Dr Christopher Solomon

Reader in Physics
Head of the Forensics Imaging Group


Dr Christopher Solomon gained a BSc in theoretical physics from the University of Durham and went on to complete a PhD in medical imaging at the Institute of Cancer Research/Royal Marsden Hospital in London in 1989. 

After spending just over four years as a post-doc in Professor Chris Dainty's astronomical imaging group at Imperial College London, Christopher was appointed Lecturer at the University of Kent in 1994. He was promoted to Senior Lecturer in 2002 and to Reader in 2005.  

Research interests

Dr Solomon's main research activities focus on image processing and evolutionary methods with particular interest in the human face:

  • image processing and reconstruction
  • facial encoding
  • facial synthesis
  • forensic image analysis.


Christopher teaches on Maxwell's equations and Fourier optics, image processing, medical physics and forensic physical methods.


Christopher directs VisionMetric Ltd, a spin-out company that is the UK's leading developer and supplier of facial composite software to the police. 


Showing 50 of 67 total publications in the Kent Academic Repository. View all publications.


  • Alsufyani, A., Hajilou, O., Zoumpoulaki, A., Filetti, M., Solomon, C., Gibson, S., Alroobaea, R. and Bowman, H. (2019). Breakthrough Percepts of Famous Faces. Psychophysiology [Online] 56:e13279. Available at: https://doi.org/10.1111/psyp.13279.
    Recently, we showed that presenting salient names (i.e. a participant’s first name) on
    the fringe of awareness (in Rapid Serial Visual Presentation) breaks through into
    awareness, resulting in the generation of a P3, which (if concealed information is
    presented) could be used to differentiate between deceivers and non-deceivers
    (Bowman et al., 2013; Bowman, Filetti, Alsufyani, Janssen, & Su, 2014). The aim of
    the present study was to explore whether face stimuli can be used in an ERP-based
    RSVP paradigm to infer recognition of broadly familiar faces. To do this, we explored
    whether famous faces differentially break into awareness when presented in RSVP and,
    importantly, whether ERPs can be used to detect these ‘breakthrough’ events on an
    individual basis. Our findings provide evidence that famous faces are differentially
    perceived and processed by participants’ brains as compared to novel (or unfamiliar)
    faces. EEG data revealed large differences in brain responses between these conditions.
  • Liu, J., Osadchy, M., Ashton, L., Foster, M., Solomon, C. and Gibson, S. (2017). Deep Convolutional Neural Networks for Raman Spectrum Recognition: A Unified Solution. Analyst [Online] 142:4067-4074. Available at: http://dx.doi.org/10.1039/C7AN01371J.
    Machine learning methods have found many applications in Raman spectroscopy, especially for the identification of chemical species. However, almost all of these methods require non-trivial preprocessing such as baseline correction and/or PCA as an essential step. Here we describe our unified solution for the identification of chemical species in which a convolutional neural network is trained to automatically identify substances according to their Raman spectrum without the need for preprocessing. We evaluated our approach using the RRUFF spectral database, comprising mineral sample data. Superior classification performance is demonstrated compared with other frequently used machine learning algorithms including the popular support vector machine method.
  • Davis, J., Thorniley, S., Gibson, S. and Solomon, C. (2016). Holistic facial composite construction and subsequent lineup identification accuracy: Comparing adults and children. Journal of Psychology [Online] 150:102-118. Available at: http://dx.doi.org/10.1080/00223980.2015.1009867.
    When the police have no suspect, they may ask an eyewitness to construct a facial composite of that suspect from memory. Faces are primarily processed holistically, and recently developed computerized holistic facial composite systems (e.g., EFIT-V) have been designed to match these processes. The reported research compared children aged 6–11 years with adults on their ability to construct a recognizable EFIT-V composite. Adult constructor's EFIT-Vs received significantly higher composite-suspect likeness ratings from assessors than children's, although there were some notable exceptions. In comparison to adults, the child constructors also overestimated the composite-suspect likeness of their own EFIT-Vs. In a second phase, there were no differences between adult controls and constructors in correct identification rates from video lineups. However, correct suspect identification rates by child constructors were lower than those of child controls, suggesting that a child's memory for the suspect can be adversely influenced by composite construction. Nevertheless, all child constructors coped with the demands of the EFIT-V system, and the implications for research, theory, and the criminal justice system practice are discussed.
  • Davis, J., Maigut, A., Jolliffe, D., Gibson, S. and Solomon, C. (2015). Holistic facial composite creation and subsequent video line-up eyewitness identification paradigm. Journal of Visualized Experiments [Online] 106. Available at: http://dx.doi.org/10.3791/53298.
    The paradigm detailed in this manuscript describes an applied experimental method based on real police investigations during which an eyewitness or victim to a crime may create from memory a holistic facial composite of the culprit with the assistance of a police operator. The aim is that the composite is recognized by someone who believes that they know the culprit. For this paradigm, participants view a culprit actor on video and following a delay, participant-witnesses construct a holistic system facial composite. Controls do not construct a composite. From a series of arrays of computer-generated, but realistic faces, the holistic system construction method primarily requires participant-witnesses to select the facial images most closely meeting their memory of the culprit. Variation between faces in successive arrays is reduced until ideally the final image possesses a close likeness to the culprit. Participant-witness directed tools can also alter facial features, configurations between features and holistic properties (e.g., age, distinctiveness, skin tone), all within a whole face context. The procedure is designed to closely match the holistic manner by which humans’ process faces. On completion, based on their memory of the culprit, ratings of composite-culprit similarity are collected from the participant-witnesses. Similar ratings are collected from culprit-acquaintance assessors, as a marker of composite recognition likelihood. Following a further delay, all participants — including the controls — attempt to identify the culprit in either a culprit-present or culprit-absent video line-up, to replicate circumstances in which the police have located the correct culprit, or an innocent suspect. Data of control and participant-witness line-up outcomes are presented, demonstrating the positive influence of holistic composite construction on identification accuracy. Correlational analyses are conducted to measure the relationship between assessor and participant-witness composite-culprit similarity ratings, delay, identification accuracy, and confidence to examine which factors influence video line-up outcomes.
  • Davis, J., Simmons, S., Sulley, L., Solomon, C. and Gibson, S. (2015). An evaluation of post-production facial composite enhancement techniques. Journal of Forensic Practice [Online] 17:307-318. Available at: http://www.emeraldinsight.com/journal/jfp.
    Purpose – The purpose of this paper is to describe four experiments evaluating post-production enhancement techniques with facial composites mainly created using the EFIT-V holistic system. Design/methodology/approach – Experiments 1–4 were conducted in two stages. In Stage 1, constructors created between one and four individual composites of unfamiliar targets. These were merged to create morphs. Additionally in Experiment 3, composites were vertically stretched. In Stage 2, participants familiar with the targets named or provided target-similarity ratings to the images. Findings – In Experiments 1–3, correct naming rates were significantly higher to between-witness 4-morphs, within-witness 4-morphs and vertically stretched composites than to individual composites. In Experiment 4, there was a positive relationship between composite-target similarity ratings and between-witness morph-size (2–, 4–, 8–, 16-morphs). Practical implications – The likelihood of a facial composite being recognised can be improved by morphing and vertical stretch. Originality/value – This paper improves knowledge of the theoretical underpinnings of these facial composite post-production enhancement techniques. This should encourage acceptance by the criminal justice system, and lead to better detection outcomes
  • Mist, J., Gibson, S. and Solomon, C. (2015). Comparing Evolutionary Operators, Search Spaces, and Evolutionary Algorithms in the Construction of Facial Composites. Informatica [Online]:135-145. Available at: http://www.informatica.si/index.php/informatica.
    Facial composite construction is one of the most successful applications of interactive evolutionary computation.
    In spite of this, previous work in the area of composite construction has not investigated the
    algorithm design options in detail. We address this issue with four experiments. In the first experiment a
    sorting task is used to identify the 12 most salient dimensions of a 30-dimensional search space. In the second
    experiment the performances of two mutation and two recombination operators for interactive genetic
    algorithms are compared. In the third experiment three search spaces are compared: a 30-dimensional
    search space, a mathematically reduced 12-dimensional search space, and a 12-dimensional search space
    formed from the 12 most salient dimensions. Finally, we compare the performances of an interactive
    genetic algorithm to interactive differential evolution. Our results show that the facial composite construction
    process is remarkably robust to the choice of evolutionary operator(s), the dimensionality of the search
    space, and the choice of interactive evolutionary algorithm. We attribute this to the imprecise nature of human
    face perception and differences between the participants in how they interact with the algorithms.

    Povzetek: Kompozitna gradnja obrazov je ena izmed najbolj uspešnih aplikacij interaktivnega evolucijskega
    ra?cunanja. Kljub temu pa do zdaj na podro?cju kompozitne gradnje niso bile podrobno raziskane
    možnosti snovanja algoritma. To vprašanje smo obravnavali s štirimi poskusi. V prvem je uporabljeno
    sortiranje za identifikacijo 12 najbolj izstopajo?cih dimenzij 30-dimenzionalnega preiskovalnega prostora.
    V drugem primerjamo u?cinkovitost dveh mutacij in dveh rekombinacijskih operaterjev za interaktivni
    genetski algoritem. V tretjem primerjamo tri preiskovalne prostore: 30-dimenzionalni, matemati?cno reducirani
    12-dimenzionalni in 12-dimenzionalni prostor sestavljen iz 12 najpomembnejših dimenzij. Na
    koncu smo primerjali uspešnost interaktivnega genetskega algoritma z interaktivno diferencialno evolucijo.
    Rezultati kažejo, da je proces kompozitne gradnje obrazov izredno robusten glede na izbiro evolucijskega
    operatorja(-ev), dimenzionalnost preiskovalnega prostora in izbiro interaktivnega evolucijskega algoritma.
    To pripisujemo nenatan?cni naravi percepcije in razlikam med interakcijami uporabnikov z algoritmom.
  • Davis, J., Gibson, S. and Solomon, C. (2014). The positive influence of creating a holistic facial composite on video lineup identification. Applied Cognitive Psychology [Online] 28:634-639. Available at: http://dx.doi.org/10.1002/acp.3045.
  • Solomon, C., Gibson, S. and Mist, J. (2013). Interactive evolutionary generation of facial composites for locating suspects in criminal investigations. Applied Soft Computing [Online] 13:3298-3306. Available at: http://dx.doi.org/10.1016/j.asoc.2013.02.010.
    Statistical appearance models have previously been used for computer face recognition applications in which
    an image patch is synthesized and morphed to match a target face image using an automated iterative fitting
    algorithm. Here we describe an alternative use for appearance models, namely for producing facial composite
    images (sometimes referred to as E-FIT or PhotoFIT images). This application poses an interesting real-
    world optimization problem because the target face exists in the mind of the witness and not in a tangible
    form such as a digital image. To solve this problem we employ an interactive evolutionary algorithm that
    allows the witness to evolve a likeness to the target face. A system based on our approach, called EFIT-V,
    is used frequently by three quarters of UK police constabularies.
  • Valentine, T., Davis, J., Thorner, K., Solomon, C. and Gibson, S. (2010). Evolving and combining facial composites: Between-witness and within-witness morphs compared. Journal of Experimental Psychology: Applied [Online] 16:72-86. Available at: http://dx.doi.org/10.1037/a0018801.
  • Gibson, S., Scandrett, C., Solomon, C., Wilkinson, C. and Maylin, M. (2009). Computer Assisted Age Progression. Journal of Forensic Science, Medicine and Pathology [Online] 5:174-181. Available at: http://dx.doi.org/10.1007/s12024-009-9102-z.
    A computer assisted method for altering the perceived age of a human face is presented. Our technique is based on calculating a trajectory or axis within a multidimensional space that captures the changes in large scale facial structure, shading and complexion associated with aging. Fine facial details associated with increasing age, such as wrinkles, are added to the aged face using a variation on a standard image processing technique called high boost filtering. The method is successfully applied to two-dimensional photographic images exhibiting uncontrolled variations in pose and illumination. Unlike our previous work on automated age progression, here the objective is to allow a certain degree of manual control over the process by the adjustment of three key progression-control-parameters. In the future this work may form the basis for a software tool to be used by forensic artists
  • Gibson, S., Solomon, C., Maylin, M. and Clark, C. (2009). New methodology in facial composite construction: from theory to practice. International Journal of Electronic Security and Digital Forensics [Online] 2:156-168. Available at: http://dx.doi.org/10.1504/IJESDF.2009.024900.
    Existing commercial, computerised techniques for constructing facial composites generated from eyewitness memory are essentially electronic versions of the original, mechanical feature-based systems such as PhotoFIT and Identikit. The effectiveness of this feature-based approach is fundamentally limited by the witness's ability to recall and verbalise accurate descriptions of facial features from memory. Recent advances in facial composite methodology have led to software systems that do not rely on this process but instead exploit a cognitively less demanding process of recognition. We provide a technical overview of the EFIT-V system, currently being used by a number of police services in the UK.
  • Scandrett (nee Hill), C., Solomon, C. and Gibson, S. (2006). A Person-Specific, Rigorous Aging Model of the Human Face. Pattern Recognition Letters [Online] 27:1776-1787. Available at: http://dx.doi.org/10.1016/j.patrec.2006.02.007.
    We present a statistically rigorous approach to the aging of digitised images of the human face. Our methodology is based on the calculation of optimised aging trajectories in a model space and aged images can be obtained through a fast, semi-automatic procedure. In addition, person-specific information about the subject at previous ages is included, allowing aging to proceed in the most appropriate direction in the model space. The theoretical basis is introduced and experimental results from our implementation are presented and discussed.
  • Hill, C., Solomon, C. and Gibson, S. (2006). Towards a semi-automatic method for the statistically rigorous aging of the human face. IEE Proceedings - Vision, Image and Signal Processing [Online] 153:639-649. Available at: http://dx.doi.org/10.1049/ip-vis:20050027.
    Forensic age progression for the purpose of aging a missing child is a discipline currently dominated by artistic methodologies. In order to improve on these techniques, we present a statistically rigorous approach to the aging of the human face. The technique is based upon a Principal Component Analysis and involves the definition of an aging direction through the model space, using an age-weighted combination of model parameters. Pose and expression compensation methods are also incorporated, allowing faces at a wide variety of pose orientations and expressions to be aged accurately. Near photo-quality images are obtained quickly and the resultant aging effects are realistic and plausible.
  • Gibson, S., Solomon, C. and Pallares-Bejarano, A. (2005). Non Linear, Near Photo-Realistic Caricatures using a Parametric Facial Appearance Model. Behavior Research Methods [Online] 37:170-181. Available at: http://dx.doi.org/10.3758/BF03206412.
    A mathematical model previously developed for use in computer vision applications is presented as an empirical model for face space. The term appearance space is used to distinguish this from previous models. Appearance space is a linear vector space that is dimensionally optimal, enables us to model and describe any human facial appearance, and possesses characteristics that are plausible for the representation of psychological face space. Randomly sampling from a multivariate distribution for a location in appearance space produces entirely plausible faces, and manipulation of a small set of defining parameters enables the automatic generation of photo-realistic caricatures. The appearance space model leads us to the new concept of nonlinear caricatures, and we show that the accepted linear method for caricature is only a special case of a more general paradigm. Nonlinear methods are also viable, and we present examples of photographic quality caricatures, using a number of different transformation functions. Results of a simple experiment are presented that suggest that nonlinear transformations can accurately capture key aspects of the caricature effect. Finally, we discuss the relationship between appearance space, caricature, and facial distinctiveness. On the basis of our new theoretical framework, we suggest an experimental approach that can yield new evidence for the plausibility of face space and its ability to explain processes of recognition.
  • Wallis, D., Burchell, M., Cook, A., Solomon, C. and McBride, N. (2005). Azimuthal impact directions from oblique impact crater morphology. Monthly Notices of the Royal Astronomical Society [Online] 359:1137-1149. Available at: http://dx.doi.org/10.1111/j.1365-2966.2005.08978.x.
    Planetary impact craters have a high degree of radial symmetry. This hampers efforts to identify the azimuthal impact direction for most craters - the radially symmetric component of an impact crater swamps any asymmetries that may be present. We demonstrate how the asymmetric component can be isolated and the direction of the asymmetries quantified using a two-dimensional eigenfunction expansion over a circular domain. The complex coefficients of expansion describe the magnitude and phase (angular alignment) of each term. From the analysis of hypervelocity impact craters formed in the laboratory, with impact angles ranging from 0° to 50° from the surface normal, we show that asymmetries which reveal the impact direction are still present at just 10° from the surface normal, and that the phase of one complex coefficient of expansion, c(11), indicates the impact direction. Analysis of the lunar crater Hadley shows bilateral symmetry in the radially asymmetric component, which may be due to oblique impact. The 31-km lunar ray crater Kepler has morphological features that indicate the azimuthal impact direction. Coefficient c(11) gives an azimuthal impact direction similar to that expected from the morphology, although post-impact gravitational collapse and slumping obscure the result to some degree. Ray craters may provide a means of testing the method for smaller 'simple' craters when data are available.
  • Johnston, V., Solomon, C., Gibson, S. and Pallares-Bejarano, A. (2003). Human facial beauty: Current theories and methodologies. Archives of Facial Plastic Surgery [Online] 5:371-377. Available at: http://dx.doi.org/10.1001/archfaci.5.5.446.
    This article examines current theories of beauty and describes recent progress in the ability to generate photorealistic faces using a computer. First, we describe a novel experimental tool, FacePrints, that allows a user to "evolve" an attractive face using a computer. We discuss the use of this program for research on human beauty and review the main experimental studies that have led to our current theoretical perspective: beauty is a product of sexual selection. Second, we outline major improvements to the FacePrints program and demonstrate the near photographic quality of facial composites that can be obtained by combining the Face-Prints algorithm with a principal components analysis-based facial appearance model. The technical basis for a possible computer-planning system that could help the patient and surgeon define reasonable and desirable surgical outcomes is also outlined. Finally, we summarize the current state of the art and examine the issues that need to be addressed for developing the current program into a practical experimental and/or clinical tool. © 2003 American Medical Association. All rights reserved.
  • Wallis, D., Solomon, C., Kearsley, A., Graham, G. and McBride, N. (2002). Modelling radially symmetric impact craters with Zernike polynomials. International Journal of Impact Engineering [Online] 27:433-457. Available at: http://dx.doi.org/10.1016/S0734-743X(01)00148-8.
    Crater morphology in a ductile target can reveal some properties of the impacting particle. Simple measurements alone, such as the crater depth and diameter are limited in potential because the complete morphology is not considered. Detailed shape measurements, made by comparing stereo Scanning Electron Micrographs, can be reduced to a parameter set based on an orthogonal expansion over a circular domain, allowing quantitative comparisons between craters that consider the complete morphology. Most high-velocity impact craters are circular (have a circular rim), enabling us to make a model using only the radially symmetric terms from the orthogonal functions set. Shape parameters can be plotted on a feature space diagram, where similar shaped craters form clusters which can be analysed statistically. The method has been applied to laboratory impacts using a two-stage light-gas gun to fire mineral grains at an aluminium alloy target and glass beads over the velocity range 1-6 km s-1. The minerals kamacite and enstatite can be distinguished from crater morphology by this method and we have shown that the shape of impact craters change over the velocity range 1-6 km s-1 as well as simply the depth to diameter ratio. © 2002 Published by Elsevier Science Ltd.
  • Solomon, C., Loos, G. and Rios, S. (2001). Variational solution for modal wave-front projection functions of minimum-error norm. Journal of the Optical Society of America A: Optics and Image Science, and Vision [Online] 18:1519-1522. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-0038457390&partnerID=40&md5=2ac24c92f90bafe4845ea23f4717fede.
    Common wave-front sensors such as the Hartmann or curvature sensor provide measurements of the local gradient or Laplacian of the wave front. The expression of wave fronts in terms of a set of orthogonal basis functions thus generally leads to a linear wave-front-estimation problem in which modal cross coupling occurs. Auxiliary vector functions may be derived that effectively restore the orthogonality of the problem and enable the modes of a wave front to be independently and directly projected from slope measurements. By using variational methods, we derive the necessary and sufficient condition for these auxiliary vector functions to have minimum-error norm. For the specific case of a slope-based sensor and a basis set comprising the Zernike circular polynomials, these functions are precisely the Gavrielides functions. © 2001 Optical Society of America.
  • Wallis, D., Solomon, C. and Kay, L. (2000). Use of Scanning Electron Microscopy for three dimensional stereo analysis of impact craters and modelling with Zernike polynomials Mukai, T. and Clark, B. eds. Advances in Space Research [Online] 25:339-344. Available at: http://dx.doi.org/10.1016/S0273-1177(99)00950-3.
    Examples will be presented to demonstrate the use of a Scanning Electron Microscope to obtain detailed measurements of hypervelocity impact craters and to obtain the shapes of craters in three dimensions by comparing stereo pairs. The depth of each crater is sampled at approximately 5000 points and the data is fitted using a set of polynomials orthogonal over a unit disc ( the Zernike polynomials). The specific application to be presented is the study of groups of craters produced under identical impact conditions by firing a quantity of projectiles as buckshot from a light gas gun. Analysis of the Zernike polynomial spectrum of the crater shapes will show quantitatively the variations in crater shapes, both within groups of identical craters and between groups corresponding to different impact conditions. In this way the potential of the method for inferring properties of impactors will be assessed, with the aim of applying the method to obtain velocity and density information from craters in retrieved space-exposed surfaces.
  • Wallis, D., Solomon, C. and Kay, L. (2000). Use of Scanning Electron Microscopy for three dimensional stereo analysis of impact craters and modelling with Zernike polynomials. Advances in Space Research [Online] 25:339-344. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-0040952808&partnerID=40&md5=967fc129d3f8501502075973f048a636.
    Examples will be presented to demonstrate the use of a Scanning Electron Microscope to obtain detailed measurements of hypervelocity impact craters and to obtain the shapes of craters in three dimensions by comparing stereo pairs. The depth of each crater is sampled at approximately 5000 points and the data is fitted using a set of polynomials orthogonal over a unit disc ( the Zernike polynomials). The specific application to be presented is the study of groups of craters produced under identical impact conditions by firing a quantity of projectiles as buckshot from a light gas gun. Analysis of the Zernike polynomial spectrum of the crater shapes will show quantitatively the variations in crater shapes, both within groups of identical craters and between groups corresponding to different impact conditions. In this way the potential of the method for inferring properties of impactors will be assessed, with the aim of applying the method to obtain velocity and density information from craters in retrieved space-exposed surfaces. ©1999 COSPAR. Published by Elsevier Science Ltd.
  • Wallis, D., Kearsley, A., McBride, N., Solomon, C. and Graham, G. (1999). The effect of mineral type on impact crater morphology. Meteoritics & Planetary Science [Online] 34:A118-A119. Available at: http://dx.doi.org/10.1111/j.1945-5100.1999.tb01769.x.
  • Solomon, C., Seeger, M., Kay, L. and Curtis, J. (1998). Automated compact parametric representation of impact craters. International Journal of Impact Engineering [Online] 21:895-904. Available at: http://dx.doi.org/10.1016/S0734-743X(98)00040-2.
    We have developed a procedure for the measurement and automated parametric representation of 3-D impact craters. The 3-D stucture of the crater is measured using a low-coherence optical interference technique and the parametric representation achieved in a two-step procedure. Sobel edge detection and morphological operations are used to define rigorously the impact region. The parameter set is defined with respect to a coordinate system whose origin and orientation are uniquely determined from the data. Subjective, arbitrary choices for this position and angle are thus eliminated and the coefficients of the Zernike polynomials are calculated by direct integration over this region to produce a parameter set. This set allows the representation of the three-dimensional geometry by a small set of numbers. Results are presented for impact craters generated at DERA Fort Halstead by the penetration of shaped charge jet particles. It is shown that the agreement between the crater profile generated by the Zernike parameter set and the real crater is very good.
  • Ash, D., Solomon, C. and Loos, G. (1998). Optimal Hartmann sensing at low light levels. Optics Communications [Online] 156:10-15. Available at: http://dx.doi.org/10.1016/S0030-4018(98)00398-8.
    A computer model has been used to investigate the performance of a Shack-Hartmann sensor operating under low light level conditions. The trade-off between sub-aperture size and slope noise is examined in detail and optimal sizes are determined for a number of imaging scenarios. Our results are discussed in relation to related literature.
  • Solomon, C., Rios, S., Acosta, E. and Bara, S. (1998). Modal wavefront projectors of minimum error norm. Optics Communications [Online] 155:251-254. Available at: http://dx.doi.org/10.1016/S0030-4018(98)00400-3.
    Sets of auxiliary vector functions may be derived which enable the modal coefficients of a wavefront expressed in terms of a given basis to be directly projected as weighted integrals of the wavefront slopes. We derive the necessary and sufficient condition for these functions to have minimum error norm and show that for the specific case of a basis set comprising the Zernike circular polynomials, they are precisely the Gavrielides functions.
  • Ball, A., Solomon, C. and Zarnecki, J. (1998). The response of gamma backscatter density gauges to spatial inhomogeneity - An extension of the single scattering model. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms [Online] 140:449-62. Available at: http://dx.doi.org/10.1016/S0168-583X(98)00188-8.
    The single scattering model for gamma backscatter density gauges has been extended to describe how the total detected count rate changes in response to localised density variations within the material. This extended model suggests there is a spatial region where density perturbations have a contradictory effect on the measured density value, an effect that has already been shown experimentally by previous workers. Here we compare their results with those predicted by application of the extended single scattering model. Since a complete description of their experimental apparatus was not available, only a crude fit could be achieved. Nevertheless, all the basic features of the data could be reproduced.
  • Ruggiu, J., Solomon, C. and Loos, G. (1998). Gram-Charlier matched filter for Shack-Hartmann sensing at low light levels. Optics Letters [Online] 23:235-237. Available at: http://dx.doi.org/10.1364/OL.23.000235.
    A study has been made of a Gram-Charlier matched filter for Shack-Hartmann sensing of wave-front slopes. The method is based on modeling the point-spread function by an expansion in terms of Gauss-Hermite polynomials. We present results for several subaperture/coherence area sizes both with and without CCD read noise. A more accurate estimation of the local slopes can be. achieved at low light levels in this way than with the standard first-moment estimator.
  • Kay, L., Podoleanu, A., Seeger, M. and Solomon, C. (1997). A new approach to the measurement and analysis of impact craters. International Journal of Impact Engineering [Online] 19:739-753. Available at: http://dx.doi.org/10.1016/S0734-743X(96)00054-1.
    We describe a low-coherence interferometric technique which we have used for 3-D mapping of impact craters produced in the laboratory. We also propose an approach to analyse the impact craters in terms of the Zernike polynomials. Preliminary results suggest that this set may provide a useful parametric representation, thereby enabling us to relate crater features to impact parameters. (C) 1997 Elsevier Science Ltd.


  • Solomon, C. and Breckon, T. (2010). Fundamentals of Digital Image Processing: A Practical Approach With Examples in Matlab. [Online]. John Wiley and Sons. Available at: http://dx.doi.org/10.1002/9780470689776.
    This is an introductory to intermediate level text on the science of image processing, which employs the Matlab programming language to illustrate some of the elementary, key concepts in modern image processing and pattern recognition. The approach taken is essentially practical and the book offers a framework within which the concepts can be understood by a series of well chosen examples, exercises and computer experiments, drawing on specific examples from within science, medicine and engineering. Clearly divided into eleven distinct chapters, the book begins with a fast-start introduction to image processing to enhance the accessibility of later topics. Subsequent chapters offer increasingly advanced discussion of topics involving more challenging concepts, with the final chapter looking at the application of automated image classification (with Matlab examples). Matlab is frequently used in the book as a tool for demonstrations, conducting experiments and for solving problems, as it is both ideally suited to this role and is widely available. Prior experience of Matlab is not required and those without access to Matlab can still benefit from the independent presentation of topics and numerous examples. Features a companion website www.wiley.com/go/solomon/fundamentals containing a Matlab fast-start primer, further exercises, examples, instructor resources and accessibility to all files corresponding to the examples and exercises within the book itself. Includes numerous examples, graded exercises and computer experiments to support both students and instructors alike. © 2011 John Wiley & Sons, Ltd.

Book section

  • Xavier, I., Pereira, M., Giraldi, G., Gibson, S., Solomon, C., Rueckert, D., Gillies, D. and Thomaz, C. (2016). A Photo-Realistic Generator of Most Expressive and Discriminant Changes in 2D Face Images. In: 2015 Sixth International Conference on Emerging Security Technologies (EST). IEEE, pp. 80-85. Available at: http://dx.doi.org/10.1109/EST.2015.17.
    This work describes a photo-realistic generator that creates semi-automatically face images of unseen subjects. Unlike previously described methods for generating face imagery, the approach described herein incorporates texture and shape information in a single computational framework based on high dimensional encoding of variance and discriminant information from sample groups. The method produces realistic, frontal pose, images with minimum manual intervention. We believe that the work presented describes a useful tool for face perception applications where privacy-preserving analysis might be an issue and the goal is not the recognition of the face itself, but rather its characteristics like gender, age or race, commonly explored in social and forensic contexts.
  • Solomon, C. and Gibson, S. (2014). Developments in Forensic Facial Composites. In: Mallett, X., Blythe, T. and Berry, R. eds. Advances in Forensic Human Identification. CRC Press.
  • Solomon, C., Gibson, S. and Maylin, M. (2012). EFIT-V: evolutionary algorithms and computer composites. In: Wilkinson, C. and Rynn, C. eds. Craniofacial Identification. Cambridge University Press, pp. 24-41.
  • Davis, J., Sulley, L., Solomon, C. and Gibson, S. (2010). A Comparison of Individual and Morphed Facial Composites Created Using Different Systems. In: 2010 International Conference on Emerging Security Technologies. Washington, DC, USA: IEEE, pp. 56-60. Available at: http://dx.doi.org/10.1109/EST.2010.29.
    An evaluation of individual and morphed composites created using the E-FIT and EFIT-V production systems was conducted. With the assistance of trained police staff, composites of unfamiliar targets were constructed from memory following a Cognitive Interview. EFIT-V composite production followed either a two-day delay, or on the same day as viewing a video of the target. E-FIT composites were created on the same day as viewing the target video. Morphs were produced from merging either two, or three composites created by the same witness, but with the assistance of a different operator. Participants familiar with the targets supplied similarity-to-target photograph ratings. No differences were found in the rated quality of composites created using E-FIT or EFIT-V, although a two-day delay in production resulted in inferior images. Morphs were rated as better likenesses than individual composites, although the benefits were greater with EFIT-Vs. Encouraging witnesses to create more than one composite image for subsequent morphing might enhance the likelihood of recognition of facial composites of criminals.
  • George, B., Gibson, S., Maylin, M. and Solomon, C. (2008). EFIT-V - : interactive evolutionary strategy for the construction of photo-realistic facial composites. In: Keijzer, M. ed. GECCO ’08 Proceedings of the 10th Annual Conference on Genetic and Evolutionary Computation. New York, USA: ACM, pp. 1485-1490. Available at: http://dx.doi.org/10.1145/1389095.1389384.
    Facial composite systems are used to create a likeness to a suspect in criminal investigations. Traditional, feature-based facial composite systems rely on the witness' ability to recall individual features, provide verbal descriptions and then select them from stored libraries of labelled features - a task which witnesses often find difficult. The EFIT-V facial composite system is based on different principles, employing a holistic (whole face) approach to construction.

    The witness is shown a number of randomly generated faces and is asked to select the one that best resembles the target. A genetic algorithm is then used to breed a new generation of faces based upon the selected individual. This process is repeated until the user is satisfied with the composite generated.

    This paper describes the main components and methodology of EFIT-V and showcases the strengths of the system.
  • Hill, C., Solomon, C. and Gibson, S. (2005). Aging the human face - a statistically rigorous approach. In: The IEE International Symposium on Imaging for Crime Detection and Prevention, 2005. ICDP 2005. IEEE, pp. 89-94. Available at: http://dx.doi.org/10.1049/ic:20050076.
    Forensic age progression for the purpose of aging a missing child is a discipline currently dominated by artistic methodologies. In order to improve on these techniques, we present a statistically rigorous approach to the aging of the human face. The technique is based upon a Principal Component Analysis and involves the definition of an aging direction through the model space, using an age-weighted combination of model parameters. Pose and expression compensation methods are also incorporated, allowing faces at a wide variety of pose orientations and expressions to be aged accurately. Near photo-quality images are obtained quickly and the resultant aging effects are realistic and plausible.
  • Ruggiu, J. and Solomon, C. (1997). Improved technique for centroiding in Shack-Hartmann sensing at low light level. In: Tyson, R. K. and Fugate, R. Q. eds. Adaptive Optics and Applications. Spie - Int Soc Optical Engineering, pp. 545-550. Available at: http://bookshop.blackwell.co.uk/jsp/id/Adaptive_Optics_and_Applications/9780819425485.
    A study has been made of a Shack-Hartman sensor utilising a Charge Coupled Device (CCD) array and of an alternative approach to estimating the centroid. This is based on modelling the Point Spread Function (PSF) by a Gram-Charlier type expansion type, we will present results for different levels of Kolmogorov turbulence with and without CCD read noise. Our high-fidelity computer simulation demonstrates that more accurate estimation of the PSF centroid may be achieved at low light level as compared to the standard center of gravity estimation.

Conference or workshop item

  • Mist, J., Gibson, S. and Solomon, C. (2014). A comparison of search spaces and evolutionary operators in facial composite construction. In: Šilc, J. and Zamuda, A. eds. Bioinspired Optimization Methods and Their Applications (BIOMA). Available at: http://bioma.ijs.si/conference/2014/?more=home.
    In this paper a series of experiments concerning the use of IEAs in the creation of facial composites are reported. A human evaluation based search space, which is itself a subspace of a larger search space, is created. The human reduced search space is used to compare two mutation operators and two recombination operators in an IEA. A mathematically reduced search space is constructed from the larger search space. The facial composite process is performed in the three search spaces. No statistically significant differences are found between the performances of the operators or the search spaces.
  • Solomon, C. and Gibson, S. (2012). EFIT-V - Interactive Evolutionary Generation of Facial Composites for Criminal Investigations. In: Gibson, S. J. ed. BIOMA 2012:The 5th International Conference on Bioinspired Optimization Methods and Their Applications.
  • Lech, A., Johnston, R., Solomon, C. and Frowd, C. (2009). Different conditions of working with facial composite systems – what works better?. In: 19th European Association of Psychology and Law Conference.
  • Lech, A., Johnston, R., Solomon, C. and Frowd, C. (2009). The influence of exposure time on facial composite construction for own-race and cross-race conditions based on two different ways of evaluating the effectiveness of facial composite software. In: 11th European Congress of Psychology.
  • Solomon, C., Gibson, S. and Maylin, M. (2009). New computational methodology for the recovery of facial images retained in human memory. In: Signal Recovery and Synthesis. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-84898078559&partnerID=40&md5=829eefc183285353c246c99c603f34c2.
    We present a new computational methodology for the construction of facial composites from eyewitness memory for application to criminal investigation. The conceptual and theoretical basis is described and results from both laboratory and real-world applications are presented. © 2009 Optical Society of America.
  • Lech, A., Johnston, R. and Solomon, C. (2008). The influence of exposure time on facial composite construction for own-race and cross-race condition. In: BPS Annual Conference.
  • Lech, A., Johnston, R. and Solomon, C. (2008). The influence of exposure time on facial composite construction for own-race and cross-race conditions. In: British Psychological Society Annual Conference.
    (1) to examine whether exposure time will significantly influence the creation of facial composite when using EigenFit and (2) to determine the relation between confidence and accuracy among eyewitness during creation and matching procedures.

    2 (exposure time: 12s x 45s) x 2 (line-up type: Target Present x Target Absent) x 15 (target faces) Methods: 30 participants were shown the target face – 15 for an exposure time of 12 seconds and 15 for 45 seconds. After a 30-minute delay, they were asked to create facial composite using EigenFit. Thirty further participants took part in the matching procedures. All photos of faces used in matching procedures were 5x5 centimetres with white background. Random line-ups were built by using 15 target pictures used in the first part and 70 distracter photos. In each trial there were 30 line-ups with 15 culprit-absent line-ups and 15 with culprit present.

    ANOVAs were performed with post-matching level of confidence ratings as the dependent measure, correlations analysis were done to show the accuracy and confidence level relation, Chi-Square was conducted to determine difference among the frequency of hits and false identifications in TP and TA line-ups and correct/mistaken answers, Signal Detection Theory (SDT) allowed separating sensitivity from response bias.

    It was also found that longer exposure significantly increases eyewitness’s confidence level. Participants working under the longer exposure condition created faces that were better identified in the matching procedure that followed.
  • Scandrett (nee Hill), C., Solomon, C. and Gibson, S. (2006). Towards a Semi-automatic Method for the Statistically Rigorous Aging of the Human Face. In: Gibson, S. J. and Solomon, C. J. eds. pp. 639-649. Available at: http://dx.doi.org/10.1049/ip-vis:20050027.
  • Gibson, S., Pallares-Bejarano, A., Maylin, M. and Solomon, C. (2006). The Generation of Facial Composites using an Evolutionary Algorithm. In: Gibson, S. J. and Solomon, C. J. eds. 6th International Conference on Recent Advances in Soft Computing (best Paper Award).
  • Gibson, S., Solomon, C., Pallares-Bejarano, A. and Maylin, M. (2006). Innovations in facial composite systems: EigenFIT. In: Workshop on Eyewitness Identification Evidence. Available at: http://www.valentinemoore.co.uk/idworkshop/abstracts.pdf.
  • Maylin, M., Solomon, C. and Gibson, S. (2005). Model-based deconvolution of the human face. In: Seventh IASTED International Conference on Signal and Image Processing. pp. 548-553. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-33644540512&partnerID=40&md5=7de3f8e32fc7ec7ba10c3a39a284faca.
    Many practical deconvolution problems arise in which explicit knowledge of both the system PSF and the spectral characteristics of the noise are unknown. We describe and present an approach to deconvolution in this situation which is specifically matched to the forensically important problem of face identification. Our approach is to model both human faces and image aberrations in a statistical appearance framework using a representative sample of faces. Deconvolution is then achieved experimentally by moving along known transition curves in a parametric face space. Our preliminary studies demonstrate that the accuracy of the method is superior to maximum-likelihood blind deconvolution at low signal-noise ratios. A hybrid method in which the noisy face image is first projected into the model space and blind deconvolution then applied yields the best overall performance.
  • Hill, C., Solomon, C. and Gibson, S. (2004). Plausible aging of the human face using a statistical model. In: Proceedings of the Seventh IASTED International Conference on Computer Graphics and Imaging. pp. 56-60. Available at: http://www.scopus.com/inward/record.url?eid=2-s2.0-10444280891&partnerID=40&md5=6ddf18b017efaa4f4a9e8cecfac38a60.
    The ability to accurately age a human face has a significant potential application in Forensic Science, in particular for attempts to locate and identify missing children. There may also be an application in the field of facial synthesis with an emphasis on facial composite construction [1]. With these uses in mind, a new technique for aging a face in an image has been developed. The technique uses Principal Components Analysis (PCA) on an equal number of male and female faces of different ages to produce separate models in shape and texture. The aging direction was identified for each gender subspace and in-sample male and female faces aged by 20 years according to their Euclidean distance from each aging axis. An out-of-sample male face was also aged. In addition, aging was computed for the male and female faces by producing a model trained only on the appropriate gender sample and using multiples of the aging axis in order to age the face.
  • Gibson, S., Solomon, C. and Pallares-Bejarano, A. (2003). Synthesis of Photographic Quality Facial Composites using Evolutionary Algorithms. In: British Machine Vision Conference 2003. pp. 221-230.


  • Solomon, C., Gibson, S. and Maylin, M. (2009). A new computational methodology for the construction of forensic, facial composites. 3rd International Workshop on Computational Forensics, IWCF 2009 [n/a] 5718 L:67-77. Available at: http://dx.doi.org/10.1007/978-3-642-03521-0_7.
    A facial composite generated from an eyewitness's memory often constitutes the first and only means available for police forces to identify a criminal suspect. To date, commercial computerised systems for constructing facial composites have relied almost exclusively on a feature-based, 'cut-andpaste' method whose effectiveness has been fundamentally limited by both the witness's limited ability to recall and verbalise facial features and by the large dimensionality of the search space. We outline a radically new approach to composite generation which combines a parametric, statistical model of facial appearance with a computational search algorithm based on interactive, evolutionary principles. We describe the fundamental principles on which the new system has been constructed, outline recent innovations in the computational search procedure and also report on the real-world experience of UK police forces who have been using a commercial version of the system. © 2009 Springer Berlin Heidelberg.


  • Thorniley, S., Davis, J., Gibson, S. and Solomon, C. (2014). The influence of creating a holistic facial composite on children’s and adult’s video lineup identifications. Applied Cognitive Psychology [Online] 30. Available at: http://eu.wiley.com/WileyCDA/WileyTitle/productCd-ACP.html.
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