According to what described on page 66 of the Radiance Cookbook, this procedure:
$ vwrays -x 400 -y 400 -vf horizontal.vf
| rcalc -e '$1=$1;$2=$2;$3=.4;$4=$4;$5=$5;$6=1
| rtrace -fac -w -I -ov
vwrays -d -vf outside.vf \ -x 400 -y 400room.oct > wp_illu.hdr
should give as a result the contour lines shown in the related picture. I assume I misinterpreted the text as I guess that the described procedure just gives the illuminance picture for the working plane defined by the .vf file.
To obtain the contour lines one should call the falsecolor function as usual so I would need to make two calculations, one with and one without the ‘-I’ option, right?
Thank you for your help.