The OED defines spectrum as "The entire range of wavelengths of electromagnetic radiation, from the longest radio waves to the shortest gamma rays of which the range of visible light is only a small part."
OED（牛津英语字典） 中“光谱”的定义是 “电磁辐射波长的 全集， 从波长最长的无线电波到波长最短的伽玛射线。 而在这其中可见光波长范围 只占了一小部分。”