Work with mobile devices in the daylight - one of the pressing problems since the start of registration (1985) Toshiba's first 9.5 "color VGA LCD (Liquid Crystal Display) monitors. Now there are technologies for obtaining high quality images in direct sunlight on the screen devices. The article by Jeff Walker (magazine "Pencomputing"), translated by Andrei Vasilyev, describes the existing markets in Europe and the U.S. color LCD technology and presents a forecast of their development in the near future.
Four-parameter LCD
There are four key parameters of any LCD device: design, technology, resolution and size.
LCD construction
The design involves the location of layers in the LCD (including the photoconductive layer) and has the highest value for the quality of the image on the screen in sunlight. Currently there are three main design color LCD: transmissive (transmissive), transflective (translucent) and reflective (reflective).
Transmissive LCD
In the transmissive-design light enters through the LCD from the backlight. Most LCDs used in handheld computers today are made using this technology. Transmissive LCD has high quality pictures indoors and are usually very low (black screen) in sunlight. Solar lighting is a thousand times stronger backlight, and thus reflected from the surface of the screen the sun's rays completely suppress the light emitted by the LCD. However, some manufacturers have developed a transmissive LCD, which can work even in direct sunlight. How do they do it? To date, there are two ways: increasing the brightness of the backlight and LCD modification to reduce the amount of reflected sunlight. We briefly describe these methods.
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The maximum brightness of currently existing notebooks is 100-150 nit (option is equivalent to one candela / candles / square meter (cd/m2)). LCD with high brightness use 200 to 1,800 nit. It is believed that the lamp of 500 nit makes transmissive LCD visible in daylight in the shade, and about 1000 nit - allows you to use it in direct sunlight. Brightness of 300 nit can be achieved by limiting increases the brightness of a lamp CCFL (Cold Cathode Fluorescent Lamp) or by adding a second tube, located across the street. Models of LCD displays with high brightness can be from 8 to 16 lamps. However, increasing the brightness increases the power consumption of batteries (one backlight consumes about 30% of energy used by the device). Consequently, the screens with high brightness can be used only if the external power source. Modification of LCD is to apply antireflection coatings on one or more layers of the display, replacing the standard polarization layer at least reflects, adding, "increases the brightness of the films, increasing the efficiency of light sources. Another way of modifying the LCD, used only in devices Fujitsu, is to fill the converter fluid with refractive index equal to the coefficient of refraction of the touch panel. This greatly reduces the amount of reflected light.
Transflective LCD
Transflective-vobschem design something similar to the previous page. But transflective-display between LCD and a layer of light is called partially reflective layer. It may be partially or silver, or completely mirror with many small holes. When a screen is used indoors, it works similarly transmissive LCD, in which some light is absorbed by the reflective layer. In daylight the sunlight reflected from a mirror layer and illuminates the LCD. In this light passes LCD twice, first inside, then outside. This leads to the fact that the image quality in daylight is lower than under artificial light in the room when the light passes LCD once.
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Balance between image quality inside and outside the premises is achieved by selecting the characteristics of transmissive and reflective layers. For example, the mirror layer can reflect 60% light and absorb 40%. If the brightness of 130 nit, only 52 nit (40% of the 130) goes to the LCD through the mirror layer. Outdoors 60% of sunlight reflected from the reflecting layer, but as the light passes through the LCD twice the picture quality. Transflective LCD is a compromise. The image on this screen is not bright either indoors or in sunlight.
Reflective LCD
Reflective-design is fully reflective mirror layer. All lighting, whether it is sunlight or light front light passes through the LCD, is reflected from a mirror layer, and again passes through the LCD. In daylight the light passes through the LCD layer and the front lights twice. In this case, the image quality of reflective LCD is usually lower than transflective (as similar technology is used in both cases). Inside the front lighting is not as effective as the back and the picture quality is also lower than the transflective.
LCD technology
To date, the production of color liquid-crystal displays used two technologies: Active matrix - TFT and passive matrix - STN. TFT - Thin Film Transistor - active matrix in which each sub-pixel is a thin film transistor (active member). STN - Super Twist Nematic - passive matrix, which consists of liquid crystal elements with variable transparency. Variety STN: CSTN (Color STN) and DSTN (Double STN) in this article are under the same notion of STN, because their differences are not significant for the conditions of sunlight. Compared with STN TFT screen more contrast, has more saturated color, works faster (moving image does not have an afterglow). The image shows a large angle. True, TFT and more expensive, so now there is a tendency to expand production of TFT by reducing the production of STN. (For example, Sanyo stopped production of STN LCD and now produces only the TFT.) Combining the three designs and two LCD technology allows six types of color LCD. Of these, all except transmissive STN, may be used in sunlight. The table shows the remaining five types of LCD, and two additional variants, the number of existing devices on the market and their manufacturers.
| Type LCD | Number of devices | Producers |
| Modified transmissive TFT | 20 | Fujitsu, Intermec, Itronix, Nokia, Panasonic, Wolkabout |
| Transmissive TFT with high brightness | 14 | Fieldworks, Microslate, Panasonic, Phoenix, Xplore |
| Transflective STN | 7 | Hitachi, Itronix, Melard, Sanyo, Two Technologies |
| Reflective TFT Front lighting | 4 | Compaq, Palm, Sony |
| Reflective TFT backlight off | 4 | Fujitsu, NEC, Strata, Via |
| Reflective STN with front lighting | 1 | Fujitsu |
| Transflective TFT | No | No |
Resolution and LCD size
Resolution and size - are important and interrelated features. We are now manufactured LCD dots per inch (dpi) ranges from 70 (for 5.7 "monitors) to 150 (for 6,7" SVGA). The most common display with a value of 90 dpi to 125.Below are the standard values of permits that are used in computers, PDA and phones. Permissions VGA - historically the resolution value for PC is 640 * 480 ¼ VGA - 240 * 320 or 320 * 240 pixels ½ VGA - 640 * 240 SVGA - Super VGA - 800 x 600 pixel XGA - Extended VGA - 1024 x 768 pixels SXGA - Super XGA - 1280 x 1024 SXGA + - Super XGA Plus - 1400 * 1050 pixels UXGA - Ultra XGA - 1600 * 1200 pixels.
The fifth important parameter LCD - aperture ratio
This option is typically not indicated in the specifications, but can be calculated from other attributes. Aperture ratio is the ratio of the square pixel (sub-pixel) to the total screen area, which includes all the elements that make up the sub-pixel (eg, "transistor" in TFT). Great importance to the aperture ratio, in which each pixel has more coverage, increases the LCD brightness and a decrease in its resolution. Therefore, in models of portable computers designed for use indoors or in sunlight, the LCD is used with different resolutions. For example, a base model Panasonic Toughbook 28 for work in the premises of a transmissive TFT with a resolution of XGA, a similar model designed for operation in sunlight, has a transflective TFT with a lower resolution SVGA. Since ambient light is transflective and reflective LCD twice, in order to achieve the brightness, similar to the transmissive LCD, through which light passes once, the aperture ratio for these displays should be more. Therefore, at the same size LCD, transflective and reflective LCD typically produced at a lower resolution than the transmissive LCD. Consider which of the combinations of characteristics of color LCD is best suited for cellular phones, PDA and PDA industry. (The author has also examined other types of devices that use LCD: Laptops, Tablet PC.)
Cell Phones
Any color LCD screen on the phone should work in sunlight (you can imagine unreadable outdoors screen on your cell phone?). Known only two models of phones with color LCD, actually sold in the U.S. and Europe. First - Sprint (US) PCS phone SCP-5000 from Sanyo, which has 2 "reflective TFT with front lighting. PC magazine describes the screen of the phone as "uniformly bright sunlight." Second - Nokia 9210, which uses a 1.3 "* 4.2" modified transmissive TFT. One of the users of the Nokia 9210 described a screen of this device is as follows: "in the sunlight the screen is so neyarok that use of any of sunglasses makes it unreadable." Thus, reflective TFT is the best option for use in cellular phones. LED (LED) is increasingly used in handsets than with a cold cathode lamp (CCFL), because of the phones do not require as much brightness as from portable computers. Due to the fact that the transflective TFT small size are not produced, transflective STN second option is to use the phones. This screen could provide a better balance between image quality indoors and in sunlight than is used in Nokia 9210, a modified transmissive TFT. Although he would not be so bright in the room.
Handhelds
Compaq iPAQ 3600 - the first handheld with color LCD, which appeared on the U.S. market. It works fine in daylight (LCD 3.8 "1/4-VGA, reflective TFT production of Sony). The great success enjoyed by that device on the market, has led to many companies (such as Philips, Sanyo, NEC, Casio and Panasonic) have announced or started deliveries of similar LCD (with a resolution of 1 / 4 VGA). Consequently, we can expect reflective TFT with front lighting and other devices on the systems Pocket PC and Palm OS (for example, the new Sony Clie PEG-N710 uses a 3.1 "320 x 320 reflective TFT with front illumination). It is unlikely that this type of LCD to be used in devices with large screen and resolution, as the latest technology lighting distribution with mikronasechek can not uniformly illuminate the LCD for more than 6 ". The technology of using the front illumination rather young and perhaps over time it will be possible to solve this problem. IPAQ really is not the first PDA, using reflective TFT.In 1999, Compaq released a device Aero 2100 c reflective TFT, in which light was used as a front light source LED. In the iPAQ uses a bright lamp CCFL. The combination of very dim (10-15 nit) light source and the imperfect reflective LCD from Sharp in 2100 Aero was the reason for the failure of this device on the market. The screen was not bright both indoors and in sunlight. Consider other types of color LCD. Transmissive STN (HP Jornada 520/540 series and Fujitsu PenCentra 200 CTM) can not be used in sunlight. Transflective STN can not yet compete with reflective TFT. Transflective TFT could be used in a PDA, but there are no manufacturers of screens 3.8 "(available screens from 10.4). Reflective TFT backlight off (as in Nintendo) does not apply to the PDA, since it requires an external source of lighting. At the moment, reflective TFT with forward illumination is the only option for LCD, which works under sunlight and is used in handheld computers.
Industrial PDA
Ceychas market no bezklaviaturnogo (PDA-size) dedicated PDA with a color screen for use in sunlight. The main products in this category are Symbol - monochrome device on the Palm OS and Windows CE (SPC-1500/1700 and PPT-2700). Among the specialized PDA screens 5 "- 8.2" The situation is very complicated - only available color transflective STN screens that can be used in sunlight. Such screens have not yet received the recognition in the market. In this category also are no devices that have good color LCD for use in sunlight. TouchLite from Two Technologies, is probably one of the best devices in this category. It uses an exceptionally large 5.7 "transflective STN. The choice of LCD in this category is limited mainly to the fact that the production of LCD does not pay for underdeveloped markets. Compared with the market of PDA and laptop market industrial PDA is very small. For manufacturers who plan to launch a new LCD (eg, 6 "VGA transflective TFT, which would be interested in many producers), the minimum annual amount should be 50K-100K, which is hardly possible for underdeveloped markets of industrial PDAs. The absence in this category, good color LCD, working under sunlight, can lead to changes in the market of specialized software. With the proliferation of color LCD 1 / 4-VGA 3.8 "and SVGA 10.4", working under sunlight, software developers will be forced to migrate their applications to these dimensions. As a result, the market will not color handheld (running in the sunlight), which would be employed specialist software written for VGA or 1 / 2 VGA.
Conclusion
Enumerate the problems limiting the production of a good color LCD working in sunlight. - LCD is a low-yield and, therefore, very sensitive to the price of the product - in most cases, laptops and PDA's are used indoors, so they can be equipped with a transmissive TFT with high brightness - The success of the production and sale of products using LCD, depends on the subjective views of users about the quality of the screen in sunlight - The market is taking on new LCD technologies, such as transflective TFT, only if the manufacturer of the system is making great efforts to promote the new device - Release of the new product is associated with greater risk, because with limited resources, cost, development time and production new LCD big enough - Choose from a matrix consisting of three structures * for two technologies (with options such as polysilicon and amorphous silicon) * * 11 permits an average of 4 size is very difficult - it's more than 500 combinations! Sources, Materials:
