当目标的光谱分布和它的背景很不一样的时候,——例如在反射阳光背景中的涡轮喷气飞机,——往往就可以提出采用双色调制盘来消除背景。只要举一个简单的例子就可以把问题说清楚了。假定太阳是一个500°K的黑体,用辐射计算尺或者用附表就可以算出,其光谱分布的平均波长为0.69 When the target's spectral distribution is very different from its background, - for example, a turbojet flying in a reflecting sun, it is often possible to propose the use of a two-tone dial to eliminate the background. Just a simple example can make the problem clear. Assuming that the sun is a blackbody at 500 ° K, it can be calculated using a radiation slide rule or using a schedule with an average spectral distribution of 0.69