A means of validating numerical simulations has been developed which utilizes chemiluminescence measurements from an internal combustion engine. By incorporating OH*, CH2O* and CO2* chemiluminescence sub-mechanisms into a detailed n-heptane reaction mechanism, excited species concentration and chemiluminescence light emission were calculated. The modeled line-of-sight chemiluminescence emission allows a direct comparison of simulation results to experimentally measured chemiluminescence images obtained during combustion in an optically accessible compression ignition engine using neat n-heptane fuel. The spray model was calibrated using in-cylinder liquid penetration length Mie scattering measurements taken from the jets of the high-pressure piezo injector. The experimental, two dimensional images of CH2O* and OH* chemiluminescence during the low and high temperature heat release period were recorded with an intensified CCD camera in a wavelength range covering emission from these species. By interpreting the color content of the images taken from a CMOS high speed camera, crank-angle resolved two dimensional CO2* chemiluminescence distributions were obtained. All the chemiluminescence images taken at the same crank angle degrees were used to generate probability density function (PDF) distributions which can then be directly compared with RANS averaged CFD simulation results. The emission spectra were recorded to confirm the existence and evolution of excited-state species throughout the various stages of combustion. Chemiluminescence from excited state OH* was found to be good proxy for ground state OH, while greater temporal variations were found to exist between excited and ground state CH2O and CO2. Ground and excited state species are predicted to have different spatial distributions during the combustion process.