Abstract: Apparatus for measuring stress at a point on an object comprising: means for applying a cyclically varying load of period T to the object; detector means for collecting and measuring thermal radiation from said point over a sample time interval t, wherein t<T; means for determining the phase of the first interval t with respect to the cyclic variation of thermal radiation produced by said point in response to said applied load and, from said collected and measured value of thermal radiation and said phase, for providing a measure of stress at said point.