The interplay between light and hand-held device screens has driven research into light-diffusing materials that obviate such undesired aspects as image blur. As the field continues to expand, the application of anisotropic thin films is a key focus.
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The use of LCDs and OLEDs as the building components of reflective tablet and phone displays is common industry practice. Requiring only low power as they don’t produce light themselves, LCDs tend to be illuminated by a combination of ambient light and backlight to provide acceptable readability in, for example, low light conditions.
Display Technology Evolution
The increases in sophistication with regard to display technology over the decades have been impressive. From televisions to computer monitors to smartphones, visually presenting information has seen a transition from cathode-ray tubes bombarding electrons at phosphor-coated screens to the advent of Liquid Crystal Displays (LCDs) with evolving complexity to solid-state devices such as organic light-emitting diodes (OLEDs).
The visibility of reflective displays is dependent on their brightness, which in turn is dependent on the brightness of the ambient light. When a display is comprised of LCDs or OLEDs, creating optimum visibility is non-trivial.
The possibilities of using light-diffusing materials to address this have attracted attention and are now a consideration in new optical designs going forward.
Effective diffused reflection is critical to the viewer observing a display as ambient light is incidental from multiple angles. Ishinabe et al. proposed particulate diffusers and surface relief-patterned diffusive mirrors as suitable light-diffusing materials, but these cannot process effectively oblique incidental light environments. What tends to result are dark images because the diffused light luminance directed towards the viewer is not sufficient.
The Use of Anisotropic Light-diffusing Films
A more recent scientific advance has been the ability to construct thin films with differing magnitudes of a certain property along different axes.
Manipulation of this property, known as anisotropy, is opening up the possibilities for innovation in material science. For example, by stacking and rotating thin layers of crystalline sheets, it is possible to create thermal conduction along a fast-axis direction while providing insulation along the slow axis.
Such a construct can eliminate hot spots within tightly packed electronics. Perhaps, a parallel analogy can be drawn with the work of Kusama et al., who have been investigating the use of anisotropic films in the context of light diffusion. Can light-diffusing films of this nature reduce or remove image blur?
This is something ultimately only realized by creating a wide viewing angle for a rotating screen such as those experienced with reflective display tablets and phones.
Altering Thickness and Tilt
Concentrating their efforts on exploring the use of refractive index profiled light diffusing films, the team has been optimizing them by studying the variances exhibited as a consequence of altering the thickness of the refractive index structure within or by changing the tilt angle.
If screen rotation, allowing landscape or portrait viewing, can be made possible in a reflective display, then applications with this additional dimension have the potential to expand the market.
What is critical in achieving this is realizing the ability to reflect and diffuse light to the front of a display, regardless of the azimuth of the incident ambient light, the azimuth being a measurement of the angle between a viewer to the point of interest.
Underpinning everything is the balance between the size of the diffusion area: wide enough to allow more light to be diffused to the front, even if the azimuth changes, but not at the expense of the overall brightness and the danger of image blur if it were to be too wide.
Taking into account lowering brightness and image blur, the team measured the diffuse reflection luminance distributions exhibited from several light-diffusing films.
Using silica filler and an acrylic pressure-sensitive adhesive, a particulate diffusion film was prepared while another was fabricated from a high refractive index monomer, a UV absorber, and a photo-initiator together with a low refractive index oligomer to make the anisotropic light-diffusing film.
Curing is undertaken using a UV lamp. Using a coating liquid on one side of a polyethylene terephthalate (PET) film with a silicone agent, the team prepared bent columnar structures within light-diffusing films.
By measuring when a point light source was incident at a polar angle of 30° for four azimuthal angles, they were able to demonstrate uniform diffuse reflection characteristics by orthogonally laminating 60-µm-thick anisotropic light-diffusing films.
They have found that diffusion performance improves as the refractive index difference between the low refractive index binder and the high refractive index, within the bent columnar structures formed inside the film, was increased.
Independence From Azimuthal
The as-prepared, anisotropic light-diffusing film with bent columnar structures formed and orthogonally laminated gives rise to diffuse reflecting capability in the front direction, regardless of the incident external light azimuthal angle. In other words, the front brightness and viewing angle do not change even when a display is rotated, presenting a step-change in what is possible when anisotropic light-diffusing films are incorporated.
Furthermore, the group has been able to ascertain that by narrowing the gap between the reflector and the diffusing film, such as by top cell glass grinding, image blurring can be suppressed, remembering that this is a potential issue related to diffusion angle region expansion.
It is being proposed that the mechanism at play in forming the structure of the diffusion film is facilitating the desired phenomena. Initially, low molecular weight monomers selectively react to form spherical lens-like polymerized aggregates with a high refractive index when the resin mix is exposed to UV light.
Continuous UV irradiation under the spherical lens grows downward while columnar structures emerge in the direction of the angle of the UV light. There is confidence in this explanation and little doubt that by applying these light-diffusing thin films to rotated reflective displays one yields good visibility, making them ideal candidates for adoption in future outdoor applications among other potential display uses.
References and Further Reading
Kusama K. et al. (2021) Design of Anisotropic Light-diffusing Film for Rotational Use of Reflective Displays ITE Trans. on Media Technology and Applications 9(4) 210 https://dl.acm.org/doi/abs/10.1145/3132272.3134137
Ma J. et al. (2015) Enhancing Interferometric Display Color View Angle Performance Using a Fiber Array Film SID Symposium Digest of Technical Papers Vol. 46(1) 469 https://www.researchgate.net/publication/280913446_323_Enhancing_Interferometric_Display_Color_View_Angle_Performance_Using_a_Fiber_Array_Film
McAleese, J. (2021) Anisotropic Thermal Conductors to Solve Electrical Component Overheating [Online] Available at: https://www.azom.com/article.aspx?ArticleID=20842
Ishinabe T. et al. (2004) Design of Light-Diffusing Film for Full-Color Reflective Liquid Crystal Display with High Contrast and Wide Viewing Angle Jpn. J. App.Phys. 43(9A) 6152 https://iopscience.iop.org/article/10.1143/JJAP.43.6152/meta
Uchida T. et al. (2004) A Novel Reflective Liquid Crystal Display with High Resolution and Full Color Capability Jpn. J. App.Phys 43(12) 8094 https://iopscience.iop.org/article/10.1143/JJAP.43.8094