICAL line is seen as darker, which corresponds to the 6-12 o’clock line on the astigmatic dial