Explore prescription transposition, spherical equivalent, principal meridians, vertex distance and vertical imbalance calculations in one place.

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Insight Optical Training · Professional Tools

Insight Rx Toolkit

Professional prescription and dispensing calculations for UK optical practice.

Enter prescription

Sphere

Cylinder

Axis

Right

Left

Sphere and cylinder use 0.25 D steps and always display a sign. Axis accepts whole numbers from 1–180° and is disabled when cylinder is plano.

Choose a calculation below. The entered Right and Left prescriptions are reused across all sections.

Transpose

Convert the entered prescription between plus-cylinder and minus-cylinder notation. Sphere, cylinder and axis are transposed automatically.

Spherical Equivalent

Calculates sphere + half the cylinder and shows the exact spherical equivalent alongside the nearest 0.25 D power.

Principal Meridians

Shows the optical power in the cylinder axis meridian and in the principal meridian 90° away.

Vertex Distance / Contact Lens Plane

Compensates the entered spectacle prescription when the back vertex distance (BVD) changes.

Tested / prescribed BVD (mm)

Dispensed BVD (mm)

For sphero-cyl prescriptions, each principal meridian is vertexed separately before the prescription is reconstructed.

Vertical Imbalance / Slab-off

Calculates vertical prism at a specified reading point using the power in the 90° meridian, then shows the resulting binocular imbalance and slab-off options.

Reading point below distance optical centre (mm)

Calculation meridian

Conventional slab-off adds base-up prism to the more-minus / less-plus lens in the 90° meridian. Reverse slab-off applies base-down prism to the opposite lens. The tool shows the calculated compensation; clinical judgement and laboratory availability still determine whether slab-off is appropriate.

Professional use

This toolkit is intended as a calculation aid for trained optical professionals. Results should be interpreted alongside the prescription, patient needs, frame/lens characteristics, laboratory requirements and professional judgement.