TruBlack displays are used in various Sony products, including digital photo frames, Cyber-shot cameras and Handycam camcorders. These displays provide superb contrast and visibility by controlling reflectance and minimizing light dispersion as well as sunlight and fluorescent light reflection. The TruBlack story begins with Sony´s efforts to create a display capable of fully reproducing the highly-advanced image quality of HD by modifying functional materials and structures.
Sony is continually developing new technologies to enhance display image quality and resolution. In practice, however, displays frequently fail to realize their full potential because of optical degradation caused by the structural characteristics of the LCD module. Till now, most LCD modules were based on the air gap structure, in which an air gap is created between the LCD module and the front panel to protect the module from external damage. Unfortunately, this design reduces both luminance and contrast because of light dispersion resulting from refraction in the air gap.
Sony used two methods to enhance the contrast ratio of its digital photo frames and minimize light dispersion and sunlight/fluorescent light reflection. First, Sony engineers filled the air gap between the liquid crystal module and the front panel with optical elasticity resin—UV curable resin capable of controlling light refraction. Second, an antireflection film was applied to the surface. The advantages over the conventional air gap structure include not only improved visibility but also enhanced panel strength. Such advances ultimately led to Sony´s successful development of the TruBlack display.
In designing the optical elasticity resin for the TruBlack display, Sony chose a material which combines excellent light-resistance with a chemical structure that would not absorb visible light. By adjusting the refraction index to a level similar to that of the end-product´s front panel, Sony was able to develop a resin with superb optical properties, including a refraction index theoretically close to zero.
When the gap between the LCD module and the front panel is filled with hardened UV curable resin, the resulting internal stress can be calculated using the following formula: Internal stress = Elasticity modulus X shrinkage during curing. If the elasticity modulus is high, warping will result in alignment errors. Substantial shrinkage during curing can result in curving. Therefore, when designing the resin, Sony strived to keep the elasticity modulus low while also minimizing shrinkage during curing to avoid optical deterioration risks associated with such problems.
The development of slimmer LCD panels has also reduced the size of the air gap. With a 4-5μm air gap, variation of just 100nm-200nm can cause alignment errors. Sony avoided this problem by reducing the elasticity modulus of the silicon gel to 5.0 X 103Pa. This was achieved in a number of ways, including developing a resin material with a specific molecular design and meticulously adjusting its composition. Shrinkage during curing was reduced to 1.8%, compared with 6-7% for conventional acrylic UV-curable adhesives. In this way, Sony succeeded in perfecting an optical elasticity resin that does not interfere with LCD performance.
A resin with a high elasticity modulus and high shrinkage during curing will cause peripheral warping. However, a resin with a low elasticity modulus and low shrinkage during curing will cause absolutely no warping and can be inserted into the panel without causing alignment errors or display defects.
Antireflection (AR) film takes advantage of the wave properties of light, i.e. utilizes wave interference to reduce reflection and improve transmittance*1. AR film is widely used to enhance image playback quality, especially in compact display devices. Reflectance is reduced by applying a thin AR film able to adjust the wavelength of light striking the front panel. In this way, light reflected at the interface with the air is cancelled out by light reflected at the interface with the substrate.
AR films are broadly divided into wet-coat types (which are applied as coatings) and dry types (which are vacuum-applied using sputter or vapor-deposition technologies).
With dry AR films, the film thickness can be controlled more precisely, and improved reflectance and physical properties can be achieved. For these reasons, the dry method tends to be used on high-end small and midsize displays, and the wet-coat method on midsize to large general-purpose displays.
Dry AR film, which enables precise thickness control, is used on TruBlack displays. Sony has further enhanced image quality over earlier products by modifying the technology to reduce reflection across a wider range while eliminating reflected color.
Light loss in TruBlack displays has been minimized by using an AR film to reduce reflection, and by filling the display with an optical elasticity resin that combines a low elasticity modulus with low shrinkage during curing. TruBlack displays provide a 15-fold improvement in contrast and visibility, even under fluorescent lighting equivalent to 600 lux*2. The ability of TruBlack to reproduce the outstanding quality of HD is the result of Sony´s intuitive pairing of optical control technology with advanced functional materials.
