Structured illumination for Rayleigh scattering imaging to eliminate the stray light interference
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摘要: 激光片光成像技术在流场测量领域的应用前景十分广阔，但该类测量技术在实际测量中都会受到杂散光或背景光干扰的影响，降低了光学成像质量。因此，开展了将结构光照明技术应用到激光片光成像测量中来消除杂散光干扰的研究。结构光照明技术是一种新型的杂散光抑制方法，可以将原始图像数据分为有效信号和杂散光（来自于激光片光焦平面外的干扰光）两部分，在后期数据处理中，有效信号会保持不变，而杂散光会因为空间频率不同而被剔除掉。首先，基于Matlab软件理论分析了该类方法消除杂散光的作用；其次，设计了一套应用于瑞利散射成像的结构光照明测量装置，主要由连续激光器、Ronchi光栅和EMCCD相机组成，其中Ronchi光栅用于产生正弦光强分布的激光片光。最后利用该测量装置在McKenna平面火焰炉上开展了瑞利散射图像测量实验，验证了结构光照明的方法具有消除杂散光影响、提高瑞利散射图像精度的作用。Abstract: The development of the laser sheet imaging technique is introduced in this paper. This technique is one of the most versatile optical imaging techniques and has been frequently applied in several different domains. However, when applied in limited operating space, the interference from indirect reflections and surrounding backgrounds is known to produce errors in the laser imaging. Therefore, a novel 2D imaging technique named Structured Laser Illumination Planar Imaging (SLIPI) is developed to solve this problem. This method is based on planar laser imaging but uses a sophisticated illumination scheme-spatial intensity modulation-to differentiate between the intensity contribution arising from useful signals and stray light. By recording and dealing with images, the SLIPI method can suppress the diffuse light and maintain the useful signals. In this paper, we first use the MATLAB software to simulate the SLIPI method, and the conclusions suggest that the stray light interference can be removed effectively. Furthermore, the SLIPI apparatus, consisted of the continuous laser, Ronchi grating and EMCCD camera is designed. And the Ronchi grating is used to produce the sinusoidal distribution of the laser sheet. At last, SLIPI apparatus is combined with the Rayleigh scattering approach in the McKenna flat burner to generate the Rayleigh scattering signal. And the thesis presents convincing experimental evidence that the SLIPI method is able to remove the diffuse light contribution, thus improving and enhancing the visualization quality.
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Key words:
- structured grating /
- stray light /
- Rayleigh scattering /
- laser sheet imaging
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图 1 SLIPI处理方法全过程图及其对应的FFT结果图
Figure 1. Simulation process of the SLIPI method and the relative FFT results
图 2 (a) 基于结构光照明的瑞利散射成像测量装置；(b) McKenna火焰炉结构示意图；(c)McKenna火焰炉及其上方实验布局尺寸图
Figure 2. Schematic of Rayleigh scattering optical arrangement for SLIPI measurement; (b) the structure of McKenna burner; (c) the size of McKenna burner and the experimental layout
图 3 3种不同测试环境下的原始瑞利散射图像((a)，(b)，(c))、光栅调制瑞利散射图像((d)，(e)，(f))和SLIPI方法处理后的图像((g)，(h)，(i))
Figure 3. (a), (b) and (c): conventional (raw data) Rayleigh scattering images without grating modulation in three different measurement cases; (d), (e) and (f): Rayleigh scattering images with grating modulation; (g), (h) and (i): images of modulated amplitude value, A, calculated by SLIPI method
图 4 2种不同测试环境下的原始瑞利散射图像((a)，(b))、光栅调制瑞利散射图像((c)，(d))、SLIPI方法处理后的图像((e)，(f))和SLIPI方法处理后的温度场分布结果((g)，(h))
Figure 4. (a) and (b): conventional (raw data) Rayleigh scattering images without grating modulation in two different measurement cases; (c) and (d): Rayleigh scattering images with grating modulation; (e) and (f): images of modulated amplitude value, A, calculated by SLIPI method; (g) and (h): combustion temperature distribution images by the SLIPI method
表 1 基于结构光照明的瑞利散射成像测量实验参数
Table 1. Experimental parameters of Rayleigh scattering measurement based on the SLIPI technique
背景分类 流场参数 相机曝光时间 相机增益 图像正弦调制周期 冷态流场 CASE 0 Air: 10 L/min 0.05 s 30 0.08495 CASE 1 0.05 s 30 0.08495 CASE 2 0.05 s 30 0.08495 甲烷/空气预混火焰 CASE 3 CH4: 0.7 L/min;
Air: 7 L/min;0.10 s 30 0.08495 CASE 4 N2: 5 L/min. 0.10 s 30 0.08495 
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