Zabudko M., Chaika A., Brik E.,
Photooptic Ltd, Russia, Obninsk , filters@photooptic.ru
Reflection from internal structures of LCD. Measurements of Vertex and NEC panels in
polarized light
Modern LCD devices consist of nematic liquid crystals sandwiched between two plates
of glass with ITO electrodes, polarizers, RGB pixel filters, TFT and etc. This complicated
layered internal structure is presented schematically on Fig. 1 [1].
Fig. 1. LCD structure and the rays of external glare light.
The mechanism of light reflection from such structure is quite complicated and poorly
studied. Investigations of this mechanism are of great practical importance, because they help to
find the optimal technology to reduce the total reflection from the front surface and to improve
the luminance contrast of the indicator.
This is especially important for high ambient light conditions when bright light makes it
difficult to read the information from the screen, the contrast and black levels on the white screen
begin to fade.
We have investigated the LCD of the world's leading manufacturers such as Vertex and
NEC.
The mirror reflection spectrum was measured in linearly polarized light in the range 400 780 nm. We used Varian Cary 300 spectrophotometer with specially designed specular
reflectance accessory, allowing to measure big panels with a diagonal of 10 inches.
LCD was settled down on accessory table horizontally. The reflected signal was changing
significantly during LCD rotation around the vertical axis.
This clearly proves that the reflection mechanisms of penetrating and non-penetrating
linear polarized light are substantially different. Results of measurements are presented in
Table 1.
The following explanation of the received results was proposed. The light penetrates the
panel and then light is multiply reflected from the internal elements in case of penetrating linear
polarized light.
This case is presented in Fig. 1. This is an inherent internal reflection (IIR).
LCD
manufacturer
Vertex
Table 1. LCD mirror reflection. Angle of incidence (AOI) = 30°
Mirror reflection
S polarization
P polarization
(S+P)/2
IIR *
Natural
(penetrating
(non penetrating
for penetrating
polarization.
polarization)
polarization)
polarization
5.6 %
8.6 %
2,7 %
2,6 %
Fresnel
reflection=6.0 %
7.7 %
Fresnel
reflection=2.7 %
2.7 %
Fresnel
Fresnel
reflection=4.3 % reflection=6.0 %
* IIR – inherent internal reflection.
Fresnel
reflection=2.7 %
NEC
Fresnel
reflection=4.3 %
5.2 %
1.7 %
In case of non-penetrating polarization the light does not penetrate the body of the LC
panel. In other words, the front polarizer does not let the light into the panel body. The light is
reflected from the front surface only by the Fresnel law.
Measurement of specular reflection of polarized light can be effectively used for input
control of the LC panels. The value of inherent internal reflection is a property of the display
which is built up during its production.
The smaller internal reflection (inherent internal reflection) of the LCD, the more it is
suitable for operation at high ambient light conditions.
Diffuse reflection of these displays was measured by special test stand equipped with
luminance & colorimeter Konica-Minolta CS-200 and solar radiation simulator with a quasiparallel beam.
Diffuse reflection for both displays is less than 0.03%. These values are small and do not
significantly affect the contrast quality of LCD in high ambient light conditions, in contrast to
the specular IIR.
It is obvious that the classical technologies of all-dielectric antireflective (AR) coatings
are not effective for improving contrast ratio for Vertex and NEC panels. AR coating on the front
panel does not reduce the IIR.
The new contrast enhancement coating (CEC) for the LCD front glass is proposed. This
is metal- dielectric AR coating with absorption.
The coating reduces the reflection from the front surface of the indicator, and also
reduces the harmful effects of internal reflection from the layered structure of the LCD.
The principle is explained schematically in Fig. 2 (Fig. taken from [2] monograph). Glare
light passes through the filter twice while signal light passes through only once. Glare light
internal reflections can be reduced by 4 times, if transmittance of CEC is equal to 50 %.
The technology was realized by the vacuum coater BALZERS BA 1400. CEC samples
with a diagonal 10’’, transmittance T= 45 % and reflectance R=0,18 %, AOI= 30 ° (RS12=0,32%,
RP12=0,03%) were made.
These samples were measured before gluing the LCD panel and computational forecast
for panel reflectance with glued contrast enhancement filter was done.
Fig. 2. The principle of an external antiglare filter. Glare light passes through the filter twice
while signal light passes through only once [2].
The calculation was made by the following formula in the case of penetrating
polarization:
RS= RS12 + RS23 * T 2,
(1)
S
where R 12 –reflectance of contrast enhancement neutral optical filter,
RS23 – inherent internal reflection in a body of LCD,
T – transmittance of contrast enhancement neutral optical filter.
Calculations for the NEC panel are the following:
RS =0.32%+(1,7%)*(45%)2=0.66%.
For non-penetrating polarization RP23=0 and the formula (1) become the following:
RP= RP12 = 0.03%.
We obtain a forecast for NEC panel reflectance with glued contrast enhancement filter.
For natural polarization (S+P)/2 reflectance R(S+P)/2 = 0,35%.
Let us consider traditional transparent anti-reflection coating (with low reflectance R =
0.15%) glued to NEC panel. In this case the specular reflectance of NEC LCD with glued AR
glass is equal to 1,00%.
The contrast neutral coating 3 times reduces the reflection from the front surface
compared to conventional anti-reflection coating.
Table 2. Specular and diffuse reflectance of experimental NEC display with contrast neutral
coating. Measurements by test stand Konica-Minolta CS-200 and spectrophotometer Cary 300
№ Measurements method
Measured parameters
Specular reflectance, %
Diffuse reflectance, %
1
Spectrophotometer Cary
—
0,33
300 with specular reflectance
(S+P)/2 - natural
accessory. AOI=30°
polarization
2
luminance & colorimeter
0,35
0,04
Konica-Minolta CS-200.
photopic reflectance
AOI=30°
The measurements of specular and diffuse reflectance of experimental NEC display with
contrast neutral coating were carried out. This display was measured by two independent
methods in order to improve the reliability of the results:
1. Varian Cary 300 spectrophotometer with specially designed specular reflectance
accessory, allowing to measure big panels with a diagonal of 10 inches. Measurements were
carried out in S and P polarizations AOI=30о. Linear polarizer was set before the entrance slit of
the Cary 300 spectrophotometer.
2. Special test stand equipped with luminance & colorimeter Konica-Minolta CS-200 and
solar radiation simulator with a quasi-parallel beam
Stand was used to measure the diffuse and specular photopic reflectance.
4
3.5
3
R, %
2.5
2
1.5
1
0.5
0
420
470
520
570
620
670
720
l, nm
Fig. 3. Mirror reflectance spectrum of experimental display with contrast neutral coating
AOI=30°. S-polarization. Cary 300
4
3.5
3
R, %
2.5
2
1.5
1
0.5
0
420
470
520
570
620
670
720
l, nm
Fig. 4. Mirror reflectance spectrum of experimental display with contrast neutral coating
AOI=30°. P-polarization. Cary 300
The measurement results are given in Table 2. Mirror reflectance spectra in S and Ppolarization of experimental display with contrast neutral coating are shown in Figures 3 and 4.
It can be seen that the measurements of specular reflectance on the Cary 300 and KonicaMinolta CS-200 give similar results. There is a good agreement between the measurements and
computational forecast.
It was determined by two independent methods that the mirror reflectance of the front
surface of the experimental display with contrast neutral coating is 0,35 % (AOI=30°).
Thereby, the problem to reduce the harmful effects of internal reflection through the use
of anti-reflective coatings with absorption (contrast neutral coating) was solved in practice.
References
1. Silverstein, L. D. (2000). Color in Electronic Displays. Society for Information Display
Seminar Lecture Notes, 1, M6/1-M6/88.
2. Macleod, H. A. (2010). Thin-film optical filters / Fourth Edition, CRC Press