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Light absorption and scattering coefficients have been measured for particles emitted by two diesel engines; one direct injection and one indirect injection, operating over a range of speeds and loads. Integrating plate absorption measurements yield a specific absorption coefficient of 9.1 square meters per gram of non-volatile particulate matter at 550 nm wavelength. This absorption coefficient is inversely proportional to wavelength and independent of engine operating conditions or type. The scattering coefficient was simultaneously measured as 1.3 square meters per gram of undifferentiated particulate matter. These experimental results are shown to be in the range predicted by theoretical absorption and scattering calculations, which have been made for elongated carbon-void particles.