Sunlight readable LCD displays for scales

So, just what are sunlight readable LCD displays?

As an engineer living in Phoenix, Arizona, sunlight readable LCD displays are a necessity. Trying to read an LCD in direct sunlight is an everyday challenge, even in the winter time.

Many times when I am outside and need to read a text or review a data sheet on my tablet, I look for shade to block the sun, so I can see the display.

A long time ago, a solar accretion disk formed and created the sun and ever since, it as been burning bright. The challenge is that no human light source can compete against it.

So, if your product contains a LCD display and it is required to work in direct sun light, you only have a few options.

Three methods of building sunlight readable LCD displays

1. SUNLIGHT READABLE LCD DISPLAYS WITH A REFLECTIVE POLARIZER.

The best option is to build a display that will cooperate with the sun. That is just what a reflective polarizer does for a LCD Display.

The Reflective polarizer is basically a mirror that sits behind the LCD glass reflecting all the light from the sun (or any other bright source) redirecting it back towards the user. The brighter the light, the easier it is to read.

An example being eBook readers which use a reflective polarizer.

The downside to a reflective polarizer is that in dark environments, you cannot place a backlight behind it. The polarizer will block any light source from reaching the user. If you must have a backlight, add an edge-lit or side-lit LED light.

An edge-lit LED light sits above the polarizer and below the top layer of LCD glass. All light from the LEDs is then reflected out towards the user.

Side-lit LCD Displays do not perform well with very large displays. The LEDs light up the edges of the LCD but not the center. Increasing the current draw to increase the brightness will only create hot spots.

2. SUNLIGHT READABLE LCD DISPLAYS WITH A TRANSFLECTIVE POLARIZER.

The second best option to compete with the sun is to use a Transflective polarizer, it is similar to a reflective polarizer in that it reflects sun light back to the user, however it does not reflect as much light as the reflective polarizer, but enough to read the display.

A recent addition to Transflective polarizers is TFT technology, which up until a few years ago, TFT (Thin Film Transistor) displays were only built with a Transmissive polarizer, which means that none of the sun light was reflected. The image would be washed out or difficult to read.

Now TFT’s can be built with a Transflective polarizer. The cost of Transflective TFTs are more expensive than the Transmissive version and currently there is a shortage of the Transflective glass.

What in the world is a NIT? Before we go on, we need to cover Nits. A Nit is a unit of measure of how bright a light is. One Nit is the amount of light produced by one candle.

3. SUNLIGHT READABLE LCD DISPLAYS WITH A TRANSMISSIVE POLARIZER.

The least desirable option for a sunlight readable display is to use a Transmissive polarizer because it does not reflect any sunlight. That means that the only way for a Transmissive display to compete against the sun is to have very, very bright LEDs. Which still may render the display is difficult to read.

There is no set value of Nits that tell you when a display is bright enough to be sunlight readable. As a general rule though, we find that 700 Nits works well.

It is possible to build a custom TFT with a very bright backlight, up to 1,000 Nit count and higher. The thing to take into consideration is that the brighter the backlight, the more power it takes.

This is a serious design concern for battery powered products.

Are you in the process of designing a product that requires a display that is sunlight readable? Give us a call. One of our friendly, coffee-drinking people can answer your questions.

Compare OLED vs TFTs vs Segmented glass displays

OLED DISPLAY:

At this time, the maximum size of an OLED is 4.3.” Along with being expensive, the display is also too small for the truck driver to read from the driver’s seat.

OLEDs are not the easiest to ready in direct sunlight, however there are some new advances taking placing that will allow them to improved performance in direct sunlight, but at this time they can ‘wash-out’ in sunlight.

TFT MODULES:

TFT displays can be manufactured in much larger sizes then OLEDs; in fact a 10.4” display would be the right size. The concern with TFTs is cost, a display this large can run from $150 and higher per display.

TFTs are not the best solution for direct sunlight, but it is possible to order the TFT with a Transflective polarizer or a very bright backlight. Both options help it to be readable in direct sunlight, but also increase their cost.

SEGMENTED DISPLAYS:

Sunlight readable segmented LCD displays offer: low cost, low power and are fully customizable.The final decision was made to integrate a segmented (also called a static or glass-only) display since the display can be built with a reflective polarizer. The reflective polarizer will reflect 100% of the suns light.

Although segmented displays cannot display multiple colors or video, they can display numbers, letters and icons; all that is necessary for this project.

ADVANTAGES OF A SEGMENTED DISPLAY INCLUDE:

Low power –
- They can operate at 3.3V or 5V, 5V is recommended for colder ambient temperatures.
Low cost –
- Segmented displays cost anywhere from $1.50 to $15.00 depending on size of glass, operating temperate range and quantity.
Low cost customization –
- The tooling cost can run between $1,000 to $3,000 depending on:
  - Type of backlight or edge-light
  - Connection method such as
    - curved pins
    - FPC cable
    - Zebra strip
  - Interface type
    - SPI
    - Parallel
    - 4:1
    - Direct drive
    - I2C
- Custom LCD Displays are designed and built to the exact dimensions and specifications requested by the engineer.

SUNLIGHT READABLE SEGMENT LCD DISPLAYS:

The best type of segment display for sunlight readable is one that implements a reflective polarizer.

Note: For more information on polarizers, please see the article on the different polarizers available.

Reflective polarizers reflect 100% of the sunlight back towards the user; this means that the brighter the sunlight, the easier it is to read the display. For example many eBook readers incorporate a reflective polarizer, cell phones and tablets use a Transflective or Transmissive polarizers which make them difficult to read in direct sun light.

The challenge of a reflective polarizer is that it cannot be integrated with a backlight since the backlight is placed behind the polarizer. Any light placed behind the polarizer will be blocked. To solve this issue, the LEDs are placed above the polarizer but below the top layer of ITO (Indium tin oxide) glass. This arrangement is called ‘edge-lit’ since the LEDs sit on the side (or edge) of the LCD and not under the polarizer.

The edge-light can be turned on at night to make the display readable, and off during the day to use the sun.

Projects requiring a sunlight readable display

The current lead time for custom prototype samples is running between six to eight weeks.

Do you have a new project or redesign that requires the display to be readable in direct sunlight? Give us a call, one of our US-based support people can help you with your design.