Tutorial: Reflections and Refractions

The viewability of most liquid crystal displays (LCDs) is severely limited in high ambient light conditions.  In this tutorial will discuss LCDs but CRT and LED suffer from the same problems.

LCDs typically use Cold Cathode Fluorescent Tubes (CCFT) to provide the light Transmitted light needed to view the LCD screen in low light conditions.  When most LCDs are taken outdoors on a sunny day "washes out" so that only about 10% of the screen image is visible to the human eye. 

Would increasing the brightness of the bulb make the screen sunlight viewable? Unfortunately no.  Increasing transmitted light often produces an increase in heat, power consumption and cost.  There will also be a point where the brightness of the bulbs with alter the contrast of the screen.

To better understand the problem let's look at a cross section of a typical LCD.  The diagram below shows the cross section of a lens in low light conditions with the CCFT transmitting light.  The diagram shows the backlight at the bottom, the LCD lens is just above the backlight.  A glass lens has also been added to the LCD assembly.  A glass or plastic lens is necessary to protect the fragile LCD and to allow the LCD to have features like a touch screen or anti-glare/anti-reflective coatings. 

The next graphic shows a cross section of the same LCD when exposed to a bright external light source thus causing multiple surface reflections and refractions.  Every flat surface will reflect (bounce back) a certain amount of light.  Optical surfaces will also refract (bend) light passing through if the density of the optical object changes. 

A good example of refraction is seen when you place a straight stick half way into a pool of water.  Since the water and air have different density the stick gives the appearance it has bent. 

Anti-Reflective Coating:  AR coating alone will not solve the problem.  As you can see in the graphic below, light passing through the A/R coating will not refract but the light will continue to reflect between the lenses and some light will escape out the back of the LCD lens.