Laboratory calibration of electro-optical sensors is preferably complemented by regular in-flight verification. This
checks whether the lab calibration parameters remain valid or recalibration is necessary. In-flight verification can
be achieved by vicarious calibration using in-flight measurements of calibration targets. We intend to identify
and design a set of suitable radiometric calibration targets. For this, we borrow from expertise gained with
the PROBA-V satellite calibration system, which uses multiple vicarious methods relying on diverse natural
on-ground targets. Besides reflectance based calibration using ground measurements, the PROBA-V calibration
methods are unproven for use in airborne calibration. The selected targets should be suitable for the calibration
of both multispectral and hyperspectral imagers.
We start from general requirements for radiometric targets and investigate their applicability to airborne
calibration. From this we identify two possible sets of natural calibration sites in Belgium. One set, located in
the Campine region, contains small water bodies and sandy lakesides. Another set is located in the Westhoek
region near the Belgian coast. It offers better suitable water bodies, as well as sandy areas, grass fields and dark
targets. Airborne calibration lends itself to the use of smaller artifical targets. We propose to complement the
natural targets with a portable target consisting of agricultural nets with different densities. The definition of
sets of calibration targets, both natural and artificial can facilitate the investigation of the usability of vicarious
targets and method for inflight radiometric verification.
Radiometric calibration often employs several independent vicarious calibration techniques to increase robustness
and accuracy. We present a statistical methodology for combining results in a hierarchical scheme. The method,
developed for the PROBA-V remote sensing mission, is based on handling and propagating of accuracies in
accordance with the ISO GUM. Robust estimation is performed and outliers removed. Results over different
sites are combined using weighted averaging. Weighted linear regression is used for temporal averaging. Results
from different methods are combined taking into account possible bias. Finally an operational update strategy
is proposed which relies on a significance criterion.
MEDUSA is the lightweight high resolution camera, designed to be operated from a solar-powered Unmanned Aerial
Vehicle (UAV) flying at stratospheric altitudes. It targets applications such as crisis management and cartography as a
technology demonstrator within the Pegasus program. From an operational altitude of 18 km MEDUSA will deliver
images with a ground sampling distance of 30 cm and cover a swath of 3 km. The innovative UAV poses high demands
on the instrument characteristics such as mass (< 2,5 kg), volume and power consumption. Furthermore the MEDUSA
instrument design needs to guarantee its optical performance within the specific environmental conditions of the
stratosphere.
Well limited chromatic aberrations require the use of anomalous partial dispersion glass. Those glasses unfortunately
suffer from large variations of their refractive index as well as their volume with temperature, causing strong focus
variations of the optical system. Furthermore, the weight constraints impose compact system (minimizing the distance
between lenses for mount weight reduction) and very thin lenses. Manufacturing each of the lenses was challenging. The
very thin aspherical lens has required few manufacture iterations to achieve sufficiently low waviness, after that it
became clear that this lens was the dominant factor for image contrast limitation.
After all subsystem tests have been completed the MEDUSA shall be integrated and finish its ground test program in
autumn 2009. This paper will describe the results of the laboratory characterization of the optical system and an outlook
on the MEDUSA instrument development.
MEDUSA is the lightweight high resolution camera, designed to be operated from a solar-powered Unmanned Aerial
Vehicle (UAV) flying at stratospheric altitudes. The instrument is a technology demonstrator within the Pegasus program
and targets applications such as crisis management and cartography. A special wide swath CMOS imager has been
developed by Cypress Semiconductor Cooperation Belgium to meet the specific sensor requirements of MEDUSA.
The CMOS sensor has a stitched design comprising a panchromatic and color sensor on the same die. Each sensor
consists of 10000*1200 square pixels (5.5μm size, novel 6T architecture) with micro-lenses. The exposure is performed
by means of a high efficiency snapshot shutter. The sensor is able to operate at a rate of 30fps in full frame readout.
Due to a novel pixel design, the sensor has low dark leakage of the memory elements (PSNL) and low parasitic light
sensitivity (PLS). Still it maintains a relative high QE (Quantum efficiency) and a FF (fill factor) of over 65%. It features
an MTF (Modulation Transfer Function) higher than 60% at Nyquist frequency in both X and Y directions The measured
optical/electrical crosstalk (expressed as MTF) of this 5.5um pixel is state-of-the art. These properties makes it possible
to acquire sharp images also in low-light conditions.
In proofing, the accurate reproduction of prints is pursued. One of the major properties which determine the appearance of prints, is the halftoning. We refer to digital proofing methods as dot for dot proofing when they try to reproduce this property.
We identify three basic requirements for a good dot for dot proof: colorimetric match, halftone match and print colorant match. They can be met simultaneously because they relate to different scales of resolution. The best starting point for a dot for dot method is the final rasterised separation data. Then, the proofing workflow
maximally shares its processing components with the printing workflow, which helps minimising differences. Since print and proof generally differ in resolution and colorants, colour and resolution conversion are basic components of dot for dot methods. We propose a general flow in which an intermediate colour space is used.
The fundamental issue is to accurately handle the colour information together with the halftone information. Normally, an image either represents the colour accurately by giving contone values, or the halftoning by giving high resolution binary data. Therefore, the choice of image representation in a processing flow becomes critical.
Solutions can be found in hybrid representations containing information about both, or in dual representations.
We study cross-media image reproduction by constructing a tonal range mapping model. We aim at making reproductions that optimally represent the overall appearance of the originals despite a reduction in dynamic range. We propose a general mapping that works for all input and output white and black points. It is described by a two parameter functional model. The parameters are chosen so that one primarily corresponds to black point variations and the other to white point variations.
We set up psychometric experiments to estimate optimal parameter values. Paired comparison was employed because of its ease of use and accurate results. To keep the number of observations down, a small pilot experiment marks out a narrower range of values first. Furthermore, the two parameter optimisation is split into a sequential single parameter optimisation. The experiments are repeated for different white and black points. The model is completed by interpolating between the experimental points and determining the correlation between the parameters. A separate verification experiment proves the validity of the model within the experimental accuracy.
A comparison of our model with the CIECAM97s colour appearance model clarifies the fundamental difference between them. The tonal mapping model aims at the best overall reproduction of images. It produces more pleasing images by giving them higher overall contrast, whereas CIECAM faithfully models the appearance.
We investigate the factors that determine the consistency of color output on digital proofing systems based on inkjet technology. Because of the multitude of factors involved, only a global solution can prove effective. We develop a complete solution consisting of two main modules. The calibration module contains the tools needed to bring a proofer into a standard condition, for which a predefined tonal response can be guaranteed. The calibration encompasses ink limitation and linearisation. It uses visual quantities, which is most sensible for proofing applications. In the case of multi density inks, the ink mixing is also calibrated in order to remain visually optimal. A convenient procedure is proposed for defining tonal responses, taking into account gamut and bleeding information. The verification module enables the user to monitor the behaviour of the proofer output. It points out problems and also prompts the user to perform suitable actions in order to restore the quality. The intelligence of the module lies in its ability to decide when and how to intervene. This gives the user a practical system that allows to ensure consistency with minimal effort.
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