IPS (in-plane switching) is a screen technology for liquid-crystal displays (LCDs). In IPS, a layer of liquid crystals is sandwiched between two glass surfaces. The liquid crystal molecules are aligned parallel to those surfaces in predetermined directions (in-plane). The molecules are reoriented by an applied electric field, whilst remaining essentially parallel to the surfaces to produce an image. It was designed to solve the strong viewing angle dependence and low-quality color reproduction of the twisted nematic field effect (TN) matrix LCDs prevalent in the late 1980s.[1]
The True depth method was the only viable technology for active matrix TFT LCDs in the late 1980s and early 1990s. Early panels showed grayscale inversion from up to down,[2] and had a high response time (for this kind of transition, 1 ms is visually better than 5 ms). In the mid-1990s new technologies were developed—typically IPS and vertical alignment (VA)—that could resolve these weaknesses and were applied to large computer monitor panels.
One approach patented in 1974 was to use inter-digitated electrodes on one glass substrate only to produce an electric field essentially parallel to the glass substrates.[3] [4] However, the inventor was not yet able to implement such IPS-LCDs superior to TN displays.
After thorough analysis, details of advantageous molecular arrangements were filed in Germany by Guenter Baur et al. and patented in various countries including the US on 9 January 1990.[5] The Fraunhofer Society in Freiburg, where the inventors worked, assigned these patents to Merck KGaA, Darmstadt, Germany.
Shortly thereafter, Hitachi of Japan filed patents to improve this technology. A leader in this field was Katsumi Kondo, who worked at the Hitachi Research Center.[6] In 1992, engineers at Hitachi worked out various practical details of the IPS technology to interconnect the thin-film transistor array as a matrix and to avoid undesirable stray fields in between pixels.[7] [8] Hitachi also improved the viewing angle dependence further by optimizing the shape of the electrodes (Super IPS). NEC and Hitachi became early manufacturers of active-matrix addressed LCDs based on the IPS technology. This is a milestone for implementing large-screen LCDs having acceptable visual performance for flat-panel computer monitors and television screens. In 1996, Samsung developed the optical patterning technique that enables multi-domain LCD. Multi-domain and in-plane switching subsequently remain the dominant LCD designs through 2006.[9]
Later, LG Display and other South Korean, Japanese, and Taiwanese LCD manufacturers adopted IPS technology.
IPS technology is widely used in panels for TVs, tablet computers, and smartphones. In particular, most IBM products were marketed as Flexview from 2004 to 2008 with IPS LCDs with CCFL backlighting, and all Apple Inc. products were marketed with the label Retina Display[10] [11] with LED backlighting since 2010.
| Name | Nickname | Year | Advantage | Transmittance or contrast ratio | Remarks | |
|---|---|---|---|---|---|---|
| Super TFT | IPS | 1996 | Wide viewing angle | 100/100 Base level | Most panels also support true 8-bit-per-channel colour. These improvements came at the cost of a lower response time, initially about 50 ms. IPS panels were also extremely expensive. | |
| Super-IPS | S-IPS | 1998 | Colour shift free | 100/137 | IPS has since been superseded by S-IPS (Super-IPS, Hitachi Ltd. in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing. | |
| Advanced Super-IPS | AS-IPS | 2002 | High transmittance | 130/250 | AS-IPS, also developed by Hitachi Ltd. in 2002, improves substantially on the contrast ratio of traditional S-IPS panels to the point where they are second only to some S-PVAs. | |
| IPS-Provectus | IPS-Pro | 2004 | High contrast ratio | 137/313 | The latest panel from IPS Alpha Technology with a wider colour gamut and contrast ratio matching PVA and ASV displays without off-angle glowing. | |
| IPS Alpha | IPS-Pro | 2008 | High contrast ratio | Next generation of IPS-Pro | ||
| IPS Alpha Next-Gen | IPS-Pro | 2010 | High contrast ratio |
| Name | Nickname | Year | Remarks | |
|---|---|---|---|---|
| Horizontal IPS | H-IPS | 2007 | Improves contrast ratio by twisting electrode plane layout. Also introduces an optional Advanced True White polarizing film from NEC, to make white look more natural. This is used in professional/photography LCDs. | |
| Enhanced IPS | E-IPS | 2009 | Wider aperture for light transmission, enabling the use of lower-power, cheaper backlights. Improves diagonal viewing angle and further reduce response time to 5 ms. | |
| Professional IPS | P-IPS | 2010 | Offer 1.07 billion colours (30-bit colour depth). More possible orientations per sub-pixel (1024 as opposed to 256) and produces a better true colour depth. | |
| Advanced High Performance IPS | AH-IPS | 2011 | Improved colour accuracy, increased resolution and PPI, and greater light transmission for lower power consumption.[14] |
In this case, both linear polarizing filters P and A have their axes of transmission in the same direction. To obtain the 90 degree twisted nematic structure of the LC layer between the two glass plates without an applied electric field (OFF state), the inner surfaces of the glass plates are treated to align the bordering LC molecules at a right angle. This molecular structure is practically the same as in TN LCDs. However, the arrangement of the electrodes e1 and e2 is different. Electrodes are in the same plane and on a single glass plate, so they generate an electric field essentially parallel to this plate. The diagram is not to scale: the LC layer is only a few micrometers thick, very thin compared with the distance between the electrodes.
The LC molecules have a positive dielectric anisotropy and align themselves with their long axis parallel to an applied electrical field. In the OFF state (shown on the left), entering light L1 becomes linearly polarized by polarizer P. The twisted nematic LC layer rotates the polarization axis of the passing light by 90 degrees, so that ideally no light passes through polarizer A. In the ON state, a sufficient voltage is applied between electrodes and a corresponding electric field E is generated that realigns the LC molecules as shown on the right of the diagram. Here, light L2 can pass through polarizer A.
In practice, other schemes of implementation exist with a different structure of the LC molecules for example without any twist in the OFF state. As both electrodes are on the same substrate, they take more space than TN matrix electrodes. This also reduces contrast and brightness.
Super-IPS was later introduced with better response times and colour reproduction.[15]
Toward the end of 2010 Samsung Electronics introduced Super PLS (Plane-to-Line Switching) with the intent of providing an alternative to the popular IPS technology which is primarily manufactured by LG Display. It is an "IPS-type" panel technology, and is very similar in performance features, specs and characteristics to LG Display's offering. Samsung adopted PLS panels instead of AMOLED panels, because in the past AMOLED panels had difficulties in realizing full HD resolution on mobile devices. PLS technology was Samsung's wide-viewing angle LCD technology, similar to LG Display's IPS technology.[24]
Samsung asserted the following benefits of Super PLS (commonly referred to as just "PLS") over IPS:[25]
In 2012 AU Optronics began investment in their own IPS-type technology, dubbed AHVA. This should not be confused with their long standing AMVA technology (which is a VA-type technology). Performance and specs remained very similar to LG Display's IPS and Samsung's PLS offerings. The first 144 Hz compatible IPS-type panels were produced in late 2014 (used first in early 2015) by AUO, beating Samsung and LG Display to providing high refresh rate IPS-type panels.[26] [27]
R. Soref, Liquid crystal electric field sensing measurement and display device, filed 28 June 1973.
K. Kondo, H. Terao, H. Abe, M. Ohta, K. Suzuki, T. Sasaki, G. Kawachi, J. Ohwada, Liquid crystal display device, filed 18 September 1992 and 20 January 1993.
