What Are Newton Rings? Causes, Prevention & Solutions
Newton rings are an optical effect that can happen when two smooth, transparent surfaces sit very close together or come into direct contact. They are especially common in display applications where a clear acrylic window is installed directly in front of an LCD.
The good news is that the issue is usually preventable. In many cases, a small change to the surface of the acrylic is enough to keep the two materials from interacting optically.
Astra Products supplies CLAREX optical-grade acrylic materials with surface finishes designed specifically for applications like this, including very light matte textures that can help reduce or eliminate Newton rings.
OVERVIEW
What are Newton Rings?
Newton rings are visible interference patterns that form when light reflects between two transparent surfaces that are extremely close together.
They often appear as rainbow-colored rings, curved lines, or uneven bands across a display. The exact pattern can vary depending on how the two surfaces are touching and how much space exists between them.
In a laboratory setting, Newton rings are often shown as perfectly round, concentric circles. In a real display assembly, they are usually less uniform.
Either way, the result is the same: a distracting pattern that can reduce the visual quality of the display.
Newton rings can affect:
- Screen readability
- Overall display uniformity
- Image clarity and color appearance
- The perceived quality of the finished product
AT A GLANCE
Caused by surface contact.
Formed when two smooth, transparent surfaces sit directly against each other, creating an optical interference that appears as rainbow-colored rings or patterns.
Commonly occurs in protective display windows.
Often occurs when a smooth acrylic window is installed directly against an LCD or other flat-panel display without a gap.
A light texture can help prevent them.
CLAREX SM90 adds a very subtle matte texture that reduces direct surface contact.
HOW IT WORKS
How Are Newton Rings Formed?
The easiest way to understand how Newton rings are formed is to think about two very smooth surfaces resting against each other.
A common example is a clear acrylic protective window placed directly over an LCD.
Both surfaces may look perfectly flat, but at a microscopic level there are tiny variations in the space between them. Light reflects off these closely spaced surfaces and travels slightly different distances.
When those reflected light waves meet, they interfere with each other. Some wavelengths reinforce each other while others cancel each other out. That interference is what creates the visible rings and color patterns.
The problem is especially common when:
- A protective window sits directly against an LCD
- There is little or no intentional gap between the surfaces
- Both surfaces are smooth and glossy
- Pressure causes the materials to contact one another unevenly
There may be nothing wrong with either material individually. The issue comes from the way the two smooth surfaces interact once they are assembled.
Where Are Newton Rings Most Common?
At Astra, one of the most common situations is a customer adding a protective acrylic window in front of an LCD or other electronic display.
Newton rings can show up in a wide range of applications, including:
LCD Protective Windows
Acrylic windows are often used to protect displays from impact, dust, chemicals, or general wear. If the rear side of the acrylic sits directly against the LCD, Newton rings can appear.
Instrumentation Displays
Meters, gauges, medical equipment, test equipment, and other electronic systems may use transparent covers where optical clarity is critical.
HMIs and Control Panels
Industrial HMIs often use protective windows on touchscreens and displays. In compact assemblies, there may not be enough space to maintain a large air gap between layers.
Flat-Panel Displays
Any layered display assembly that places two smooth transparent surfaces very close together can potentially develop Newton rings.
How to Get Rid of Newton Rings
If you are trying to figure out how to get rid of Newton rings, the main goal is simple: keep two perfectly smooth surfaces from forming close, uniform contact. There are a few ways to do that.
1. Maintain a Small Gap Between the Surfaces
If the design allows it, one solution is to keep the protective window physically separated from the display.
Spacers, gaskets, bezels, or other mechanical features can be used to maintain a consistent gap.
This is often effective, but it is not always practical. Many modern display assemblies have very limited space, and designers may want the protective window to sit as close to the LCD as possible.
2. Add a Very Light Surface Texture
Another option is to add a subtle texture to the side of the acrylic that faces the display.
This is often the most practical solution.
The texture prevents the two surfaces from sitting completely flat against each other. Instead of forming a broad optical bond, they only make contact at very small points.
That small difference can be enough to prevent Newton rings from forming.
THE SOLUTION
How CLAREX SM90 Helps Prevent Newton Rings
When acrylic needs to sit very close to a display, Astra will often recommend CLAREX SM90, a very fine non-glare texture applied to one of the mating surfaces.
SM90 is a subtle surface finish. It is much lighter than what most people would picture when they hear “matte acrylic.”
The texture is barely noticeable, but it gives the surface enough variation to keep it from bonding optically with the smooth display underneath. There is technically a small increase in haze because of the texture, but the effect is minimal compared with more aggressive non-glare finishes.
That makes SM90 a good option when the goal is to prevent Newton rings without noticeably changing the appearance of the display.
Birefringence Under Polarized Light
Comparing stressed clear plastic vs low-stress optical acrylic.
Smooth Surface
Direct contact creates an optical bond.
Full surface-to-surface contact traps a thing firm of air or moisture.
Newton rings/interference pattern
Interference causes visible rainbow rings.
SM90 Textured surface
Micro-texture prevents self-bonding.
The micro-texture reduces real contact area, preventing full surface-to-surface bonding.
Helps eliminate Newton rings.
The air gap and reduced contact prevent interference, keeping the view clear.
Choosing the Right Surface Finish
There is no single surface finish that works for every display.
A heavily textured non-glare surface may be useful when reducing reflections and glare is the main priority. But if the goal is simply to stop Newton rings, a much lighter texture may be the better choice.
The key is using enough texture to prevent the surfaces from bonding without adding more haze than the application can tolerate.
Astra can also help account for other requirements such as:
- Optical clarity
- Light transmission
- Glare reduction
- Scratch resistance
- Chemical resistance
- Thickness
- Color
- Custom fabrication
- Additional optical coatings
See Astra’s CLAREX customization options for more information
Why Start With Optical-Grade Acrylic?
Fixing Newton rings should not come at the expense of overall display quality.
CLAREX optical-grade cell-cast acrylic provides a high-quality starting point for protective windows and other optical applications.
From there, the material can be customized with different textures, coatings, thicknesses, and fabrication options depending on what the final assembly needs.
That is important because Newton ring prevention is usually only one part of the specification. A protective window may also need to withstand abrasion, reduce reflections, filter certain wavelengths, or meet tight dimensional requirements.
Looking at the complete display stack early in the design process usually produces a better result than trying to solve each issue separately.
Need Help Preventing Newton Rings in Your Display?
Newton rings are frustrating, but the solution is often relatively simple.
If a protective window is sitting directly against a display, adding a very light surface texture can prevent the two smooth surfaces from forming the optical contact that creates the interference pattern.
For many applications, CLAREX SM90 provides that texture while keeping haze and visual impact to a minimum.
Astra Products can help review your display requirements, recommend the right CLAREX material and surface finish, and provide samples for testing.