【维基百科】光的漫反射(翻译)


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原文地址:https://en.wikipedia.org/wiki/Diffuse_reflection

1 Mechanism

Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection. An illuminated ideal diffuse reflecting surface will have equal luminance from all directions which lie in the half-space adjacent to the surface (Lambertian reflectance). 漫反射是光的一种反射形式:即在一个表面上,构成光线的光子,从某一个角度入射(incident)进来,与构成物质的微观粒子发生了碰撞,然后从多个方向,而不仅仅只是一个方向反射(reflect)回来。一个理想的漫反射表面(ideal diffuse reflection surface)是:一束入射光从以某个角度入射进来,照射在这个漫反射表面上,然后在这个表面上,构成光线的光子,向四面八方反射出去。这些反射光照射在一个覆盖在这个漫反射表面上的一个半球空间(half-space)上,(想象一个非常非常薄, 其厚度的极限为0的透明半球状玻璃罩,覆盖在这个表面上)。这个半球空间上的每一个点,被反射光照明的亮度,都是相等的。这种理想的漫反射模型,又称为朗伯特反射比(Lambertian reflectance)。
A surface built from a non-absorbing powder such as plaster, or from fibers such as paper, or from a polycrystalline material such as white marble, reflects light diffusely with great efficiency. Many common materials exhibit a mixture of specular and diffuse reflection. 一个表面如果是由一些不吸收光的粉末,比如纯白石膏;或者是由各种纤维,比如白纸;又或者由多晶体材料(polycrystalline material),比如汉白玉等等制作而成的话,对入射光线的漫反射效果将会十分的明显。而大部分常见的物质则是对入射光的反射,则是由镜面(specular)反射和漫反射混合而成。
The visibility of objects, excluding light-emitting ones, is primarily caused by diffuse reflection of light: it is diffusely-scattered light that forms the image of the object in the observer's eye. 人们之所以能够看得到物质,除了自发光的物质之外,主要是由于这些物质对光进行了漫反射,这些遵循了漫反射规律,向四面八方反射出来的光又称为扩散性散射光(diffusely-scattered)。这些扩散性散射光进入人眼之后为大脑神经系统所感知,便形成了人眼所看到的物质的形象。
The most general mechanism by which a surface gives diffuse reflection does not involve exactly the surface: most of the light is contributed by scattering centers beneath the surface,[2][3] as illustrated in Figure 1. If one were to imagine that the figure represents snow, and that the polygons are its (transparent) ice crystallites, an impinging ray is partially reflected (a few percent) by the first particle, enters in it, is again reflected by the interface with the second particle, enters in it, impinges on the third, and so on, generating a series of "primary" scattered rays in random directions, which, in turn, through the same mechanism, generate a large number of "secondary" scattered rays, which generate "tertiary" rays, and so forth.[4] All these rays walk through the snow crystallytes, which do not absorb light, until they arrive at the surface and exit in random directions.[5] The result is that the light that was sent out is returned in all directions, so that snow is white despite being made of transparent material (ice crystals). 当大部分的提供了漫反射的表面的漫反射机制,并不涉及到表面内部的构造:在图1所示举的例子中:大部分的反射光,是在表面之下的散射中心点(scattering)贡献的。假如我们把这个图1想象为雪花的话,图中的多边形表示构成雪花的透明的微小冰粒晶体。入射光线中的一部分光子在第一个冰粒中直接被反弹回来(这些光子占有很小的比例);剩余的光子穿过第一个冰粒,然后其中的一小部分和第二个冰粒发生碰撞,被反弹回来,接着剩余的光子以同样的方式穿过或反弹其他的冰粒。和第一层冰粒发生交互时,将在随机方向上生成一系列的“首次”散射,然后和第二层冰粒发生交互时,将在随机方向上生成“第二次”散射,和第三层交互时发生“第三次”散射,和其他层交互时则依次生成各次散射。这些光线将穿过这些不吸收光的冰粒,直到他们到达表面,以随机的方向逃逸出来。这些过程的结果是照向表面的入射光的光子将会从四面八方被反弹回来。所以,雪花看起来时白色的——尽管构成雪花的冰粒晶体时透明的。
For simplicity, "reflections" are spoken of here, but more generally the interface between the small particles that constitute many materials is irregular on a scale comparable with light wavelength, so diffuse light is generated at each interface, rather than a single reflected ray, but the story can be told the same way. 为简单起见。本文所谈论的“反射”,,所以在每一个入射点(interface)上都会生成漫反射,而不是单独的某一处才有。虽然简化讨论,但依然同理。
This mechanism is very general, because almost all common materials are made of "small things" held together. Mineral materials are generally polycrystalline: one can describe them as made of a 3D mosaic of small, irregularly shaped defective crystals. Organic materials are usually composed of fibers or cells, with their membranes and their complex internal structure. And each interface, inhomogeneity or imperfection can deviate, reflect or scatter light, reproducing the above mechanism. 这个机制是很普通的,因为机会所有的常见材质都是由“小颗粒”聚合而成的。矿物质由多晶体聚合而成。可以把它们视为由一系列的不规则的“晶体3D马赛克”拼图而成。有机物则通常由纤维和细胞以及其他复杂的内部结构构成。每一个复杂多异的入射点将会偏移、反射、散射光线,凡此种种构成了上述的机制。
Few materials do not cause diffuse reflection: among these are metals, which do not allow light to enter; gases, liquids, glass, and transparent plastics (which have a liquid-like amorphous microscopic structure); single crystals, such as some gems or a salt crystal; and some very special materials, such as the tissues which make the cornea and the lens of an eye. These materials can reflect diffusely, however, if their surface is microscopically rough, like in a frost glass (Figure 2), or, of course, if their homogeneous structure deteriorates, as in cataracts of the eye lens. 有小部分材料不会引起漫反射。例如不能让光通过的金属,气体,液体,玻璃,透明塑料,单晶体,宝石,盐晶体,以及一些特殊的材料。比如眼角膜和眼的晶状体。。所有的这些材质能够扩展性地反射,但是如果通过显微镜观察,它们就像一块磨砂玻璃。
A surface may also exhibit both specular and diffuse reflection, as is the case, for example, of glossy paints as used in home painting, which give also a fraction of specular reflection, while matte paints give almost exclusively diffuse reflection. 一个表面同时会产生镜面反射和漫反射。在这种情况下,例如在装修房子时涂抹墙壁,使用无光油漆进行涂抹,大部分情况下都是漫反射,但同时依然会有一定的镜面反射

2 Specular vs. diffuse reflection

Virtually all materials can give specular reflection, provided that their surface can be polished to eliminate irregularities comparable with light wavelength (a fraction of a micrometer). A few materials, like liquids and glasses, lack the internal subdivisions which give the subsurface scattering mechanism described above, so they can be clear and give only specular reflection (not great, however), while, among common materials, only polished metals can reflect light specularly with great efficiency (the reflecting material of mirrors usually is aluminum or silver). All other common materials, even when perfectly polished, usually give not more than a few percent specular reflection, except in particular cases, such as grazing angle reflection by a lake, or the total reflection of a glass prism, or when structured in certain complex configurations such as the silvery skin of many fish species or the reflective surface of a dielectric mirror. 事实上所有的材料能够产生镜面反射。这些材料的表面能够被抛光,从而消除它的不规则表面,使得它能够匹配入射光的波长的话。有一部分材料,比如液体和玻璃,它们缺乏“内部细分”,所以它们没有如上面所述的在子表面上对光的散射机制。因此,他们可以只有镜面反射。同时,在常见的材质中,只有被抛光的金属才能比较好地产生镜面反射机制。(例如玻璃镜的反射材料通常使用铝或者银)。所有的其他常见材料。除了上面提及到的例外的之外,就算是经过抛光处理,也不能产生太明显的镜面反射效果。
Diffuse reflection from white materials, instead, can be highly efficient in giving back all the light they receive, due to the summing up of the many subsurface reflections. 纯白材料的漫反射,因为对照射在它上面的光完全反射,所以漫反射效果是最明显的。因为这些材料表面有许多小的漫反射表面产生这种效果

3 Colored objects

Up to this point white objects have been discussed, which do not absorb light. But the above scheme continues to be valid in the case that the material is absorbent. In this case, diffused rays will lose some wavelengths during their walk in the material, and will emerge colored. 上面提及到原理是用完全不吸收光的纯白的物质来说明的,但这个原理对于能吸收光的物质也是有效的。在这种情况下,当照射在这些物质时,发生漫反射的光线,将会损失掉其中一部分波长的光。同时,这些物质也将呈现出某种颜色。
Diffusion affects the color of objects in a substantial manner because it determines the average path of light in the material, and hence to which extent the various wavelengths are absorbed.[6] Red ink looks black when it stays in its bottle. Its vivid color is only perceived when it is placed on a scattering material (e.g. paper). This is so because light's path through the paper fibers (and through the ink) is only a fraction of millimeter long. However, light from the bottle has crossed several centimeters of ink and has been heavily absorbed, even in its red wavelengths. 待翻译
And, when a colored object has both diffuse and specular reflection, usually only the diffuse component is colored. A cherry reflects diffusely red light, absorbs all other colors and has a specular reflection which is essentially white. This is quite general, because, except for metals, the reflectivity of most materials depends on their refraction index, which varies little with the wavelength (though it is this variation that causes the chromatic dispersion in a prism), so that all colors are reflected nearly with the same intensity. Reflections from different origin, instead, may be colored: metallic reflections, such as in gold or copper, or interferential reflections: iridescences, peacock feathers, butterfly wings, beetle elytra, or the antireflection coating of a lens. 待翻译

4 Importance for vision

Looking at one's surrounding environment, the vast majority of visible objects are seen primarily by diffuse reflection from their surface. This holds with few exceptions, such as glass, reflective liquids, polished or smooth metals, glossy objects, and objects that themselves emit light: the Sun, lamps, and computer screens (which, however, emit diffuse light). Outdoors it is the same, with perhaps the exception of a transparent water stream or of the iridescent colors of a beetle. Additionally, Rayleigh scattering is responsible for the blue color of the sky, and Mie scattering for the white color of the water droplets of clouds. 待翻译
Light scattered from the surfaces of objects is by far the primary light which humans visually observe. 待翻译

5 Interreflection

Diffuse interreflection is a process whereby light reflected from an object strikes other objects in the surrounding area, illuminating them. Diffuse interreflection specifically describes light reflected from objects which are not shiny or specular. In real life terms what this means is that light is reflected off non-shiny surfaces such as the ground, walls, or fabric, to reach areas not directly in view of a light source. If the diffuse surface is colored, the reflected light is also colored, resulting in similar coloration of surrounding objects. 待翻译
In 3D computer graphics, diffuse interreflection is an important component of global illumination. There are a number of ways to model diffuse interreflection when rendering a scene. Radiosity and photon mapping are two commonly used methods. 待翻译

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