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Firmness is an important internal quality attribute of fruits. Nondestructive measurement of fruit firmness will help the industry provide better fruit for the consumer. The objective of this research was to improve the multispectral imaging system used in our previous studies and refine scattering analysis methods for more effectively measuring apple fruit firmness. An improved multispectral imaging system equipped with a light intensity controller was used to measure light scattering from Red Delicious apples at seven wavelengths and Golden Delicious apples at eight wavelengths. A filtering method was applied to reduce noise signals in scattering images during radial averaging of image pixels. Apple size/shape affected scattering intensity and distance, and two methods were proposed for correcting their effect. Firmness prediction models were developed by multi-linear regression against Lorentzian parameters. The improved system yielded better firmness predictions with the correlation (r) of 0.898 and the standard error of validation (SEV) of 6.41 N for Red Delicious and r = 0.897 and SEV = 6.14 N for Golden Delicious.