Sphero-cylinder to Cross cylinder form

Another way of writing lens prescriptions is cross-cylinder form. Usually, this form is never used to write spectacle lens prescriptions. However, it gives you an in-depth understanding of the power of each meridian without the need to add or subtract.  Or it divides Sphero-cylinder prescription into two Plano cylinders with their axis 90 degrees apart from one another. This cross-cylinder form of prescription writing is one of the methods of calculating gross and net retinoscopy values. It is also a way to represent keratometer readings.

Steps of Sphero cylinder to Cross cylinder

Step 1: To find the first cross-cylinder power, write the spherical power of the sphero-cylinder prescription as it is, but the axis will be 90 apart from the axis of the cylinder in sphero-cylinder form.

Step 2: To find the second cross-cylinder power, add the spherical and cylindrical power of the sphero-cylinder form and the axis will be the same as the axis of the cylinder in sphero-cylinder form.

Example 1:

+5.00 DS / -2.00 DC X 90

Step 1:

1^st cylinder = Power similar to sphere power of sphero-cylinder form X axis 90 apart from the axis of the cylinder in sphero-cylinder

1^st cylinder = +5.00 DC X 180

Step 2:

2^nd cylinder = Algebraic sum of sphere and cylinder power of sphero-cylinder form X axis same as the axis of the cylinder in sphero-cylinder form.

2^nd cylinder = +3.00 DC X 90

Final = +5.00DC X 180 / +3.00 DC X 90

Alternate cross-cylinder form

Another way of writing cross-cylinder power is instead of mentioning the axis of the cylindrical meridian will mention the power meridian. Remember to use @ which means the power of that particular meridian. Don’t use, X which means axis meridian.

Steps of Sphero cylinder to Alternate cross-cylinder form

Step 1: To find the first cross-cylinder power, write the spherical power of the sphero-cylinder prescription as it is, power meridian will be the same as the axis of the cylinder in sphero-cylinder form.

Step 2: To find the second cross-cylinder power, add spherical and cylindrical power of sphero-cylinder form, but the power meridian will be 90 apart from the axis of the cylinder in sphero-cylinder form.

Example 2:

+5.00 DS / -2.00 X 90

Step 1:

1^st cylinder = Power similar to sphere power of sphero-cylinder form, @ power meridian same as the axis of the cylinder in sphero-cylinder form.  

1^st cylinder = +5.00 DC @ 90

Step 2:

2^nd cylinder = Algebraic sum of sphere and cylinder power of sphero-cylinder form, @ power meridian 90 apart from the axis of the cylinder in sphero-cylinder.

2^nd cylinder = +3.00 DC @ 180

Final = +5.00DC @ 90 / +3.00 DC @ 180

Power cross

Cross-cylinder form is easier when the cylinder components are visualized on a power cross. It is similar to alternate cross cylinder form but instead of writing in cross form, it will be drawn as a power cross.

For more information watch our YouTube video