Tech Briefs Magazine - March 2021 - PIT-24

Final radiometric calibration at the end of FAT. It is important that the calibration is traceable and stable.

tion with sharp optics. On the other
hand, sampling the PSF with more
than one pixel gives more information
about the PSF, but for a given detector,
it will reduce the resolution of the system. Any given optical system will
always output more information, the
more pixels you have per point spread
function (PSF)[4].
There are many trade-offs when
deciding what system to make - or for
the user, what systems to buy. The sharpness of the optics is one of them.
Another key-quality parameter is the
straylight of the optical system. In general, straylight is light that reaches the
detector from places where it should
not. This can be due to reflections
inside the optics, scattering from optical surfaces, reflections on mechanical
surfaces, and so on. The straylight
effect can also be different for different spectral ranges. There is no industry standard on how to measure the
straylight of hyperspectral systems
today and different suppliers do it differently.
The light gathering capability of the
optics is usually a very important
parameter (low F#). The more spatial
pixels and spectral bands in the system,
the less light that reaches each
pixel/band. This means that a high-resolution hyperspectral imaging system is
required to be very light sensitive. This
parameter can vary in importance
depending on the platform. If the
hyperspectral system is deployed on an
aircraft or in an industrial setting,
speed is usually an important parameter and a low F# is then very important
for getting a good signal-to-noise ratio
(SNR) for the whole spectral range.
Conversely, lab applications can typically collect more light by using slower
image capture rates and by integrating
longer, which allows for more flexibility
on the aperture.
High SNR for the whole spectral
range is a very important parameter of
the hyperspectral system and how high
SNR is depends on the spectral response
function of the whole system. The peak
SNR only specifies the maximum SNR
from a band that is close to saturation
and, therefore, does not give the whole
story. One would also need to know what
the total quantum efficiency of the
whole system is as a function of wavelength. To provide useful information,
the SNR curve needs to be specified for
a given input radiance and a given (and

24

Photonics & Imaging Technology, March 2021

Maximum errors (scene A)

0.6
0.5

Keystone
PSF width
PSF shape

5

4

0.4
Error

3
10

0.3
2
9

0.2
8
1

0.1
6

0
0

30

7
28
26

1

29

27

2

3

4

5

6

Maximum errors in a scene caused by keystone (red), by differences in PSF width (blue), and by differences in PSF shape (green) for 15 simulated cameras. [2]

Hyperspectral camera integrated with a LiDAR solution.

Cov

ToC



Tech Briefs Magazine - March 2021

Table of Contents for the Digital Edition of Tech Briefs Magazine - March 2021

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