Global climate, local weather, and human economic activity all depend on the balance of energy gained from the sun and lost to space. Accurate radiation measurements are crucial because minute changes in this balance could cause dramatic shifts in climate. Because of limited stability and accuracy, current atmospheric radiation instrumentation cannot make routine measurement with uncertainties less than about +/- 5%. This project develops and applies a new approach to radiation measurement in which the energy received by the sensor surface is actively removed and measured to maintain the surface at a constant temperature, instead of allowing it to heat up as in current instruments. The isothermal detector thus eliminates inaccuracies due to temperature differences and reduces uncertainties in field measurements.