辐射计，又称“放射计”，是一种测量电磁辐射的辐射通量的装置。“放射计”这一术语有时特指红外辐射检测计，但也可指检测其它各种波长的电磁辐射的检测计。

较常见的辐射计是克鲁克斯辐射计（1873年由威廉·克鲁克斯发明），它是一个内有转子（带有颜色深浅不同的叶片）的处在在半真空中的早期模型，在受到光照时叶片会转动。

尼克斯辐射计（1901年发明）的原理与克鲁克斯辐射计不同，这类辐射计更加灵敏。

微波辐射计用于检测微波波段的电磁辐射。微波辐射计内充有氩气以使其旋转。

MEMS（Micro-electromechanical Systems，微电子机械系统）辐射计，由帕特里克·简柯维克（Patrick Jankowiak）发明，可以按尼克斯辐射计或克鲁克斯辐射计的原理运作，而且可以检测更宽的波段和粒子的能级。

A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or ultraviolet detector.

The name Radiometer is frequently used to refer to a Crookes radiometer ("light-mill"), an early model device wherein a rotor (having vanes which are dark on one side, and light on the other) in a partial vacuum spins when exposed to light. A common myth (one originally held even by Crookes) is that the momentum of the absorbed light on the black faces makes the radiometer operate. If this were true however, the radiometer would spin away from the non-black faces, since the photons bouncing off those faces impart even more momentum than the photons absorbed on the black faces. Photons do exert radiation pressure on the faces, but those forces are dwarfed by other effects. The final explanation depends on having just the right degree of vacuum, and is more related to the flow of heat than the direct effect of photons.