You should change the document type to positive film as follows:
  VSTwain1.StartDevice()
  VSTwain1.OpenDataSource()
  VSTwain1.Capability = 4382     ' 0x111e
  VSTwain1.CapType = 0           ' One value
  VSTwain1.CapValue = 1          ' TWLP_TRANSMISSIVE
  VSTwain1.SetCap()              ' Set cap value
  VSTwain1.Acquire()
This turns on the top light for transparencies or XRays.