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The svelte screen

Jan 1, 2003

A radar in the nav station gives the voyager many advantages. But if there is one drawback to mounting a radar screen in a small boat it is finding a place to put the radar screenandmdash;even small cathode ray tubes (CRTs) can occupy substantial space in a small nav station.

One way around the space problem would be to use a screen that uses a flat liquid crystal display (LCD). Without the need for a bulky CRT, a flat panel unit can be put just about anywhere. Several radar manufacturers have used this approach and this type of screen is getting better with each new model.

Due to their use in products ranging from digital watches to loran displays, LCDs have become an increasingly common method for showing alphanumeric information. It turns out that they also work well for radar screens since a radar picture doesn't need extremely fast updatingandmdash;once every two seconds or so. (CRTs are still the best bet for television displays, however, since a TV picture is updated at the swifter pace of 30 times a second. Flat-panel TV displays are catching up, however: The first class seats on some new airliners are equipped with a small, color LCD screens for watching the in-flight movie.) And LCD screens don't need as much power as CRT displaysandmdash;a definite plus on a voyaging sailboat. Another plus stems from the fact that CRTs can interfere with other electronic equipment at the nav station.

Any LCD works by taking advantage of a property of light (and most forms of electromagnetic radiation) called polarization. If one has a pair of sunglasses with polarized lenses, one has experienced this effect firsthandandmdash;even if unknowingly!

Electromagnetic radiation propagating through space can be defined by alternating electric and magnetic fields which are at right angles to each other. If we look at the electric field vector of nonpolarized light, it's random and unpredictable. Plane-polarized light, however, has its electric vector oriented in a predictable direction; all the electric vectors are pointed along the same plane. (This effect is not to be confused with laser light where all the particles/waves are andquot;in phaseandquot;andmdash;that's a different kettle of fish.)

When ordinary, non-polarized light strikes a reflective dielectric surface such as glass, or even a polished cabintop, a large percentage of the reflected light is plane polarized at 90anddeg; to the angle of incidence. This reflective polarization occurs from light striking on all sorts of surfaces.

However, there are other ways to get polarized light. One way is via something called a transmissive film. Polarized sunglasses use a type of plastic film whose molecules allow light propagating on a given plane to pass through unhindered. Light on any other plane is absorbed. The glasses filter out all light except that with the desired polarization. While a wearer can't tell that he or she is seeing anything special (the human eye is incapable of sensing polarized light), it is obvious that the sunglasses are reducing annoying glare.

To understand how polarization is used in an LCD, we first need to look at the liquid crystals themselves. Liquid crystals are a general name for a group of compounds that have useful electro-optical properties. Left on their own, these crystals will align themselves in a characteristic way. Then, if a magnetic field or an electric current is applied, they will assume a different orientation.

The liquid crystals used for displays align themselves in a corkscrew fashion when the power is off, but line up like patrons at a popular movie when a current is applied. This on/off alignment, and the polarization effect discussed earlier can be brought together to construct an LCD.

Most LCDs are made up of many individual elements. Even though it looks like a single piece of glass on which numbers magically appear, an LCD is actually a sandwich of several layers. The outside layers are thin sheets of polarizing film. The polarizing axes of these sheets are set at an angle to each other. Inside, there is another very thin sandwich that holds a small amount of liquid crystal. Alphanumeric displays will have various elements etched onto this layer so the display can form letters and numbers.

Light encounters the first polarizing filter and only that light with the proper polarization is allowed though. Next, the light encounters the liquid crystal, which, due to the power being off, is in its natural twisted orientation. Light actually travels along the twist of the crystal and is conveyed to the filter at the back of the sandwich. This filter, remember, is set at an angle to the front filter. This angle has been chosen to coincide with the twist of the crystal. In other words, by the time the light reaches this filter, it has been twisted into a new polarization plane that allows it to pass through the second polarizing filter. Since all the polarized light is passing through the sandwich, the screen is clear, and no numbers or letters are displayed.

When a current is applied to the sandwich, the crystal rearrange themselves. The front filter still passes light of the correct polarity to the crystal, but now the crystal is no longer twisted. Now the light travels straight along the crystal until it hits the second filter. But it has the wrong polarity and does not pass through it. Since the majority of the light is absorbed by the second filter, the display now looks dark. By applying current to the various elements in the right sequence, we can make numbers and letters appear.

Some liquid crystals twist 90anddeg; when no current is applied. However, newer displays use a type of crystal that twists through 270anddeg;. This andquot;supertwistandquot; screen provides better contrast. Some screens also have a control for changing the contrast angle of the screen. This allows one to see the screen at maximum contrast depending on whether the unit is mounted at a chart table or on the overhead.

One can see that an LCD works fine just so long as there is light available to illuminate it; the crystals themselves produce no light (unlike CRT screens or light emitting diode [LED] displays which can easily be read in low light or at night). To see an LCD at night, some form of backlighting is required. The new Furuno 1621 LCD radar tackles this problem with a electroluminescent panel behind its supertwist liquid crystal elements.

One of the problems with earlier attempts at an LCD screen has been the low number of picture elements, or pixels. The 1621 has a more readable screen due to its 80 horizontal and 106 vertical pixelsandmdash;for a total of 8,480. (By comparison, the LCD screen on a VGA-equipped notebook computer has 307,200 pixels. Of course, a computer screen is called on to display more complicated graphics than are required for a radar picture.)

LCD screens have improved in the past few years and they make sense for small vessels with cramped quarters. They will, no doubt, continue to increase in picture quality. (Actually, color LCD screens are already made with very good resolution, their only drawback is high price.) For better resolution and ease of plotting, though, a good, old-fashioned CRT is still the best.