Enabling crisp full-motion video

With the recent widespread use of PC and online games, and PCs equipped with DVD drives give users more opportunities to see moving images such as those in 3D games or action movies on screen. This increase in motion picture content means computer monitors must be able to display not only still images, but moving ones as well.

Manufacturers and IT publications often cite a fast response time as an indication that a display can play videos or games with little or no blurring. Hence, we would like to share with you what response time is, and how helpful it is in determining how well an individual LCD display can portray moving images.

Response time:
Why is it increasingly important for LCD applications?

If response time is slow, the transition from one picture (or frame) to another can produce an afterimage or blurring effect. This problem occurs not only when looking at motion pictures, but also during scrolling. For this reason, panels with faster response times are typically recommended for displaying moving images. Listed below are calculations for the liquid crystal response times that LCD displays meet, with consistent reliability, for various application standards. Response time is measured in milliseconds (ms, 1/1000 second). The shorter the time frame, the better the display quality.

 
Crisp Low Response Rate at Left and High Response Rate at Right.
Notice the blurring that occurs (right image) with High Response Rates.

LCD RESPONSE RATES
30 ms:1/0.030 = 33 fps meets specs of NTSC (30 fps), PAL (25 fps) or movie (24 fps) standards
16 ms:1/0.016 = 63 fps meets the spec of HDTV (60 fps) standards
12 ms:1/0.012 = 83 fps meets VESA flicker-free display with CRT of 72 fps and human-eye perception
8 ms:1/0.008 = 125 fps 3D PC games requirement
4 ms:1/0.004 = 250 fps Professional 3D PC games requirement
fps = frame (picture) per second

What is response time?

The transition time when LC materials are rotating on each of the required white/black or gray levels is called “rise time” and “fall time,” respectively. Normally, the transition time of 256 x 256 LC rotation levels needs to be measured. However, some companies don’t measure degree levels due to limitations of equipment capability.

Liquid crystals are rarely completely turned on or off. Instead, they cycle in between gray states. The following are two common methods some manufacturers use to measure response time:

On-Off response time Refers to the change time for screen pixels to turn from white to black (Tr) and from black to white (Tf) when the screen receives the signal. However, it does not indicate the transit time between gray levels.

Gray-to-Gray response time:

Since virtually all moving images include gray levels, and the frequency of gray-to-gray transitions is typically far greater than black-and-white transitions, we use the Gray-to-Gray response time definition to address the gray-to-gray transition time, allowing us to make an accurate assessment of a displays’ suitability to portray moving images.

At present, there is no accepted standard for the computation of Gray-to-Gray response time. However, as a company that emphasizes product reliability, most manufacturers insist on using the average to gauge performance, delivering better value to the end user.

How some manufacturers accelerate response times and guarantees reliable products:

Lower rotational viscosity liquid crystal materials and reduced cell gap thickness enhance “On-Off Response Time” performance.

To rapidly improve liquid crystal on-off response time, some manufacturers have developed products with lower rotational viscosity liquid crystal materials and reduced cell gap thickness during the first stage.

Many manufacturers overcome technical challenges such as non-uniformity and side effects caused by new LC materials in the LC-cell manufacturing process. Furthermore, new products undergo strict testing before launch.

Higher voltage with overdriving technology reduces the moving image’s “Gray-to-Gray response time.”

These quick response times modeled with new LC materials and a thick cell gap have earned such products much praise in the market in terms of capability and reliability, encouraging their makers to keep seeking new technologies for product upgrades. Models with overdriving technology have been integrated into many LCD displays, from manufacturers such as Acer, accelerating response times, especially for gray-to-gray.

Faster gray-to-gray response time via overdrive (OD) technology

The key benefit of OD technology is the clear improvement of the gray-to-gray level, which is the most important factor in the moving-picture viewing experience. Liquid crystal molecules respond faster to the high voltage that’s needed for black-white transitions than to the low voltage that’s needed for transitions between gray areas. Therefore, even though going from one grayscale level to another is less of a jump than going from black to white, the gray-to-gray transition time can actually take longer. Two LCD panels with the same black-white response times but with different gray-to-gray response times will have different moving picture playback capabilities.

As the figures below show, using an overdriving algorithm, LCD displays can reduce the deviation in the transition time and approach ideal performance. This significant improvement allows LCDs to deliver high-quality moving pictures for 3D games and videos.

If you are a graphics professional you need accurate, predictable color. If you use an LCD display, then no matter how carefully you calibrate your display, you could have trouble seeing your work consistently.

The facts

The pixels in LCD panels work by passing polarized light through filter layers. To light up a pixel, the liquid crystal component in each pixel applies a ‘twist’ to the light after it passes through the first polarizing layer, making it able to pass through the second. Unlike CRT displays, where what you see is caused by the phosphor coating inside the glass tube being excited by electron beams, the light emitted from an LCD screen makes its way through multiple layers, channeled out through each pixel. This is the root cause of this technology’s biggest Achilles’ heel: view an LCD screen from straight on and you’ll see each pixel exactly as intended. But view it from far enough to one side – or above or below for that matter – and you won’t get the direct, face-on strength of the light beaming out of the pixels. To use a very crude analogy, it is a little like the difference between viewing a light at the base of a tube from face on or from off to one side. Only one viewpoint gets the full strength of the light as it shines out. The result is changes in the values of what’s shown on the screen depending on where you sit. This is clearly a disaster for color proofing, and something that no amount of calibration can help.

This problem has plagued LCD manufacturers for years, but the situation has improved immensely. The best modern LCD panels have all but eliminated this problem. Note the qualifier: the problem still exists, but at the top end of the market it is effectively negligible. Most decent modern displays are dramatically better than models from just a few years ago. In today’s desktop LCD screens at least you’re unlikely to see color and contrast inversions just by leaning to one side or looking down from a standing position, but you will still see a slight shift in hue and contrast if you move far enough away from straight on to the display. The problems now tend to show themselves as a minor contrast drop and a faint yellowing of whites instead. How far you have to move to see this is as critical a point as how much change there is, which is why most manufacturers give viewing angle specifications for their monitors. It is common to see figures of 140° or more, but this will be the point at which the display shows obvious, unmistakable changes rather than where visually critical users might first spot color drifts.

Older LCD screens had such a narrow field of view that merely leaning over a bit or just sitting up straight in the chair would produce obvious visual changes. This isn’t the case with newer displays, but it is worth noting that if you sit quite close to today’s larger panels your angle of view from one side to the other and from top to bottom can be great enough to produce colour shifts in objects simply though being in different parts of the screen. Although LCD technology is constantly improving, the increase in display sizes tends to make this somewhat of a ‘two steps forward, one step back’ situation, particularly with the very largest LCD panels now on offer. For example, despite using the the very best quality of LCD hardware available, this effect can still be seen in Apple’s 30in Cinema display. Making sure you sit at a reasonable distance from your screen can help reduce or even eliminate this, but it is something to remember if you do critical color work on your Mac.

Finally, make sure you keep everything in perspective. This shouldn’t be seen as a reason why you should stick with your old CRT if it is getting a bit long in the tooth. The unstoppable problem of phosphor aging means that CRTs will loose sharpness and brightness as time goes by, and a top-quality LCD will always beat all but the best CRTs.

ColorSync and color management

Calibration of LCD screens can’t be performed to the same degree as with CRT displays, but the best way to manage color lies more in the use of a well-managed color workflow. Use calibration hardware such as the Eye-One from Gretag Macbeth to produce full, accurate ColorSync profiles from your monitor’s display characteristics, then use those to define your display as part of your ColorSync settings. Once in place, ColorSync will manage how images are shown on that display, adjusting the presented appearance according to the particular slight variances in color presentation that the profile lists.

If you don’t have a hardware calibration device you can actually perform a limited calibration to produce a ColorSync profile with nothing but software and your own eyes. In the Displays pane in System Preferences, click the Color tab, then the Calibrate button and walk through the simple process. After making a few selections with sliders you can save and then select your new profile. Alternatively, SuperCal from bergdesign.com provides excellent display calibration including corrected gamma tables. Whichever route you choose, whether hardware-based or performed entirely in software, having an accurate ColorSync profile created specifically for your monitor is one of the keys to an efficient colour workflow.

source: http://www.thesmallest.com/lessonettes/lcdscreensandcol.html

How to Compare LCD Monitors Based on Specifications To Find The Right One

With manufacturing improving, LCD panel sizes continue to get larger all while prices keep dropping. Retailers and manufacturers throw around a lot of numbers and terms to describe their products. So, how does one know what all these mean? This article looks to cover the basics so one can make an informed decision when buying an LCD monitor.

Screen Size

The screen size is the measurement of the displayable area of the screen from the lower corner to the opposite upper corner of the display. LCD’s typically gave their actual measurements but they are now rounding those numbers. Be sure to find the real dimensions typically referred to as the actual screen size the whenever looking at a LCD.

Aspect Ratio

The aspect ratio refers to the number of horizontal pixels to vertical pixels in a display. Traditional displays used a 4:3 aspect ratio. Most new widescreen monitors use either a 16:10 or 16:9 aspect ratio. The 16:9 is the ratio typically used for HDTVs. Now a new breed of ultra wide monitors is coming to market. These have a near 2:1 width to high measurements.

Native Resolutions

All LCD screens can actually display only a single given resolution referred to as the native resolution. This is the physical number of horizontal and vertical pixels that make up the LCD matrix of the display. Setting a computer display to a resolution lower than this resolution will either cause extrapolation. This extrapolation attempts to blend multiple pixels together to produce a similar image to what you would see if the monitor were to display it at the given resolution but it can result in fuzzy images.

Here are some of the common native resolutions found in LCD monitors:

* 17-19″: 1280×1024 (SXGA)
* 20″+: 1600×1200 (UXGA)
* 17″ (Widescreen): 1280×800 (WXGA)
* 19″ (Widescreen): 1440×900 (WXGA+)
* 22″ (Widescreen): 1680×1050 (WSXGA+)
* 23.6″ (Widescreen): 1920×1080 (WUXGA)
* 23″ (Ultra-Widescreen): 2048×1152 (QWXGA)
* 24″ (Widescreen): 1920×1200 (WUXGA)
* 30″ (Widescreen): 2560×1600

Contrast Ratio

Contrast ratios are a big marketing tool by the manufacturers and one that is not easy for consumers to grasp. Essentially, this is the measurement of the difference in brightness from the darkest to brightest portion on the screen. The problem is that this measurement will vary throughout the screen. This is due to the slight variations in the lighting behind the panel. Manufacturers will use the highest contrast ratio they can find on a screen, so it is somewhat deceptive. Basically a higher contrast ratio will mean that the screen will tend to have deeper blacks and brighter whites.

Color Gamut

Each LCD panel will vary slightly in how well they can reproduce color. When an LCD is being used for tasks that require a high level of color accuracy, it is important to find out what the panel’s color gamut is. This is a description that lets you know how wide a range of color the screen can display. The larger the percentage of NTSC, the greater level of color a monitor can display.

Response Times

In order to achieve the color on a pixel in an LCD panel, a current is applied to the crystals at that pixel to change the state of the crystals. Response times refer to the amount of time it takes for the crystals in the panel to move from an on to off state. A rising response time refers to the amount of time it takes to turn on the crystals and the falling time is the amount of time it takes for the crystals to move from an on to off state. Rising times tend to be very fast on LCDs, but the falling time tends to be much slower. This tends to cause a slight blurring effect on bright moving images on black backgrounds. The lower the response time, the less of a blurring effect there will be on the screen. Most response times now refer to a gray to gray rating that generates a lower time than the traditional full on to off state response times.

Viewing Angles

LCD’s produce their image by having a film that when a current runs through the pixel, it turns on that shade of color. The problem with the LCD film is that this color can only be accurately represented when viewed straight on. The further away from a perpendicular viewing angle, the color will tend to wash out. The LCD monitors are generally rated for their visible viewing angle for both horizontal and vertical. This is rated in degrees and is the arc of a semicircle whose center is at the perpendicular to the screen. A theoretical viewing angle of 180 degrees would mean that it is fully visible from any angle in front of the screen. A higher viewing angle is preferred over a lower angle unless you happen to want some security with your screen.

Connectors

Most LCD panels have an analog and a digital connector on them. The analog connector is the VGA or DSUB-15. The common digital interface is the DVI connector. This is a digital interface that is supposed to allow for a cleaner and brighter picture compared to standard VGA connectors. HDMI and DisplayPort are two other digital interfaces that are becoming common. Check to see what type of connector your video card can use before buying a monitor to ensure you get a compatible monitor. Some monitors may also come with home theater connectors including component, composite and S-video.

Stands

Many people don’t consider the stand when purchasing a monitor but it can make a huge difference. There are typically four different types of adjustment: height, tilt, swivel and pivot. Many less expensive monitors only feature the tilt adjustment. Height, tilt and swivel are generally the critical types of adjustments allowing for the greatest flexibility when using the monitor in the most ergonomic fashion.

source: http://compreviews.about.com/od/monitors/a/LCD-Monitor-Buyers-Guide.htm

Determining How Well an LCD Monitor is at Reproducing Color

What is Color Gamut?

Color gamut refers to the various levels of colors that can potentially be displayed by a device. There are actually two types of color gamuts, additive and subtractive. Additive refers to color that is generate by mixing together colored light to generate a final color. This is the style used by computers, televisions and other devices. It is more often referred to as RGB based on the red, green and blue colors used to generate the colors. Subtractive color is that used by mixing together dyes that prevent reflection of light that then produce a color. This is the style used for all printed media such as photos, magazines and books. It is also generally referred to as CMYK based on the cyan, magenta, yellow and black colors used.

Since we are talking about LCD monitors in this article, we will be looking at the RGB color gamuts and how various monitors are rated for their color. The problem is that there are a variety of different color gamuts that a screen can be rated by.

sRGB, AdobeRGB and NTSC

In order to quantify how much color a device can handle, it uses one of the standardized color gamuts that define a particular range of color. The most common of the RGB based color gamuts is sRGB. This is the typical color gamut used for all computer displays, TVs, cameras, video recorders and other consumer electronics. It is one of the oldest and therefore narrowest of the color gamuts that is used in reference for computer and consumer electronics.

AdobeRGB was developed by Adobe as a color gamut to provide a wider range of colors than sRGB. They developed this to be used with their various graphics programs including Photoshop as a means to give professionals a greater level of color when they work on graphics and photos before converting for print. CMYK has a much greater color range compared to RGB gamuts, thus the wider AdobeRGB gamut gives a better translation of colors to print than sRGB.

NTSC was the color space developed for the widest range of colors that can be represented to the human eye. Many may think that this has to do with the television standard group that it is named after, but it is not. Most real world devices to date do not have the ability to actually reach this level of color in a display.

So, to quantify the various color gamuts in terms of their relative range of color of narrowest to widest would be: sRGB < AdobeRGB < NTSC. In general, displays are generally referred to compared to the NTSC color standard unless they state a different standard.

What is the Typical Color Gamut of a Display?

Monitors are generally rated on their color by the percentage of colors out of a color gamut that are possible. Thus, a monitor that is rated at 100% NTSC can display all of the colors within the NTSC color gamut. A screen with a 50% NTSC color gamut can only represent half of those colors.

The average computer monitor will display around 70 to 75% of the NTSC color gamut. This is fine for most people as they are used to the color they have seen over the years from television and video sources. (72% of NTSC is roughly equivalent to 100% of the sRGB color gamut.) The CRTs used in most televisions and color monitors also produced roughly a 70% color gamut.

Those that are looking to use a display for graphical work for either a hobby or profession will probably want something that has a greater range of color. This is where many of the newer high color or wide gamut displays have come into play. In order for a display to be listed as a wide gamut, it generally needs to produce at least a 92% NTSC color gamut.

An LCD monitor’s backlight is the key factor in determining its overall color gamut. The most common backlight used in an LCD is a CCFL (Cold-Cathode Fluorescent Light). These can generally produce around the 75% NTSC color gamut. Improved CCFL lights can be used to generate roughly 100% NTSC. Newer white LED backlighting has been able to actually generate greater than 100% NTSC color gamuts.

Summary

If an LCD monitor’s color is an important feature for your computer, it is important to find out how much color it can actually represent. Manufacturer specs that list the number of colors are generally not useful and typically inaccurate when it comes to what they actually display versus what they theoretically can display. Because of this, consumers should really learn what the monitors color gamut is. This will give consumers a much better representation of what the monitor is capable in terms of color. Be sure to know what the percentage is as well as the color gamut that percentage is based off of.

Here is a quick list of the common ranges for different levels of displays:

* Average LCD: 70 to 75% of NTSC
* Professional non-Wide Gamut LCD: 80 to 90% of NTSC
* Wide Gamut CCFL LCD: 92 to 100% of NTSC
* Wide Gamut LED LCD: 100%+ of NTSC

source: http://compreviews.about.com/od/monitors/a/LCDColorGamut.htm