Techniques for improving the recording quality during heat assisted magnetic recording (HAMR) by monitoring the power of a source used to heat a storage medium are described. In one example, a source emits electromagnetic radiation. A waveguide transmits the electromagnetic radiation onto a surface of a magnetic media. A photoresistive material is proximately located to the waveguide. The resistance of the photoresistive material varies based on the intensity of electromagnetic radiation propagating through the waveguide. The power of the source is determined by measuring the resistance of the photoresitive material. The power of the source is adjusted based on the determined power.