Image material and comparability · Assessment standard
Image suitability
The suitability of a questioned image cannot be disposed of by a single figure. Technical resolution is an important marker, but no generally valid threshold for expert usability follows from it. Images of similar size may differ greatly in usefulness depending on contrast, sharpness, artefacts, camera characteristics, covering, and the feature constellation.
For practice this means: a measurement image is not suitable simply because it is digitally large. Nor is it unsuitable simply because at first glance it appears technically limited. What matters is whether the structures important for the evidential question are recognisable and can in fact be compared meaningfully in a later juxtaposition. Particular examination is required of sharpness, contrast, illumination, perspective, compression, covering, and the number of actually visible features.
The examination of image suitability is therefore not a formal preliminary act, but a professional step in its own right. It helps determine whether a full assessment is sensible or whether narrow methodological limits already become apparent at this stage.
Image material and comparability · Comparative basis
Comparability
Material is comparable only where the same, or at least corresponding, feature regions are actually visible on both sides. Differences in head position, gaze direction, focal length, illumination, sharpness, compression, or covering may alter individual regions to such an extent that an apparent finding becomes nothing more than a technical artefact. Comparability therefore means more than the mere presence of two images of a person.
Forensic practice must therefore distinguish carefully between recognisability and comparability. A feature may be visible on both sides and still not be comparable in a methodologically sound way. Especially in low-grade source material, there is otherwise a risk that differences caused only by imaging conditions are interpreted as personal characteristics, or that actual differences are smoothed away.
That is why comparability requires a sober examination of which regions can really be juxtaposed. Where this is not the case, the limitation itself forms part of the conclusion. The absence of comparability is not a defect of the opinion, but a factual property of the material.
Comparison images are therefore not useful merely because they exist. They should also approximate the questioned image as closely as possible in viewing direction, head position, distance, focal length, and illumination. In addition, there is the prior question whether the comparison image can in fact be assigned securely to the named person. Only this combination makes comparison material genuinely reliable for later feature analysis.
Image material and comparability · Technical limits
Image suitability and the limits of technical thresholds
The suitability of an image for a morphological identity opinion cannot be reduced to a single technical threshold. Resolution and facial image size are important factors, but they do not by themselves determine usability. Even at lower resolution, material may still be assessable in an individual case, whereas images with apparently favourable technical values may remain only of limited use because of low contrast, covering, shadowing, perspective, or artefact-related disturbances.
For expert assessment, several considerations must therefore always be examined together: the number of recognisable features, their recognisability, their individual character, and the technical and recording-related conditions of the image. A methodologically careful opinion addresses these influences expressly and states the resulting limits of the conclusion.
In practice, a graded assessment of usability is therefore more appropriate than a simple either-or. Image material may be fully usable, usable only with restrictions, informative only for exclusion-oriented considerations, or merely orienting. It may equally become clear that any further expert assessment is no longer methodologically defensible. For courts and other parties to the proceedings, this graduated approach is often more useful than a premature yes-or-no answer.
Typical disruptive factors and artefacts
Disruptive factors alter individual features, create apparent dissimilarities or conceal true differences.
Typical limitations
- focal length and object distance
- illumination and shadowing
- motion blur and low sharpness
- pixel artefacts and compression
- facial expression and head position
- covering of facial regions
- age-related change and soft-tissue alterations
- disease- or lifestyle-related changes
Each observed feature is examined as to whether it is morphologically reliable or better explained technically.
Image material and comparability · Traceability
Documentation
Documentation concerns not only the later opinion, but already the material chain. Important are original files, metadata, known exports, post-processing steps, where applicable sequence information, and the exact designation of the comparison images used. Only in this way does it remain traceable on what basis the later conclusion rests.
This is especially important where questioned material has already passed through several stations before the instruction: police export, messenger forwarding, screenshot, printout, or scan. Every such step may alter quality, crop, colour impression, or the visibility of individual regions. Without clean documentation, it later becomes difficult to distinguish original properties of the image from later alterations.
Documentation is therefore not an appendix but part of the professional basis. It secures the distinction between source material, technical preparation, and expert interpretation, and thereby also the traceability of the later evidential assessment.
Image material and comparability · Additional foundations
Comparison images and 3D
Standardised comparison images or supplementary 3D acquisitions make perspective, surface course and comparison position easier to control. Their use depends on the evidential question, the material situation and the expected improvement of the comparison conditions.
Technical acquisition creates material. The expert report evaluates this material.
Image material and comparability · Technical starting point
3D data and file formats
Supplementary 3D data may be provided in different output formats. They support documentation, comparability and controlled further processing.
Formats at a glance
VRML 1.0
.wrl
VRML 2.0
.wrl
Softimage
.hrc
Wavefront
.obj
DXF
.dxf
ASCII
.asc
STL
.stl
ASCII, binary
MGF
.mgf
Image material and comparability · Further orientation