N À
AT TE SO TS ET
4.3 Averaging with respect to the image field
This is an important parameter for aerial Survey lenses. For
an image quality criterion, the MTF needs to be averaged with
respect to the field angle w. The weights p(w) are similar to
those used for the AWAR and are considered to take account of
the appropriate area belonging to the chosen image point con-
[
(
sidered. Therefore averaging the areas of the MTF Q(w) with
respect to the image field can be written:
"max : Wmax
Q = pw) Q (w) dw, with ( Pp (v) aw = 1
vo 0 In
Re : (0
: S 1/2
where Q (w) = ( (rs (R) T+ (R)) / dR
0
Tg - (Ts (R) Ty (RR = geometrical MTF from Tg(sag.MTF)
and TX (tan.MTF)
R = spatial frequency (Lp/mm)
Rg - limiting spatial frequency (inter-
Section of the threshold curve with
the lower MTV. of T.-, TQ)
For practical applications the integral is written as a sum,
namely:
m ces N (9
Q = à Pk Qk , where = Pk = l1 and k = 152... 359.500 1
k=1 k=1
N = number of image points
For the analysi
the two images
small variations of image quality with respect to image field
are acceptable. To take into account Some information on the
variation of image quality over the field we can write:
AT
N
Qx = [2
k=]
Dk a] £02 | ila 50
For a perfectly uniform image quality over the whole image
field, Q for ali L