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 Optical Society of America
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 . 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 . 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 [3–6]. 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 . Assuming the rotating center of structure is z axis and point Ac is a point on the structure surface, the equation can be written
According to Eq. (1), each structure on the SSD can be designed for the obtainment of uniform light pattern. The structure and its corresponding simulation results of SSD are shown in Fig. 1 , where the length (L), width (W), and height (H) of SSD’s structure size are 2000μm, 400μm, and 1000μm respectively. We named this kind of diffuser as Generation 0 type (G0-type). In the simulation, a collimating beam with a diameter of 1 cm was used as incident beam. The collimating beam passed through the structured surface of SSD under the condition of normal incidence. When we made an observed plane behind the output plane of the SSD with a distance of 20 cm in simulation, most of energy were directed to form an illuminated area of 36.6 cm × 8.4 cm on the observation plane. The illuminated area formed a rectangular and uniform light pattern, as shown in Fig. 1(b). Because the transmittance of SSD is only restricted by Fresnel loss, optical transmittance as high as 90.2% can be achieved by our design. About the diverging angle of SSD, we use a value of full width at half maximum (FWHM) to describe it. The angular distribution of FWHM in two orthogonal directions are 84° and 22° respectively, as shown in Fig. 1(c).
According to the design of SSD, we proceeded to make a real mold of G0-type SSD by CNC machining. The real mold of G0-type SSD and its related measurement results are shown in Fig. 2 . We first used polymethylmethacrylate (PMMA) of which the refractive index was about 1.49 as the molding material. For the mold of G0-type SSD, we made measurements corresponding to the parameters in the simulation to make a comparison. In the Experiment, a collimating beam from a He-Ne laser with a diameter of 1 cm was used and the wavelength was 632.8 nm. In Fig. 2(b), the length and width of light pattern are about 34 cm and 8 cm respectively. The FWHM values in the two orthogonal directions are 72° and 16° respectively, as shown in Fig. 2(c). The transmittance of G0-type SSD static mold was 89%. Compared with the results in simulation, the results of the light pattern and diverging angle in measurement were slightly narrower than the simulation results. To realize the reason, we used high resolution optical microscope to observe the structured surface of G0-type SSD. Some pictures in different depths which are grabbed by high resolution optical microscope are shown in Fig. 3 .
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 . 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 . 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 . Figure 4 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 . 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]. 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 .
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 . 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 . 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 , 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 . 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.
The measurement of the light pattern of the SSDs show that the G1-type and G2-type SSD illuminated an area of 34 cm × 8 cm, and the angular spreading of FWHM for three SSDs in horizontal and vertical directions were 72° and 14° respectively. Although the light patterns were not as uniform as the those in the design, the illuminated area and the angular spreading were the same as the measurement result of G0-type SSD. This means that we successfully reduced the volume of G0-type with into a half and quarter dimension by using high temperature sintering technology. However, the illuminating area of the G3-type SSD of 34 cm × 6 cm was slightly smaller than the design value and some blurry parts were appeared at the center of light pattern. We tried to figure out this problem by using high resolution optical microscope. As shown in Fig. 10 , the surface of G3-type SSD became rougher than the surfaces of G0-type, G1-type, and G2-type SSD. The rough surface caused the occurrence of scattering behavior. The reason is that G3-type SSD cannot duplicate the structure of the G2-type by the size limit of the sintering powder. Therefore, a rough surface replaced the knife-mark in the G2-type, so that more scattering lights occurred at the center of the light pattern. Although the problem of blurry region appeared in experiment result, the G3-type SSD still offered similar function about transmittance, light pattern, and diverging angle as good as other generation SSD. In Fig. 11 , the similar light patterns in simulation and measurement are shown when the structure of the SSD faced outward the light source. The light pattern by G1-type or G2-type was similar to the designed pattern except blurred boundary of the center rectangle because of knife mark. The light pattern by the G3-type was slightly different with more scattering lights at the center. It again shows that the limited size of the sintering powder could cause a limit in the proposed method for making smaller structure, where the dimensions were 250μm (length), 50μm (width), and 125μm (height).
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 , 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 , 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 , 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 . 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 . 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 . Through introducing the SSD, the illumination uniformity in the meeting room was improved obviously while the energy efficiency holds at a high level.
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.
This study was sponsored by the Ministry of Economic Affairs of the Republic of China andthe National Science Council with contract no. NSC 99-2623-E-008-002-ET, NSC 99-2221-E-008-047, and NSC 100-3113-E-008-001. Furthermore, the authors thank Evonik Industries AG, Evonik Cristal Material Corporation, and Regatech Corporation for the technological cooperation and assistance.
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