宝石内部的光路长度对其颜色呈现的影响

The Influence of the Light Path Length through a Gem on its Color Presentation

宝石颜色的呈现受很多因素影响,例如光源、背景、观察者等,此外宝石内部的光路长度也会影响宝石颜色的色调、明度和彩度。在探究光路长度对宝石颜色的影响时,可以结合CIE 1976 L*a*b*均匀色空间进行定量描述和分析。少数宝石的色调会随光路长度增加出现明显变化,其中色调由绿色变为红色的现象称为乌桑巴拉效应,这是由于宝石的光谱在可见光红区和绿区存在两个透射窗,厚度增加使得绿区透光率减小的速度大于红区。而大多数宝石仅会出现明度和彩度的变化,厚度的增加使透射光谱的透射峰宽度变窄,彩度增大,明度降低。分析光路长度与颜色三要素的关系,寻找颜色呈现的最佳路径长度可用于指导切割,体色较深的宝石可以通过减小厚度使颜色变得更美观。

There are many factors that affect the color of gemstones, such as light sources, backgrounds, observers, etc. In addition, the path length of light through a gemstone will also affects its color, including the hue, lightness and chroma. When exploring the influence of light path length, quantitative description and analysis can be carried out based on CIE 1976 L*a*b*uniform color space. The hue of a few gemstones will change with the increase of light path length obviously, and the phenomenon that the hue changes from green to red is called the Usambara effect. This is because that there are two transmission windows in the green and red area of the visible light spectrum, and the light transmittance in the green area will decrease faster than that in the red area with the increase of the light path length. However, most gemstones only show changes in lightness and chroma. Generally, with increasing thickness, the transmission peak becomes narrower and the chroma increases while the lightness decreases. Analyzing the relationship between the light path length and a gem’s color to find the best thickness of color presentation can be used for cutting, and the darker gemstones can have more attractive color by reducing the thickness.