Light Guide Film for Mobile Phone

Light Guide Film for Mobile Phone
Takashi Edo1, Tomosada Inada1, Masaki Oyama1, Takayuki Imai1, Shimpei Sato2,
Ken^ichiro Asano2, and Kenji Nishiwaki3
Recently, thin LGF that illuminates mobile phone keys uniformly has been attracting
attention. We have succeeded in developing and commercializing it. Base films with high
optical transparency and flexibility and ink that diffuses light effectively and possesses high
durability are used for LGFs. We established the important technologies of optical simulation
and evaluation of brightness. We report the development of these fundamental technologies and
introduce the variations of products developed on the basis of the technology.
1. Introduction
LEDs emitting light at the top are used for lighting
the keys of mobile phones. However, in this structure,
LEDs cannot be placed below each key, which leads to
the problem of light uniformity. If many LEDs are used
to improve the uniformity, both the cost and battery
drain speed increase. Hence, inorganic EL films were
used instead of LEDs. However, due to the problems
of decrease in brightness due to humidity, need for
DC/AC converter, and generation of audible noise,
the use of inorganic films was not widespread. We
have succeeded in developing thin LGFs for lighting
keys uniformly with less number of LEDs. A thin LGF
consists of a base film and light diffusion ink printed
on the base film. The base film is required to possess
high transparency and flexibility so as not to worsen
the tactile feeling, while diffusion ink is required to diffuse the light effectively and prevent damage by knocking. Moreover, technologies are needed for fine dot
printing, simulation of lighting, and evaluations of the
lighting. We will explain these developments in the following sections.
2. Structure of LGF
Figure 1 shows the mechanism of lighting by thin
LGF. The light from an LED goes through the 0.2 mm
thickness film and diffuses in the printed area. Diffused light goes out from the film and lights the keys.
Thus, following items are needed to light the keys effectively.
ase film with low absorbance and diffusion of light
iffusion ink of low absorbance and effective diffusion of light
ptical design and simulation to light the keys uniformly
1 Functional devices R&D Department of Electronics Components R&D Center
2 Electronics Technology Department of Optics and Electronics Laboratory
3 Key Devices Engineering Department of Printed Circuit Board Division
Fujikura Technical Review, 2010
Light diffusion area
Fig. 1. The mechanism of thin LGF.
Key mat
Fig. 2. Cross section of LGF assemble product.
Figure 2 shows a cross-sectional view of LGF products. The LGF is sandwiched between a MDS and the
key mat. The LGF requires the following items.
• Retention of the tactile feeling
• High durability for the numbers of knocking
3. Development of a base film
Base film requires the following characteristics.
mall absorbance and diffusion of light in the visible
Panel 1. Abbreviations, Acronyms, and Terms.
LGF–Light Guide Film
LED–Light-emitting Diode
DC/AC–Direct Current/Altenating Current
MDS–Metal Dome Sheet (Metal Dome with Adhesive Film)
PET–Polyethylene Terephthalate
• High durability for mobile phone application
• Mechanical flexibility to minimize the bad influence
on tactile feeling
3.2 Heat and humidity test result
Figure 4 shows the outgoing light after a 1000 hour
heat and humidity test. The test was carried out under
the conditions of 85°C and 90% humidity. Some films
turned yellow because damaged polymers caused increased absorbance at a wavelength of about 400 nm.
In contrast, our base film demonstrated high durability to withstand heat and humidity and the outgoing
light remained white.
3.3 Tactile feeling
It is possible for an LGF to make the tactile feeling
of keys worse because the LGF is sandwiched between
the MDS and the key mat. The tactile feeling is expressed as the click ratio, which is calculated from the
following formula.
(Click ratio) =100 ¥ (P1 - P2)/P1
P1 and P2 are described in load stroke curve (Fig. 5)
and we measured these values by the system of Fig.
The result is shown in Table 1. While the click ratio
of the MDS itself was 46%, it decreased merely to 26%
when a hard LGF was put on the MDS. On the other
wavelength (nm)
Fig. 3. Transparency of each material.
Reference A
Reference B
Fig. 4. Color change of each material after heat & humid test.
Figure 3 shows the light transmission spectrum of
various base films. We made an evaluation system to
measure their spectrums. The film thickness is 0.2
mm and the length is 50 mm. The transparency of light
emitted from LED, incident from the edge of the film,
was measured by integrating sphere and the spectrophotometer. It means that this measurement result is
affected not only by the absorption and diffusion in the
film but also by the diffusion at the surface and edge of
the film. The transmittance of the light through each
film was over 90% when measured on the backside of
the film of 0.2 mm thickness by shining the light on
the surface. When the transmittance of the light
through the film of 50 mm in length was measured
along the direction of the length, the ratio significantly
decreased. Especially, the transmittance of PET film
decreased noticeably because the light diffused at the
edge of the film.
transparency (%)
3.1 Light transmission spectrum
Fig. 5. Typical load stroke curve.
hand, the click ratio was 41% when our elastomeric
LGF was put on the MDS, slightly lower than the result of the MDS itself. Generally, click ratio is measured using an actuator of 1-2 mm diameter. The harder the LGF materials, the wider the area on the MDS
Load cell
Fig. 6. Tactile feeling measurement system.
Table 1. Load stroke data for each material.
Soft LGF
Hard LGF
Click ratio(%)
(a) Non damaged
(b) Damaged
Actual lighting
Fig. 8. Luminance of simulation and actual lighting.
Fig. 7. Peeling ink after knocking test.
to which load is applied is, and the greater the decrease in the click ratio.
Thus, our base film has high transparency, high
flexibility that prevents the tactile feeling from getting
worse, and high durability against knocking.
4. Development of light diffusion ink
LGF diffusion area is required to diffuse the light
with high efficiency to let out the light from the film.
Special diffusion ink is printed on our LGF to achieve
highly efficient diffusion. This ink consists of a binder
and filler whose refractive index and transparency are
optimized. As the diffusion ink printed area is knocked
repeatedly, the ink should have high durability to withstand knocking. The damaged ink is shown in Fig.
7(b). If the ink is damaged or peeled off the film, its
lighting characteristics worsen. The binder of our diffusion ink is made from special urethane with high
durability to withstand knocking. In addition, by pretreating the film, we have succeeded in developing the
highly durable light diffusion pattern enduring 1 million knocks. There is no damage as shown in Fig. 7(a)
and no change in the performance of light diffusion
even after frequent knocking.
5. Optical simulation
The denser the printed area , the more is the light
diffused in an upward direction in the lighting area of
an LGF. Accordingly, the luminance of the LGF increases with a rise in the density of the printed area.
Fujikura Technical Review, 2010
Fig. 9. Segmentation LGF.
Thus, we evaluated the correlation between the density of the printed area and the luminance. We made
printing dot samples of different diameters, that is, different densities of the printed area. We measured the
luminance with a two-dimension luminance measurement machine made by Konica Minolta. We confirmed
that the bigger the dot diameter, the higher the luminance.
We performed an optical simulation to light all the
keys uniformly on the basis of the result of the transparency of the film and the correlation between the
density of the printed area and the luminance. As Fig.
8 shows, the simulation results coincide with the actual LGF lighting. For the luminance of each key, the
maximum and minimum luminance remained within
20% of the mean value. The uniformity of the lumi29
1.8 mm
1.0 mm
Current structure
New structure
Fig. 13. Cross section of each segment width.
Fig. 10. Multi color LGF.
Fig. 14. Fine segment LGF.
Table 2. Specification of Fujikura LGF/MDS product.
Fig. 11. Multi layer LGF.
F 0.2
F 0.05
Test conditions and specifications
After 8 N ¥ 1,000,000 cycles
Dot will not peel off
Heat and humid
After 85 °C, 85 % ¥ 1000 hours
Transparency decrease by under 20%
from original one
MD click ratio
Over 35 %
Luminance uniformity
Min / max = over 60 %
Film hardness
90~96 (JIS A)
Film thickness
0.125, 0.15, 0.2 mm
Dot diameter
Min 0.2 mm
(0.05 mm under development)
Segment width
Min 1.8 mm
(1.0 mm under development)
Dot color
White, red, blue, yellow, green
LED/LGF distance
0.35 ± 0.2 mm
was blocked. We verified that segmented lighting was
achieved. The sample is shown in Fig. 9.
Current dot
Fine dot
nance satisfies the requirement of the mobile phone.
Regarding normal LGFs, they are lit by diffusing
white color lights with light diffusion ink. We can make
multi color light with the ink that absorbs the light
with specific wavelength as shown in Fig.10.
6. Applications being developed
6.3 Multi layer LGF
6.1 Segmentation lighting LGF
By stacking several LGFs, different areas can be lit
selectively. Figure 11 shows that numbers and characters on the keys emit the light selectively.
Fig. 12. Fine dot LGF.
By blocking the light propagating in the film with
the gasket, light segmentation can be realized. We
made an LGF sample that can light navigation key independently. This sample is made by the following
procedure. First, a slit was made in the film. Second,
the film was laminated with an MDS. Third, a gasket
was inserted in the slit area. We measured the luminance of this sample and observed that 99% of light
6.2 Multicolor lighting LGF
6.4 Fine dot LGF
Regarding the current LGF, the minimum dot diameter is 0.2 mm and this dot size is visible. With a key
mat with high transmittance, dots can be seen even
through the key mat. That is why we are developing an
LGF with invisible dots with a diameter of 0.05 mm or
less. The dots are made by gravure offset printing instead of screen-printing. Figure 12 shows the LGF
with fine dots.
6.5 Fine segment LGF
We explained the LGF segment method in Section
6.1. One of the disadvantages of this method is the
wide area of the borders. To overcome the disadvantage, we are developing new segmentation method
shown in Fig. 13. In this method, first, grooves are
made, and, second, black ink is printed on these
grooves. We can decrease the light block width by
about 50% by this method. Figure 14 shows the actual
lighting. Light blocking ratio is over 80 % by this new
method and it is enough for actual mobile phones.
Fujikura Technical Review, 2010
7. Conclusion
We developed two key materials for LGFs. One is a
base film that is superior in transparency, durability,
and flexibility. Second is light diffusion ink that is superior in light diffusion performance and durability.
Moreover, we established the optical design of the
LGF, optical simulation, and luminance measurement
method. We have commercialized the LGF for mobile
phone key lighting. Table 2 shows the specifications
and characteristics of LGFs. We are developing a variety of lighting technologies such as light segmentation, multicolor lighting, and fine dot printing. LGF
technology can be applied not only to mobile phones
but also to other kinds of digital equipment.