Circular Polarizers

If you’ve compared American Polarizers Inc’s circular polarizer film datasheets, you’ve probably noticed retardation values like 125nm, 140nm, or 165nm listed alongside the product name. These numbers aren’t arbitrary, they’re the key spec that determines how well a circular polarizer performs at a given wavelength, and choosing the wrong one can mean the difference between excellent performance and a film that has, shall we say, less than excellent performance.

A circular polarizer is built from two layers: a linear polarizer and a quarter-wave retarder laminated with its fast axis at 45° axis to the linear polarizer’s axis. The retarder’s job is to introduce a phase shift between the two orthogonal components of the light passing through it. True circular polarization only happens when that phase shift equals exactly one quarter of the wavelength of the light involved, hence “quarter-wave.” Those numbers (such as 140nm) represent that quarter wave shift. Often that shift is referred to as the Optical Plane Difference or OPD. If you want to know where the performance is centered, multiply that 140nm by four and you get 560nm.

Because different applications often use light centered on different wavelengths, the physical retardation needed to hit that quarter-wave condition changes too. A retarder tuned for 560nm (roughly the center of the visible spectrum, where human eyes are most sensitive), thus a OPD value at 140nm. Shift the target wavelength toward blue or toward red, or design for a specific laser line instead of broadband visible light, and the ideal retardation value moves with it. Which is why we offer circular polarizers films not only for 560nm such as APNCP37-010-STD (OPD 140nm), but also ones centered at 660nm APNCP37-010-165NM (OPD 165nm, special order item). Note that all our circular polarizer films can also be laminated onto acrylic and glass.

For example:

https://www.apioptics.com/product/apncp37-010t-std/
https://www.apioptics.com/product/apncp37-072a-std/
https://www.apioptics.com/product/apncp37-105g-ar2/

This is also where broadband, or “achromatic,” circular polarizers come in. A single quarter-wave layer only hits the ideal phase shift at one specific wavelength; everywhere else in the spectrum, the polarization is slightly elliptical rather than perfectly circular, which shows up as color shift or incomplete glare rejection. Achromatic designs stack multiple retarder layers with different orientations to hold performance more consistent across the full visible band, at the cost of added thickness and complexity. We have recently developed a very high, multi-layer wave plate that has amazing performance across the visible spectrum. Also, our Near Achromatic Quarter Wave Plate APQW92-002-PC-143NMHE offers excellent performance from a single layer and can be combined with a linear polarizer to make a very cost effective circular polarizer.

https://www.apioptics.com/product/apqw92-002-pc-143nmhe/
https://www.apioptics.com/wp-content/uploads/2018/06/Quarter-Wave-high-and-standard-extinction-polycarbonate-140nm-12-15-20.pdf

The practical takeaway: if your application operates across the full visible spectrum, such as photography or general display anti-reflection, a broadband-optimized retardation value is usually the right call. If it’s tied to a specific wavelength, such as a sensor, a monochrome sensor, or a specific LED output, a retarder tuned closer to the peak system wavelength will perform better, since it hits the quarter-wave at its optimized OPD.

Photographers reach for circular polarizer filters constantly to cut reflections off water and glass, deepen blue skies, and reduce haze. But there’s a reason cameras use circular polarizers specifically, rather than the simpler (and cheaper) linear polarizer: linearly polarized light interferes with autofocus and metering systems.

Modern cameras rely on beam-splitters inside the autofocus and metering path to divert some incoming light to sensors that aren’t the main image sensor. Beam-splitters are polarization-sensitive, they respond differently depending on the orientation of linearly polarized light hitting them. If you mounted a linear polarizer filter on the lens, rotating it to control glare would simultaneously change how much light reaches the autofocus and metering sensors, causing inconsistent exposure or autofocus issues.

A circular polarizer filter, which is made of a linear polarizer and quarter wave plate, solves this by having the quarter wave plate facing the camera and the polarizer facing out at the subject matter. The added wave plate converts the linearly polarized light into circularly polarized light before it enters the camera body. Circularly polarized light behaves uniformly regardless of orientation, so the beam-splitter sees a consistent signal no matter how the filter is rotate, meaning you get full control over glare and reflections without disrupting autofocus or metering accuracy.

The color-shift problem is closely related. Because the quarter-wave retarder’s performance depends on wavelength (as covered in our retardation post), a well-matched, broadband-optimized circular polarizer maintains consistent color rendition across the visible spectrum as it’s rotated. For all our camera filters, we use a broad band 560nm (140nm OPD) wave plate to make the circular polarizers. This helps give a broad performance across the visible spectrum and can help reduce color shift issues, often seen a blue or amber tint, which becomes more noticeable at certain filter rotation angles.

For lens manufacturers and camera system integrators, this means the retardation layer’s spectral performance is just as important as the linear polarizing layer’s extinction ratio. A film optimized for broadband visible performance, laminated with a durable, scratch-resistant anti reflective coating suited to front-of-lens use, is the right specification target for imaging applications.

Look at almost any OLED smartphone or television screen and you’re looking through a circular polarizer, whether you realize it or not. It’s one of the most widely deployed uses of circular polarizer film in consumer electronics, and it solves a problem that’s specific to how OLED panels are built.

OLED pixels sit on top of a reflective metal layer used for the electrode structure underneath. Without any mitigation, ambient light hitting the display would reflect off that metal layer and back out toward the viewer, washing out contrast and making the screen look washed-out or mirror-like in bright environments, especially outdoors.

A circular polarizer laminated to the front of the panel fixes this through a clever one-way trick. Incoming ambient light passes through the linear polarizer and quarter wave retarder layer, becoming circularly polarized. When that light reflects off the metal layer beneath the OLED pixels, its circular polarization handedness reverses. On the way back out through the retarder the light becomes linearly polarized again but it is 90º to the polarizer’s linear axis, thus the rotated linear polarized light is now blocked rather than transmitted, effectively trapping the reflection and preventing it from reaching the viewer’s eye.

Critically, this only affects reflected ambient light. Light generated by the OLED pixels themselves travels outward through the polarizer stack in the forward direction and is transmitted normally, so the actual image isn’t blocked, only the unwanted reflections are.

This same principle extends to other applications, such as sensors, where reducing glare and improving contrast matters, and it’s a major reason circular polarizer film demand has grown alongside the expansion of OLED manufacturing and machine automation. We offer a number of different transmissions and laminates for our customers unique image capture requirements.