After editing the initialization files, the user chooses a number of photons to simulate, and runs the main Matlab program AO3D_MonteCarlo.m. When finished, this program saves the data in binary files. If the chosen observations include a radiance grid, two files are saved, one for upwelling radiance and one for downwelling radiance. These files contain all the information about how many photons exited each cell at each zenith and azimuth angle, limited to the chosen resolutions. The time it takes to simulate a given number of photons depends heavily on the spatial and angular resolutions chosen for the radiance grid. There is also an effective limit on the resolutions based on the available RAM in the computer upon which the program is run. A separate program read_radiance.m, is used to examine the radiance data in both 3D volume plots and 2D horizontal (x-y plane) slices. Sample plots are shown below.