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Author = Sheridan, J. T.;
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Displaying Results 1 - 23 of 23 on page 1 of 1
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A Numerical analysis of double random phase encryption
(2006)
Monaghan, David S.; Gopinathan, Unnikrishnan; Naughton, Thomas J.; Sheridan, J. T.
A Numerical analysis of double random phase encryption
(2006)
Monaghan, David S.; Gopinathan, Unnikrishnan; Naughton, Thomas J.; Sheridan, J. T.
Abstract:
We consider a Double Random Phase Encoding (DRPE) Encryption/Decryption system in which the image encryption/decryption process is performed numerically. In this paper we present a key-space analysis of the (DRPE) algorithm which is used to encrypt two dimensional (2-D) images. We map the assiocated error for every phase-key in the key-space of a particular system to get a visual representation of the spread of phase-keys for the system and so assess it’s security from a key-space perspective.
http://mural.maynoothuniversity.ie/8424/
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Applications of Spatial Light Modulators in Optical Signal Processing Systems
(2005)
Monaghan, David S.; Gopinathan, Unnikrishnan; Hennelly, Bryan M.; Kelly, Damien P.; Nau...
Applications of Spatial Light Modulators in Optical Signal Processing Systems
(2005)
Monaghan, David S.; Gopinathan, Unnikrishnan; Hennelly, Bryan M.; Kelly, Damien P.; Naughton, Thomas J.; Sheridan, J. T.
Abstract:
Coherent optical signal processors, due to their ability to process and relay information in two dimensions, have been receiving increasing attention in recent years. These systems involve a coherent field being propagated through some bulk optical system consisting of thin lenses and sections of free space (such paraxial systems being described mathematically using the Linear Canonical Transformation). A Spatial Light Modulator (SLM) may be used to modulate the input digital data onto a coherent wave-field as well as to modulate the amplitude and/or phase of the complex wave-field at any desired plane. The complex field (amplitude and phase) at any desired plane may be recorded quantitatively using a CCD camera, using digital holographic techniques allowing the further processing of data digitally. Such hybrid optoelectronic systems have applications for 2D and 3D data processing covering fields as diverse as data storage, data security and pattern recognition. But devices such as ...
http://mural.maynoothuniversity.ie/8393/
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Comparison of a new self developing photopolymer with AA/PVA based photopolymer utilizing the NPDD model
(2011)
Gleeson, M. R.; Sheridan, J. T.
Comparison of a new self developing photopolymer with AA/PVA based photopolymer utilizing the NPDD model
(2011)
Gleeson, M. R.; Sheridan, J. T.
Abstract:
The development of suitable recording media for applications such as holographic optical elements and holographic data storage are of significant research and commercial interest. In this paper, a photopolymer material developed by Bayer MaterialScience is examined using various optical techniques and then characterised using the Non-local Photopolymerization Driven Diffusion model. This material demonstrates the capabilities of a new class of photopolymer offering high index modulation, full colour recording, high light sensitivity and environmental stability. One key result of this study is the material’s high spatial frequency resolution, indicating a very low non-local effect, thus qualifying it as a very good storage medium.
http://mural.maynoothuniversity.ie/3598/
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Fractional Fourier transform-based image encryption: phase retrieval algorithm
(2003)
Hennelly, Bryan M.; Sheridan, J. T.
Fractional Fourier transform-based image encryption: phase retrieval algorithm
(2003)
Hennelly, Bryan M.; Sheridan, J. T.
Abstract:
A number of methods have been proposed in the literature for the encryption of two-dimensional image information using optical systems based on the fractional Fourier transform (FRT). We present an image encryption technique, which is based on a recently proposed method of phase retrieval using the FRT. The optical implementation of the method is mentioned and digital simulations are presented. The technique is shown to be a powerful method of digital data and image encryption. In this paper, we outline the implementation of the algorithm and examine the sensitivities of the various encryption keys. We also compare the performance of the new technique to digital implementations of other FRT-based optical encryption schemes.
http://mural.maynoothuniversity.ie/5808/
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Image encryption and the fractional Fourier transform
(2003)
Hennelly, Bryan M.; Sheridan, J. T.
Image encryption and the fractional Fourier transform
(2003)
Hennelly, Bryan M.; Sheridan, J. T.
Abstract:
A number of method have been recently proposed in the literature for the encryption of 2-D information using optical systems based on the fractional Fourier fransform, FRT. In this paper a brief review of the methods proposed to date is presented. A measure of the strength/robustness of the level of encryption of the various techniques is proposed and a comparison is carried out between the methods. Optical implementations are discussed. Robustness of system with respect to misalignment and blind decryption are also discussed.
http://mural.maynoothuniversity.ie/5809/
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Kinetics of Chain Transfer Agents in Photopolymer Material
(2012)
Guo, J.; Gleeson, M. R. ; Sheridan, J. T.
Kinetics of Chain Transfer Agents in Photopolymer Material
(2012)
Guo, J.; Gleeson, M. R. ; Sheridan, J. T.
Abstract:
The Non-local Photo-Polymerization Driven Diffusion (NPDD) model was introduced to describe the observed drop-off in the material’s response for higher exposing spatial frequencies. Recent work carried out on the modeling of the mechanisms which occur in photopolymers during- and post-exposure, has led to the development of a tool, which can be used to predict the behaviour of these materials under a wide range of conditions. In this article, based on the chemical reactions of chain transfer agents, we explore this extended NPDD model, illustrating some of the useful trends, which the model predicts and we analyse their implications on the improvement of photopolymer material performance.
http://mural.maynoothuniversity.ie/3593/
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Monomer diffusion rates in photopolymer material: Part I. Low spatial frequency holographic gratings: reply
(2012)
Sheridan, J. T.; Gleeson, M. R.; Close, C. E.
Monomer diffusion rates in photopolymer material: Part I. Low spatial frequency holographic gratings: reply
(2012)
Sheridan, J. T.; Gleeson, M. R.; Close, C. E.
Abstract:
In [1,2] an error (by a factor of 1000) in the diffusion rate of monomer in a photopolymer material used by the authors of [3], is presented. In [3] no errors are identified in our analysis and our physical evidence is not addressed. It is implied that our model and our results are disproven by the results in the papers referenced in [3]. In fact these papers do not provide any such quantitative evidence. The observations made regarding the significance of the authors’ contributions, in particular the validity of their model and the practical importance of their material are also discussed. © 2012 Optical Society of America
http://mural.maynoothuniversity.ie/3585/
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Monomer diffusion rates in photopolymer material. Part I. Low spatial frequency holographic gratings
(2011)
Close, C.E.; Gleeson, M. R. ; Sheridan, J. T.
Monomer diffusion rates in photopolymer material. Part I. Low spatial frequency holographic gratings
(2011)
Close, C.E.; Gleeson, M. R. ; Sheridan, J. T.
Abstract:
For photopolymers, knowing the rate of diffusion of the active monomer is important when modeling the material evolution during recording in order to understand and optimize their performance. Unfortunately, a confusingly wide range of values have been reported in the literature. Re-examining these results, experiments are carried out for both coverplated (sealed) and uncoverplated material layers and the measurements are analyzed using appropriate models. In this way, a more detailed analysis of the diffraction processes taking place for large-period gratings is provided. These results, combined with those in Part II, provide unambiguous evidence that the monomer diffusion rate in a commonly used acrylamide polyvinyl alcohol-based material is of the order of 10−10 cm2=s. This value closely agrees with the predictions of the nonlocal polymerization-driven diffusion model.
http://mural.maynoothuniversity.ie/3590/
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Monomer diffusion rates in photopolymer material. Part II. High-frequency gratings and bulk diffusion
(2011)
Close, C. E. ; Gleeson, M. R.; Mooney, D.; Sheridan, J. T.
Monomer diffusion rates in photopolymer material. Part II. High-frequency gratings and bulk diffusion
(2011)
Close, C. E. ; Gleeson, M. R.; Mooney, D.; Sheridan, J. T.
Abstract:
Photosensitive polymers are of practical importance, and mass transport within such materials plays a critical role in their behavior. Building on the work in Part I [J. Opt. Soc. Am. B doc. ID 136413 (posted 5 January 2011, in press)], the diffusion constants of a number of materials (i.e., acrylamide, polyacrylamide, water, propanol, and acetone) within a photosensitive layer are measured. A combination of optical and physical chemistry techniques is applied under different conditions. Determining the rates of diffusion is beneficial as it: (i) indicates material stability over time and (ii) supports material characterization, modeling, and performance optimization.
http://mural.maynoothuniversity.ie/3591/
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Non-local spatial frequency response of photopolymer materials containing chain transfer agents: I. Theoretical modelling
(2011)
Guo, J; Gleeson, M. R.; Sheridan, J. T.
Non-local spatial frequency response of photopolymer materials containing chain transfer agents: I. Theoretical modelling
(2011)
Guo, J; Gleeson, M. R.; Sheridan, J. T.
Abstract:
The non-local photopolymerization driven diffusion (NPDD) model predicts that a reduction in the non-local response length within a photopolymer material will improve its high spatial frequency response. The introduction of a chain transfer agent reduces the average molecular weight of polymer chains formed during free radical polymerization. Therefore a chain transfer agent (CTA) provides a practical method to reduce the non-local response length. An extended NPDD model is presented, which includes the chain transfer reaction and most major photochemical processes. The addition of a chain transfer agent into an acrylamide/polyvinyl alcohol photopolymer material is simulated and the predictions of the model are examined. The predictions of the model are experimentally examined in part II of this paper.
http://mural.maynoothuniversity.ie/3586/
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Non-local spatial frequency response of photopolymer materials containing chain transfer agents: II. Experimental results
(2011)
Guo, J.; Gleeson, M. R.; Sheridan, J. T.
Non-local spatial frequency response of photopolymer materials containing chain transfer agents: II. Experimental results
(2011)
Guo, J.; Gleeson, M. R.; Sheridan, J. T.
Abstract:
In part I of this paper the non-local photo-polymerization driven diffusion model was extended to include the kinetics of chain transfer and re-initiation, in order to analyse the effects of chain transfer agents on the system kinetics and to study their use in reducing the average polymer chain length in free-radical based photopolymer materials. Based on these results, it is proposed that one possible way to improve the material response at high spatial frequency is the addition of chain transfer agents. In this paper, the validity of the proposed model is examined by applying it to fit experimental data for an acrylamide/polyvinyl alcohol (AA/PVA) layer containing two different types of chain transfer agent (CTA): sodium formate (HCOONa) and 1-mercapto-2-propanol (CH3CH(OH)CH2SH). The effects on decreasing the average polymer chain length formed, by the addition of chain transfer agent, which in turn reduces the non-local response of the material, are demonstrated. These reductio...
http://mural.maynoothuniversity.ie/3587/
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NPDD model: A tool for photopolymer enhancement
(2011)
Gleeson, M. R.; Guo, J.; Sheridan, J. T.
NPDD model: A tool for photopolymer enhancement
(2011)
Gleeson, M. R.; Guo, J.; Sheridan, J. T.
Abstract:
The use of theoretical models to represent the photochemical effects present during the formation of spatially and temporally varying index structures in photopolymers, is critical in order to maximise a material’s potential. One such model is the Non-local Photo-Polymerization Driven Diffusion (NPDD) model. Upon application of appropriate physical constraints for a given photopolymer material, this model can accurately quantify all major photochemical processes. These include i) non-steady state kinetics, (ii) non-linearity iii) spatially non-local polymer chain growth, iv) time varying primary radical production, v) diffusion controlled effects, vi) multiple termination mechanisms, vii) inhibition, (viii) polymer diffusion and ix) post-exposure effects. In this paper, we examine a number of predictions made by the NPDD model. The model is then applied to an acrylamide/polyvinylalcohol based photopolymer under various recording conditions. The experimentally obtained results are th...
http://mural.maynoothuniversity.ie/3588/
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Numerical analysis of systematic errors in an optical encryption system
(2006)
Monaghan, David S.; Gopinathan, Unnikrishnan; Kelly, Damien P.; Naughton, Thomas J.; Sh...
Numerical analysis of systematic errors in an optical encryption system
(2006)
Monaghan, David S.; Gopinathan, Unnikrishnan; Kelly, Damien P.; Naughton, Thomas J.; Sheridan, J. T.
Abstract:
We consider a double random phase encoding Encryption/Decryption system in which the image encryption/decryption process is performed numerically. In this paper we look at the effect of quantisation in the decryption process due to the discrete values which a spatial light modulator can display. We look at the characterisation of a transmissive spatial light modulator and we present results from simulations of the system.
http://mural.maynoothuniversity.ie/8423/
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Optical encryption and the space bandwidth product
(2005)
Hennelly, Bryan M.; Sheridan, J. T.
Optical encryption and the space bandwidth product
(2005)
Hennelly, Bryan M.; Sheridan, J. T.
Abstract:
The use of optical signal processing in the field of image encryption typically involves the use of optical transforms representing quadratic phase systems (QPS), to implement the optical Fourier transform (OFT), the optical fractional Fourier transform (OFRT), the Fresnel transform (FST) and the general linear Canonical transform (LCT). Random phase keys (RPK) or random shifting stages are applied after transformation and the process is repeated for deeper encryption. Each such stage may change the spatial extent of the complex distribution and may also change the spatial frequency bandwidth of the signal. Therefore, the space bandwidth product (SBP), which is equal to the number of discrete samples that are required to fully represent the encrypted signal (and are required to recover the original), will also change. In general the encrypted field is complex and recording must be carried out using a holographic material or using digital holographic methods. We show how the matrices...
http://mural.maynoothuniversity.ie/5810/
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Optimisation of photopolymers for holographic applications using the Non-local Photopolymerization Driven Diffusion model
(2011)
Gleeson, M. R. ; Guo, J.; Sheridan, J. T.
Optimisation of photopolymers for holographic applications using the Non-local Photopolymerization Driven Diffusion model
(2011)
Gleeson, M. R. ; Guo, J.; Sheridan, J. T.
Abstract:
An understanding of the photochemical and photo-physical processes, which occur during photopolymerization is of extreme importance when attempting to improve a photopolymer material’s performance for a given application. Recent work carried out on the modelling of the mechanisms which occur in photopolymers during- and post-exposure, has led to the development of a tool, which can be used to predict the behaviour of these materials under a wide range of conditions. In this paper, we explore this Non-local Photo-polymerisation Driven Diffusion model, illustrating some of the useful trends, which the model predicts and we analyse their implications on the improvement of photopolymer material performance.
http://mural.maynoothuniversity.ie/3583/
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Paraxial speckle-based metrology systems with an aperture
(2006)
Kelly, Damien P.; Ward, Jennifer E.; Hennelly, Bryan M.; Gopinathan, Unnikrishnan; O...
Paraxial speckle-based metrology systems with an aperture
(2006)
Kelly, Damien P.; Ward, Jennifer E.; Hennelly, Bryan M.; Gopinathan, Unnikrishnan; O'Neill, Feidhlim T.; Sheridan, J. T.
Abstract:
Digital speckle photography can be used in the analysis of surface motion in combination with an optical linear canonical transform (LCT). Previously [ D. P. Kelly et al.Appl. Opt. 44, 2720 (2005) ] it has been shown that optical fractional Fourier transforms (OFRTs) can be used to vary the range and sensitivity of speckle-based metrology systems, allowing the measurement of both the magnitude and direction of tilting (rotation) and translation motion simultaneously, provided that the motion is captured in two separate OFRT domains. This requires two bulk optical systems. We extend the OFRT analysis to more general LCT systems with a single limiting aperture. The effect of a limiting aperture in LCT systems is examined in more detail by deriving a generalized Yamaguchi correlation factor. We demonstrate the benefits of using an LCT approach to metrology design. Using this technique, we show that by varying the curvature of the illuminating field, we can effectively change the output...
http://mural.maynoothuniversity.ie/8720/
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Photoinitiation study of Irgacure 784 in an epoxy resin photopolymer
(2010)
Sabol, D; Gleeson, M. R.; Liu, S.; Sheridan, J. T.
Photoinitiation study of Irgacure 784 in an epoxy resin photopolymer
(2010)
Sabol, D; Gleeson, M. R.; Liu, S.; Sheridan, J. T.
Abstract:
A deeper understanding of the processes, which occur during free radical photopolymerization, is necessary in order to develop a fully comprehensive model, which represents their behavior during exposure. One of these processes is photoinitiation, whereby a photon is absorbed by a photosensitizer producing free radicals, which can initiate polymerization. These free radicals can also participate in polymer chain termination primary termination, and it is therefore necessary to understand their generation in order to predict the temporally varying kinetic effects present during holographic grating formation. In this paper, a study of the photoinitiation mechanisms of Irgacure 784 photosensitizer, in an epoxy resin matrix, is presented. We report our experimental results and present a theoretical model to predict the physically observed behavior.
http://mural.maynoothuniversity.ie/3599/
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Photopolymers for use as Holographic Media
(2012)
Gleeson, M. R.; Guo, J.; Sheridan, J. T.
Photopolymers for use as Holographic Media
(2012)
Gleeson, M. R.; Guo, J.; Sheridan, J. T.
Abstract:
In this chapter we briefly review some of the developments which have been made in the area of photopolymer material development for applications such as holography. We also examine some of the extensions which have been made to the theoretical models of the processes which occur during free radical photo-polymerisation, namely the Non-local Photo-polymerisation Driven Diffusion (NPDD) model. An understanding of these photochemical and photo-physical processes is of extreme importance when attempting to improve a photopolymer material’s performance for a given application. We will explore the most recent developments made and illustrate some of the useful trends, which this NPDD model predicts and then analyze their implications on photopolymer improvement. Among the effects examined are; the temporal and spatial primary radical generation (the driving force of photo-polymerisation), multiple termination mechanisms, non-local polymer chain growth, the effects of oxygen inhibitio...
http://mural.maynoothuniversity.ie/3600/
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Quantifying the 2.5D imaging performance of digital holographic systems
(2011)
Kelly, D.P.; Healy, J.J.; Hennelly, Bryan M.; Sheridan, J. T.
Quantifying the 2.5D imaging performance of digital holographic systems
(2011)
Kelly, D.P.; Healy, J.J.; Hennelly, Bryan M.; Sheridan, J. T.
Abstract:
Digital holographic systems are a class of two step, opto-numerical, three-dimensional imaging techniques. The role of the digital camera in limiting the resolution and field of view of the reconstructed image, and the interaction of these limits with a general optical system is poorly understood. The linear canonical transform describes any optical system consisting of lenses and/or free space in a unified manner. Expressions derived using it are parametrised in terms of the parameters of the optical system, as well as those of the digital camera: aperture size, pixel size and pixel pitch. We develop rules of thumb for selecting an optical system to minimise mean squared error for given input and digital camera parameters. In the limit, our results constitute a point spread function analysis. The results presented in this paper will allow digital holography practitioners to select an optical system to maximise the quality of their reconstructed image using a priori knowledge of the c...
http://mural.maynoothuniversity.ie/5786/
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Recent developments in the NPDD model
(2012)
Gleeson, M. R. ; Guo, J.; Sheridan, J. T.
Recent developments in the NPDD model
(2012)
Gleeson, M. R. ; Guo, J.; Sheridan, J. T.
Abstract:
An understanding of the photochemical and photo-physical processes, which occur during photo-polymerization, is of extreme importance when attempting to improve a photopolymer material’s performance for a given application. Recent work carried out on the modeling of photopolymers during- and post-exposure, has led to the development of a tool, which can be used to predict the behavior of a number of photopolymers subject to a range of physical conditions. In this paper, we explore the most recent developments made to the Non-local Photo-polymerization Driven Diffusion model, and illustrate some of the useful trends, which the model predicts and then analyze their implications on photopolymer improvement.
http://mural.maynoothuniversity.ie/3592/
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Spread-space spread-spectrum technique for secure multiplexing
(2007)
Hennelly, Bryan M.; Naughton, Thomas J.; McDonald, John; Sheridan, J. T.; Unnikrishnan,...
Spread-space spread-spectrum technique for secure multiplexing
(2007)
Hennelly, Bryan M.; Naughton, Thomas J.; McDonald, John; Sheridan, J. T.; Unnikrishnan, G.; Kelly, D.P.; Javidi, B.
Abstract:
A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals’ energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.
http://mural.maynoothuniversity.ie/5791/
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Strengths and weaknesses of optical encryption algorithms
(2005)
Gopinathan, Unnikrishnan; Monaghan, David S.; Naughton, Thomas J.; Sheridan, J. T.; Jav...
Strengths and weaknesses of optical encryption algorithms
(2005)
Gopinathan, Unnikrishnan; Monaghan, David S.; Naughton, Thomas J.; Sheridan, J. T.; Javidi, Bahram
Abstract:
Though many optical encryption techniques have been proposed in the recent years, a systematic analysis of the strengths and weaknesses of these algorithms has not been undertaken yet. This work attempts to systematically study the strengths of some of the well known image encryption algorithms.
http://mural.maynoothuniversity.ie/572/
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Theoretical and Experimental Analysis of Chain Transfer Agents Behaviors in Photopolymer Material
(2011)
Guo, J. ; Gleeson, M. R. ; Sheridan, J. T.
Theoretical and Experimental Analysis of Chain Transfer Agents Behaviors in Photopolymer Material
(2011)
Guo, J. ; Gleeson, M. R. ; Sheridan, J. T.
Abstract:
The Non-local Photo-Polymerization Driven Diffusion (NPDD) model indicates how a material’s performance might be improved, and provides a tool for quantitive comparison of different material compositions and to predict their fundamental limits. In order to reduce the non-locality of polymer chain growth (i.e the non-local response parameter, σ) and to improve the spatial frequency response of a photopolymer material, we introduce the chain transfer agent (CTA). In the literature, extensive studies have been carried out on the improvements of the non-local response modifying by the CTA, sodium formate, in the polyvinyl alcohol-acrylamide (PVA/AA) material. In this article, i) based on the chemical reactions of CTA, we extended the CTA model in the literature; ii) we compare two different CTA materials, sodium formate and 1-mercapto-2-propanol without cross-linker in order to obtain the experimental confirmation of the reduction in the average polymer molecular weight is provided usin...
http://mural.maynoothuniversity.ie/3589/
Displaying Results 1 - 23 of 23 on page 1 of 1
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