FUNCTION dustem_rap_xisrf,R,choice=choice

;choice=1 : you enter R=I160/I100 and you obtain the Xisrf associated
;choice=2 : you enter Xisrf and you obtain the ratio I160/I100 associated


choice_run=1
IF keyword_set(choice) THEN BEGIN
  choice_run=choice
ENDIF


CASE choice_run OF
  1:BEGIN
      a0=1.6104103 & a1=-2.9885436 & a2=0.54573055  & a3= -0.30773096 & a4=0.33125862  & a5= -0.19978200
      xisrf=exp(a0+a1*alog(R)+a2*alog(R)^2+a3*alog(R)^3+a4*alog(R)^4+a5*alog(R)^5)
return,xisrf
  END

  2: BEGIN 
      a0=0.59136133 & a1=-0.39529777 & a2=0.011704696 & a3=0.0051459595 & a4=-0.0013039416 & a5=0.00014532588
      rap=exp(a0+a1*alog(R)+a2*alog(R)^2+a3*alog(R)^3+a4*alog(R)^4+a5*alog(R)^5)
return,rap
  END
ENDCASE

END