Some equations have a property called conformal invariance, which means that a conformal rescaling of the metric can be accompanied by another rescaling of the field and the rescaled field satisfies the same equation on the rescaled metric. This is true of all zero rest-mass field equations (Dirac for spin 1/2, Maxwell for spin 1) except for spin 0 which corresponds to the wave equation; in this case, we have a zero order modification of the wave equation, called the conformal wave equation, involving the scalar curvature of the spacetime, that is conformally invariant. We describe this conformal invariance precisely, and use it to define the notion of radiation field, which encodes the asymptotic behaviour of the field along outgoing null geodesics.