LCD’s and Optical Physics
LCD modules currently manufactured by NEC, Samsung, LG, Optrex, AUO and many others are engineered to be highly optimized optical devices specifically designed for human vision. Active matrix liquid crystal displays are standard on most laptop computers as well as commercial and industrial grade display systems. Two properties of liquid crystal are used as tiny switches to turn picture elements (pixels) off and on. First the crystals are transparent but can alter the orientation of polarized light passing through them. Second, the alignment of their molecules (and their polarization properties) is changed by applying an electric field.
In a color display the liquid crystals are held between two glass plates or transparent plastics. These plates are usually manufactured with transparent electrodes, typically made of indium tin oxide, that makes it possible to apply an electric field across small areas of the film of liquid crystal.
The outsides are coated with polarizing filters. Only light with a perpendicular polarization can pass through these filters. (a). See figure 1.1 below.
Inside the plates are transparent electrodes and color filters, which form very small picture element regions called subpixels. A grouping of a red, a green and a blue subpixels defines the color that the pixel transmits. Fluorescent (or LED) backlighting illuminates a display from the rear. In pixels that are off, light passes through the rear polarizing filter, the crystals (b) and the color filters, only to be blocked (absorbed) by the front polarizing filter. To the eye , these pixels appear dark. When a pixel is turned on, the liquid crystals reorient their position, and they in turn repolarize the light so that it can pass through the front polarizing filter (c).
The active matrix provides a superior method of electronically addressing (turning on ) an array of pixels. For an image to appear on screen, one row of pixels receives the appropriate voltage. At the same time, software in the computer dictates that voltage be applied to those columns holding active subpixels. Where an activated row and column intersect, a transistor turns on a subpixel electrode, generating an electrical field that controls the orientation of the liquid crystal. This process repeats sequentially for each of the rows which can take 16 to 33 milliseconds.
One of the fundamental problems and inherent limitations with all LCDs in real-world environments is the delicate nature of the polarizer material. The frontal polarizer is easily scratched and physically damaged which will permanently destroy the quality of the display image. Another problem is that this polarizer material is as well very hydrophilic (absorbs water) and can be damaged with prolonged exposure to moisture, such as rain, melting snow, dew, etc. It is because of these fragile characteristics of the front polarizer that system manufacturers, such as VarTech, deem it a necessity to protect the delicate frontal LCD surface with some type of protective window, be it cover glass or polycarbonate or touch screen. And here’s where everything starts to fall apart. Once a cover window or touch screen is placed in front of the LCD, an “air gap” is formed between the front polarizer of the LCD and the overlaying protective cover window. This ‘air gap’, regardless of thickness, causes undesirable optical and performance conditions. From an ‘optical’ standpoint, this ‘gapped’ cover window causes reduction in display contrast, decreases in visible luminance from the LCD, and increases both specular and diffuse reflection levels.
Reflections can be divided into two types: Specular reflection and Diffuse reflection. Specular reflection describes glossy surfaces such as mirrors or LCD cover glass, which reflect light in a simple, predictable way. This allows for production of reflected images that can be associated with an actual (real) or extrapolated (virtual) location in space. Diffuse reflection describes matte surfaces, such as paper or rock.
So, in the end, the protective cover window or touch screen is the direct cause of reduced display viewability….and environmental performance, which will be discussed later on in this article.
The ‘air gap’ has such an adverse effect on the quality of the LCD image because of the optics of Index of Refraction (Refractive Index) of transparent surfaces. Transparent materials, such as touch screens, Lexan overlays, glass protective windows, heater and/or EMI windows, etc. transmit light at slightly different rates. This variation is measured on the Refractive Index (RI) scale. The polarizer material typically used by the Original Equipment Manufacturer (OEM) has Refractive Index of 1.45; air has a value of 1.00; and the various types of glass substrates such as borosilicate and soda-glass have an average Refractive Index of closer to 1.50. The existence of an Refractive Index mismatch of more than 0.10 units between contacting surfaces is enough to cause significant light reflection to occur at the interface between those substances.
The greater the Refractive Index discord, the greater the interface light reflection levels. Consider this, the three reflective layers of Refractive Index mismatches typical of most LCD “air gap” monitors have several optical effects. First, is external ambient light shining on the display surface at an angle of incidence greater than zero degrees. A subsequent result is specular reflections at each of the three traditional interfaces; (1) Polarizer to Air, (2) Air to Glass and (3) Glass to Air. The second effect is light generated by the LCD’s backlight. This generated light causes internal specular reflections at the interfaces of (A) Air to Glass and (B) Glass to Air. The third effect is external ambient light shining onto the display surface at zero degrees of incidence resulting in ‘diffuse’ reflections (generally referred to as “glare”) at the interfaces of (X) Air to Glass and (Y) Glass to Air.
The cumulative effect of the internal specular reflections of A & B alone result in an average loss of 9.0% light transmission (luminance) from the display’s backlight(s). Depending on the angle of incidence and intensity of external light, both specular and diffuse reflections can cause image “washout” (see “Reflection Washout” image below). This is the point where reflected light intensity is greater than the emitted light intensity from the display image. As the level of reflected light increases, the contrast ratio of the display image decreases below the level of 5:1, and no longer is visible to the human eye. It is for these reasons that ‘air gapped’ LCD products are not considered as sunlight viewable. The use of anti-reflective (AR) coatings on the front and back surfaces of the cover glass substrate, and even on the surface of the front polarizer, serves to help minimize these reflection levels by index matching the glass and polarizer surfaces closer to the 1.00 Refractive Index of air. Although the usage of multiple anti-reflective (AR) coatings (commonly referred to as ‘passive enhancements’) improves the viewability of ‘air gapped’ display products, the limited efficiency of these AR coatings (see Image 2 below) still permits reflections to occur at all interface surfaces. So, in the end, passively enhanced only displays are marginal at best for achieving LCD image readability in direct sunlight or very high ambient lighting conditions.