FUNCTION intensity_h_alpha,em,T,case_ab=case_ab

;em is emission measure in cm^-6*pc
;T is gas the temperature in K
;intensity is in erg/cm2/s/sr

use_case='B'
IF keyword_set(case_ab) THEN use_case=case_ab

T4=T/1.e4

CASE use_case OF
  ;This is caseB eq 9 of Walls-Gabaud 1998
  'B':intensity=9.41*1.e-8*T4^(-1.017)*10.^(-0.029/T4)*em
  ;This is caseA eq 10 of Walls-Gabaud 198
  'A':intensity=6.36*1.e-8*T4^(-1.134)*10.^(-0.038/T4)*em
ENDCASE

RETURN,intensity

END