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四波长成像测温仪设计的参数要求见表1。
Parameter Value Waveband/nm 400-900 Field of view/(°) 5±0.1 Frequency/Hz 30 Temperature range/℃ 800-2500 Table 1. Imaging thermometer design parametes
基于光阑处四色分光原理的多波长成像测温仪设计方案如图3所示。四波长成像测温仪主要由窗口、中性衰减片、四色滤光片、光学物镜镜头、可见/近红外集成传感器、测量控制单元及软件等部分组成。通过可见和红外复用的成像系统,经过多色分光后,由软件采集目标图像并进行图像融合和对准处理,提取出四个波长的单色成像图像,根据每个通道的信号质量(非饱和且在探测器线性响应区内),由软件智能选择相应的多波长测温算法,得到目标的真温温场分布。将测量主机放置在远端,即可远距离遥测得到目标的快速温场分布。
四色滤光片每个象限的中心波长设计与CCD响应光谱曲线中心波长相同,分别为460、533、605、800 nm,带宽约20 nm。CCD每个单色像元本身也有一个接收带宽,两者组合之后,每个单色通道的带宽约为10 nm,保证了四色成像的单色性和独立性。
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光学镜头的设计图如图4所示,需要满足在四个波段消色差且在CCD的耐奎斯特频率处清晰成像等要求。物镜镜头成像视场角2
$\omega $ 设计为5°,CCD尺寸为4.76 mm×3.57 mm,对角线长度为5.95 mm,在CCD传感器上的半像高$y$ 为5.95/2=2.975 mm,则根据物像关系式:计算得到镜头的焦距设计为68.1387 mm。CCD像元尺寸为4.65 μm,耐奎斯特频率为1/(2×4.65 μm)=107.52 lp/mm,要求乃奎斯特频率处传递函数不低于0.2。
设计的光学镜头光学传递函数接近衍射极限,满足设计要求。镜头首先固定在内部套筒上,内部套筒与外部套筒用螺纹相结合,外部套筒用C接口与CCD相机接口连接,通过调整内部套筒的前后位置实现调焦。
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四色滤光片主要是将目标辐射中其他波长辐射滤除,保留成像用四个波段辐射信号。由于在镜头光阑处进行滤色,为保证滤色性能,滤色片设计的尽量薄。考虑到测量的目标温度最低800 ℃,最高2500 ℃,假设目标为灰体,滤光片带宽为20 nm,选取目标温度为适中的1200 ℃,计算红外波段(605 nm)与蓝光波段(460 nm)的辐射能量比为:
由公式(10)可知,测温仪红光波段信号远远高于蓝光波段信号,为保证探测器都处在最佳的线性输出区,保证测量的动态范围能覆盖更宽的温度范围,将滤光片设计成特殊的不等比透光型的滤光片,如图5所示。
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选用的CCD传感器是一款可见和近红外集成的CCD传感器,其工作光谱范围为可见光和近红外波段,每个波段的带宽较宽,约在70~110 nm范围内。
四色滤光片设计的中心波长与传感器的中心波长相同,与传感器耦合后,总的单色滤光带宽约为10 nm,可获得较为窄带的目标单色辐射,提高了多波长测温数据处理的准确度。
Non-scanning wide range multi-wavelength imaging temperature measurement technology
doi: 10.3788/IRLA20200394
- Received Date: 2020-10-10
- Rev Recd Date: 2021-02-03
- Publish Date: 2021-05-21
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Key words:
- imaging temperature measurement /
- multi-wavelength /
- non-scanning /
- wide range
Abstract: Multi-wavelength temperature measurement technology is an advanced radiation temperature measurement technology, which assumes a functional relationship between emissivity and spectrum. The measured multi-wavelength radiation signals and the function relationship between emissivity and spectral is used to obtain the true temperature and emissivity of the target. Multi-wavelength imaging temperature measurement technology detects the multi-wavelength radiation image information of the target and inversely calculates the temperature field distribution of the target. Aiming at the inadequacy of color CCD-based multi-wavelength imaging temperature measurement, such as adaptability of the nonlinear spectral emissivity model, and the narrow dynamic range of temperature measurement, and so on, a four-wavelength non-scanning imaging temperature measurement method based on non-equal ratio filter color splitting at the diaphragm was proposed. This method could effectively compress the band imaging bandwidth to form four narrow band imaging detections, and was suitable for wide dynamic range temperature field measurement of nonlinear spectral emissivity model targets. A four-wavelength imaging thermometer was developed according to the proposed temperature measurement method. The thermometer was mainly composed of a window, a neutral optical attenuator, a four-color filter, an optical objective lens, a visible/near infrared integrated sensor, a measurement control unit, and software. The real temperature distribution of the target was obtained according to the multi-wavelength warming algorithm by the thermometer software. The target high-temperature field (800-2500 ℃) was tested under laser heating conditions. The comparison between the measurement result and the thermocouple data shows that the error is less than 1%, and the method has higher accuracy and better dynamic range adaptability.