What Are Blocking and Layout?
Layout and blocking are the preparatory steps that occur before a lens is placed in the edger. Layout determines the exact position and orientation of the optical center relative to the frame shape. Blocking physically secures the lens so the edger can cut it while maintaining that precise positioning. Together, these steps ensure that the finished lens has its optical center aligned with the patient's pupil and the cylinder axis oriented correctly.
The Layout Process
Layout involves marking the lens with critical reference points:
- Identify the optical center: Using the lensmeter, find the point on the lens where the sphere and cylinder powers are centered and there is no prismatic effect (unless prism is prescribed).
- Mark the optical center: Use the lensmeter's built-in marking device to place three dots on the lens: one at the optical center and two on either side along the horizontal axis line.
- Verify cylinder axis: Ensure the horizontal line connecting the dots aligns with the correct axis orientation.
- Calculate decentration: Determine how far the optical center must be shifted from the geometric center of the frame to align with the patient's PD.
- Mark the layout position: Position the lens so the optical center will be in the correct location after edging, accounting for the decentration.
Decentration in Layout
Decentration is the amount the optical center is moved from the geometric center of the lens opening. The calculation:
- Per-eye decentration = (Frame PD - Patient PD) / 2
- The optical center is moved inward (nasally) when the frame PD is larger than the patient PD (most common)
For progressive lenses, the fitting cross (not the distance optical center) must be positioned at the patient's pupil center, and the horizontal reference line must be level.
The Blocking Process
Once layout is complete, the lens is blocked for the edger:
- Apply a block pad: A sticky pad (often called a lens pad or lap pad) is placed on the front surface of the lens at the layout position
- Attach the block: A metal or plastic block (also called a chuck) is pressed onto the pad, adhering firmly to the lens
- Verify alignment: Confirm the block is centered on the correct layout position and the axis alignment marks are properly oriented
- Load into edger: The block fits into the edger's chuck, holding the lens in the exact position needed for cutting
- Layout determines the optical center position and axis orientation on the lens
- Blocking secures the lens for edging using an adhesive pad and block chuck
- Decentration moves the optical center to align with the patient's pupil position
- Per-eye decentration = (Frame PD - Patient PD) / 2
- For progressives, the fitting cross, not the distance OC, is positioned at the pupil center
- Verify block position and axis alignment before starting the edger
The block serves as the lens's "handle" during edging. It holds the lens securely while the edger grinds around it. If the block shifts during edging, the entire lens will be cut off-center, ruining the job.
Blocking for Different Lens Types
| Lens Type | Block Position Reference | Special Consideration |
|---|---|---|
| Single vision | Optical center | Simple decentration for PD |
| Bifocal | Optical center + seg position | Seg height must match order |
| Progressive | Fitting cross | Use lens engravings for positioning |
| Prism Rx | Prism reference point | Account for prism in layout |
Automated Layout and Blocking
Modern optical labs use automated blockers that receive data from the lensmeter and frame tracer electronically. The machine calculates the correct block position based on the prescription, PD, seg height, and frame shape, then places the block automatically. This reduces human error and speeds up production.
Even with automation, understanding the manual process is important for troubleshooting and for the ABO exam.
Clinical Relevance
Blocking and layout errors are among the most common causes of lens remakes. A lens with the wrong optical center position, incorrect axis orientation, or misplaced seg height must be redone entirely. Understanding the process helps opticians communicate effectively with the lab and identify the root cause when a patient reports visual discomfort with new glasses.