Aerosol Modulation Transfer Function Research Articles

Effects of aerosol modulation transfer function on target identification

Atmospheric aerosol effects are often overlooked in target acquisition studies. Typically, performance models only consider extinction and turbulence within the prediction processes. The aerosol modulation transfer function (MTF) is included in range acquisition algorithms to determine how scattering and absorption effects change the target identification predictions. We modeled the aerosols as monodisperse water droplets comparable to a tenuous fog or mist. Integrating the aerosol MTF into the system MTF gives the opportunity to utilize the night vision integrated performance model to predict the target identification range with aerosol contributions. The aerosol MTF is a function of range, water droplet composition, wavelength, and aperture size. The analysis focuses on these variables with an emphasis on wavelength dependence to characterize mid-wave and long-wave performance. Results show that the mid-wave systems have a substantial diffraction advantage over long-wave systems. Only in the limit of increasing optical depths do the mid-wave and long-wave performance models begin to converge, verifying that the aerosols can be the limiting factor for target identification.

Aerosol modulation transfer function model for passive long-range imaging over a nonuniform atmospheric path

An aerosol modulation transfer function (MTF) model is developed to assess the impact of aerosol scattering on passive long-range imaging sensors. The methodology extends from previous work to explicitly address imaging scenarios with a nonuniform distribution of scattering characteristics over the propagation path and incorporates the moderate resolution transfer code database of aerosol cross-section and phase function characteristics in order to provide an empirical foundation for realistic quantitative MTF assessments. The resulting model is compared with both predictions from a Monte-Carlo scattering simulation and a scene-derived MTF estimate from an empirical image, with reasonable agreement in both cases. Application to long-range imaging situations at both visible and infrared wavelengths indicates that the magnitude and functional form of the aerosol MTF differ significantly from other contributors to the composite system MTF. Furthermore, the image-quality impact is largely radiometric in the sense that the contrast reduction is approximately independent of spatial frequency, and image blur is practically negligible.

Removing atmospheric MTF and establishing an MTF compensation filter for the HJ-1A CCD camera

The total image modulation transfer function (MTF) comprises imaging system MTF and atmospheric MTF, both of which can produce blurring effects in remote-sensing images. Imaging system MTF, one of the important specifications of the imaging system, is used to characterize the radiometric response of the sensor. Atmospheric MTF, which consists of aerosol MTF and turbulence MTF, usually changes with external atmospheric conditions. To obtain the MTF of the imaging system, the atmospheric MTF has to be removed from the total image MTF. This study aims to determine a method for removing the atmospheric MTF for the charge-coupled device (CCD) camera on-board the Huanjing 1A satellite (HJ-1A). The aerosol MTF and the turbulence MTF in the CCD images were first predicted based on the aerosol optical depth (AOD) derived by dark target and reanalysis of data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). Then, the MTFs of the CCD camera were retrieved by removing the aerosol and turbulence MTFs from the total on-orbit image MTFs that were calculated from the linear features on CCD images. For the HJ-1A CCD camera, results show that the MTF values at Nyquist frequency increased by about 10% after the atmospheric MTFs were removed. The total image MTF became degraded by about 50% when AOD was high or turbulence was strong. Furthermore, this study has established an MTF compensation (MTFC) filter for the HJ-1A by modifying the ‘modified inverse filter (MIF)’ approach. Results illustrate that the effective instantaneous field of view (EIFOV) of the restored image improved from 78 to 41 m, while the noise level of the model was well controlled. This research demonstrates that the modified approach is effective for sensors with low MTF values.

Atmospheric scattering and turbulence modulation transfer function for CCD cameras on CBERS-02b and HJ-1A/1B

Atmospheric turbulence and aerosol scattering can produce the blurring of the remotely sensed image. The degrading effect is usually quantified by atmospheric modulation transfer function (MTF). However, this effect behaves differently with different remote sensors. The effort of this article is to study the different degrading effects of aerosol MTF and turbulence MTF between charge-coupled device (CCD) camera on China–Brazil Earth Resources Satellite-02b (CBERS-02b) and on Huan Jing-1A/1B satellite (HJ-1A/1B). Specifically, a corrected aerosol MTF model is established based on classical solution of small-angle approximation (SAA) model by considering the MTF and the effective instantaneous field of view (EIFOV) of CCD cameras. By assuming many different atmospheric conditions, the aerosol MTF and turbulence MTF for two CCD cameras are evaluated. It is found that the output aerosol MTF of CCD camera on HJ-1A/1B causes more degrading effect than that of CBERS-02b under the same atmospheric condition. However, the situation reverses for the turbulence MTF. Furthermore, CCD images acquired over Beijing, China, by CBERS-02b and HJ-1A/1B on four different dates are selected. The overall atmospheric MTF for these images are determined based on the aerosol products from Aerosol Robotic Network (Aeronet) and radiosounding data. Results indicate that the overall atmospheric MTF of CBERS-02b CCD camera reduces image quality more seriously than that of HJ-1A/1B CCD camera. Additionally, the atmospheric MTF compensation is performed and evaluated for these CCD images based on the overall atmospheric MTF.

Atmospheric modulation transfer function in the infrared.

In high-resolution ultranarrow field-of-view thermal imagers, image quality over relatively long path lengths is typically limited by atmospheric degradation, especially atmospheric blur. We report our results and analyses of infrared images from two sites, Fort A. P. Hill and Aberdeen Proving Ground. The images are influenced by the various atmospheric phenomena: scattering, absorption, and turbulence. A series of experiments with high-resolution equipment in both the 3-5- and 8-13-microm regions at the two locations indicate that, as in the visible, image quality is limited much more by atmosphere than by the instrumentation for ranges even of the order of only a few kilometers. For paths close to the ground, turbulence is more dominant, whereas for paths involving higher average elevation, aerosol modulation transfer function (MTF) is dominant. As wavelength increases, turbulence MTF also increases, thus permitting aerosol MTF to become more dominant. A critical role in aerosol MTF in the thermal infrared is attributed to absorption, which noticeably decreases atmospheric transmission much more than in the visible, thereby reducing high-spatial-frequency aerosol MTF. These measurements indicate that atmospheric MTF should be a basic component in imaging system design and analysis even in the infrared, especially as higher-resolution hardware becomes available.

Criteria for satellite image restoration success

Many properties of the atmosphere affect the quality of images propagating through it by blurring and reducing their contrast. The atmospheric path involves several limitations, such as scattering, absorption of light, and turbulence, which degrade the image. Recently developed atmospheric filters, which correct for turbulence blur, aerosol blur, and path radiance simultaneously, are implemented here in the digital restoration of Landsat thematic mapper (TM) imagery. The turbulence modulation transfer function (MTF) is calculated from meteorological data or estimated if no meteorological data were measured. Aerosol MTF is consistent with optical depth. The product of the two yields atmospheric MTF, which is implemented in the atmospheric filter. Restoration improves smallness of size of both resolvable detail and contrast. Restorations are quite apparent even under clear weather conditions. A way to evaluate restoration improvement is presented here by the use of quantitative criteria as well as subjective opinions of human observers in perception experiments. Not all the restoration criteria represent improvement in the same tested image under the same restoration conditions. When one criterion suggests an enhancement, there is a chance that another one might represent a lower value for restoration success.

Effects of image restoration on target acquisition

We focus on modeled atmospheric and image restoration effects on the human acquisition of a target. The image restoration is by filtering atmospheric effects, such as blur caused by absorption, scattering from aerosols, and distortion caused by turbulence that changes the wave front angle, moves the image on the image plane, and blurs it. The restoration method using an atmospheric wiener filter is intended to correct the atmospheric effects. This filter is based on the fact that the modulation transfer function (MTF) of the turbulence is composed of a mean value and a random component while the aerosol MTF changes slowly. The atmospheric wiener filter is more advantageous than an ordinary wiener filter in high turbulence situations. Using the atmospheric wiener filter, we consider the target acquisition limitations of the filter through perception experiments and statistical analysis of the results. We found that the atmospheric wiener filter can noticeably improve target acquisition probability at low noise levels. As the noise increases, however, the improvement becomes more limited because the restoration increases the noise level in the image.

Myopic deconvolution of adaptive optics images by use of object and point-spread function power spectra: comment

It is suggested here that the lack of total image correction that is typical in adaptive optics (AO) imaging can be attributed in part to blur derived from small-angle scatter of light by aerosols, known also as the adjacency effect, especially as it is a well-established fact that such atmospheric blur is dominant in satellite imagery and the shape of the modulation transfer function after AO correction is strikingly similar to the unique shape of the aerosol modulation transfer function. Further investigation of AO systems to confirm this would aid in and improve image restoration.

Experimental investigation of the influence of the relative position of the scattering layer on image quality: the shower curtain effect

The imaging quality of optical systems in a turbid environment is influenced not only by the content of the turbid layer between the object and the optical receiver but also by the inhomogeneity of that medium. This is important, particularly when imaging is performed through clouds, nonhomogeneous layers of dust, or over vertical or slant paths through the atmosphere. Forward small-angle scattering influences image quality and blur more severely when the scattering layer is closer to the receiver. In this study it is the influence of the relative position of the scattering layer on the image quality and modulation transfer function (MTF) that is investigated. The scattering layer in controlled laboratory experiments consists of calibrated polystyrene particles of known size and quantity in a small cuvette. A point source was imaged by a computerized imaging system through a layer containing polystyrene particles, and the point-spread function (PSF) was recorded. The aerosol MTF was calculated using the measured PSF. The MTF was measured as a function of changing relative distance of the scattering layer from the receiver, whereas the object-plane-to-receiver distance was constant. The experimental results were compared to theoretical shower curtain effect models based on the solution from radiative transfer theory under the small-angle approximation. Although the general trend of the experimental results certainly agrees with the theoretical models, it could be that the small-angle approximation method might be of limited validity at such low spatial frequencies. Aggregation also causes some disagreement with predictions from theory.

Restoration of atmospherically blurred images according to weather-predicted atmospheric modulation transfer functions

Restoration for actual atmospherically blurred images is per- formed using an atmospheric Wiener filter that corrects simultaneously for both turbulence and aerosol blur by enhancing the image spectrum primarily at those high frequencies least affected by the jitter or random- ness in a turbulence modulation transfer function (MTF). The correction is based on weather-predicted rather than measured atmospheric MTFs. Both turbulence and aerosol MTFs are predicted using meteorological parameters measured with standard weather stations at the time and location where the image was recorded. A variety of weather conditions and seasons are considered. Past results have shown good correlation between measured and predicted atmospheric MTFs. Here, the pre- dicted MTFs are implemented in actual image restoration and quantita- tive analysis of the MTF improvement is presented. Corrections are shown also for turbulence blur alone, for aerosol blur alone, and for both together. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)01511-0)

Effect of the inner scale of turbulence on the atmospheric modulation transfer function

The effect of the inner scale of turbulence on the atmospheric modulation transfer function (MTF) is analyzed. It is shown that owing to this effect the turbulence MTF has all the main features of the overall measured atmospheric MTF, including different behaviors in the low- and high-frequency ranges, a knee, and dependence on the wavelength. It is also shown that the mismatches between the overall measured atmospheric MTF and the predicted turbulence MTF might be eliminated by use of a theoretical model with a finite inner scale of turbulence for the turbulence MTF and an accurate interpretation of the optical measurements of the refractive-index structure characteristic. Therefore no revision of the classical aerosol MTF theory is required to interpret the measured data.

Prediction of overall atmospheric modulation transfer function with standard weather parameters: comparison with measurements with two imaging systems

The overall atmospheric modulation transfer function (MTF) is essentially the product of the turbulence and aerosol MTFs. Models describing meteorological dependences of both <i>C</i>²<i>n</i> (where <i>Cn</i> is the refractive index structure coefficient) and the coarse aerosol size distribution have been developed previously. Here, they are used to predict the turbulence MTF, aerosol MTF, and overall atmospheric MTF according to the weather. Comparison of predictions with measurements yields very low mean squared normalized error and suggests that such prediction models can also be very useful in image restoration based on weather data at the time and general location in which the image was recorded. An interesting aspect of this work is that measurements of the aerosol MTF with different imaging instrumentation are very different, as expected from theory developed previously concerning the practical aerosol MTF actually recorded in the image. This is dependent on instrumentation parameters. This experimental verification with two different imaging systems supports the model that the "practical" aerosol MTF is very dependent on instrumentation.

High-resolution restoration of images distorted by the atmosphere, based on an average atmospheric modulation transfer function

A new method of real-time high-resolution imaging through the atmosphere is presented. This technique is based on knowledge of average atmospheric modulation transfer function (MTF) at the time the image is received. Atmospheric effects are modeled by a noisy spatial frequency filter including an average component described by the average atmospheric MTF and a noisy component modeled by the atmospheric point spread function's power spectral density. The noisy component represents random changes in the atmospheric MTF. Analytical results are accompanied by experimental image restoration examples, indicating significant image quality improvement based on knowledge of average atmospheric MTF, which includes both turbulence and aerosol MTF components. This method can be used to help overcome the jitter characteristics of turbulence, and is capable of yielding real-time image restoration with resolution limited essentially only by the hardware itself. Turbulence blur, aerosol blur, and contrast degradation are all corrected simultaneously, unlike adaptive optics, which corrects for turbulence only.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function: reply to comment

New exact numerical calculations as well as exact calculations in J. Opt. Soc. Am. A10, 172 ( 1993) indicate that the approximations in the Comment in J. Opt. Soc. Am. A11, 1175 ( 1994) are based on faulty perceptions of basic scattering phenomena and on a misreading of our paper. The Comment does not sufficiently consider effects of instrumentation. Whereas our model refers to the aerosol modulation transfer function (MTF) in the image plane, the values of maximum angles for scattered (θS) and unscattered (θ0) light used in the Comment refer to the optics plane. The Comment does not consider the fact that dynamic range limits (θS) in the image, whereas limited spatial-frequency bandwidth broadens θ0 in the image, each by orders of magnitude. Therefore the Comment’s conclusion contradicts experimental results obtained by numerous researchers. The Comment’s conclusion that aerosol MTF is insignificant is based on that author’s own experiments, in which clear weather and haze atmospheric MTF (composed of turbulence and aerosol MTF’s) could not be measured, the reason being insufficient equipment resolution, rather than insignificant turbulence or aerosol MTF. Furthermore, the claim in our original paper that aerosol MTF is extremely significant has since been supported by many different types of experiments that included short and long exposures, thermal imaging, and image restoration based on atmospheric MTF, including a highly significant practical, uniquely shaped, aerosol MTF.

Experimental comparison of turbulence modulation transfer function and aerosol modulation transfer function through the open atmosphere

Although turbulence is usually considered to be the primary cause of image blur, simultaneous and independent measurements of overall atmospheric modulation transfer function (MTF) and turbulence MTF over fairly long horizontal paths at 15-m average elevation indicate that, even at midday, aerosol MTF deriving from forward scatter is usually more dominant than turbulence MTF. Three different experimental techniques are used, two passive and one active. Aerosol MTF measurements are accompanied by actual meteorological and coarse aerosol size distributions and scattering parameters at the times of MTF measurements. The wavelength dependence of aerosol and therefore of overall atmospheric MTF can be significant. This wavelength dependence and a usually well-defined knee can in no way be due to turbulence but can be explained by a significant aerosol MTF deriving from typical aerosol size distributions representative of other climates as well. Measurements confirm that the narrower the open-atmosphere aerosol scattering patterns and the greater the scattering densities, the greater the degradations of aerosol MTF and, consequently, of overall atmospheric MTF. Results imply that system design and image-restoration algorithms based on atmospheric turbulence only may often lead to image quality that is much poorer than if aerosol MTF is considered, too, with its proper proportional effect on imaging through the atmosphere. Whereas adaptive optics cannot correct for aerosol-derived blur, digital image restoration can.

Restoration of thermal images distorted by the atmosphere, using predicted atmospheric modulation transfer function

Restoration of thermal images distorted by the atmosphere, detected with a focal plane array (FPA) Pt-Si thermal imaging system, is presented. The restoration method is based upon atmospheric modulation transfer function (MTF) analysis. Using turbulence and aerosol MTF prediction models, atmospheric distortions and image degradation are modeled. Restoration results indicate significant improvement in image quality. However, it is critical to include the unique shape of aerosol MTF when modeling atmospheric MTF in order to obtain good restoration.

Effects of practical aerosol forward scatter of infrared and visible light on atmospheric coherence diameter

A correction to the definition of the atmospheric coherence diameter is suggested here, based on the existence of a practical instrumentation-based aerosol modulation transfer function (MTF), which is often the dominant ingredient of the atmospheric MTF. As defined classically by Fried about 25 yr ago, atmospheric MTF and coherence diameter were related to turbulence MTF only. Lutomirski considered diftractive aerosols, too, but did not consider effects of instrumentation on scattering angles actually recorded in the image. These are limited in the real world by instrumentation to milliradians, rather than by the broad angular spread of diffraction to radians. In the case of a Gaussian approximation of the practical aerosol MTF, an analytical expression is derived for the practical aerosol-derived coherence diameter. This parameter is related to the practical aerosol MTF's cutoff frequency, and to its asymptotic value at high spatial frequencies. Thus, a more general concept of atmospheric coherence diameter is proposed here, which is relevant to actual real-world imaging systems, whether they are passive or active. Quantitative validation of the theory is presented, based on both simulations and actually measured atmospheric MTFs in both the visible and thermal infrared spectral ranges. Overall atmospheric coherence diameter is determined generally by the smaller of the turbulence and practical aerosol coherence diameters, depending on optical depth. The results here appear applicable particularly to cost-effective thermal imaging system design, although applications are considered, too, for the visible and near infrared. For example, blur deriving from aerosol scatter should have much less effect in coherent detection laser radar (LIDAR) than in direct detection imaging.

Effects of absorption on image quality through a particulate medium

A new approach to studying the effects of absorption by aerosols and molecular particulates of electromagnetic radiation is presented. In contradiction to the conventional concept that absorption gives rise to constant attenuation, it is shown here that the particulate-absorbed irradiance is spatial frequency dependent. An analytically corrected model of the aerosol modulation transfer function and the aerosol mutual coherence function is presented. An important application of this model is in thermal imaging, in which particulate-absorption effects are very significant.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function: comment

Simple order-of-magnitude expressions derived from the forward-scattering small-angle approximation show that the scattering modulation transfer function (MTF) contributes insignificantly to the overall atmospheric MTF in most practical imaging applications except for precipitation, ice-crystal clouds, and some rare advection-fog or high-wind desert storm events with particles in the size range of tens of micrometers.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function

Recent experimental measurements of overall atmospheric modulation transfer function (MTF) indicate a significant difference between overall atmospheric and turbulence MTF’s except often at midday, when turbulence is strong. We suggest a physical explanation for these results that essentially relates to a practical instrumentation-based atmospheric aerosol MTF that is a modification of the classical aerosol MTF theory. Based on radiative transfer theory, this practical approach takes into account the effect of finite field of view, finite dynamic sensitivity, and finite spatial bandwidth of every existing imaging system. These generally limit the scattering angles of received light to values far less than the diffraction limit for aerosols, thereby decreasing blur radius and increasing spatial frequency bandwidth. This can explain the broadening of the aerosol MTF from that theoretically expected. We discuss the asymptote value that the measured aerosol MTF approaches at high spatial frequencies, which is significantly higher than the theoretical prediction of turbid medium transmittance. An important conclusion that we derive is that the aerosol MTF is often the dominant part in the actual overall atmospheric MTF. In addition, there seems to be an inescapable trade-off between image resolution and image irradiance. The system designer must choose between imaging of faint and bright objects at the expense of image quality or imaging of either faint or bright objects with improved image quality. The concepts here are basic to all long-range imaging through the atmosphere.