--- ray/src/cal/cal/norm.cal	2017/04/19 22:09:12	1.3
+++ ray/src/cal/cal/norm.cal	2021/09/12 17:04:41	1.4
@@ -1,4 +1,4 @@
-{ RCSid $Id: norm.cal,v 1.3 2017/04/19 22:09:12 greg Exp $ }
+{ RCSid $Id: norm.cal,v 1.4 2021/09/12 17:04:41 greg Exp $ }
 {
 	Normal Distribution Functions
 
@@ -15,12 +15,18 @@ NORMF : 1/sqrt(2*PI) ;
 
 Z(u) : NORMF * exp(-u*u/2) ;
 
+{ ### Old approximation:
+
 Q2(t) : t*(.31938153+t*(-.356563782+t*(1.781477937+
 		t*(-1.821255978+t*1.330274429)))) ;
 
 Q1(u) : Z(u) * Q2(1/(1+.2316419*u)) ;
 
 Q(u) : if( u, Q1(u), 1-Q1(-u) ) ;
+
+### erf() gives us exact value: }
+
+Q(u) : .5 - .5*erf(u/sqrt(2)) ;
 
 u2(t) : t - (2.515517+t*(.802853+t*.010328))/
 		(1+t*(1.432788+t*(.189269+t*.001308))) ;