Unnikrishnan Gopinathan

Universität Stuttgart, Stuttgart, Baden-Wuerttemberg, Germany

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Publications (42)36.77 Total impact

  • U. Gopinathan, G. Pedrini, B. Javidi, W. Osten
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    ABSTRACT: We demonstrate imaging of microscopic objects using a lensless in-line 3D digital holographic microscope at vacuum UV wavelength of 193 nm. The absence of microscope objective and a separate reference beam simplifies the setup and reduces the number of UV components required. We also discuss the space-bandwidth considerations in the design of such a system. Experimental results using human blood cells and latex beads are presented.
    Journal of Display Technology 11/2010; · 1.66 Impact Factor
  • G. Pedrini, D. Hopp, U. Gopinathan, W. Osten
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    ABSTRACT: We present different systems based on digital holography suitable for the investigation of microscopic objects. A deep UV laser source (193 nm) and high numerical aperture geometry have been used to increase the resolution of the reconstructed amplitude and phase. Furthermore we show how digital holographic interferometry can be used to measure the deformation of microsystems down to the nanometer scale.
    Proc SPIE 10/2009;
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    ABSTRACT: The amplitude-encoding case of the double random phase encoding technique is examined by defining a cost function as a metric to compare an attempted decryption against the corresponding original input image. For the case when a cipher-text pair has been obtained and the correct decryption key is unknown, an iterative attack technique can be employed to ascertain the key. During such an attack the noise in the output field for an attempted decryption can be used as a measure of a possible decryption key's correctness. For relatively small systems, i.e., systems involving fewer than 5x5 pixels, the output decryption of every possible key can be examined to evaluate the distribution of the keys in key space in relation to their relative performance when carrying out decryption. However, in order to do this for large systems, checking every single key is currently impractical. One metric used to quantify the correctness of a decryption key is the normalized root mean squared (NRMS) error. The NRMS is a measure of the cumulative intensity difference between the input and decrypted images. We identify a core term in the NRMS, which we refer to as the difference parameter, d. Expressions for the expected value (or mean) and variance of d are derived in terms of the mean and variance of the output field noise, which is shown to be circular Gaussian. These expressions assume a large sample set (number of pixels and keys). We show that as we increase the number of samples used, the decryption error obeys the statistically predicted characteristic values. Finally, we corroborate previously reported simulations in the literature by using the statistically derived expressions.
    Journal of the Optical Society of America A 10/2009; 26(9):2033-42. · 1.67 Impact Factor
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    ABSTRACT: Ocular microtremor (OMT) is a physiological high frequency (up to 150Hz) low amplitude (150-2500nm) involuntary tremor of the human eye. It is one of the three fixational ocular motions described by Adler and Fliegelman in 1934 as well as microsaccades and drift. Clinical OMT investigations to date have used eye-contacting piezoelectric probes or piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetised. In some cases, this induces eyelid spasms (blepharospasm) making it impossible to measure OMT. Using the contact probe method, the eye motion is mechanically damped. In addition to this, it is not possible to obtain exact information about the displacement. Results from clinical studies to date have given electrical signal amplitudes from the probe. Recent studies suggest a number of clinical applications for OMT, these include monitoring the depth of anaesthesia of a patient in surgery, prediction of outcome in coma, diagnosis of brainstem death. In addition to this, abnormal OMT frequency content is present in patients with neurological disorders such as Multiple sclerosis and Parkinson's disease. However for ongoing clinical investigations the contact probe method falls short of a non-contact accurate measurement solution. In this paper, we design a compact non contact phase modulating optical fiber speckle interferometer to measure eye motions. We present our calibration results using a calibrated piezoelectric vibration simulator. Digital signal processing is then performed to extract the low amplitude high frequency displacement information.
    Proc SPIE 08/2009;
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    ABSTRACT: An optical implementation of the amplitude encoded double random phase encryption/decryption technique is implemented, and both numerical and experimental results are presented. In particular, we examine the effect of quantization in the decryption process due to the discrete values and quantized levels, which a spatial light modulator SLM can physically display. To do this, we characterize a transmissive SLM using Jones matrices and then map a complex image to the physi-cally achievable levels of the SLM using the pseudorandom encoding technique. We present both numerical and experimental results that quantify the performance of the system.
    Optical Engineering 01/2009; 48. · 0.88 Impact Factor
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    ABSTRACT: a b s t r a c t A phase encoded image is encrypted using the double random phase encoding technique, (DRPE). The effects of using a variable dynamic range of phase distribution during phase encoding (pre-encryption) are examined. We begin by phase encoding the input image using the full phase range, from Àp to p. We perform numerically perfect encryption and we then introduce errors into the decrypting phase-keys in the form of a pseudo-random distribution (position and phase) of incorrect pixels values. By quantify-ing the resulting error in the attempted decryptions, for increasing amounts of error in the decrypting phase-keys, we examine the effects of reducing the phase range to, +/À (p À D). In this way we attempt to improve the phase encoding procedure for use with the DRPE technique. When the pixel values calcu-lated, during an attempted decryption, fall outside the phase range used to phase encode the assigned input image, we examine different methods of redistributing the value of that pixel to an assigned value within the allowed phase range. The effects of the phase quantisation used in the keys are also examined.
    Optics Communications - OPT COMMUN. 01/2009; 282(4).
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    ABSTRACT: Ocular microtremor (OMT) is a physiological high-frequency (up to 150 Hz) low-amplitude (25-2500 nm peak-to-peak) involuntary motion of the human eye. Recent studies suggest a number of clinical applications for OMT that include monitoring the depth of anesthesia of a patient in surgery, prediction of outcome in coma, and diagnosis of brain stem death. Clinical OMT investigations to date have used mechanical piezoelectric probes or piezoelectric strain gauges that have many drawbacks which arise from the fact that the probe is in contact with the eye. We describe the design of a compact noncontact sensing device to measure OMT that addresses some of the above drawbacks. We evaluate the system performance using a calibrated piezoelectric vibrator that simulates OMT signals under conditions that can occur in practice, i.e., wet eye conditions. We also test the device at low light levels well within the eye safety range.
    Journal of Biomedical Optics 01/2009; 14(1):014021. · 2.88 Impact Factor
  • Unnikrishnan Gopinathan, Giancarlo Pedrini, Wolfgang Osten
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    ABSTRACT: We analyze the effects of partial coherence in the image formation of a digital in-line holographic microscope (DIHM). The impulse response is described as a function of cross-spectral density of the light used in the space-frequency domain. Numerical simulation based on the applied model shows that a reduction in coherence of light leads to broadening of the impulse response. This is also validated by results from experiments wherein a DIHM is used to image latex beads using light with different spatial and temporal coherence.
    Journal of the Optical Society of America A 11/2008; 25(10):2459-66. · 1.67 Impact Factor
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    ABSTRACT: A compact optical system has been developed capable of measuring minute movements of the eye. Eye movement is simulated through the application an electrical signal to a piezoelectric material which acts as the eye’s surface.
    10/2008;
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    ABSTRACT: We perform a numerical analysis of the double random phase encryption-decryption technique to determine how, in the case of both amplitude and phase encoding, the two decryption keys (the image- and Fourier-plane keys) affect the output gray-scale image when they are in error. We perform perfect encryption and imperfect decryption. We introduce errors into the decrypting keys that correspond to the use of random distributions of incorrect pixel values. We quantify the effects that increasing amounts of error in the image-plane key, the Fourier-plane key, and both keys simultaneously have on the decrypted image. Quantization effects are also examined.
    Applied Optics 07/2008; 47(21):3808-16. · 1.69 Impact Factor
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    ABSTRACT: We discuss a projection system for real world three-dimensional objects using spatial light modulators (SLM). An algorithm to encode the digital holograms of real world objects on to an SLM is presented. We present results from experiments to project holograms of real world holograms using a nematic liquid crystal SLM. We discuss the case when the pixel sizes of the charge-coupled device (CCD) and SLM used for recording the hologram and projection are different.
    Journal of Display Technology 07/2008; 4(2):254-261. · 1.66 Impact Factor
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    ABSTRACT: We discuss a projection system for real world three-dimensional objects using spatial light modulators (SLM). An algorithm to encode the digital holograms of real world objects on to an SLM is presented. We present results from experiments to project holograms of real world holograms using a nematic liquid crystal SLM. We discuss the case when the pixel sizes of the charge-coupled device (CCD) and SLM used for recording the hologram and projection are different.
    Journal of Display Technology 06/2008; 4(2). · 1.66 Impact Factor
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    ABSTRACT: Contact techniques exist to measure low amplitude low frequency mechanical vibration, however, by mechanically loading the system of interest, they affect the measured results. In this paper, we design a compact non-contact optical fiber speckle interferometer to measure inplane displacements. We implement this under laboratory conditions, and present our calibration results, measuring low-amplitude microvibrations from 0.34 nm to 1.5 mum over a frequency range from 10 Hz to 150 Hz.
    Proc SPIE 05/2008;
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    ABSTRACT: Ocular microtremor (OMT) is a biological high frequency (up to 150Hz) low amplitude (25-2500nm peak to peak) involuntary motion of the human eye. Clinical OMT investigations to date have used eye-contacting mechanical piezoelectric probes or piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetized. In some cases, this eyelid spasms occur making it impossible to measure OMT. Using the contact probe method, the eye motion is mechanically loaded. Results from clinical studies with this method to date have given electrical signal amplitudes from the probe proportional to the displacement, but not the exact displacement information. Recent studies suggest a number of clinical applications for OMT, these include monitoring the depth of anesthesia of a patient in surgery, prediction of outcome in coma, diagnosis of brain stem death. In addition to this, in patients with neurological disorders such as Multiple Sclerosis and Parkinson's disease, abnormal OMT frequency content is present. In this paper, we design a compact non-contact phase modulating optical fiber speckle interferometer to measure eye motions. We simulate OMT motion using a calibrated piezoelectric vibration simulator and compare results produced using a contact method with those using our optical non-contact method.
    Proc SPIE 05/2008;
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    Guohai Situ, James P Ryle, Unnikrishnan Gopinathan, John T Sheridan
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    ABSTRACT: In-line digital holography based on two-intensity measurements [Zhang Opt. Lett. 29, 1787 (2004)], is modified by introducing a pi shifting in the reference phase. Such an improvement avoids the assumption that the object beam must be much weaker than the reference beam in strength and results in a simplified experimental implementation. Computer simulations and optical experiments are carried out to validate the method, which we refer to as position-phase-shifting digital holography.
    Applied Optics 03/2008; 47(5):711-7. · 1.69 Impact Factor
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    Unnikrishnan Gopinathan, Guohai Situ, Thomas J Naughton, John T Sheridan
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    ABSTRACT: The signal extraction method based on intensity measurements in two close fractional Fourier domains is examined by using the phase space formalism. The fractional order separation has a lower bound and an upper bound that depend on the signal at hand and the noise in the optical system used for measurement. On the basis of a theoretical analysis, it is shown that for a given optical system a judicious choice of fractional order separation requires some a priori knowledge of the signal bandwidth. We also present some experimental results in support of the analysis.
    Journal of the Optical Society of America A 02/2008; 25(1):108-15. · 1.67 Impact Factor
  • Damien P Kelly, Jennifer E Ward, Unnikrishnan Gopinathan, John T Sheridan
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    ABSTRACT: The correlation properties of speckle fields are studied for general paraxial systems. The previous studies on lateral and longitudinal speckle size for the case of free-space propagation (Fresnel transform) are generalized to the case of the linear canonical transform. These results have implications for the control of speckle size, through appropriate design of optical systems, with particular relevance for speckle interferometry.
    Optics Letters 01/2008; 32(23):3394-6. · 3.39 Impact Factor
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    ABSTRACT: We examine the Amplitude-Encoding (AE) case of the Double Random Phase Encoding (DRPE) technique. A cost function is the function we use to evaluate an attempted decryption with our original input image. For systems with a relatively small key-space we can evaluate the output of every key to get an overall idea of the spread of these keys in key-space. However for larger systems this is not practical. Based on a normalised root mean squared cost function we wish to identify expressions for the mean and variance of the output (decrypted) intensity for a sample set of keys in a large system (256x256 pixels).
    Proc SPIE 01/2008;
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    ABSTRACT: We perform a numerical analysis on the double random phase encryption/decryption technique. The key-space of an encryption technique is the set of possible keys that can be used to encode data using that technique. In the case of a strong encryption scheme, many keys must be tried in any brute-force attack on that technique. Traditionally, designers of optical image encryption systems demonstrate only how a small number of arbitrary keys cannot decrypt a chosen encrypted image in their system. However, this type of demonstration does not discuss the properties of the key-space nor refute the feasibility of an efficient brute-force attack. To clarify these issues we present a key-space analysis of the technique. For a range of problem instances we plot the distribution of decryption errors in the key-space indicating the lack of feasibility of a simple brute-force attack.
    Applied Optics 10/2007; 46(26):6641-7. · 1.69 Impact Factor
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    ABSTRACT: The main advantage of the double random phase encryption technique is its physical implementation however to allow us to analyse its behaviour we perform the encryption/decryption numerically. A typically strong encryption scheme will have an extremely large key-space, which will make the probable success of any brute force attack on that algorithm miniscule. Traditionally, designers of optical image encryption systems only demonstrate how a small number of arbitrary keys cannot decrypt a chosen encrypted image in their system. We analyse this algorithm from a key-space perspective. The key-space of an encryption algorithm can be defined as the set of possible keys that can be used to encode data using that algorithm. For a range of problem instances we plot the distribution of decryption errors in the key-space indicating the lack of feasibility of a simple brute force attack.
    Proc SPIE 09/2007;

Publication Stats

264 Citations
36.77 Total Impact Points

Institutions

  • 2010
    • Universität Stuttgart
      • Institute of Laser Technologies
      Stuttgart, Baden-Wuerttemberg, Germany
  • 2005–2009
    • University College Dublin
      • School of Electrical, Electronic and Mechanical Engineering
      Dublin, Leinster, Ireland
    • University College Cork
      • Department of Electrical and Electronic Engineering
      Cork, M, Ireland
  • 2008
    • Vienna University of Technology
      Wien, Vienna, Austria
  • 2005–2006
    • Indian Institute of Technology Delhi
      • Department of Physics
      New Delhi, NCT, India
    • National University of Ireland, Maynooth
      • Department of Computer Sciences
      Maynooth, L, Ireland