Comparative study on lens focal length measured with planar and spherical reference wave

In order to study influence of the surface shape of reference wave on lens focal length, digital holography method was used for theoretical analysis and experimental verification. Focal length was measured respectively with planar reference wave and spherical reference wave at different wavelengths. And then, the measured values were compared with the nominal values and the theoretical calculation values. The results show that the relative errors are above 5% between the measured value, the nominal value and the theoretical calculation value with planar reference wave. The relative errors are blow 2% with spherical reference wave. Therefore, the precision of focal length measured by spherical reference wave is higher. After calculation, focal lengths measured by two different reference lights are within the range of theoretical depth of focal. Therefore, the reconstructed image obtained by the measured value is equivalent to the reconstructed image obtained by the nominal value and the theoretical value. So the spherical reference wave has higher precision to the measurement results. Both kinds of reference wave are within the scope of the theoretical depth of focus. So the quality of the reconstructed images which are calculated with measured values, the nominal values and the theoretical calculating values is about the same. The study provides guidance for measurement of lens focal length and wavefront reconstruction of digital holography.