To use X-ray diffraction as an example, a single X-ray photon diffracting off of one electron cloud will not generate a strong enough signal for the equipment to detect.

• 以X射线衍射为例，被某个固体原子（或离子）的外层电子散射的X射线光子太少，构成的辐射强度不足以被仪器检测到。