Thanks Koos PA0KDF <kfockens@worldonline.nl> we can count duty cycle 
for Feld-Hell mode. Duty cycle for several modes:

RTTY, FM	100 %
PSK31   	80 %
CW      	46 % for standard dash/dot/pause
SSB     	50-80 % by compression
HELL    	22 %


Duty cycle in Feld -Hell mode can be count exactly as sum of dark pixels
divided by total sum of all pixels multiplied by 100 (%).
Number of pixels is 7 horizontally x 14 vertically = 98.
Koos counted these values for each character:

(sum of pixels by columns:)
A  9+5+4+5+9   = 32 >> 32.7 %
B  4+10+4+4+8  = 30 >> 30.6 %
C  10+4+4+4+4  = 26 >> 26.5 %
D  4+10+4+4+8  = 30 >> 30.6 %
E  10+6+6+4+4  = 30 >> 30.6 %
F  10+4+4+2+2  = 22 >> 22.4 %
G  10+4+4+6+8  = 32 >> 32.7 %
H  10+2+2+2+10 = 26 >> 26.5 %
I  0+0+10+0+0  = 10 >> 10.2 %
J  4+2+2+2+10  = 20 >> 20.4 %
K  10+2+4+5+5  = 26 >> 26.5 %
L  10+2+2+2+2  = 18 >> 18.4 %
M  10+2+2+2+10 = 26 >> 26.5 %
N  10+3+3+3+10 = 29 >> 29.6 %
O  10+4+4+4+10 = 32 >> 32.7 %
P  4+10+6+4+6  = 30 >> 30.6 %
Q  10+4+6+10+4 = 34 >> 34.7 %
R  10+4+4+6+8  = 32 >> 32.7 %
S  6+6+6+6+6   = 30 >> 30.6 %
T  2+2+10+2+2  = 18 >> 18.4 %
U  10+2+2+2+10 = 26 >> 26.5 %
V  10+3+3+3+3  = 22 >> 22.4 %
W  9+2+4+2+9   = 26 >> 26.5 %
X  6+4+4+4+6   = 24 >> 24.5 %
Y  3+2+7+2+3   = 17 >> 17.3 %
Z  5+7+6+7+5   = 30 >> 30.6 %
1  0+2+2+10+0  = 12 >> 12.2 %
2  5+6+6+6+2   = 25 >> 25.5 %
3  2+4+6+8+4   = 24 >> 24.5 %
4  7+2+2+10+2  = 23 >> 23.5 %
5  2+2+8+6+6   = 24 >> 24.5 %
6  9+6+6+4+5   = 30 >> 30.6 %
7  5+5+5+5+3   = 23 >> 23.5 %
8  8+8+6+8+8   = 38 >> 38.8 %
9  5+6+6+4+8   = 29 >> 29.6 %
0  8+4+4+4+8   = 28 >> 28.6 %
/  3+3+3+3+3   = 15 >> 15.3 %
+  2+2+8+2+2   = 16 >> 16.3 %
-  2+2+2+2+2   = 10 >> 10.2 %
?  2+2+8+5+0   = 17 >> 17.3 %
.  0+3+3+0+0   =  6 >> 6.1 %
,  2+5+0+0+0   =  7 >> 7.1 %
'  3+3+0+0+0   =  6 >> 6.1 %
=  4+4+4+4+4   = 20 >> 20.4 %
)  6+6+4+0+0   = 16 >> 16.3 %
(  0+0+4+6+6   = 16 >> 16.3 %
:  6+6+0+0+0   = 12 >> 12.2 %
Average value for all characters is 23.8 %

Special characters:
%  5+5+3+5+5   = 23 >> 23.5 %
!  0+9+9+0+0   = 18 >> 18.4 %
@  10+4+9+8+12 = 43 >> 43.9 %
#  4+10+4+10+4 = 32 >> 32.7 %
$  6+6+12+6+6  = 36 >> 36.7 %
^  2+2+2+2+0   =  8 >>  8.2 %
_  2+2+2+2+2+2+2 = 14 >> 14.3 %

"88" (kiss) 6+5+4+6+5+6 = 32 >> 32.7 %

Average value for word "PARIS" incl. one additional pause between words:
30 + 32 + 32 + 10 + 30 + 0 = 134 / (6*98) = 22.8 %
=========================================================================

You see how it is advantageous for QSO under difficult conditions and for
"RF ecology".