This dissertation aims at measuring the photon energy scale combining specialized Monte Carlo simulation with data taken during the combined ATLAS test beam in 2004. This work explains the steps taken to arrive at the photon energy scale, starting from the knowl- edge acquired for electrons. The chapters are structured as follows: Chapters 1 and 2 briefly introduce this work and the motivation behind it. Chapter 3 gives an overview of the LHC ex- periment and the ATLAS detector as a whole. Chapters 4 and 5 address in detail the ATLAS electromagnetic calorimeter and signal reconstruction at the cell level. Chapter 6 concentrates on the setup for the combined test beam with emphasis on the photon run. Chapter 7 details the event selection strategy used for the photon run analysis. Chapter 8 describes the generation and tuning of the special Monte Carlo for the photon run. Chapter 9 focuses on the highly special- ized Monte Carlo studies that employed special calibration objects known as calibration hits. Chapter 10 details the methodology behind the measurement of the photon scale and evaluates it in terms of the electromagnetic calorimeter resolution. Chapters 11 and 12 present a summary of the results and the conclusions, respectively
