Articles: Monitors
 

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This review will cover two very new monitors from Samsung: SyncMaster F2080 and F2380. Most of the company’s new products released in the last months are meant for home use and feature shiny plastic, bright buttons and other eye-catching elements, but the F2080 and F2380 are work-oriented semiprofessional monitors.

But the most interesting thing about these models is that they are based on a new type of PVA matrix called C-PVA. Samsung says C-PVA will bring the pricing of PVA-based monitors down to the level of TN-based ones while retaining all of the benefits of PVA technologies such as high contrast ratio and good viewing angles.

C-PVA Matrix

Although Samsung did not keep the matrix type secret since the very announcement of the new monitors and has even touted C-PVA technology, it is still unclear what C-PVA really is and what differentiates it from the familiar PVA and S-PVA technologies. Samsung does not even say what the letter C stands for, giving rise to numerous guesses from the derisive “Crap PVA” to the incidental coincidence with the CP-VA abbreviation that was used a few years ago in research papers on VA matrix modernization.

I will dwell upon the latter version for a while because it is discussed widely on the Internet. The CP-VA abbreviation can be found in the work called Optical Analysis of Vertical Aligned Mode on Color Filter Liquid-Crystal-on-Silicon Microdisplay and is spelled out as Circularly Polarized Vertical Alignment. As you probably know, an LCD panel is a sandwich made out of a backlight unit that emits non-polarized white light, a polarizer that transforms it into linearly polarized light, a liquid crystal layer that turns the plane of polarization of the passing light by the angle which depends on the voltage applied to the crystals, and a second polarizer that is often called analyzer.

As you may know from a course of physics, the portion of polarized light that passes through the analyzer equals sin²(α/2), where α is the angle between the polarization planes of the light and the analyzer. Again, the angle can be changed by applying different voltage to the liquid crystals and thus the brightness of the LCD panel can be changed. Different types of crystals can be used in specific LCD panels, phase-shifting plates or some other additions can be implemented, but the general principle remains the same.

In one LCD panel type called MVA (Multidomain Vertical Alignment), each pixel is divided into four parts called domains in which the crystals turn at different angles. Without delving deep into detail, I can just say that this is necessary to ensure good viewing angles: the light of each individual domain depends on the angle of view, but the four differently oriented domains yield the same average brightness irrespective of whether you are looking at them directly, from a side, or from above.

One of insignificant, yet persistent problems of MVA matrixes is the dark line crossing each pixel at the junction point of the neighboring domains. It lowers the overall transparency of the pixel and reduces the efficiency of the LCD panel, which is itself very low to start with – about 4% (that is, if the whole panel shows pure white, only 4% of the light produced by the backlight lamps pass through it).

The mentioned work described a solution of this problem using circularly polarized light. The issue is discussed at more detail in the works of two Japanese authors published in the Japan Journal of Applied Physics: “Transmittance Enhancement for Randomly Aligned Liquid Crystal Displays with Circular Polarizers” and “Transmitted Light Enhancement of Electric-Field-Controlled Multidomain Vertically Aligned Liquid Crystal Displays Using Circular Polarizers and a Cholesteric Liquid Crystal Film”. I want to note that the CP-VA abbreviation does not actually occur in these works.

From a technical point of view, a matrix with circular polarization differs from an ordinary one with two quarter-wave plates (i.e. plates that produce a phase difference of λ/4 between differently polarized light beams, where λ is the wavelength of the light passing through; λ is usually equal to 550 nanometers if the entire visible range is to be covered) located on the interior sides of the polarizer and analyzer. A λ/4 plate has such properties that it transforms linearly polarized light into circularly polarized light and vice versa. By the way, photographers often deal with such plates. Every circular polarization filter is made from a linear polarizer and a λ/4 plate. As a result, its output light has circular polarization instead of linear polarization which might confuse the auto-focus system of many cameras.


Matrixes with linear (a) and circular (b) polarization
(Jpn. J. Appl. Phys. Vol. 42 (2003), L 51 — L 53)

Thus, the light from the backlight passes through the polarizer, transforming into linearly polarized light. Then it goes through the λ/4 plate, which changes its polarization into circular. And then it goes through a layer of liquid crystals and through one more λ/4 plate which changes circular polarization back into linear. And the analyzer comes last. From the end viewer’s standpoint, there is only one difference. This LCD panel does not have dark lines at the junction points of the neighboring domains, which increases the maximum transparency and efficiency of the LCD panel. The mentioned papers prove that the efficiency of CP panels is about one and a half that of LP panels (LP stands for Linear Polarization), so a backlight unit with fewer or less intensive lamps can be used to achieve the same resulting brightness.

 
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