植被光谱是如何形成的?

遥感是20世纪60年代发展起来的对地观测综合性技术,是指通过某种装置,不直接接触被研究目标、区域或现象来获取其有关数据,并对所获取的有关数据进行分析从而得到所需要信息的一种科学和技术。

 

遥感按照光谱分辨率通常分为多光谱遥感、高光谱遥感、超光谱遥感。以a表示波长,一般认为,光谱分辨率在a/10数量级范围内的遥感称为多光谱遥感;光谱分辨率在a/100数量级范围内的遥感称为高光谱遥感,光谱分辨率在a/1000数量级范围内的遥感称为超光谱遥感。目前应用最为广泛的就是多光谱遥感。

 

根据麦克斯韦的电磁场理论,变化的电场在其周围产生变化的磁场,而变化的磁场又在其周围产生变化的电场。变化的电磁场在空间以一定的速度传播,就形成了电磁波。电磁波的传播不依赖任何媒介,可以在真空中以光速传播。电磁波的范围很广,但其本质是相似的,都遵守基本波动理论,只是由于频率的不同而显示不同的特性。电磁波根据波长由长到短包含无线电(长波、中波、短波、超短波、微波)、红外(超远红外、远红外或称热红外、中红外、近红外)、可见光(红、橙、黄、绿、青、蓝、紫)、紫外线、X射线、Gamma射线等。其中红外又可分为短波红外、中波红外和长波红外。下图为太阳辐射照度分布曲线,可以看出太阳辐射从紫外线开始,到中红外结束。

 

 

 

平常我们人眼所能感受到的只是包含380nm-760nm之间的可见光部分。但是地物的完整的反射光谱中包含着更多的信息,如果单从一个波段的图像是没办法解析出这些信息的。下图是欧空局哥白尼计划Sentinel-2A星的光谱响应函数图像,该传感器共包含十三个波段,其中8和8A波段的光谱响应函数范围重叠,中心波长接近,因此在使用时往往根据需要选取一个使用。

 

 

下图是典型的植被的光谱。

 

 

  1. 400nm-700nm:在550nm处有一个反射峰,该峰形成于叶子内部的叶绿素a,b以及在450nm与640nm-680nm为中心有两个强烈的吸收带,以及胡萝卜素叶黄素在430nm-480nm有强烈的吸收带所致,这也是叶片表现为绿色的原因。在这波段范围内,植被的波谱特征基本上被叶绿素、胡萝卜素所控制,前者占总吸收量的65%-75%,后者占总吸收量的25%-35%。

 

  1. 700nm-800nm:该部分形成一个强烈的陡坡,是由于叶片的栅栏组织结构导致的强反射形成的,是植被特有的光谱特征,称为“红边”,红边的陡峭程度决定了植被叶片的健康程度。Sentinel-2也是目前唯一一个在植被红边处有三个波段的传感器。

 

  1. 800nm-1100nm:这一波段叶片的吸收作用基本可以忽略,散射作用占据了主导地位,投入叶子内部的光线,因细胞壁与细胞孔腔的折射率有明显的差异,而造成光线在叶子内部的多次反射与折射,向上折射穿出上表皮层便构成反射光,向下折射穿出下表皮层便形成透射光,这一过程具有明显的随机性质,因此反射率与透射率是相近的。这一波段范围内具有很宽很强的反射峰是植被所独有的光谱特征,在叶子发生枯萎或因缺水而凋谢,便意味着细胞的萎缩,折射率差异的减少,其宏观表现为这一波段的反射率值明显下降。

 

  1. 1100nm-2500nm:这一波段的波谱特征基本上被液态水的吸收特性所控制,对可见光而言,液态水是相当透明的,但在近红外波段它存在两个强烈的吸收峰,中心分别为1420nm和1960nm,这就造成了叶子在这两个中心带上存在两个强烈的吸收谷,其谷深与液态水的含量有关。除两个大吸收谷外,在980nm和1140nm处还存在两个小吸收谷,也与叶片的含水量有关。

 

高光谱由于光谱分辨率高,波段数多,因此可以绘制出较为完整的光谱曲线,在矿物研究领域有着较广泛的应用。根据不同地物的吸收反射特性,进行不同的RGB波段组合,我们就可以轻松的对地物进行识别了。

 

 

最后,放一张假彩色合成的TM图像,图中亮红色部分和暗红色部分均为植被。

 

 

 

其真彩色对照图如下所示:

 

 

模拟真彩色(看起来更鲜艳,但不是真实颜色)对照图如下所示

 

 

本文授权转载于知乎用户“遥感巫师”,感谢其辛勤创作!原文链接:https://www.zhihu.com/question/417888052/answer/1904002964。

 

 

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吾爱地理 » 植被光谱是如何形成的?
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