In order to solve the problem of handwriting identification in the field of scientific document examination in court,we usedreflectance transformation imaging to observe the three-dimensional characteristics of sister lines and quantitatively analyze them for the first time.In this experiment,the writing robot was employed to regulate the writing strength, writing angle,azimuth angle,and pen and paper clamp angle.A total of 21 common domes- tic pens were utilized to create“Lin”samples under the pen and paper clamp angles of 30°, 45°,and 60°.The experimental samples were produced at azimuth angles of 0°,30°,45°, and 60°,and at 0°,30°,and 45°with respect to the line of sight.In total,336 sets of sam- ples were produced and imaged under the normal line visualization mode.The sister line characteristics and quantitative analysis were then conducted.A total of 336 sets of experi- mental samples were imaged and analyzed in the normal visualization mode,and the sister line  characteristics  of each  set  of samples were  analyzed  to  derive the  optimal  conditions  for the   appearance   of   sister    lines.Subsequently,under   the    optimal   conditions,the    factors   af- fecting the  sister  lines were  investigated by  3D reconstruction  and  quantitative  analysis based on  RTI  technology.Finally,the  probability  of  sister   line   appearance   of  the   samples  under different  pen  and  paper  clip  angles  was  analyzed  by  ANOVA.Preliminary  experimental  re- sults  show  that  the  sister  line  feature  is  most  obvious  when  the  azimuth  angle  is  45°and  the pen and paper clamp angle is 45°,and the probability of the sister line appearing is as high as 99%when the  experiment  is  repeated;the pen  and  paper  clamp  angle  has  a  decisive  influence on  the  formation  of  the  sister  line.The  combination  of  RTI  technology  and  sister  lines  pro- vides  new  possibilities  for  the  supplementation  and  enrichment  of the  handwriting  inspection method  system,and  has  a  broad  development  prospect  in  the  field  of  document  inspection.