Display lighting unit - Part 1-4: Glass light guide plate

IEC TR 62595-1-4:2020(E), which is a Technical Report, provides general information for judging the necessity of future standardization of glass light guide plates for display lighting units, which include backlight units for transmissive displays such as LCDs, and frontlight units for reflective displays.

General Information

Status
Published
Publication Date
12-Jul-2020
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
13-Jul-2020
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IEC TR 62595-1-4
Edition 1.0 2020-07
TECHNICAL
REPORT
colour
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Display lighting unit –
Part 1-4: Glass light guide plate
IEC TR 62595-1-4:2020-07(en)
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IEC TR 62595-1-4
Edition 1.0 2020-07
TECHNICAL
REPORT
colour
inside
Display lighting unit –
Part 1-4: Glass light guide plate
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 31.120; 31.260 ISBN 978-2-8322-8618-0

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® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 62595-1-4:2020 © IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms, definitions and abbreviated terms ........................................................................ 6

3.1 Terms and definitions .............................................................................................. 6

3.2 Abbreviated terms ................................................................................................... 6

4 Overview ......................................................................................................................... 7

4.1 General ................................................................................................................... 7

4.2 Light guide plate technologies and its typical materials ........................................... 7

4.3 Advantages of and issues with GLGP ..................................................................... 8

5 Optical characteristics ..................................................................................................... 9

5.1 Factors affecting optical characteristics of GLGP .................................................... 9

5.2 Optical absorption of the glass materials for LGP .................................................... 9

5.3 Optical absorption and scattering loss caused by the dot pattern .......................... 11

5.4 Incident loss ......................................................................................................... 11

5.5 Effect of the reflection tapes ................................................................................. 12

5.6 Discussions for possible future standardization ..................................................... 12

5.6.1 Applicability of existing standards .................................................................. 12

5.6.2 Mechanical structure and interface ................................................................ 13

5.6.3 Hotspot influence caused by LED light source ............................................... 13

5.6.4 Non uniformity around edge ........................................................................... 13

5.6.5 Optical absorption of glass materials for LGP ................................................ 13

6 Mechanical and environmental characteristics ............................................................... 13

6.1 General ................................................................................................................. 13

6.2 Rigidity ................................................................................................................. 14

6.3 Thermal expansion and heat resistance/noninflammability .................................... 15

6.4 Humidity absorption .............................................................................................. 17

6.5 Impact resistance .................................................................................................. 18

6.6 Discussions for possible future standardization ..................................................... 19

7 Additional functions and possible future standardization ................................................ 19

7.1 General ................................................................................................................. 19

7.2 Local dimming for HDR TV .................................................................................... 19

7.3 Curved GLGP for curved LCD ............................................................................... 20

7.4 Quantum dot coating and quantum dot coated film LCD ........................................ 21

7.5 Frontlight .............................................................................................................. 21

7.6 Transparent LCD .................................................................................................. 21

7.7 Combination with PDLC ........................................................................................ 22

Bibliography .......................................................................................................................... 23

Figure 1 – Structure of edge-lit BLU and LGP ......................................................................... 7

Figure 2 – Light propagation in LGP ........................................................................................ 8

Figure 3 – Examples of internal transmittance spectra at 50 cm in optical path length ........... 10

Figure 4 – Chromaticity gradient against the distance from incident edge ............................. 10

Figure 5 – Variation of the relative BLU luminance against the thickness of the GLGP .......... 12

Figure 6 – Weight /thickness dependence of the rigidity of PMMA and glass for LGPs .......... 15

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IEC TR 62595-1-4:2020 © IEC 2020 – 3 –

Figure 7 – Schematics of the simulation setup for the deformation calculation of the

LGP by pulling up one corner and fixing the other three corners ........................................... 15

Figure 8 – Horizontal bowing of polymeric LGPs under elevated temperature ....................... 16

Figure 9 – Simulated temperature distribution of (a) GLGP and (b) PMMA LGP ................... 17

Figure 10 – Simulated thermal deformation of (a) GLGP and (b) PMMA LGP due to

LED lighting .......................................................................................................................... 17

Figure 11 – Increase in the horizontal length of LGP with temperature change for a 65”

diagonal LGP ........................................................................................................................ 18

Figure 12 – Example of curved LCD using a curved GLGP .................................................... 20

Figure 13 – Example of transparent LCD .............................................................................. 21

Figure 14 – Example of transparent LCD with GLGP including PDLC .................................... 22

Table 1 – Comparison between polymers and glasses for LGP ............................................... 8

Table 2 – Physical properties of commercial glass for LGP and PMMA ................................. 14

Table 3 – Comparison of thickness, weight, and calculated deformation between

GLGP, PMMA LGP, and PMMA combined with steel plate .................................................... 15

Table 4 – Comparison of GLGP and polymer LGP in confined structure under humid

condition ............................................................................................................................... 18

Table 5 – Impact resistance with different machining ............................................................ 19

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– 4 – IEC TR 62595-1-4:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DISPLAY LIGHTING UNIT –
Part 1-4: Glass light guide plate
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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The main task of IEC technical committees is to prepare International Standards. However, a

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data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 62595-1-4, which is a Technical Report, has been prepared by IEC technical

committee 110: Electronic displays.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
110/1174/DTR 110/1200/RVDTR

Full information on the voting for the approval of this technical report can be found in the

report on voting indicated in the above table.
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IEC TR 62595-1-4:2020 © IEC 2020 – 5 –

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 62595 series, published under the general title Display lighting

unit, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

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– 6 – IEC TR 62595-1-4:2020 © IEC 2020
DISPLAY LIGHTING UNIT –
Part 1-4: Glass light guide plate
1 Scope

This part of IEC 62595, which is a Technical Report, provides general information for judging

the necessity of future standardization of glass light guide plates for display lighting units,

which include backlight units for transmissive displays such as LCDs, and frontlight units for

reflective displays.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.

IEC 62595-1-2:2016, Display lighting unit – Part 1-2: Terminology and letter symbols

3 Terms, definitions and abbreviated terms

For the purposes of this document, the following terms and definitions given in IEC 62595-1-2

and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 Terms and definitions
3.1.1
glass light guide plate
GLGP
light guide plate whose optically transparent medium is made of glass material

Note 1 to entry: See IEC 62595-1-2:2016, 3.3.1. A GLGP includes optical elements for light guide plates, such as

diffusion patterns, in addition to a glass sheet for light guide plates.
3.2 Abbreviated terms
BLU backlight unit
CTE coefficient of thermal expansion
DLU display lighting unit
FLU front lighting unit
FPC flexible printed circuits
GLGP glass light guide plate
HDR high dynamic range
LC liquid crystal
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IEC TR 62595-1-4:2020 © IEC 2020 – 7 –
LCD liquid crystal display
LED light emitting diode
LGP light guide plate
MCPCB metal core printed circuit board
MS methyl-methacrylate styrene copolymer
PDLC polymer dispersed liquid crystal
PMMA polymethyl methacrylate
S/N signal/noise ratio
4 Overview
4.1 General

Glass light guide plate (GLGP) enables distinctive display product features such as thinner,

lighter, larger and narrower bezel design with several additional considerations of material

properties and stabilities compared to conventional polymer light guide plate. This document

intends to investigate display product features enabled by GLGP and to identify possible

future standardization.
4.2 Light guide plate technologies and its typical materials

An LGP is a component of an edge-lit backlight unit (BLU) as shown in Figure 1 and in

IEC 62595-1-2:2016, Annex A. This edge-lit BLU has been widely used for thin LCDs. In the

BLU, the light emitted from LEDs positioned in close proximity to the edges of the LGP is

optically coupled into the LGP to illuminate an LC device. Figure 2 shows the schematics of

the cross-section view of the LGP. The light from the LEDs propagates in the LGP by means

of total internal reflection, and the patterned reflection dots at the surface disrupt the total

internal reflection to couple out light, resulting in uniform light output for surface illumination.

Figure 1 – Structure of edge-lit BLU and LGP
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– 8 – IEC TR 62595-1-4:2020 © IEC 2020
Figure 2 – Light propagation in an LGP
Generally, polymer materials, such as polymethyl methacrylate (PMMA) and methyl-

methacrylate styrene copolymer (MS), have been applied for the LGP due to their excellent

optical properties. However, the polymer LGP has apparent disadvantages: lower stiffness,

deformation by humidity, higher thermal expansion, and lower chemical and thermal stability

(see Table 1). Because of its lower stiffness, the polymer LGP is difficult to apply for extra-

large size displays, that is, larger than 65 inches with ultra-thin design less than 5 mm. Easier

deformation by humidity and thermal expansion result in the limitation of TV sets design to

keep the optical clearance between LEDs and LGPs. In addition, the thermally unstable

nature is not suitable for future high power LEDs that also generate more heat and introduce

higher temperature; it potentially limits the brightness improvement of the BLU [10] .

Table 1 – Comparison between polymers and glasses for LGP
Polymer Glass
Young’s modulus (GPa) Low (~5) High (≈70)
Thermal conductivity (W/m/K) Low (≈0,2) High (≈1,1)
High (> 400) Low (< 100)
Thermal expansion (×10 1/K)
Water/humidity absorption (vol %) High (< 0,1) None
Flammability Yes No
4.3 Advantages of and issues with GLGP

Glass materials have been gathering much attention these days as the candidates for novel

LGP materials because they have better chemical durability, thermal stability, and mechanical

properties in comparison with polymers. GLGPs have been already mass produced [1] to [3],

and GLGP installed LCD TVs and monitors have been on the market [4], [5].

Although anticipation has increased, various major hurdles have to be overcome before

GLGPs become popular. One is the facility asset: existing production lines, supply chains of

BLUs are basically optimized to use polymer LGPs, and are not easy to convert to use

GLGPs. Another big issue is the lack of appropriate information: most of the documentation

related to the LGP was prepared with the use of polymers in mind, therefore the appropriate

information is difficult to reach. Evaluation methods are also designed with the use of

polymers in mind, hence some of these, such as optical properties, mechanical and

environmental properties, seem inappropriate to the glass. If the correct recognition of the

difference between these two materials is not sufficient, biased knowledge and experiences of

the polymer LGP can prevent the adoption of the glass materials. In addition, the current

structure explained in Figure 1 would be based on polymer LGPs, and for GLGPs a new

structure might be applied according to the feature of the GLGP. The current standards for

BLUs need to be checked considering whether they are based on only polymer LGPs or not.

—————————
Numbers in square brackets refer to the Bibliography.
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IEC TR 62595-1-4:2020 © IEC 2020 – 9 –

As an additional point of view, compared with polymer LGPs, GLGPs may be suitable not only

for the BLUs mentioned above but also for other DLUs such as FLUs, transparent LCDs, and

so on, which seems attractive for the future. According to this situation, this document

summarizes the basic information of GLGPs and the desirable characters for GLGP

application, in order to discuss the necessity of revising the current BLU standards [6] to [9]

and proposing new standards.
5 Optical characteristics
5.1 Factors affecting optical characteristics of GLGPs

The main function of the GLGP is the light propagation from the incident edge to the output

surface, and both the radiant or luminous flux and chromaticity are expected to become

uniform in the whole output surface. Applying the reflection tapes around one or three side

(non-incident) surfaces of the GLGP can increase the luminance and uniformity. Uniformity of

the illuminant power and chromaticity depend on the optical absorption, scattering loss during

the propagation, and loss of the LED coupling at the incident edge.

1) Optical absorption: the absorption of the glass material itself is the major factor to

determine the optical performance of the GLGP.

2) Scattering loss: the GLGP generally uses ink-based light extraction. It uses scattering as

its mechanism to control light. This scattering by the reflection dot pattern has spectral

and spatial dispersion, thus it also causes the similar effect of optical absorption. This

effect is not unique to GLGP, but the GLGP is expected to use a thinner thickness

compared to polymer LGPs, such as less than 3 mm in thickness, so the light hits the ink

more often than on a thicker LGP, and it amplifies the ink's deleterious effects.

3) Loss of the LED coupling: from the viewpoint of GLGPs, the loss at the LED coupling is

affected by the distance between the LED and the LGP and the edge surface condition of

the GLGP such as edge straightness, edge surface waviness, incident area width,
chamfering shape and roughness.
5.2 Optical absorption of the glass materials for LGPs

The optical path length of the LGP in LCD TVs is longer than several tens of centimetres,

whereas that in general usage is several millimetres at the most. Therefore, lower optical

absorption, that is, higher internal transmittance, is mandatory for the glass for LGPs. These

distinguishing characteristics are reported in the references [10] and [11]. Figure 3 shows

examples of internal transmittance spectra of the commercial glass for LGPs. The spectra of

MS, PMMA and conventional extra clear glass for solar cells are shown as a reference. Note

that the optical path length of the spectra in Figure 3 is 50 cm, in contrast with the length of

the normal spectra which is 1 cm at the most. As shown in Figure 3, the glass for LGPs

showed significantly higher internal transmittance than conventional glasses; the internal

transmittance of the glass for LGPs is higher than 80 % even if the optical path length is as

long as 50 cm.
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– 10 – IEC TR 62595-1-4:2020 © IEC 2020

NOTE The solid, dashed, dotted, long-dashed lines represent the commercial glass for LGP, MS, PMMA, and

conventional extra clear glass for solar cell, respectively.

Figure 3 – Examples of internal transmittance spectra at 50 cm in the optical path

length

In addition, the spectral shape of the glass for LGPs in the visible wavelength region is flatter,

that is, there is less wavelength dispersion than the conventional glasses. It is important to

suppress the chromaticity change of GLGPs.

Figure 4 shows an example of the chromaticity gradient against the distance from the incident

edge by using the conventional glass as LGP. dx and dy in Figure 4 represent the chromaticity

difference between the measured position and the incident edge which is calculated from the

difference of x and y at the measured points and at incident edge, respectively, where x and

y are the chromaticity parameters derived from tristimulus values which are defined in

ISO/CIE 11664-1 [30]. The variation of dx and dy as shown in Figure 4 indicates the

occurrence of the colour shift. The large spectral dispersion of the transmittance easily

causes the colour gradation.
Figure 4 – Chromaticity gradient against the distance from the incident edge
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IEC TR 62595-1-4:2020 © IEC 2020 – 11 –

For the evaluation of the optical absorption of the glass materials for LGPs, precise optical

absorption data of the material for LGPs is mandatory information for designing the BLU. The

optical path length of the GLGPs is several ten times longer than that for conventional usage,

thus the accuracy needs to improve in comparison with the available data. The conventional

evaluation is inappropriate because the optical path length is too short to gain enough S/N,

and the effect of the surface scattering is not negligible. Therefore, the evaluation by using

the sample of a longer optical path length is preferable. One example of the precise

evaluation for the purpose is to use a spectral optical setup with a coherent light source, such

as super continuum white light, to avoid losses caused by total internal reflections during

propagation. By using the setup, a sample of about 50 cm in length can be measured. Another

example is to use an available optical spectrophotometer with a combination of a well

collimated light source and a special attachment to ensure the repeatability of the sample’s

precise position. A sample of 5 cm to 15 cm in length can be measured by using the setup.

Both examples seem to show a good correlation with each other, if the measurement setup is

done under the appropriate conditions. The detailed measurement conditions and possibility

of future standardization may need to be discussed.
5.3 Optical absorption and scattering loss caused by the dot pattern

There are various techniques for dot patterning for polymer LGPs such as molding during

sheet formation process, screen printing, inkjet printing, CO laser patterning or imprinting.

However, the dot pattern for GLGP is basically formed by dot printing techniques due to the

limitation of the cost and optical performance. UV cured or IR cured inks are widely used, and

using the inks with less colouring is important especially for GLGPs A GLGP is basically

expected to have a thinner thickness compared to polymer LGPs, and in a thinner condition

the light hits the ink more often than on a thicker LGP, amplifying the ink’s deleterious effects.

The dot pattern can be formed by engraving the patterned pits at the surface of the glass.

Mechanical holing, chemical etching and laser patterning techniques can be applied for this

process. In these types, controlling the spatial and spectral dispersion of the scattering by

controlling the pit's size and shape is the key to suppressing the optical degradation.

The influence of this degradation needs to be measured and evaluated.
5.4 Incident loss

The LED bar and LGP coupling straightness can affect the coupling alignment between LEDs

and LGPs, and thus the coupling efficiency. In addition, the variation of the LED bar (peak to

peak about 100 µm) and the waviness of the coupling edge surface of the LGP can change

the gap between the LED and LGP, and thus the coupling efficiency. The surface roughness

of the incident plane also likely affects the coupling efficiency between LEDs and LGPs.

Although the loss is not unique to a GLGP and is likely also present in a plastic LGP, the

difference caused by the brittleness of the glass material should be taken into account. The

cutting and breaking method for the glass materials
...

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