Re: A Comparison of 14 LCD Monitors

The review posted yesterday is a good introduction to LC flat panel displays but doesn't go into enough detail for the (very) few of us who really get into LCDs. There are also misconceptions about LCDs perpetuated by the display and monitor manufacturers. Some of which the author has, unfortunately, fallen for.

The first really big headache everyone gets trying to compare LCDs with CRTs is contrast ratio. CRTs work by emission and scattering of light by phosphors excited by an electron beam. LCDs work by the phase (velocity) retardation of polarized light. It's all science so what's the big deal? Well, for starters, that phase retardation effect is very directionally sensitive.

A CRT's contrast ratio is not very directionally sensitive. The phosphor dot's scattering ability means the excited dot rolls off in luminance slowly until the critical angle between glass and air. In other words, it looks nice until you're looking at it more sideways than strait on and there's no point in testing because no one wants to look at it like that.

LCDs are not nearly so straightforward. Twisted Nematic displays not only loose contrast quickly as the viewer moves away from the normal angle, but gray scales also invert in one direction (typically at a negative vertical angle). So, even though a LCD's specification may say 300:1 contrast ratio, what you get is usually a lot less depending on a number of design factors.

Twisted Nematic Active Matrix Liquid Crystal Displays are old technology (relatively speaking). The TN-AMLCD manufacturing process is fairly mature compared with In-Plane Switching (IPS) and Vertical Alignment (VA) manufacturing processes. This is not to say IPS and VA technology isn't old, too. It's just that design and manufacturing IPS and VA displays is more difficult. But it's worth it because both LC modes offer advantages over TN modes. Now, back to contrast ratio.

Both IPS and VA mode LCDs are what is referred to as normally black displays. That is, when the Thin-Film Transistor (TFT) is electrically off, the dot is black. Not only does this have the effect of virtually eliminating bright dot defects, but this also means the display as a whole is really, really black; something very hard to do with normally white, TN-AMLCDs. That's how some LCD monitors actually exhibit better than 500:1 contrast ratios.

Here's the number one thing to remember about comparing CRTs and LCDs: CRT's loose contrast badly as the viewing environment brightens. I'm sure everyone's had trouble at some point with sunlight on their CRT. That's because the sunlight scatters backward off the phosphor. A CRT may claim a 700:1 contrast ratio, but that's probably a measurement inside a dark room. CRTs used in projection TVs have very high contrast ratio but you don't look at them. They're hidden to protect them from backscattered light.

LCDs, on the other hand are easier to improve luminance than CRTs and don't suffer nearly as much from reflected light even though the analyzer (front polarizing film) has a matte finish. In fact, most LCDs use only reflected light. Watches, calculators, cash registers, telephones and lots of other devices all use reflected light. Just check out Nintendo's Game Boy Advance to see some advanced reflective LCD technology. The point being, "traditional" testing developed for CRTs and LCD manufacturer's specifications are almost intentional in its effort to confuse the customer with biased numbers.

When you see a CRT specification for a 300:1 contrast ratio, what you should be thinking is "Either the tube isn't dark enough (shadow mask/glass) or it's really dim".

When you see a LCD specification for a 300:1 contrast ratio, what you should be thinking is "That doesn't tell me anything. Where's the damn LCD panel spec?" Because, inside the spec should be how it's measured and what LC mode it's using. There, you'll find out that the display is guaranteed better than 300:1 at normal incidence for indoor lighting conditions.

You'll also find that the viewing angle is specified as a region guaranteed better than 10:1 contrast ratio. THAT's when you know something's not right. And different manufacturers use different "minimum" CR numbers. Some use 15:1 or 20:1 depending on what they feel safe with and what other companies are saying.

Fortunately, IPS and VA AMLCDs behave more like CRTs than TN AMLCDs. So when the spec says up/down, left/right 160 degrees, then you can assume it's not a TN display. One other big plus for IPS and VA displays is they don't have gray scale inversions. That is, the colors don't suddenly reverse if you look at it wrong.

Oh, and just one note about inversions. The VA mode has BAD inversions. The inversions are even worse than the TN mode but it's very difficult in practice to create a multi-domain TN so everyone gave up MTN in favor of MVA. However, the "M" in MVA stands for Multi-domain. Meaning each dot is divided into sections where the LC molecules point in opposite directions. If the sections are small enough, you don't see two different gray-scale pieces of a dot but one interpolated gray-scale (spatial modulation). IPS displays are also multi-domain because of the electrodes needed for the transverse electric field and manufacturers like NEC are dividing the dot into even more sections (called domains) to make the performance even more CRT-like.

Overall, while I agree contrast ratio is an issue with LCD monitors, it’s not because of the maximum specified, but how quickly contrast ratio decreases away from normal incidence. TN displays may be acceptable for laptop displays (for now) because of the price pressure on that market and because of the lower transmission (percent of light reaching the viewer) but TN is quickly approaching the end of its life in the desktop monitor market because of its poor performance and the rapidly maturing IPS and MVA technology.

As an additional FYI, IPS has a slower CR roll-off than the MVA mode but has typically had a lower maximum peak contrast and slower LC response time than both MVA and TN. MVA displays have a faster LC response time than TN. Manufacturers are divided over whether IPS or MVA will dominate. Fujitsu and Sharp are fully on the MVA side while NEC has long been an IPS supporter. Samsung has experimented with both types and is currently leaning toward IPS.

To Tom?fs Hardware:
Please review Sharp?fs ASV (MVA) monitors, Fujitsu?fs MVA monitors, Samsung?fs PVA (MVA) monitors, IBM's IPS monitors, Chi Mei Optronics?f MVA displays and Hyundai?fs FFS (IPS) monitors to get a feel for the next wave of desktop monitors.