Slab off lenses Calculation, Slab off Prism

What are Slab off lenses?

Slab Off Meaning:

Slab off prism, or bicentric grind, is a valuable technique in the field of optics, addressing the unique needs of individuals with significant differences in prescription between their eyes.

Eye care professionals use precise calculations and grinding techniques to create lenses that provide improved visual comfort and binocular vision. Understanding the principles behind slab off prism can help both practitioners and patients appreciate the intricacies involved in achieving optimal vision correction.

We have written a detailed article on what are slab off prism? In this article we will do some more Slab off lenses calculation

Bicentric grind method for slab off Prism

One specialized technique used in lens crafting is the slab off prism is known as bicentric grind. This method involves inducing a vertical prismatic effect in the lower half of a lens to correct certain visual conditions. In this article, we will explore the slab off prism, its applications, and the process involved in its implementation.

Image credit- Zeiss.com

The Basics of Slab Off Prism:

The primary purpose of slab off prism is to address the optical imbalance that may occur in individuals with prescriptions that differ significantly between their eyes. This discrepancy can lead to difficulties in binocular vision, causing discomfort and visual disturbances. Slab off prism is particularly useful in cases where there is a significant difference in the prescription between the two eyes.

Process of Slab Off Prism calculation:

1. Identifying Lens Power and Meridian: The first step in the slab off prism process is to determine the lens power at the 90-degree meridian. This involves assessing the prescription and identifying the optical needs of the individual.
2. Determining Reading Depth: The reading depth, often referred to as the “seg drop,” is the distance from the optical center to the reading portion of the lens. This parameter is crucial in calculating the slab off prism effect.
3. Applying Prentice’s Rule: Prentice’s Rule is a fundamental principle in optics that relates prism power to lens power and the distance from the optical center. By applying this rule, eye care professionals can determine the amount of prism required to achieve the desired correction.
4. Grinding Base-Up: Once the necessary calculations have been made, the lens is ground away at the bottom back surface, leaving a base-up effect. This adjustment helps balance the prescription and ensures that the eyes work together more harmoniously.

Slab Off Prism Range:

Slab off prism typically ranges from 1.5ΔBU (Base-Up) to 6ΔBU, progressing in half-diopter increments. The selection of the specific prism power depends on the individual’s prescription and the magnitude of the difference between the two eyes.

Slab Off formula

Patient Prescription: OD: -3.50, OS: -2.00, ADD: +2.25

Reading Parameters: Distance below seg line: 7 mm Frame height: 45 mm Seg height: 18 mm

Calculations:

• Lens power: OD -3.50, OS -2.00
• Seg drop: (45/2) – 18 = 9 mm
• Reading depth from optical center: 9 mm + 7 mm = 16 mm
• Prentice’s Rule:
  • OD: 1(3.50) = 3.50ΔBD (base down)
  • OS: 1(2.00) = 2.00ΔBD (base down)
• Imbalance: 3.50 – 2.00 = 1.50Δ
• Slab off correction: OD 1.50ΔBU (base up)

Step by step method-

Distance below seg line: 7 mm Frame height: 45 mm Seg height: 18 mm

Step 1: Identify Lens Power

• Lens power: OD -3.50, OS -2.00

Step 2: Determine Reading Depth

• Calculate seg drop: (45/2) – 18 = 9 mm
• Reading depth from optical center: 9 mm + 7 mm = 16 mm

Step 3: Apply Prentice’s Rule

• Prentice’s Rule: Δ = cF (c is the distance from the optical center in centimeters, F is the lens power in diopters)
  • OD: 1(3.50) = 3.50ΔBD (base down)
  • OS: 1(2.00) = 2.00ΔBD (base down)

Step 4: Find Imbalance

• Imbalance: 3.50 – 2.00 = 1.50Δ

Step 5: Apply Slab Off Correction

• Since both eyes have the same base direction (base down), subtract the values to find the correction:
  • Slab off correction: OD 1.50ΔBU (base up)

For this patient, a slab off correction of OD 1.50ΔBU is recommended to address the vertical imbalance induced by the significant difference in prescription between the two eyes. This correction helps achieve better binocular vision and visual comfort for the patient.

Example 2:

Patient Prescription: OD: -6.00, OS: -4.75, ADD: +2.50

Reading Parameters: Distance below seg line: 6 mm Frame height: 55 mm Seg height: 22 mm

Calculations:

• Lens power: OD -6.00, OS -4.75
• Seg drop: (55/2) – 22 = 6.5 mm
• Reading depth from optical center: 6.5 mm + 6 mm = 12.5 mm
• Prentice’s Rule:
  • OD: 1(6.00) = 6.00ΔBD (base down)
  • OS: 1(4.75) = 4.75ΔBD (base down)
• Imbalance: 6.00 – 4.75 = 1.25Δ
• Slab off correction: OD 1.25ΔBU (base up)

Example 3:

Patient Prescription: OD: -2.25, OS: -3.75, ADD: +2.00

Reading Parameters: Distance below seg line: 8 mm Frame height: 50 mm Seg height: 17 mm

Calculations:

• Lens power: OD -2.25, OS -3.75
• Seg drop: (50/2) – 17 = 8.5 mm
• Reading depth from optical center: 8.5 mm + 8 mm = 16.5 mm
• Prentice’s Rule:
  • OD: 1(2.25) = 2.25ΔBD (base down)
  • OS: 1(3.75) = 3.75ΔBD (base down)
• Imbalance: 3.75 – 2.25 = 1.50Δ
• Slab off correction: OS 1.50ΔBU (base up)

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