The principle, mechanism, accuracy and reliability of 19-bit and 21-bit absolute photoelectric shaft angle encoder -with high accuracy are described. The accuracy and reliability of the absolute encoder is enchanced significantly by means of interpolation of phase-shift resistance and sectionally and repeatedly correcting. The results measured are as follows: resolution accuracy σ = ±0.6 angle second, angle measurement accuracyσ2= ±1.2 angle second for the encoder of 19-bit; andσ1= ±0.2, σ22 =±0.62 respesti...