threshold of luminescence

例句：

1. 1.The threshold of light is the point at which objects begin to appear.
   光的阈值是物体开始显现的地方。
2. 2. The threshold of human emotions is often difficult to define.
   人类情感的阈值往往难以定义。
3. 3. The threshold for a new product launch is set at $50 million.
   新产品发布的阈值设定为5000万美元。