users to stitch together the images to
view and monitor the status of the entire
field, construction site or area of interest.
However, these processes can take hours
to complete and use significant processing power without the correct software.
Alternatively, there are a few software
packages available to tile the images
together within minutes on site to allow
immediate viewing of the entire area.
Low lens distortion improves the overall
appearance of the final images with these
software packages. With this in mind,
imaging lenses with two percent or less
geometric distortion across the field of
view are ideal for these applications.
For these software tools to work as
intended, the images need to easily
stitch together to generate 2D mosaic
images or 3D point clouds. To simplify
the programming required to stitch the
images together, the motion blur needs to
be minimized, as well as the distortion.
The loss in resolution that motion blur
introduces can reduce the accuracy of the
stitching algorithm and, in turn, limit the
accuracy of the final data set obtained. To
control motion blur, the aperture setting
for the lens must be considered — along
with the dynamic range and sensitivity of
the sensor — to ensure shorter exposure
times. All specifications relating to minimizing motion blur should be considered
when optimizing the exposure time for
any given application.
Stability in the aperture setting itself is
also crucial to UAV performance; therefore, fixed aperture lenses are preferable
to traditional irises for UAV applications.
Depending on the sensitivity of the system and image-processing algorithm to
fluctuations in light intensity, even small
size variations in the aperture setting
from a variable iris can be problematic.
Additionally, using a lens with a fixed
aperture offers the added benefit of reducing the size and weight of the overall lens
assembly. In UAV applications where the
overall payload and geometric dimensions
are critical to how aerodynamic the entire
system is and the maximum time-of-flight offered, the weight and size of the
imaging lens have greater importance.
Particularly in situations where a camera
Spectral signatures tell all
with a large sensor format is being used,
the size of the imaging lens and its weight
can be untenable, and the ability to drop
out subcomponents in a fixed aperture
lens can be invaluable in meeting these
requirements. Simplicity is paramount in
UAVs; as such, anything that can be done
to make the equipment smaller, lighter
and composed of fewer moving compo-
nents is always preferred. Having a fixed
aperture lens design covers all of these
In addition to aperture stability, the
coatings and glass types chosen can
impact the light throughput of the system.
Specifically for agriculture applica-
tions, the near-infrared (NIR) region is
of particular interest. Lenses with high
transmission in the NIR wavelength range
are required3 because in many applica-
tions, the Normalized Difference Vegeta-
tion Index (NDVI) must be quantified.
The NDVI is a method of determining
the density of green on a patch of land by
comparing the amount of visible and NIR
light reflected by the plants within it4.
When taking a true NDVI measurement, raw data is collected in both the
visible and NIR spectrums. In some circumstances, different imaging lenses with
their own separate imagers can be used
so that each channel can be optimized for
its particular spectral region. To ensure
this, the coatings of each imaging lens
for each spectral channel are optimized
for the specified wavelength range to
increase the transmission. Shortpass and
longpass filters can also be incorporated
to create an exact spectral separation for
each channel. In some cases, different
lenses designed for the best resolution in
each wavelength range are used. Conversely, there are some companies that
offer NDVI systems that only capture a
synthetic NDVI measurement. Instead of
collecting raw data in both the visible and
NIR, these systems estimate the nonvisible portion of the spectrum using the
visible data. But this does not guarantee a
Narrow bandpass filters with high
transmissions in the passband are often
also in multispectral imaging. In agriculture, the optical vegetation index (OVI)
Images courtesy of Sentera LLC.
Sentera AgVault software interface uses its Quick Tile software to use georeference data to compile composite images within minutes on site to allow immediate viewing of the entire areas.
In addition to aperture
stability, the coatings
and glass types chosen
can impact the light
throughput of the