We developed a new measurement system for bidirectional reflectance distribution functions (BRDF). The system can obtain simultaneously isotropic BRDF of all scattering angles utilizing a semicircular ring and an image sensor. First, we predicted the performance of our measurement system using integrated ray tracing simulation. The light path is as follows: the light from the light source at 635 nm is reflected off the target material, and the light is reflected back at the semi-circular ring. The image sensor records the light reflected from the semicircular ring. The results show good agreement with original and simulation BRDF, but detailed analysis suggested. The system improves significantly measurement time and resolution of reflection angles. Furthermore, the system is not only more cost effective than other traditional measurement systems, but also eliminated the temporal fluctuation of the light source intensity.
Micro-Lens Array (MLA) for diffusing incident light has been widely used for 3D depth measurement. Especially in Time-of-Flight (TOF) 3D sensor, output beam uniformity through the MLA is one of the most important factors to determine the sensor accuracy. This performance is highly related to a MLA lens structure. Randomized MLA, a substitute for regular periodic MLA, does not overcome a low diffusing efficiency due to the leaking light. To tackle this issue, we propose a new MLA design methodology which is comprised of various unit lenses having different curvatures and sizes (hereafter referred to as the “Mini-Lens"). The design method is to randomly overlap a sub lens having different curvatures and sizes into the regular periodic arrange. The Mini-Lens type MLA shows a non-periodic structure and no height difference. The experiments with our design show ~30% improvements in uniformity and efficiency compared to the regular periodic MLA and the randomly distributed MLA, respectively. New Mini-Lens can achieve a fill factor of 100% over various designed base and sub lens. In addition, the suggested methodology will be useful for designing all the structures that are difficult to arrange apart from rectangular and hexagonal type. Our methodology will be a new design method to overcome the limitation of its structure.
Development and operational planning for ocean color satellite requires lots of careful consideration of the spatial and radiometric performance, which are represented by modulation transfer function (MTF) and signal-to-noise ratio (SNR) respectively. Those representative values are crucial indicator of sensor performance so that small changes of ocean properties (e.g., remote sensed reflectance (Rrs), surface chlorophyll-a concentrations (Chl-a), and so on) can be quantified and directly related with those values. MTF is affected from a performance of instrument itself and environmental conditions, and its variation leads to change the final products. The goal of this study is to simulate and to analyze the relationship between MTF parameter and ocean product variations, and then to provide a reference for the design of future ocean color sensors. In this study, we used the Geostationary Ocean Color Imager (GOCI) data to generate the simulated atmospheric correction band image. And then Rrs data and ocean products were generated with imagery from two different locations and acquisition times, and we analyzed and compared the statistical results with study area having different characteristics. For ocean products relationships, we notify the linear variation of the absolute percentage difference (APD) according to the changeable MTF value. Especially, Case-II water (turbid water) area shows more sensitive variation than Case-I water (clear water) area. Even though the same area was applied in the simulation, it was 1-2 times higher sensitivity of variation when a specific ocean phenomena such as red tide. The suggested simulation can be confirmed the relationship between blurred NIR band image and ocean products. And statistical results with MTF values were able to help estimating ocean product precision and designing a future mission such as the Geostationary Ocean Color Imager-II (GOCI-II) mission currently being progressed.
In this study, we performed alignment state estimation simulations and compared the performance of two Computer Aided Alignment (hereafter CAA) algorithms i.e. ‘Merit Function Regression (MFR)’ and ‘Multiple Design Configuration Optimization (MDCO)’ for a TMA optical system. The former minimizes the merit function using multi-field wavefront error measurements from single configuration, while the latter minimizes the merit function using single-field measured wavefront error from multiple configurations. The optical system used is an unobscured three-mirror anastigmat (TMA) optical system of 70mm in diameter, and F/5.0. It is designed for an unmanned aerial vehicle for coastal water remote sensing. The TMA consists of two aspherical mirrors, a spherical mirror and a flat folding mirror. Based on the sensitivity analysis, we set the tilt x, y of tertiary mirror as a compensator, and not considered decenter of tertiary mirror because of its spherical characteristic. For the simulation, we introduced Gaussian distribution of initial misalignment to M3. It has the mean value of zero and standard deviation of 0.5 mrad. The initial simulation result of alignment state estimation shows that both algorithms can meet the alignment requirement, λ/10 RMS WFE at 633nm. However, when we includes measurement noise, the simulation result of MFR shows greater standard deviation in RMS WFE than that of MDCO. As for the measurement, the MDCO requires single on-axis field while the MFR requires multiple fields, we concluded that the MDCO is more practical method to align the off-axis TMA optics than MFR.
The design and performance analysis of a new sensor is introduced which is on board a small unmanned aerial vehicle (UAV) for coastal water remote sensing. The top level requirements of sensor are to have at least 4cm spatial resolution at 500m operating height, and 4° field of view (FOV) and 100 signal-to-noise ratio (SNR) value at 660nm. We determined the design requirements that its entrance pupil diameter is 70mm, and F-ratio is 5.0 as an optical design requirement. The three-mirror system is designed including aspheric primary and secondary mirrors, which optical performance are 1/15 λRMS wavefront error and 0.75 MTF value at 660nm. Considering the manufacturing and assembling phase, we performed the sensitivity, tolerance, and stray-light analysis. From these analysis we confirmed this optical system, which is having 4cm spatial resolution at 500m operating height, will be applied with remote sensing researches.
Image mosaic technique is widely used in a field of remote sensing research. However, in case of Geostationary Ocean
Color Imager’s (GOCI’s) mosaic image which is consist of 16 slot images, the radiance level discrepancy was noticed in
the cloudy circumstance next to each other slot when acquiring the imagery data in the low Sun elevation angle. We
provided, in this study, the in-depth stray light analysis results in order to find out this discrepancy phenomenon, and
performed to compare the stray light pattern via a bright target movement.
Stray light analysis as the first step was completed with ray tracing technique based on ASAP program, and we
suggested that unwanted radiations from the Earth bright target or the atmosphere such as cloud are major candidates of
stray light in the problematic images. For embodying GOCI operational concept, we constructed the Integrated Ray
Tracing model consisting of the Sun model as a light source, a target Earth model, and the GOCI optical system model.
In the second step, we investigated the stray light pattern at each slot image including unwanted random source from out
of field, and then constructed the simulated mosaic bias image reached at the detector plane. In the simulated bias, the
ray path followed the procedures that light travels from the Sun and it is then reflected from the Earth section of roughly
2500km * 2500km in size around the Korea peninsula with 16 slots.
Lastly, we analyzed stray light pattern in the third step for the real image environment acquired at UTC-03 16th, October,
2011. In addition, verification was performed to compare the difference among slot boundaries for moving bright target.
In the remote sensing researches, the reflected bright source from out of FOV has effects on the image quality of wanted signal. Even though those signal from bright source are adjusted in corresponding pixel level with atmospheric correction algorithm or radiometric correction, those can be problem to the nearby signal as one of the stray light source. Especially, in the step and stare observational method which makes one mosaic image with several snap shots, one of target area can affect next to the other snap shot each other. Presented in this paper focused on the stray light analysis from unwanted reflected source for geostationary ocean color sensor. The stray light effect for total 16 slot images to each other were analyzed from the unwanted surrounding slot sources. For the realistic simulation, we constructed system modeling with integrated ray tracing (IRT) technique which realizes the same space time in the remote sensing observation among the Sun, the Earth, and the satellite. Computed stray light effect in the results of paper demonstrates the distinguishable radiance value at the specific time and space.
Geostationary Ocean Color Imager(GOCI) is one of three payloads on board the Communication, Ocean, and
Meteorological Satellite(COMS) launched 27th, June, 2010. For understanding GOCI imaging performance, we
constructed the Integrated Ray Tracing model consisting of the Sun model as a light source, a target Earth model,
and the GOCI optical system model. We then combined them in Monte Carlo based ray tracing computation.
Light travels from the Sun and it is then reflected from the Earth section of roughly 2500km * 2500km in
size around the Korea peninsula with 40km in spatial resolution. It is then fed into the instrument before reaching to the detector plane. Trial simulation runs for the GOCI imaging performance were focused on the combined slot images and MTF. First, we used modified pointing mirror mechanism to acquire the slot images, and then mosaiced them. Their image performance from the GOCI measurement were compared to the ray tracing simulation results. Second, we investigated GOCI in-orbit MTF performance with the slanted knife edge method applied to an East coastline image of the Korea peninsula covering from 38.04N, 128.40E to 38.01N, 128.43E. The ray tracing simulation results showed 0.34 in MTF mean for near IR band image while the GOCI image obtained 9th Sep, 2010 and 15th Sep, 2010, were used to produce 0.34 at Nyquist frequency in MTF. This study results prove that the GOCI image performance is well within the target performance requirement, and that the IRT end-to-end simulation technique introduced here can be applicable for high accuracy simulation of in-orbit performances of GOCI and of other earth observing satellite instruments.
Geostationary Ocean Color Imager (GOCI), a payload of the Communication, Ocean and Meteorology Satellite
(COMS), is the world's first ocean color observation satellite in geostationary orbit. It was launched at Kourou Space
Center in French Guiana in June 2010. The detector array in GOCI is custom CMOS Image sensor about 2 Mega-pixels,
featuring rectangular pixel size to compensate for the Earth oblique projection.
This satellite is being operated on geostationary orbit about 36,500km far from the earth; hence it can be more influenced
by sun activities than the other on low Earth orbit. Especially, the detector is sensitive of heat and it may give rise to
increasing the defective pixels. In this paper, radiometric performance variations have been analyzed through the time
series analysis, using the offset parameters and detector temperature estimated in GOCI radiometric model. It is essential
to monitor the overall sensitivity of GOCI sensor, and it will helpful to the radiometric calibration.
In the result, we notified there was no great variation in time series of offset parameters after operating the GOCI in July
2010, but we monitored an anomaly by an operational event. One of them related to thermal electron showed slightly
increasing trend and the diurnal variation by the sun energy. Although sun interferences are occurred sometimes, any
significant anomaly isn't found. With these results of characterization, we find that GOCI has been carrying out stably in
the aspect of radiometric performance, and expect that it will be kept during the mission life.
In our earlier study[12], we suggested a new alignment algorithm called Multiple Design Configuration Optimization
(MDCO hereafter) method combining the merit function regression (MFR) computation with the differential
wavefront sampling method (DWS). In this study, we report alignment state estimation performances of
the method for three target optical systems (i.e. i) a two-mirror Cassegrain telescope of 58mm in diameter for
deep space earth observation, ii) a three-mirror anastigmat of 210mm in aperture for ocean monitoring from the
geostationary orbit, and iii) on-axis/off-axis pairs of a extremely large telescope of 27.4m in aperture). First
we introduced known amounts of alignment state disturbances to the target optical system elements. Example
alignment parameter ranges may include, but not limited to, from 800microns to 10mm in decenter, and from
0.1 to 1.0 degree in tilt. We then ran alignment state estimation simulation using MDCO, MFR and DWS. The
simulation results show that MDCO yields much better estimation performance than MFR and DWS over the
alignment disturbance level of up to 150 times larger than the required tolerances. In particular, with its simple
single field measurement, MDCO exhibits greater practicality and application potentials for shop floor optical
testing environment than MFR and DWS.
We first studied the characteristics of alignment performances of two computer-aided alignment algorithms i.e. merit
function regression (MFR) and differential wavefront sampling (DWS). The initial study shows i) that, utilizing damped
least square algorithm, MFR offers accurate alignment estimation to the optical systems with non-linear wavefront
sensitivity to changes in alignment parameters, but at the expense of neglecting the coupling effects among multiple
optical components, and ii) that DWS can estimate the alignment state while taking the inter-element coupling effects
into consideration, but at the expense of increased sensitivity to measurement error associated with experiment apparatus.
Following the aforementioned study, we report a new improved alignment computation technique benefitted from
modified MFR computation incorporating the concept of standard DWS method. The optical system used in this study is
a three-mirror anastignmat (TMA) based optical design for the next generation geostationary ocean color instrument
(GOCI-II). Using an aspheric primary mirror of 210 mm in diameter, the F/7.3 TMA design offers good imaging
performance such as 80% in 4 um in GEE, MTF of 0.65 at 65.02 in Nyquist frequency. The optical system is designed to
be packaged into a compact dimension of 0.25m × 0.55m × 1.050m. The trial simulation runs demonstrate that this
integrated alignment method show much better alignment estimation accuracies than those of standard MFR and DWS
methods, especially when in presence of measurement errors. The underlying concept, computational details and trial
simulation results are presented together with implications to potential applications.
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