The human eye "sees" contrast while it "perceives" brightness. The eye is edge triggered at the transition edge of light and dark images. The reason why you can read this text is that your eye can see the differences of the dark text on the white background at the edge of the text. The human eye is incredibly capable at this task and can actually see billions of levels of contrast. In display enhancement, there is often a request to make a display brighter. A bright display merely means that there is a high output of light on the white level. If black of the display is not saturated, the contrast of a display can be surprisingly low. Low contrast, sub-3 :1, means that the display may be readable, but not easily read. So, how does a display lose its contrast ratio?
Index of Refraction-
A display's viewability is a function of a phenomena of "signal to noise" ratio. The "signal" is the image of the display; while the "noise" is reflected light off the display. If the "signal" or display's brightness is greater that the reflected light then the display is viewable. If the "noise" off the display is greater than the displays brightness, then the loss of contrast will make the display unacceptably viewable. As with any electromagnetic energy, as it transfers from one conductor to another, the energy will either transfer, get absorbed or reflect. Air is a conductor of light. Its ability to transfer light, referred to its Refractive Index. All other medium that can conduct light, glass or plastic or even water for example; have their own unique Refractive Index. The difference in the refractive index of the new medium compared to glass means that a set amount of light will not transfer into the new medium and will instead reflect. In an example, we will use glass as the medium of light.
The difference in the refractive index of glass compared to air is 1.45 : 1 This means that 4.5% of the light striking the front of the glass will reflected backwards. This spurious amount of light is essentially "garbage" white light or optical "noise". The reflected white light is added to the image of the display. This means that the white level will increase by 4.5% of the ambient light but it also means that the black level and colors of the image will also add the white light which dilutes the saturation of the display. Essentially, it will dilute black and colors and as they are compared to white, the ratio between them becomes smaller. This dilution by reflected light is a phenomena at each of the transfer points of light. Light entering the glass reflects, as will light exiting the rear of the glass, as will light entering the front of a display. All total, a plain piece of glass in front of a LCD will reflect 13.5% of the ambient light. When 13.5% of ambient light is added to a display image, the dilution of colors lower the contrast ratio to unacceptable levels. This washout is referred to as Reflective Loss.
Index Matching-
Since a mismatch in the refractive index causes washout, matching the index at the transition points will eliminate most of the reflective loss. Special coatings, AR or anti-relfective, are deposited on the front layer of the glass to give the glass the same index of refraction of air. As light transfers from air to glass, all but .3% of the light will transfer since there is essentially no difference in the index of refraction between air and glass. To complete the steps, the backside of the glass and the front of the display must also be corrected. There are two approaches to index-match these two surfaces. You can optically bond the glass to the display or one can add AR coating to the back of the glass and AR film to the front of the display. These two approaches will maintain reflected light to less than 1%, enough to prevent reflective loss or washout.
Still not enough contrast?
In some extreme cases, the internal brightness of a display, even with index-matching AR coatings, is not enough for the display to have an acceptable level of contrast. As you can see by the table below, the lower a display's brightness even with AR, its contrast ratio could fall below acceptable levels. For this reason the display will need to made brighter. There are only two ways to make a display brighter. You can add more light (photons) to the display; referred to as Active Brightness; or you can collect the light (photons) already produced by the display and channel them out the front; referred to Passive Brightness.
Types of Display Enhancements Contrast Improvements- Eliminating reflective loss G-Bond: Direct optical bonding Air Gap: AR film on display and double sided AR coated cover glass
Brightness Improvements- Making the display brighter Active Brightness - Backlight / Edgelight installation: LED rails and backlights Passive Brightness - Film Enhancements: Brightness films; DBEF, BEF and ESR films
