The operating range of lean-burn spark-ignition engines is limited by the level of cyclic variability in the early flame development that typically corresponds to the 0-5% mass fraction burned duration. An experimental investigation was undertaken to study the levels of flame chemiluminescence in an optical stratified-charge spark-ignition engine, using a Cassegrain optical system with high spatial resolution. Measurements of OH and CH-radical intensities were simultaneously acquired with double flame images per cycle for a range of air-to-fuel ratios (A/F = 12-22). These signals of chemiluminescence were used to evaluate the in-cylinder equivalence ratio of the reacting mixture and to further examine its contribution to the flame growth speed and the cyclic variability in the crank angle by which 5% mass fraction was burned (theta(Xb5%)). Specifically, the ratio of the OH/CH chemiluminescence signals was calibrated in the engine and tested extensively for different injection strategies and spark advances, to measure the "global" and "local" in-cylinder A/F ratio around the spark plug. The complications encountered towards this goal are discussed in detail. The results showed that the equivalence ratio exhibited large variations on a cycle-by-cycle basis and consistently produced negative correlation coefficients with theta(Xb5%), especially for lean-set operating conditions (A/F = 20-22). Particularly, for open-valve injection strategy that yielded a stratified mixture, the degree of this correlation tied in the range approximate to -0.4 to -0.8, being lower for the locally measured A/F ratio and higher for the globally evaluated one. Some issues related to the opposite gradients of the calibration curves deduced for the measurement of the global and local in-cylinder A/F ratios need to be examined outside the engine using a combustion facility with controlled conditions of pressure, temperature, turbulence intensity, and dilution by combustion residuals. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.