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In this paper, we have demonstrated an efficient speckle-suppressed white light source generation when a blue laser diode illuminates on a micro-vibrated phosphor paper. Both micro-vibrated and color-mixing techniques are used in this system. With only micro-vibrated technique, the speckle contrast can be reduced from about 50% to 7.4% for the scattering blue image for a 16-ms integration time. Together with color-mixing technique, mixing speckle contrast is defined for laser diode pumped phosphor and almost speckle-free result is achieved. For color temperature lower than 5000 K, almost speckle-free mixed white can be obtained even without vibration technique.
© 2015 Optical Society of America
1. Introduction
In 2014, Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki were awarded the Nobel Prize in Physics. Their main contribution is the development of high-brightness and energy-saving blue light emitting diode and white light source [1–4]. The Nobel successful experience provoked us more confidence to develop the laser based light source. Since blue laser diodes (LD) can share the same basic material with blue light emitting diodes (LED) [5]. This will lead to the opportunity of effectively reducing cost and commercialization for blue laser products. The 21st century will be an important century for developing laser-based lighting products for human life applications. From science and education to life, the laser products have quietly entered to our everyday lives. In the past decade, laser seemed to become more and more popular for life applications, and its market was taking shape such as laser based Blu-ray DVD, large size projection TV, data projector and high power cinema projector [6–13]. In the recent years, scholars were interested in the development and study of blue-laser excited light sources to generate white light for illumination applications [14–16]. The researchers have used blue lasers to pump fluorescent powder for broadband yellow and red light generation, and then mixed RGB into a white light source to provide the daily needs of lighting applications for human beings.
Laser pumped fluorescence light (FL) source has its potential for long distance, such as several hundred-meter or kilo-meter levels, of illumination. People may use this illumination equipment for high-speed transportation vehicles. Among them, an interesting subject is about white light generation by using blue light excited phosphor powder for car headlamp. In 2001, BMW and Audi have delivered a new core concept for car headlamp by using blue LD and fluorescent powder for white light generation and it opened the prelude for lasers in car lamp applications [17]. BMW has used a blue LD to stimulate phosphor, which converts blue to broadband yellow and red light, thus achieving white lighting. Laser pumped FL source has the advantages of compact structure and lightweight, and the appearance is so cool and fascinated for us. In addition, it has advantages of simple structure, broad emission spectrum, energy saving, high brightness, stable color output, stable power output, easy for output control and design, high power output and flexible substrate and those make laser be a potential candidate for solid-state light source (SSLS) development. However, for laser lamp, there are still speckle, optics, and efficiency issues needed to be solved. In this study, we will focus on solving the speckle issue. The speckle issue is that the speckle patterns are generated when a coherent light source illuminates on rough objects, and the perceived image will become blurred, low contrast, and cause less safety. To obtain an illumination light with good quality, speckle is an important issue to be solved. J. Kinoshita et al. has coupled laser light from multi-LDs into a fiber to pump YAG:Ce phosphor for speckle reduction [18]. F. P. Shevlin et al. has configured a deformable mirror with random distributed surface for speckle reduction [19]. In addition, there are various speckle reduction methods to be applied for lighting applications such as laser array [7], broadband light source [20], diffraction optical element (DOE) [21, 22], polarization-rotation [23, 24], dynamic deformable mirror [25], stationary multimode optical fiber [26, 27], fast scanning micromirror [28], moving diffuser [29, 30], rotating diffuser [31], encoding multiple holograms [32]. In this paper, we have developed micro-vibrated and color-mixing techniques for blue laser pumped phosphor white light source. We demonstrate the speckle reduction results with a static and micro-vibrated YAG:Ce phosphor paper. The YAG:Ce phosphor was coated on the paper for FL generation. The phosphor paper was equipped on the voice coil motor (VCM) vibration device for longitudinal vibration and speckle suppression. The speckle contrast (SC) could be reduced from about 50% to about 7.4%. It shows almost speckle-free result useful for the illumination applications. The mixing speckle contrast is defined for the mixed white light source. This efficient speckle reduction technique makes the white illumination light smoother and more comfortable for human eyes. The long distance illumination system will be the potential application area to be explored.
2. Principle for speckle reduction
When the coherent laser is incident on the rough interface or rough screen, the speckle will be generated [33, 34]. The concept is also suitable for the blue laser illuminating on a YAG:Ce phosphor paper. Since the YAG:Ce phosphor paper is configured by the randomly distributed phosphor particles on a flexible transparent substrate, the speckle patterns will be generated when the blue laser light is incident onto the phosphor paper. Figure 1 shows the diagram of phosphor paper on the VCM which we used for light conversion and speckle reduction. When the blue laser illuminates on the YAG:Ce phosphor paper, the blue light will pass through the phosphor particles in the paper and be converted to yellow photons through the transition between the YAG:Ce energy levels which has a range from 480 to 700 nm or longer and centered at a yellow wavelength [35]. Then the residual pumping light and the converted yellow light will be scattered from the interface of the phosphor particle boundary and then be separated to forward and backward scattered lights leaving from the phosphor paper. Since partial of the residual blue laser light which does not participate in the photon conversion by the phosphor particles will still maintain its coherent laser performance. When the blue laser is incident on the rough interface, the incident and reflective waves will interfere and construct the random distributed speckle patterns on the phosphor paper, and then be perceived by our eyes.
So if the speckle patterns are all caused by the non-converted residual blue laser light, the measured speckle contrast (SC) will still be as high as 50%. The speckle reduction theory will be the same as non-converted green laser light in our previous paper [33]. By the micro-vibrated speckle reduction technique along the longitudinal direction, the speckle contrast should be lowered down by turning on the VCM motor. The only difference will come from the different divergence angles from the transparent phosphor paper used. The divergence angle of the scattered blue laser could be around 100 degrees at least. The calculated de-correlation length will be around 1 μm. With the same type of VCM device, the estimated independent speckle pattern number will be 1.4x104 #/sec. The estimated speckle contrast will be as low as 3.3% for a 16-ms integration time [33]. The speckle contrast for monochromatic light is shown by Eq. (1) [34, 36–39]. σ is the standard deviation of I. I is the intensity distribution of sampling signal and is its average. Although the speckle reduction concept is the same as before, the speckle contrast should be affected and need to be re-defined for a light conversion case when the conversion efficiency of the generated yellow light is high enough. So here we define a mixing speckle contrast (MSC) for this light converted system based on Eq. (2). I1 is the intensity distribution of the excited light pattern. I2 is the intensity distribution of the converted FL pattern. is the average total intensity of I, which is the summation of I1 and I2. σ(I1) is the standard deviation of I1 intensity which is the source of speckle. For blue LD pumped YAG:Ce phosphor, a uniform image of yellow light intensity is assumed for I2 and added for the MSC calculated. MSC value will be more suitable than the original SC value to describe the mixed speckle patterns for human eyes. This is the most simplified case to describe the MSC for the two-wavelengths case. Here, we do not consider eye sensitivity function or V-parameter and multi-wavelength effects in this formula. For more advanced analysis, the V-parameter should be considered. We may relate the formula Eqs. (3) and (4) to Eq. (2). The charge coupled device (CCD) response is neglected here since the affection is quite smaller than that for V-parameter in the visible range. In our case, the CCD response is only ~1.5 for Y/B intensity ratio [40].
3. Experimental setup
For light conversion, a blue LD is used for pumping a phosphor paper, and the general relation of emission spectrum versus LD current is shown in Fig. 2. The blue LD bandwidth is of around 4 nm and hence the speckle issue still exists. The generated light can be distributed from 480 to 700 nm or longer and centered at a yellow wavelength. Here, the static blue LD is used for pumping phosphor paper to demonstrate the speckle reduction (SR) results for SSLS. This SSLS can also be applied for light source of a digital light processor (DLP) or a liquid crystal on silicon (LCoS) projector. The CIE 1931 diagram is shown in the inset figure in Fig. 2. The (x, y) coordinate is located at (x = 0.34, y = 0.36) as the LD is operated at 1.0-A current for white light generation. The color of mixed white light may change its color temperature for different driving currents because of different composition of blue and yellow. For example in Fig. 3, the color temperature may range from 5250 to 4200 K as the current varies from 0.3 to 1.0 A. The color rendering index (CRI) ranges from 56.6% to 64.8% for this current range. The mixed white color is varied from “Day white light” to “Natural white light”. Accordingly, the Y/B intensity ratio I2/I1 based on Eqs. (3) and (4) can be obtained as from 3.2 to 8.0 for this color temperature range by integrating the spectrum curves in Fig. 2. For the blue and yellow wavelengths, we can obtain the V-parameter ratio varied from 10 to 30 for Y/B, from the eye sensitivity function [41].
Fig. 2 General relation of emission spectrum versus LD current. Inset figure: CIE diagram and the measured point (x, y) = (0.34, 0.36) for the LD current = 1.0 A.
Fig. 3 Measured color temperatures (blue curve) and calculated Y/B intensity ratios (red curve) versus LD current.
For general lighting, we also need to consider Duv and color rendering index (CRI). With small Duv and high CRI conditions, and spectra having the chromaticities on the blackbody locus, the mixed white light source will have better color performance for lighting applications. Chromaticities of YAG:Ce phosphor can be obtained from Nakamura and Fasol’s study in 1997 [42]. Only partial of points having chromaticities on the blackbody locus by optimizing the Ce3+ concentration and spectra distribution [43]. Our present color temperature results are measured from the general YAG:Ce paper without optimizing the CRI and Duv. Thus, not all of the chromaticities of these spectra are located on the blackbody locus. The chromaticity of the spectrum at current of 1.0 A is well located on the blackbody locus. The color temperature is 5250 K. The CRI and Duv are 64.8% and 0.0057, respectively. We will use this color temperature around 5000 K to classify the speckle-free boundary with and without vibration technique.
For speckle measurement, a commercial blue LD is used for pumping phosphor paper as shown in Fig. 4(a). The blue laser beam is diverged by a lens and illuminating on the phosphor paper screen. In this study, YAG:Ce yellow phosphor is used, which is the most popularly applied phosphor in white LEDs. The measured particle size is of a Gaussian distribution ranged from 2 to 10 μm and with the peak at around 6 μm. This blue LD excites the phosphors, and generates the broadband florescent source, with B, Y, and R lights included. The blue laser light is incident on the phosphor, and then reflected and transmitted from the phosphor material to generate the florescent light. The residual blue laser light is scattered from the phosphor and formed the forward and backward scatterings, and finally formed the speckle patterns on the phosphor sample. For low power pumping, the resulted beam pattern is taken by a camera and as shown in Fig. 4(b). This illuminated area looks uniform and comfortable for our eyes. It seems no speckle patterns on the phosphor paper. Since the converted yellow light is considered as all FL and without stimulated emission inside, there should be no speckle patterns generated in this case. The total speckle patterns will be caused from the residual non-converted blue laser light. To confirm and obtain the actual speckle contrast for the blue laser light, a 10-nm band-pass filter is used for filtering the yellow laser image for speckle measurement. Then we find a serious speckle issue for the residual non-converted blue laser light. The speckle pattern seems to exist in the mixed warm white light source. It raises us interest to explore the reason.
Fig. 4 (a) Setup diagram of speckle reduction measurement and (b) speckle suppressed white light beam pattern on the YAG:Ce paper for low pumping power.
For quantitatively evaluating the speckle patterns, a model type of WinCam CCD D-UCD23-1310 and image system are used to capture the image for speckle analysis [30]. The image system is tilted with a 30° angle for SC measurement and with a good image lens system inside. The f-number of our lens system is f/4. The calculated average speckle size Ac = 4.31 um [44]. By using this system, a fine speckle with an average size of 4.3 um can be relayed onto the CCD camera. A 16-ms integration time is set. The image plane is located at the phosphor particle interface. The 1x magnification ratio is an important parameter for our SC measurement system. The speckle image resolution is 6.4 um which is the same as the CCD resolution. Based on the present parameters, our measured image should be better than human eye’s resolution if the observer to screen distance is set as 2 m and with a magnification ratio 0.01x. An image of 1064 x 1064 pixels is recorded and partitioned into 50x50-pixel data arrays in the SC calculation. The SC can be calculated by Eq. (1) [34, 36–39] for blue light. For further analyzing the light-converted speckle contrast, the MSC can be calculated by Eq. (2) for both blue and yellow light intensities. We find that with different yellow/blue conversion ratios, the MSC values will be changed and reduced along with the increased yellow component. We find that the MSC variation will make us further understand the blue speckle issue in the color-mixing technique for speckle reduction.
4. Experimental results
The efficient speckle reduction can be obtained in the experiment results. The blue laser image is captured by a CCD and shown in Fig. 5. Figures 5(a) and 5(b) show respectively the results for a static and a micro-vibrated phosphor paper at 400 Hz VCM frequency. For the static phosphor paper, the measured SC is 48% ± 3% for a 16-ms integration time with the BP filter. A ± 6% percentage error value exists for a single shot SC measurement system. After vibrating the phosphor paper, the SC value can be reduced from ~50% to 7.4%. Roelandt et al. [44] have studied speckle test for human perception of laser projector system. Their study showed that 70% people cannot confirm the speckle for R (SC = 3-3.5%), G (SC = 3.5-4.0%), and B (SC = 3.7-7.6%). Thus, SC = 4% for blue can be set as a speckle-free condition for human perception and it is an important parameter in our present study. When SC or MSC for blue color is smaller than 4%, it is defined as speckle-free for lighting application. The range from 4% to 8% can be defined as the almost speckle-free condition. It shows almost speckle-free result for blue image generation as the blue LD illuminating on the micro-vibrated phosphor paper. The SC background (BG) for our SC measurement system is around 3%-5% when a uniform LED white light source illuminates on a Double A paper. By observing the measured speckle patterns, the fine speckle patterns are disappeared and local non-uniformity image still exists. The 3% larger SC BG should be attributed to the large grain sizes of phosphor particles located on the surface of phosphor paper. The measured minimum SC value was limited by bright/dark image caused by phosphor surface and non-uniformity of LD transverse modes.
The SCs versus VCM frequency relation is measured and shown as in Fig. 6. The SC is reduced from ~50% to about 7.4% when the VCM frequency is changed from 0 to about 400 Hz and then gradually increased up to 1 kHz. The minimum 7.4% SC is obtained at around 400 Hz. The result is well matched for our constructed speckle suppression model [33]. We have constructed a speckle suppression model with a longitudinally micro-vibrated system in our previous paper [33] and the maximum displacement of the VCM was measured [33]. The working principle of a VCM can be referred to [45]. Based on the previous study, the maximum total displacement per second is still located at VCM frequency at around 350-380 Hz. In this study, we replace the Double A paper by a YAG:Ce paper on a new VCM device. The maximum total displacement per second is shifted a little to the VCM frequency at around 400 Hz. The best vibration result is still at around 400 Hz. The reason of frequency shift might be attributed to the new VCM device, and the variation of the paper dimension and weight. The maximum total displacement per second will generate the largest independent speckle patterns which lead to the minimum speckle contrast at around 400 Hz for the present system. At 1 kHz, the vibrated distance is only several micro-meters and the vibrating distance is small enough for real speckle suppression applications.
Here, we choose a simple numerical method to analyze the MSC value statistically for speckle contrast analysis of mixed white light. We assume the converted yellow light is a Lambertian distribution and the intensity distribution is uniform for each image area. A uniform image for yellow light intensity is added for the MSC calculation. The calculated speckle patterns and MSCs results are shown in Fig. 7(a)-7(d). The initial SC for pure blue image is set as 45% for a static phosphor paper. After adding the yellow intensity image into the MSC calculation, the MSC results can be changed from 45% to about 44%, 22%, 4% and 0.43% with Y/B intensity ratio = 0.1, 1, 10, and 120, respectively. We can see the speckle patterns disappear gradually when the Y/B intensity ratio is increased and the MSC is reduced. The MSC is proved to be a suitable parameter for evaluating speckle for mixed white light.
With more detailed information, the calculated MSCs versus Y/B intensity ratio are shown as in Fig. 8. The MSC is reduced from 45% to 0.43% when Y/B intensity ratio increases from 0.1 up to 120 for static phosphor paper. When the ratio is larger than 10, the MSC is smaller than 4% and almost speckle-free result is achieved without vibrating technique. When intensity ratio is smaller than 10, the blue intensity dominates, the speckle issue become important and VCM vibration is required for speckle-free reduction. The MSC is reduced from 7.4% to 0.1% when Y/B intensity ratio increases up to 120 and the VCM frequency is operated at 400 Hz. With both micro-vibrated and color mixing techniques, the MSC can reach as low as 0.1%. This will result in an excellent contrast ratio and uniformity for the white light source in illumination and image formation. When the Y/B intensity ratio is larger than 3, the MSC is smaller than 3% and almost speckle-free result can be achieved. The MSC is about 5% and 3% when Y/B intensity ratio equals 1 and 2. The final results shows that speckle issue is solved by employing both micro-vibrated technique and color-mixing technology simultaneously. So when Y/B intensity ratio is set at 3, the color temperature of illumination light is around 5000 K and almost speckle-free mixed white can be obtained without micro-vibration technique beyond this color temperature. The results above do not include eye sensitivity function or V-parameter. Since human eyes are more sensitive to yellow than blue. Based on the eye sensitivity function, assume the ratio of V-parameters for Y/B is 10. A weighting factor of 10 should be added to the MSC calculation. With such a modification, the Y/B intensity ratio = 1 can reduce MSC to about 4% as shown in Fig. 9. Almost speckle-free can be satisfied for a lower Y/B intensity ratio value of 1 and the corresponding color temperature will be shifted toward cool white light.
A simple verification is done by mixing experiment. A low pass filter for blue is used and replace the BP filter in Fig. 4. When rotating the filter angles from −6 to ~85 degrees, the measured Y/B intensity ratios from 0.1 to ~92 are obtained by integrating the spectrum curves from a broadband green-yellow-red curve and a narrow band blue curve. One of the output spectrum curves for MSC measurement is shown in Fig. 10. Partial spectrum curve from 480 to 560 nm is cut out by this filter. The measured Y/B intensity ratio is around 8.5 and the measured MSC is about 5%. Based on such operation, we obtained the measured MSC from 45% to 3% by rotating the filter angles. Figure 11 shows the measured MSCs versus Y/B intensity ratio. The result shows good match with the theoretical MSCs for a static phosphor paper. This result verifies that our proposal of Eq. (2) is practical for a general speckle evaluation of mixed white light. Simultaneously, the experimental result verify that there exists a about 3% BG level even though a theoretic curve become lower and lower while increasing the Y/B intensity ratio.
Figure 12 shows the color temperature chart from LED illumination data sheet [46]. From this figure, we classify the speckle-free results with and without micro-vibrated technique, where the boundary color temperature is roughly located at 5000 K for Y/B intensity ratio = 3 without considering the eye sensitivity function. When the color temperature is higher than 5000 K, the white color becomes cooler and the micro-vibrated technique is required for speckle-free results. When the color temperature is lower than 5000 K, the white color becomes warmer and only color-mixing technique can reach speckle-free results for illumination applications. This result can be used to verify the reason why we can directly observe no obvious speckle patterns under low blue laser light intensity on the phosphor paper. When considering the eye sensitivity function with an averaged V-parameter ratio = 10 for Y/B, it will make the boundary color temperature, for speckle-free with and without vibration technique, be shifted toward a higher color temperature (10000 K). Finally, all the results show that we should take care of speckle issue for cool white light applications. For some illumination applications, they need cool white or blue dominated colors, the micro-vibrated technique will become more important for such illumination systems. It can be seen that for a large part of color temperature, the speckle-free white color could be achieved by static color-mixing technique. So choose suitable converted spectrum or color temperature for mixed white light will lead to speckle-free results. Based on the V-parameter modification discussed above, a blue-converted-to-yellow mixing system will be an efficient speckle suppression system for mixed white color. When comparing with a up-conversion mixing system from yellow to blue [47], we can infer that the down-conversion mixed white light system from blue to yellow is better than a up-conversion mixing light system for speckle suppression.
The study is aimed to develop a static speckle reduction method by adding yellow phosphor-converted light to a blue laser light for speckle suppressed white light. When adding too much yellow light, the chromaticities of the mixed white light will leave the blackbody locus. Having better CRI with small Duv and chromaticities on the blackbody locus can be obtained by designing the phosphor components with higher red intensity output [48]. By adding the red phosphor or other color phosphor into the YAG:Ce phosphor paper will increase CRI, decrease Duv, have the chromaticities on the blackbody locus, and reach the speckle-free result by the same method of this study. Based on our calculation for MSC with V-parameter, we can also obtain a boundary color temperature to classify the speckle-free result for the red-rich phosphor paper. This classification method can also be suitable for a blue laser pumped red-rich or orange-rich phosphor or other phosphor for mixed white light source applications. The only difference is that both yellow and red/orange/other spectra with individual intensity and V-parameter should be considered.
Speckle effect is governed by many factors, including human eye perception. It is a parameter related to the psychophysics of vision, similar to color shift or image flicker. When an observer changes his/her position, the speckle varies it distribution and make image flickering. To reduce the speckle contrast will also improve the image flicker phenomena. The SC can be reduced to 7.4% by VCM vibration for a 16-ms integration time. When increasing Y/B ratio, the MSC can be reduced to less than 4%. People cannot see the speckle. Thus, the image flicker should also be improved. Without VCM vibration, the SC can be measured as 50% for a 16-ms integration time and Y/B ratio should be larger than 10-11 for MSC being less than 4%. Image flicker will also be reduced as the mixed white light color become warmer.
5. Discussion and Conclusion
The efficient speckle suppressed results are achieved by both static and micro-vibrated phosphor paper. The almost speckle-free result SC = 7.4% can be obtained for pure blue image with the micro-vibrated condition. For white light generation, the re-defined MSCs are calculated for Y/B intensity ratios increasing from 0.1 to 120. When the eye sensitivity function is not considered, larger than 3 and 10 of Y/B intensity ratios are required for speckle-free results with a micro-vibrated and a static phosphor paper, respectively. We find that 5000 K is the boundary color temperature to classify the system as the speckle-free white light system with and without vibration mechanism. After considering the eye sensitivity function, the Y/B intensity ratio will be shifted to 1 and boundary color temperature will be shifted toward a cool white color. The almost speckle-free results are achieved with both micro-vibrated and color-mixing techniques. The speckle-suppressed white light source can be applied for illumination light source, display and projector backlighting sources. This study is helpful for understanding the speckle-free laser based white light source and developing such a system into our daily life application. When we use a highly collimated blue LD to pump a phosphor paper to generate a white light source, the residual reflective blue light is primarily with 100° scattering angle and minor part of it is with Fresnel reflection. The scattering angle may be designed by adjusting the phosphor particle size distribution. The angular color uniformity will be affected. The optical characteristics of the residual blue light and hence the mixed white light can be further evaluated by angular correlated color temperature deviation (ACCTD) [49, 50]. We do focus on speckle effect and image flicker from psychophysic of vision, however color shift will be a good direction and need future studies [51].
Acknowledgments
The authors gratefully acknowledge the financial support by the Ministry of Science and Technology of Taiwan for the financial support of this research under Contract No. MOST 102-2221-E-002-205-MY3 and MOST 104-3113-E-155-001, and National Taiwan University under the Aim for Top University Projects 104R7607-4 and 104R8908.
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