What is the Maltese cross pattern in a polariscope?

The Maltese cross pattern is a distinctive four-armed cross shape that appears at the center of a properly heat-tempered glass lens when viewed through a polariscope. It indicates that the lens has uniform compressive surface stress, which provides impact resistance. A clear, centered, symmetrical Maltese cross confirms the lens meets tempering standards. Its absence or distortion indicates improper tempering.

What causes stress patterns in eyeglass lenses?

Stress patterns in lenses can be caused by improper manufacturing (molding defects), frame pressure from an overtight eyewire, forced insertion during mounting, excessive heat during processing, or concentrated stress at drill points in rimless frames. Internal stress can degrade optical quality and reduce impact resistance.

How do you use a polariscope to check lenses?

Place the lens between the two polarizing filters of the polariscope. For tempered glass lenses, look for the Maltese cross pattern at center. For plastic lenses, look for stress patterns that indicate manufacturing defects or frame-induced stress. Colored bands or fringes indicate areas of stress. Minimal or no pattern is ideal for plastic lenses.

Can a polariscope identify polarized lenses?

Yes. Hold the lens in front of the polariscope and rotate it slowly. A polarized lens will show dramatic alternation between bright and dark as its polarization axis aligns with or opposes the analyzer filter. A non-polarized lens does not show this alternating effect. This is a quick, reliable way to identify polarized lenses.

Polariscope: Detecting Stress and Verifying Tempered Glass Lenses

What Is a Polariscope?

A polariscope (also called a colmascope or strain viewer) is an optical instrument that uses polarized light to reveal internal stress in transparent materials. When polarized light passes through a stressed material, the stress alters the light's polarization, creating visible color patterns. These patterns indicate where and how much stress is present in the lens or frame material.

How the Polariscope Works

The instrument consists of two polarizing filters:

The polarizer (bottom filter): Creates the polarized light beam
The analyzer (top filter): Set perpendicular to the polarizer so that no light passes through when no specimen is present

When a lens or frame is placed between these two filters, any internal stress rotates the polarization of the light passing through it, allowing some light to pass through the analyzer. The resulting visible patterns (called isochromatic fringes) appear as colored bands that indicate the location and intensity of stress.

Verifying Tempered Glass Lenses

One of the most important uses of the polariscope is verifying that glass lenses have been properly heat-tempered for impact resistance. When a glass lens has been correctly tempered, viewing it through the polariscope reveals a distinctive Maltese cross pattern (also described as a butterfly or four-leaf clover shape) at the center of the lens.

The Maltese cross pattern in a tempered glass lens confirms that the lens has uniform compressive stress on its surface, which is what provides impact resistance. A glass lens without this pattern may not have been properly tempered and may not meet FDA impact resistance requirements. This is a patient safety issue.

The Maltese cross pattern should be:

Centered in the lens
Symmetrical
Clearly visible (distinct pattern, not faint or distorted)

An off-center, distorted, or absent pattern indicates improper tempering. The lens should be returned to the lab for re-tempering or replacement.

Detecting Stress in Plastic Lenses

Plastic lenses should ideally show minimal stress patterns on the polariscope. Excessive stress in a plastic lens can indicate:

Improper molding: The lens blank was not manufactured correctly
Frame pressure: The frame eyewire is too tight, squeezing the lens
Improper insertion: The lens was forced into the frame during edging or mounting
Thermal stress: The lens was exposed to excessive heat during coating or processing

Stress in a plastic lens can cause:

Optical distortion that degrades image quality
Reduced impact resistance
Potential for cracking or fracturing

Ignoring stress patterns in a finished pair of glasses. If a patient complains of visual distortion or discomfort that cannot be explained by the prescription, check the lenses on a polariscope. Frame-induced stress from an overtight eyewire is a surprisingly common cause of visual complaints that goes undetected without the polariscope.

Checking Rimless and Semi-Rimless Frames

The polariscope is particularly useful for checking stress at drill points in rimless frames and groove areas in semi-rimless frames. These mounting points concentrate mechanical stress, and excessive stress at these locations can lead to lens cracking.

After mounting a lens in a rimless frame, viewing the drill areas through the polariscope reveals whether the screws are too tight or the mounting hardware is causing excessive localized stress.

Identifying Polarized Lenses

A secondary use of the polariscope is quickly identifying polarized lenses. When a polarized lens is rotated in front of the polariscope, the transmitted light alternates between bright and dark as the lens's polarization axis aligns with or opposes the instrument's filters. A non-polarized lens does not show this alternating pattern.

To quickly check if a lens is polarized, hold it in front of a polariscope (or even another known polarized lens) and rotate it. Polarized lenses will show a dramatic brightening and darkening effect at 90-degree intervals. This is useful when a patient brings in unknown lenses and you need to identify whether they are polarized.

Clinical Relevance

The polariscope serves as a quality assurance tool for both lens safety and optical quality. Checking tempered glass lenses verifies impact resistance compliance. Checking plastic lenses after frame insertion catches stress-related problems before they affect the patient. It is a quick, non-destructive test that adds a valuable layer of quality control to the dispensing process.

Key Takeaways

The polariscope uses crossed polarizing filters to reveal internal stress in transparent materials
A properly tempered glass lens shows a Maltese cross pattern at center
Absent or distorted Maltese cross indicates improper tempering (safety concern)
Excessive stress in plastic lenses can cause optical distortion and reduced impact resistance
Check drill points and groove areas in rimless/semi-rimless frames for localized stress
The polariscope can also identify polarized lenses through the brightening/darkening rotation effect