2020-02-24 16:47:40

Design Basics

This introduction is designed to give you a basic working understanding of the design issues you should consider when designing a display product. Feel free to contact JIYA to work through your requirements.

Type of Display (Module or Glass Only)

The most fundamental decision to make is if the display will have the drive circuitry attached (module), or not (glass only). There are advantages and disadvantages to each approach.

When purchasing a Custom Module, the basic electronic design work will be done by the module supplier. This obviously saves design time and reduces the manpower needed to bring a product to market. You can benefit from the experience we have gained during our previous designs to shorten the design cycle and deliver an optimized product. The major decisions you need to make are the Interface Type, with standard serial or parallel interfaces being the most common, and the Type of Backlighting desired. The Temperature Range, Viewing Angle, Viewing Mode, and Contrast will need to be considered, but those decisions are common to both approaches.

A Glass Only Design puts the design burden on the end user. You will need to learn a great deal about LCD's in order to complete your design. Fortunately, there is an enormous amount of data available online to help.

The main benefits to buy the LCD glass only are to reduce costs and provide design flexibility. The total cost of the components necessary to build the drive circuitry is less than the cost of a pre-built module. As long as you have space on your existing PC board, you won't have to pay for an extra PC board on which to mount the display. By doing a little homework, your design will work just as well as a module, and will allow the flexibility most designers need to adapt their design to ever-changing demands.

Technology - TN or STN

The type of technology used is determined by the specific performance characteristics of the display you are designing. For a display with a low multiplex rate, i.e. about 8:1 or less, it is possible to use a standard TN cell and still get an acceptable viewing angle and contrast ratio. Above this level, the contrast falls off quickly.

For displays with multiplex rates greater than 8:1, it has been found that increasing the twist angle to 180 to 240 degrees, gives superior results. By using a higher twist angle, one can achieve a larger number of multiplexed lines, with a much higher contrast ratio.

By designing displays with higher multiplex rates, a few problems, which could be tolerated at low multiplex rates, come into play. The first problem is the color. Normal STN cells have a greenish background which is objectionable in some designs. The green color can be corrected. However the correction adds cost to the display. The operating temperature range of the displays also suffers, which may require a heater to overcome.

Viewing Mode - Reflective, Transflective and Transmissive. A Reflective Display has the brightest appearance with the highest contrast ratio possible. However, it will be difficult to read at night or under changing lighting conditions. If your display must be readable under a wide range of lighting conditions, you will generally want a Transflective Display so that it will look very good in the bright sunlight, but will also be backlightable at twilight and at night. A Transmissive Display must always have a working backlight, and is therefore unacceptable in applications where power consumption is a problem.

The tradeoff with a Transflective Display is that it will not look as good as a Reflective Display during the day, and it will not look as good as a Transmissive Display at night. It will however enable you to have an acceptable compromise between the two, and provides a very acceptable appearance. A further discussion of viewing modes can be found in or technical Q & A section under What is Reflective? Transflective?

The display can also be Negative Image or Positive Image. What is Viewing Mode? in our Technical Q & A area discussed these terms.

Viewing Angle (Direction) - The viewing angle of a part is discussed on the What is Viewing Angle page in the Technical Q & A area. In general, the face of the display is divided into quadrants. A display with a 12:00 viewing angle has optimum contrast from above the normal. One with optimum contrast below the normal has a 6:00 viewing angle. Similarly, a display which will be viewed mostly from the left or right should have either a 9:00 or 3:00 optimum viewing angle.

Drive Method - In general, any LCD will have better contrast and viewing angle the lower the mux rate. For parts which are to be viewed under changing lighting conditions, a direct drive part is desirable. Any design which has 14-segment digits will have to be multiplexed to some level, and the mux rate kept as low as possible. This item ties into the next item directly, and the two must be balanced to give the best looking display possible. Direct Drive and Multiplexing are discussed extensively in the Technical Q & A, and Application Notes Sections.

Indoor / Outdoor - In general, the two natural elements that destroy displays are high heat and humidity. The parts of a display which are most sensitive to heat and humidity are the polarizers. Under extreme conditions, the seals which hold the glass together can fail. However, at LXD we have developed materials which effectively eliminate this failure mode. When designing a display for an extremely rugged environment, it is important to make sure that seals will not be a problem. In general then, the environmental considerations comes down to the polarizers.

Displays which will be indoors, or mostly indoors, can use commercial grade polarizers, These polarizers will hold up very well when used in most instrumentation, office and home products, and other applications where the products will be protected from high temperature and humidity.

For harsh environments, a polarizer specifically designed for outdoor and extremely humid conditions should be used. We call these polarizers industrial grade. A wide variety of different outdoor materials is available, with varying contrast, coloration, and efficiency. Because of the many different options, we can supply samples of displays with different polarizers so that you can choose the one which best fits your application.

Color Filters - LXD can add a color decal to the back of the display. This decal can have different colors in different areas so as to give different annunciators different colored backgrounds. This method has the lowest cost, but also gives the dimmest colors.

Screen Print - Similar to the color filters, LXD can screen print different colors onto either the front or rear of the display. Each color can be applied in an intricate pattern, thus giving the same flexibility as the decals but with much brighter colors. This method usually costs more than a color decal, but is the preferred method for most applications.

Color Polarizers - LXD can add a colored polarizer to the front of the display. This method will cause the individual segments to be colored when they are activated, i.e. all segments will be red, green or other color on a silver background. This method will add only slightly to the cost of the display. These colored polarizers are not as efficient as normal polarizers, and the resulting segments do not have the same contrast as a normal black on silver display.

Filters - It is possible to put anti-reflective or anti-glare filters over the front of a display to improve viewability in harsh lighting conditions. These filters are usually bonded directly to the front polarizer of the display.

An anti-glare filter is one that has its front surface either physically or chemically roughened. This surface scatters specular reflections over a wide area. It produces diffuse, bright reflections that will reduce the overall contrast, but can improve the display readability.

A better, but more expensive solution, is to use a high efficiency anti-reflective material. These materials do not scatter the light like the anti-glare materials, but rather re-direct the light waves so that they continue traveling forward instead of reflecting back toward the observer. New anti-reflective materials can reduce the front surface reflections to less than 0.3% or less.

2020-02-24 16:47:40
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