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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 6 — Mar. 12, 2012
  • pp: 6135–6145
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Surface-structured diffuser by iterative down-size molding with glass sintering technology

Xuan-Hao Lee, Jung-Lin Tsai, Shih-Hsin Ma, and Ching-Cherng Sun  »View Author Affiliations


Optics Express, Vol. 20, Issue 6, pp. 6135-6145 (2012)
http://dx.doi.org/10.1364/OE.20.006135


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Abstract

In this paper, a down-size sintering scheme for making high-performance diffusers with micro structure to perform beam shaping is presented and demonstrated. By using down-size sintering method, a surface-structure film is designed and fabricated to verify the feasibility of the sintering technology, in which up to 1/8 dimension reduction has been achieved. Besides, a special impressing technology has been applied to fabricate diffuser film with various materials and the transmission efficiency is as high as 85% and above. By introducing the diffuser into possible lighting applications, the diffusers have been shown high performance in glare reduction, beam shaping and energy saving.

© 2012 OSA

1. Introduction

The recent demand for environmental protection has precipitated a large number of interests in the topics of energy saving and human factors. According to a released publication of International Energy Agency (IEA), 19% of total electricity global power was consumed in the use of lighting for 2005 [1

1. International Energy Agency, Light’s Labour’s Lost: Policies for Energy-Efficient Lighting (OECD/IEA, Paris, 2006).

]. Therefore, the topics of energy-saving and human-factor in lighting have attracted much attention. However, energy wastefulness and human-factor problem in lighting such as light pollution, glare, and non-uniform illumination can be observed usually. To solve these problems, an appropriate design for luminaire will be one of the most important issues in modern lighting.

At present, more and more luminaires adopt light-emitting diode (LED) as the light source to replace the traditional ones such as fluorescent lamp and halogen lamp for the factors of energy-saving and environmental protection. Based on the selection of light source, a concept of second optics must be introduced into the design of luminaire to obtain an optimum result and a component of third order optics is suggested to prevent glare but with high energy efficiency [2

2. C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). [CrossRef] [PubMed]

]. Therefore a diffuser gradually plays an important role in the third order optics because a diffuser may offer the advantage effectively making the light pattern be more uniform to reduce some effects by glare.

In the traditional diffusers, the introduction of particles in the volume of a diffuser is a common approach to perform diffusing effect. Such kind of diffuser is called volume scattering diffuser. When light passes through a volume scattering diffuser, different light scattering behaviors will appear according to some important factors such as particle sizes, adulterating densities, the refractive index of material, and the wavelength of incident light. Volume scattering diffusers perform good effect upon the functions of diffusion and anti-glare, but hard to control the scattering behavior and lower transmittance are major shortages for practical applications. Also, the problems of light pollution, non-uniform illuminated region, and energy wastefulness still exist observably when this kind of diffuser is used in a luminaire without an appropriate design.

For the improvement of the problems about light pollution, glare, non-uniform illuminated region, and energy wastefulness, a development of high performance diffuser performing the ability of controllable diverging angle will be a good solution. With the high-performance diffuser, light can be effectively concentrated and limited at a target region so that light pollution can be reduced and optical utilization factor can be increased. The kind of high performance diffuser can be obtained by use of a diffuser of surface structure. However, the structure must be small for the purpose of homogenization. It is difficult in traditional metalworking industry because the structure is too small to manufacture by computer numerical control (CNC) machining or other technologies. Some researchers tried to replace volume scattering diffuser by making structured micro-lens array to reach the purpose of high performance [3

3. T. R. M. Sales, “Structured microlens arrays for beam shaping,” Opt. Eng. 42(11), 3084–3085 (2003). [CrossRef]

6

6. R. P. C. Photonics, Inc., http://www.rpcphotonics.com/.

]. They fabricated the structured micro-lens array by using laser writing system, photoresist, or point-by-point exposure method to produce a continuous surface-relief profile. Although high-performance structured micro-lens array can be obtained by the technologies, the expensive equipments including laser writing system, photoresist, developer, etc. and the requirement of high resolution automatic alignment instrument produce problems in higher cost and the complexity of manufacturing process.

In this paper, we propose a down-size sintering technology for volume reduction to improve traditional metalworking problem in small structure manufacture. We design a high performance surface-structure film to verify the feasibility of the technology at the same time. The theoretical simulation and corresponding experiment will be demonstrated and presented as follows.

2. Design of high performance diffuser

For the verification of the feasibility of the sintering technology, we designed a specific one-dimension surface-structure film at first. Here, the specific surface-structure film is called Surface Structure Diffuser (SSD). To achieve a uniform light pattern, each structure must be designed to obtain a controllable intensity-profile. Based on the purpose, we use a mathematical equation about the sag of a conic surface to obtain the structure that we need [7

7. V. N. Mahajan, Optical Imaging and Aberrations: Part I. Ray Geometrical Optics (SPIE Press, 1998).

]. Assuming the rotating center of structure is z axis and point Ac is a point on the structure surface, the equation can be written
Zc=rc2/R1+[1(1e2)rc2/R2]1/2,
(1)
where, R is the vertex radius of curvature and e is the eccentricity of a conic surface. The coordinates of point Ac are (xc, yc, zc), and rc is the distance of point Ac from the z axis and can be written as

rc=(xc2+yc2)1/2.
(2)

According to Fig. 3, some defects on the bottom of structure are discovered. The reason for the formation of defects is that the tiny structure of G0-type SSD is difficult to manufacture by CNC machining. In making the tiny structure of G0-type SSD, the larger knife of CNC machining easily caused some abrasion and knife-mark on the bottom of G0-type SSD’s structure. The problem of abrasion and knife-mark caused that the curvature of structure of G0-type SSD had a little difference with the design. Such a difference induced distortion of the light pattern by the G0-type SSD. Besides, such abrasion and knife-mark might cause degradation of the transmittance in measurement by the scattering loss.

3. Sintering method and large-area impression technology

According to midfield model, light patterns changes from a distance to another in the midfield region [8

8. C. C. Sun, T. X. Lee, S. H. Ma, Y. L. Lee, and S. M. Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Opt. Lett. 31(14), 2193–2195 (2006). [CrossRef] [PubMed]

]. Reduction of diffuser’s structure will be helpful to achieve far-field region (Fraunhofer region) in a shorter distance which is between output plane and the position of light pattern [9

9. J. W. Goodman, Introduction to Fourier optics, 2nd ed. (McGraw-Hill, 1996).

]. More widespreading applications for designers can be obtained if far-field region (Fraunhofer region) is obtained at a shorter distance. Besides, reduction of diffuser’s structure will be also helpful in the thickness reduction of diffuser to obtain a slim luminaire [2

2. C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). [CrossRef] [PubMed]

]. Figure 4
Fig. 4 The comparison of the simulation results of light pattern of G0-type SSD before and after the process of volume reduction at the (a) 2 cm, (b) 5 cm, and (c) 10 cm distances between output plane of SSD and the observed plane. (Volume reduction with a multiple of 2 from up to down in turn.)
shows a comparison of the simulation results of light pattern of G0-type SSD before and after the process of reduction of the diffuser’s structure. In the simulation, we used the same incident collimating beam with a diameter of 1 cm and the same observed plane at distances of 2 cm, 5 cm, and 10 cm respectively from the output plane. According to Fig. 4, at the position of 2 cm, obvious fringe patterns appeared in different size structure except 1/8 size reduction of the G0-type SSD. Compared with the results with the process of reduction of the diffuser’s structure, the smaller diffuser’s structure was helpful to achieve far-field region (Fraunhofer region) at a shorter distance. Besides, a better result of uniformity of light pattern using smaller structure was also obtained at a same position.

The proposed process of down-size sintering method is illustrated in Fig. 5
Fig. 5 The process of volume reduction by down-size sintering method.
. The mold with larger structure size (G0-type SSD) which may be fabricated by CNC machining was used as the mold in the first step of the reduction process. We injected specific silica glass material, SAVOSILTM silica glass which was made by Evonik Industries AG, into G0-type SSD mold and shaped silica glass material to form an inverse replica at room temperature [10,11

11. F. Costa, L. Costa, and L. Gini, “Optical articles and sol-gel process for their manufacture,” World Intellectual Property Organization WIPO, WO 2004/083137, A1 (2004).

]. Then a development step and a drying process were also introduced to the process of precise shaping. By means of high temperature sintering process and some specific manufacture technologies, the glass replica of the G0-type SSD will be equally reduced to a half size in all dimensions. We could use this replica to make the next-generation mold with half-size reduction. After the procedure of mold re-making, a sample with half dimension of G0-type SSD could be obtained and it was named Generation 1 type (G1-type) SSD. Similarly, we could use the same procedure on the sintering, and mold re-making to successively produce smaller SSD which would reduce the dimension by 1/4 in dimension for Generation 2 type (G2-type), by 1/8 for Generation 3 type (G3-type), and so on. The samples of G1-type, G2-type, and G3-type SSD are shown in Fig. 6
Fig. 6 The practical products of G1-type, G2-type, and G3-type SSD.
.

In order to fit practical applications, a large-area impressing technology, which is a synthesizing process for forming multi-segment SSD in a glass mold and for large-area impressing, was introduced to produce a large-area SSD [12]. By using the large-area impressing technology, a plate of SSD with large-area replication can be obtained under a condition of invariable structure, and offers the possibility of a practical application in different materials, as shown in Fig. 7
Fig. 7 Large-area SSD. (a) The diagram for large-area SSD manufacturing method. (b) The practical products of large-area G1-type, G2-type, and G3-type SSD. (c) The largest SSD in the current manufacture process.
. The schematic diagram of large-area manufacturing method is shown in Fig. 7(a). Figure 7(b) shows large-area diffusers such as G1-type, G2-type, and G3-type with different materials, including polyurethane (PU), silica gel, and polyethylene terephthalate (PET), were successfully applied to the fabrication of the structured film in the large-area SSD by the large-area impressing technology. Figure 7(c) shows a SSD with dimensions of 470 mm × 470 mm. The largest dimensions of SSD were 650 mm × 650 mm in current manufacture process. Measurement of the transmission efficiency for the PU film is shown in Fig. 8
Fig. 8 The measured transmission efficiency for three types of SSD.
, where the transmission efficiency for the structure outward the light source is obviously lower than that toward the light source because of more lights are reflected back through total internal reflection by the structure.

4. Measurement and analyses

The characteristic analysis will be focused on the transmission efficiency and the lighting pattern. For the lighting patterns, the simulation and measurement result of G1-type, G2-type, and G3-type SSD are shown in Fig. 9
Fig. 9 Light pattern of (a) G1-type, (b) G2-type, and (c) G3-type SSD in simulation (the left) and measurement (the right).
. Here, we used a collimating beam with a diameter of 1 cm under normal incidence to pass through the SSDs. As in the design, the collimating beam was incident on the structured surface of the SSDs with PMMA. The distance between the SSDs and the observation plane was 20 cm in both simulation and measurement. In the simulation results, the light patterns of three kinds of SSDs illuminate an area of 36.6 cm × 8.4 cm, and the angular spreading of FWHM in two orthogonal directions are all 84° and 22° respectively.

5. Practical applications

In addition to the design and manufacture of the SSD, some possible applications are presented. First, we tried to change the arrangement of SSD’s structure by the developed impressing technology. The new practical SSDs named as G2C-type SSD and G2H-type SSD respectively are shown in Fig. 12
Fig. 12 The SSD which structures with different arrangements. (a) A sample of the G2C-type SSD. (b) The corresponding light pattern of the G2C-type SSD. (c) A sample of the G2H-type SSD. (d) The corresponding light pattern of the G2H-type SSD.
, where the G2C-type SSD performed a cross light pattern and the G2H-type SSD performed a star light pattern. According to the characteristic of these SSDs, we used these SSDs to make some applications in lighting further.

We introduced the SSDs into a commercial down light and replaced the original v-cut diffuser by the SSD in the lighting cavity, as shown in Fig. 13
Fig. 13 Commercial down lamp with different diffuser. (a) Without diffuser. (b) Common V-cut diffuser, the structure surface toward light source. (c) Common V-cut diffuser, the structure surface outward the light source. (d) G2-type SSD, the structure surface toward light source. (e) G2-type SSD, the structure surface outward light source. (f) G2C-type SSD, the structure surface toward light source. (g) G2C-type SSD, the structure surface outward light source. (h) G2H-type SSD, the structure surface toward light source. (i) G2H-type SSD, the structure surface outward light source.
, where high transmission efficiency as well as glare reduction was achieved simultaneously. With the definition of cavity efficiency as the ratio between the flux of down lamp without diffuser and with diffuser [2

2. C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). [CrossRef] [PubMed]

], the cavity efficiency was increased to around 87% in comparison to 84% in the v-cut diffuser which was also a structured diffuser.

Except for the application of down light, we also introduced SSDs into an application of LED light bar. In this application, three kinds of SSDs of G2-type, G2C-type, and G2H-type respectively were used, as shown in Fig. 14
Fig. 14 LED light bar with different SSD. (a) G2-type SSD, the structure surface faces toward the light source. (b) G2-type SSD, the structure surface faces outward the light source. (c) G2H-type SSD, the structure surface faces toward the light source. (d) G2H-type SSD, the structure surface faces outward the light source. (e) G2H-type SSD, the structure surface faces toward the light source. (f) G2H-type SSD, the structure surface faces outward the light source.
. By the combination of SSD and LED light bar, we could keep the LED light bar to be a line-type light source as same as traditional fluorescent lamp or other novel types. Wider diverging angle and higher cavity efficiency could be obtained at the same time, as shown in Fig. 15
Fig. 15 The intensity distribution of LED light bar with different SSD. (a)-(f) correspond to the cases in Fig. 14.
. In such application, Fig. 15 shows that various light patterns could be provided and the cavity efficiencies were measured among 85% to 87%. In addition, we applied G2C-type SSD in a real case for smoothing the light pattern in a meeting room shown in Fig. 16
Fig. 16 The demonstration of SSD which was applied in a meeting room. (a) The lamps without diffuser. (b) The lamps with the G2C-type SSDs.
. Through introducing the SSD, the illumination uniformity in the meeting room was improved obviously while the energy efficiency holds at a high level.

6. Conclusion

In this paper, we propose a novel way with high temperature down-size sintering technology and large-area impressing technology to perform a structured diffuser with specific light patterns and high transmission efficiency. The down-size sintering technology can reduce the dimension to half of the original one. Through several sequential processes, a diffuser with micro-structure can be fabricated. In this paper, we present a study with the down-size sintering technology to reduce the dimension to 1/2, 1/4, and 1/8. All of SSDs with different generations, such as G0-type, G1-type, G2-type, and G3-type performed similar light patterns and the transmission efficiency were higher than 85%. Except for the defect by the CNC machining and the caused blur in light pattern, the G1-type and G2-type performed similar lighting characteristics to the design. Through sequential down-size process to the third generation with the dimensions of 250μm (length), 50μm (width), and 125μm (height), the light patterns show that the fine structure cannot be easily reproduced because of the size limit of the sintering powder. This is why more scattering lights can be observed in G3-type SSD. However, the proposed scheme is a useful and effective way to fabricate an SSD with specific light pattern and high transmission efficiency.

The G2-type and G3-type SSDs were applied to real products with the impressing technology. The combination of multi segments with the diffuser with different orientation in each segment can produce various light patterns to practical applications such as down light, light bar, and others. The proposed technology will be useful in light pattern control to reduce glare and increase optical utilization efficiency and leads energy saving in general lighting and especially helpful in LED solid-state lighting.

Acknowledgments

References and links

1.

International Energy Agency, Light’s Labour’s Lost: Policies for Energy-Efficient Lighting (OECD/IEA, Paris, 2006).

2.

C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). [CrossRef] [PubMed]

3.

T. R. M. Sales, “Structured microlens arrays for beam shaping,” Opt. Eng. 42(11), 3084–3085 (2003). [CrossRef]

4.

T. R. M. Sales, “Structured microlens arrays for beam shaping,” Proc. SPIE 5175, 109–120 (2003). [CrossRef]

5.

T. R. M. Sales, “High-contrast screen with random microlens array,” US Patent No. 6,700,702 (2004).

6.

R. P. C. Photonics, Inc., http://www.rpcphotonics.com/.

7.

V. N. Mahajan, Optical Imaging and Aberrations: Part I. Ray Geometrical Optics (SPIE Press, 1998).

8.

C. C. Sun, T. X. Lee, S. H. Ma, Y. L. Lee, and S. M. Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Opt. Lett. 31(14), 2193–2195 (2006). [CrossRef] [PubMed]

9.

J. W. Goodman, Introduction to Fourier optics, 2nd ed. (McGraw-Hill, 1996).

10.

A. G. Evonik Industries, http://www.savosil.com/product/savosil/en/Pages/default.aspx.

11.

F. Costa, L. Costa, and L. Gini, “Optical articles and sol-gel process for their manufacture,” World Intellectual Property Organization WIPO, WO 2004/083137, A1 (2004).

12.

Regatech Co, http://www.regatech.com/e1.htm.

OCIS Codes
(150.2950) Machine vision : Illumination
(220.0220) Optical design and fabrication : Optical design and fabrication
(230.3670) Optical devices : Light-emitting diodes
(080.4298) Geometric optics : Nonimaging optics

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: February 2, 2012
Revised Manuscript: February 27, 2012
Manuscript Accepted: February 27, 2012
Published: February 29, 2012

Citation
Xuan-Hao Lee, Jung-Lin Tsai, Shih-Hsin Ma, and Ching-Cherng Sun, "Surface-structured diffuser by iterative down-size molding with glass sintering technology," Opt. Express 20, 6135-6145 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-6-6135


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References

  1. International Energy Agency, Light’s Labour’s Lost: Policies for Energy-Efficient Lighting (OECD/IEA, Paris, 2006).
  2. C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express18(6), 6137–6148 (2010). [CrossRef] [PubMed]
  3. T. R. M. Sales, “Structured microlens arrays for beam shaping,” Opt. Eng.42(11), 3084–3085 (2003). [CrossRef]
  4. T. R. M. Sales, “Structured microlens arrays for beam shaping,” Proc. SPIE5175, 109–120 (2003). [CrossRef]
  5. T. R. M. Sales, “High-contrast screen with random microlens array,” US Patent No. 6,700,702 (2004).
  6. R. P. C. Photonics, Inc., http://www.rpcphotonics.com/ .
  7. V. N. Mahajan, Optical Imaging and Aberrations: Part I. Ray Geometrical Optics (SPIE Press, 1998).
  8. C. C. Sun, T. X. Lee, S. H. Ma, Y. L. Lee, and S. M. Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Opt. Lett.31(14), 2193–2195 (2006). [CrossRef] [PubMed]
  9. J. W. Goodman, Introduction to Fourier optics, 2nd ed. (McGraw-Hill, 1996).
  10. A. G. Evonik Industries, http://www.savosil.com/product/savosil/en/Pages/default.aspx .
  11. F. Costa, L. Costa, and L. Gini, “Optical articles and sol-gel process for their manufacture,” World Intellectual Property Organization WIPO, WO2004/083137, A1 (2004).
  12. Regatech Co, http://www.regatech.com/e1.htm .

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