Sunlight Readable TFT LCD


Most of TFT LCDs are hard to read under the sunlight. Why they are “washed” out under the sunlight? We will start with the basic understanding of the contrast.

  • 10:1: Readability is good
  • 5:1: Readability is acceptable
  • 2:1: It is the minimum readability which human eyes can see

Let’s do some math work. We assume sunlight is 10,000 nits; indoor environment is 200 nits, on state backlight is 250 nits; off state backlight is 1 nit, cover glass surface reflection is 4%; optical bonded boundary between cover glass and OCA reflection is 0.2%.  According to the calculation below, the contrast indoor is 27.49; while the contrast under the sunlight is only 1.59. We can conclude that the LCD is good to use indoor while it is not readable under the sunlight.

Orient Display: Contrast Calculation under Sunlight and Indoor

Fig.1 Contrast Calculation under Sunlight and Indoor

How to get a sunlight readable LCD?  There are several ways to do it.

1. High Brightness TFT LCD

Let’s see the reflection rate for different surface.

Orient Display: Reflection on Different Surfaces

Fig.2 Reflection on Different Surfaces

Let’s again do some math here.

Example 1

  • Assume a LCD brightness is 200 nits, contrast: 100:1, so the off state brightness is 2 nits.
  • Sunlight brightness: 10,000 nits, suppose LCD reflection rate is 8%, the sunlight reflection brightness is 800 nits.
  • This LCD contrast under the sun will be: (200+800)/(2+800)=1.3, very poor, can’t see.

Example 2

  • If the LCD brightness is 500 nits. Off state brightness is 5 nits.
  • This LCD contrast under the sun will be:(500+800)/(5+800)=1.6 Still difficult to see.

Example 3

  • If the LCD brightness is 1000 nits. Off state brightness is 10 nits.
  • This LCD contrast under the sun will be: (1000+800)/(10+800)=2.2 Achieve the min. readability

Disadvantages for High Brightness LCD

  • Too much power
  • Generate heat
  • Bigger size or thicker

Please find the table below for the general guideline to choose LCD brightness under the different surrounding brightness.

Orient Display: General guideline to choose LCD brightness under the different surrounding brightness

 

2. Transflective TFT LCD

Orient Display: Reflective TFT LCD

Fig. 3 Reflective TFT LCD

Transflective TFT has a transparent reflector inside TFT LCD cell that is in front of the backlight. The incoming light is reflected and used to illuminate the TFT display. Transflective TFT have both “transmissive” and “reflective” modes. Both display modes work together for enhanced performance, as a result, the performance of the display content in very bright light is perfect even without any backlight on the transmissive TFT LCD. Customers using a transflective TFT LCD module can save power by reducing or turning the display’s backlight off during bright environments. Transflective TFT becomes more popular since the demand of requiring good performance in bright environments and direct sunlight. The transflective TFT displays are perfectly readable under almost any ambient light conditions. In addition, they are very energy efficient, making them ideal for battery powered devices and mobile applications.

Unfortunately, the process of building a reflector inside TFT LCD is complicated and transflective TFT LCD is normally several times higher cost compared with normal transmissive TFT LCD.

 

3. Surface Treatment

Surface anti-reflection (AR) treatment greatly reduces the surface reflection and increases the readability. Let’s re-calculate the contrast with AR coating as below.

Orient Display: AR Coating to Reduce the Surface Reflection

Fig. 4 AR Coating to Reduce the Surface Reflection

Example 4

  • If the LCD brightness is 1000 nits. Off state brightness is 10 nits.
  • AR Coating reduces surface reflection from 4% to 0.2%, so the total LCD reflection is 420 nits
  • This LCD contrast under the sun will be: (1000+420)/(10+420)= 3.3. Contrast improves 50% compared with no AR coating.

 

4. Optical Bonding

Optical bonding is the use of an optical-grade adhesive to glue a glass to the top surface of a display. The main goal of optical bonding is to improve the display performance under outdoor environments. This method eliminates the air gap between the cover glass and the display.

Orient Display: Optical Bonding to Reduce Reflection

Fig. 5 Optical Bonding to Reduce Reflection

Example 5

  • If the LCD brightness is 1000 nits. Off state brightness is 10 nits.
  • AR Coating reduces surface reflection from 4% to 0.2%, optical bonding reduces the boundary reflection from 4% to 0%, so the total LCD reflection is 20 nits
  • This LCD contrast under the sun will be: (1000+20)/(10+20)= 34 Contrast is greatly improved and the good contrast and real sunlight readability can be achieved.

Optical Bonding also has the following advantages which are equally important for outdoor applications

  • Improved ruggedness
  • Increased scratch resistance
  • Improved safety (less chance of broken glass)
  • Increased shock/vibration resistance
  • Prevents condensation/internal fogging

Orient Display: Without Optical Bonding vs with Optical Bonding

Fig. 6 Reflectivity and Delta E of Different Generations OGS

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Orient Display: Comparison between Example 3, 4 and 5

Fig. 7 Comparison between Example 3, 4 and 5

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