(ms
-1
), by the image capture time(s), and then dividing the
product by the
GSD
of the image in (meters per pixel). We
estimated
AIM
for various exposure settings that were deter-
mined to be optimal radiometric fidelity and image acuity,
based on platform altitudes and camera lens focal lengths
used in imagery collections associated with our
RSI
change
detection research (Coulter
et al
., 2008; Stow
et al
., 2016).
Images with an
AIM
of greater than one have the potential for
acuity loss resulting from ground movement that occurred
during the shutter action. Similarly,
ISO
has an effect on image
acuity, with image shot noise increasing as the camera gain is
increased, resulting in poorer image acuity.
The second research objective pertains to within frame
trends in brightness response and how across-frame bright-
ness variation can be minimized, with relative aperture and
ISO
being the variables of relevance to this question. Lens
aperture has an effect on vignetting within an image, with
lower
f
-
stop
numbers resulting in more pronounced vignett-
ing effects. Also, variable radiance to digital number response
may occur across an image frame as a function of detector
sensitivity (
ISO
).
The third research objective involves minimizing differ-
ences in image brightness intensity or digital number (
DN
)
values for spatially corresponding pixels of multitemporal
image sets, with light meter measurement zones and white
balance being the variables of relevance. The light meter
measurement zone selected may have an impact on how the
camera assesses the scene brightness between imagery collec-
tions over time. On account of changes to atmospheric optical
and illumination conditions, and land cover or ground feature
changes within the scene,
WB
is a relevant variable to consider
when seeking to minimize temporal differences in brightness.
Having laid out the variables of interest, and how they pertain
to the individual research questions, the experimental design
used to collect image data for testing these variables follows.
Images were collected with combinations of five general
exposure variables:
EV
,
WB
, light metering,
ISO
, and aperture
(
f/stop
). The collections were all performed from fixed camera
stations and a tripod platform. The image center was fixed on
the same landmark for image collected of the same scene, to
insure consistency in the perspective of the camera. None of
the scenes imaged had objects within them closer than 350 m
from the camera. The controlled constants for all images, un-
less explicitly stated otherwise were:
EV
= 0,
WB
= automatic
(
AWB
), light metering = overall,
ISO
=100, and focal length (
f
)
= 55 mm. Images with varying
EV
s were captured at
⅓
step
Figure 2. Oblique intensity image of Mt. Soledad scene (San Diego, California), used for the light meter testing.
Figure 3. Location and position of the camera and tripod used for all of the imaging, positioned at the imaging site for the La
Jolla Business Park (see Figure 1).
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March 2018
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