I think finally I understand how roatory Risley prism is working with using Von Grafe technique for measurement phoria .
For lateral (horizental) phoria , we should use vertical prim to dissociate and use horizental prism to measure . The arrow of Risely shows the base . Therfore for a typical set up :
OD : 12 prism diopter base in (BI) (Measurement prism)
OS : 6prism diopter base up(BU) ( dissociated prism) fixed
Move OD prism until patient sees target lin up like buttons on a shirt, and patient should instructed to watch upper , non moving image( which belongs to right eye)
and for neutralize Base out for esophoria and Base In for exophoria .
The same procedure for vertical phoria , use horizental for dissociate & vertical for measurement .
OD: 12 BI & OS : 6 BUp and move OS until patient sees target lin up ,
Base up for hypophoria and base down for hyperphoria .
This video clip is useful .
I have a confusion everytime , I used Risly prism , It looks I nearly forgot all of my optometry knowledge , but for teaching to students, I should learn and remember all the basic and essential things . It's strange I'm teaching this module for the third year now , but anybody didn't ask me about rotary prim and I didn't feel I need to do the test.Some tests like Fan & Block or Von Grafe don't have much application in the real professional clinics , but they 're essential for students to learn them .