prEN ISO 21227-1

SLOVENSKI STANDARD
01-april-2026
Nadomešča:
SIST EN ISO 21227-1:2004
Barve in laki - Vrednotenje napak na premazanih površinah z uporabo digitalne
obdelave slik - 1. del: Splošno navodilo (ISO/DIS 21227-1:2026)
Paints and varnishes - Evaluation of defects on coated surfaces using digital image
processing - Part 1: General guidance (ISO/DIS 21227-1:2026)
Beschichtungsstoffe - Beurteilung von Beschichtungsschäden mittels digitaler
Bildverarbeitung - Teil 1: Allgemeine Anleitung (ISO/DIS 21227-1:2026)
Peintures et vernis - Évaluation des défauts sur des surfaces revêtues par traitement
numérique d’images - Partie 1: Lignes directrices générales (ISO/DIS 21227-1:2026)
Ta slovenski standard je istoveten z: prEN ISO 21227-1
ICS:
87.040 Barve in laki Paints and varnishes
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 21227-1
ISO/TC 35/SC 9
Paints and varnishes — Evaluation
Secretariat: BSI
of defects on coated surfaces using
Voting begins on:
digital image processing —
2026-02-27
Part 1:
Voting terminates on:
2026-05-22
General guidance
ICS: 87.040
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FOR COMMENTS AND APPROVAL. IT
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Reference number
ISO/DIS 21227-1:2026(en)
DRAFT
ISO/DIS 21227-1:2026(en)
International
Standard
ISO/DIS 21227-1
ISO/TC 35/SC 9
Paints and varnishes — Evaluation
Secretariat: BSI
of defects on coated surfaces using
Voting begins on:
digital image processing —
Part 1:
Voting terminates on:
General guidance
ICS: 87.040
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
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Published in Switzerland Reference number
ISO/DIS 21227-1:2025(en)
ii
ISO/DIS 21227-1:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 4
4.1 Methods in the spectrum of visible light .5
4.1.1 Principle .5
4.1.2 Types of illumination .5
4.1.3 Uniformity and constancy .5
4.1.4 Panel positioning and focussing .6
4.1.5 Image acquisition .6
4.1.6 Image processing .6
4.1.7 Image analysis .6
4.1.8 Image evaluation .6
4.2 Optical imaging methods .7
4.2.1 Introduction .7
4.2.2 System for acquiring images using an area scan camera.7
4.2.3 System for acquiring images using a line scan camera .10
4.2.4 Limits of digital image processing methods . 15
4.3 Chromatic focus line sensor . . 15
4.3.1 Chromatic focus line sensor (CFL) for contactless surface measurement of
defects (chromatic aberration, colour fringing) . 15
4.4 Methods in the spectrum of long-wave light (infrared spectrum) .19
4.4.1 Image acquisition .19
4.4.2 Generation of images: . . . 20
4.4.3 Processing of measured values .21
4.4.4 Test report/documentation .21
4.4.5 Radiation sources .21
4.4.6 Sensor systems . 23
4.4.7 Movement mechanics . 23
4.4.8 Procedure . 23
4.4.9 Analysis . 23
5 Calibration .24
6 Documentation .24
Annex A (informative) Standards for visual assessment of coating defects .25
Bibliography .26

iii
ISO/DIS 21227-1:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee
SC 9, General test methods for paints and varnishes in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 139, Paints and varnishes, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 21227-1:2008), which has been technically
revised.
The main changes are as follows:
— new methods for digital image processing have been added;
— the definitions for “bright-field illumination” (3.2.2), “coaxial bright-field illumination” (3.2.3),
“brightness” (3.5.4) and “contrast” (3.5.5) have been changed;
— a new definition for “metric calibration of a digital measuring system” (3.10) has been added;
— calibration has been added as Clause 5;
— the normative references have been updated.
A list of all parts in the ISO 21227 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/DIS 21227-1:2025(en)
Introduction
This document describes various methods that enable damage patterns to be evaluated on the basis of
images that are generated in different ways. Here, different physical principles are used to help identify
zones with different levels/types of damage and to determine the distances between the boundaries that
mark the extent of the damage or the proportional areas of the damaged regions on the basis of the image
data. The technology described in the various parts of this International Standard can yield more objective,
accurate, quantitative and reproducible results when compared to the human visual evaluation techniques.
This document is an instruction for creating analyzable images for the application of digital image processing
and image processing systems for the quantitative characterization of defects on coated surfaces that occur
after exposure in various test methods, e.g. stone chipping, weathering or cross-cut testing. One aim of the
ISO 21227 series of standards is to use digital image processing to reproduce the results of existing methods
for visual assessment. Additionally, digital image processing provides further information which can be
used for a more detailed evaluation of coating defects.

v
DRAFT International Standard ISO/DIS 21227-1:2025(en)
Paints and varnishes — Evaluation of defects on coated
surfaces using digital image processing —
Part 1:
General guidance
1 Scope
This document contains a general introduction to the methods and definitions of digital image processing.
The performance of individual test methods and requirements for precision are described in other parts of
the standard.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 4618, Paints and varnishes — Vocabulary
CIE Publication No. 17.4:1987, International lightning vocabulary/IEC 60050‑845:1987, International
Electrotechnical Vocabulary — Lightning
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4618 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
digital image processing
method for acquiring, digitizing, processing and analysing images using optical components and computer
systems
Note 1 to entry: It results in a set of features that describe the degree of damage to a test panel. In addition, a further
digital image is generated in which different areas of damage are marked.
3.2
illumination
application of light to a scene, objects or their surroundings so that they may be seen
[SOURCE: CIE 17.4:1987/IEC 60050-845:1987]

ISO/DIS 21227-1:2025(en)
3.2.1
reflection illumination
illumination whereby light source and image acquisition device are both arranged on the same side of the
object
Note 1 to entry: The image quality depends greatly on the reflective properties of the object under testing. Smooth
surfaces reflect the light more and deliver images with more contrast than matt and porous surfaces.
3.2.2
bright-field illumination
special kind of reflection illumination where the image acquisition device is positioned perpendicularly
above the object and the object surface reflects the incident light directly into the image acquisition device
3.2.3
coaxial bright-field illumination
method of image acquisition in which the light source and the image acquisition device are arranged
vertically above the object, and the image acquisition device and the light source have a common optical axis
Note 1 to entry: The optical axis of the light from the light source is matched to the optical axis of the sensor with the
aid of half mirrors. Flat surfaces reflect the light directly to the sensor and are shown as bright areas in the image.
Edges that are at an angle reflect the light away from the sensor and remain dark in the image as a result. Coaxial
bright-field illumination is used for example for the visualization of filiform corrosion on coated test panels.
3.2.4
dark-field illumination
method of image acquisition in which the light source and the image acquisition device are arranged at an
angle to each other
Note 1 to entry: Only the light scattered by object structures travels to the image acquisition device; flat surfaces
remain dark in the image. Dark-field illumination is used for example in order to highlight raised contours.
3.2.5
directional lighting
lighting method in which the light on the object plane is incident predominantly from one particular
direction
3.2.6
diffused lighting
lighting method in which the light on the object plane is not incident predominantly from one particular
direction
3.3
objective
combination of multiple optical imaging components (lenses) with light beam-collating properties in a
shared enclosure
Note 1 to entry: Objectives/lenses provide real imaging. The technical properties of the objective/lens determine the
resolution of the image.
3.4
image acquisition
image capture
process in which a two-dimensional image of an object is created
3.4.1
original image
digitized image taken by the image acquisition system, without having carried out any image processing

ISO/DIS 21227-1:2025(en)
3.4.2
camera
device that uses light-sensitive semiconductor materials as an optical sensor
Note 1 to entry: CMOS or CCD sensors are used in digital cameras. A CMOS or CCD chip is subdivided into very fine
elements, each of which corresponds to a pixel of the digitized image. The light-sensitive elements can be arranged as
a two-dimensional array (area scan camera) or in a one-dimensional row (line scan camera). Among others, there are
colour cameras, monochrome cameras and IR cameras.
3.4.3
scanner
image acquisition device with a one-dimensional optical sensor (line scan camera)
Note 1 to entry: The overall image is built up by line scanning of the surface of an object.
3.4.4
pixel
smallest element of an image, to which a colour value or greyscale value is assigned
3.4.5
resolution
number of pixels per unit length on the surface of an object
Note 1 to entry: If the resolution is different in the X-direction and Y-direction then both values shall be stated.
3.5
image processing
software manipulation of the original image in order to prepare for subsequent image analysis
Note 1 to entry: For example, image processing can be used to eliminate mistakes generated during image acquisition
or to reduce image information to the essential.
3.5.1
binary image
image in which each pixel is either 0 (black) or 1 (white)
3.5.2
gamma correction
conversion of the greyscale values of the image into a new image based on a non-linear characteristic curve
³
using the function YX=
Note 1 to entry: The exponent γ determines the profile of the characteristic curve and therefore the effect on the
image.
γ = 2,2 – convex, brightens the image and increases the dynamic range in dark regions.
γ = 0,5 – concave, generates increased contrast in the image.
3.5.3
interface
connection point between camera and PC defined by its electrical and mechanical design
Note 1 to entry: In the case of digital cameras, digitization of the image is performed directly in the camera. For
the transmission of the digital image points to the PC, the following interfaces are commonly used, for example, in
industrial image processing:
— Gigabit Ethernet (GigE Vision: 1 000 Mbit/s, cable lengths up to 100 m)
— USB 3.x. (5 Gbit/s, cable lengths up to 3 m)

ISO/DIS 21227-1:2025(en)
3.5.4
brightness
subjective perception of the light intensity of an image, which is objectively determined by the frequency
distribution of the pixel values in the image (histogram)
Note 1 to entry: A large number of pixels with high intensity make the image appear bright, while many pixels with
low intensity make an image appear dark.
3.5.5
contrast
pixel lightness difference between the brightest and darkest areas in the image
Note 1 to entry: Low contrast produces a dull image. High contrast means large differences in brightness in the image
and produces an image with high dynamic.
3.5.6
shading correction
correction of uneven illumination of the object
Note 1 to entry: The correction is performed within the hardware of the camera or via a software function.
3.6
image analysis
reduction of the image information to a set of values which are specific to the application
3.6.1
reference panel
specified panel that has already been assessed and therefore has a known rating
Note 1 to entry: This is used to check the reproducibility and repeatability of an image processing method.
Note 2 to entry: Panels used in this document are flat plates. They are not curved or deformed.
3.7
image assessment
process of relating the set of values resulting from image analysis to one or more characteristic values or
categories via a classification or rating scheme
3.8
pulsed infrared emitter
emitter that outputs radiation as a sequence of pulses with one or more frequencies
3.9
continuous infrared emitter
emitter that outputs continuous radiation
3.10
metric calibration of a digital measuring system
determining the valid relationship between pixels and geometric dimensions for the measuring system used
4 Principle
Different methods of excitation and different detectors can be used for detection of the damage limits. The
methods are described below.
ISO/DIS 21227-1:2025(en)
4.1 Methods in the spectrum of visible light
4.1.1 Principle
Perception and evaluation of visual information is an outstanding ability of the human mind. The ISO 21227
series of standards is intended to imitate and, in some cases, exceed this ability through use of digital image
processing.
Before defects on a coated surface can be analyzed using digital image processing, an image first needs to
be generated and converted into a digital format. Image acquisition is performed using an image acquisition
device, which is normally based on a CCD or CMOS camera. In a second step, the spatial image information
is digitized and processed through manipulation of, for example, the greyscale values. Using image analysis,
the image information is reduced to a set of values that are specific to the application.
In the paint industry, coated panels are generally used for testing. These are exposed in accordance with a
standard test method. In order to obtain results that are comparable with these existing methods of visual
assessment, the parameters of image acquisition as well as image processing and image analysis shall be
adapted to the specific application (see subsequent parts of this document). In paint testing, the following
typical defects can be distinguished:
— individual coating defects less than 5 mm in size, e.g. craters, blisters, inclusions;
— larger individual coating defects, e.g. delamination, corrosion, flaking and cross-cuts;
— multiple coating defects spread over a large area, e.g. defects due to stone chipping, multiple blisters;
— imperfections which extend over a large area, e.g. loss in gloss, brush marks.
4.1.2 Types of illumination
Different materials and surface conditions and different kinds of defects will require different optics and
illumination to show adequately the features in an image that are to be analyzed. Therefore the optimum
combination of optics and illumination for the specific application has to be used. Generally, the following
types of illumination are used:
— directional lighting;
— diffuse lighting;
— mixture of directional and diffuse lighting;
— bright-field illumination;
— dark-field illumination.
All the above types of illumination can be either reflection or transmission illumination.
4.1.3 Uniformity and constancy
Generally, the illuminance can be adjusted.
An appropriate combination of light source and optical devices shall be used to ensure that the illumination
at the surface of the panel being evaluated is uniform. The illuminance measured at the point on the panel
surface which receives the highest illuminance shall not be more than 10 % greater than the illuminance at
the point on the surface receiving the lowest illuminance. Improvements can be made using software. This
requirement is not valid for an inclined illumination geometry.
The illuminance and spectral power distribution shall be kept constant during the whole measurement
period.
ISO/DIS 21227-1:2025(en)
4.1.4 Panel positioning and focussing
Panels are positioned in two mutually perpendicular axes (X, Y). The third axis (Z) is the axis of the
optical image acquisition system. In most cases the Z-axis is perpendicular to the X-Y plane, and its point
of intersection with the panel surface defines the point of focus. The distance between the front side of
the image acquisition optics and the panel surface shall be constant, independently of the thickness of the
panels, so that focussing and magnification are not adversely affected.
4.1.5 Image acquisition
The image acquisition system is typically comprised of a light-sensitive sensor, the resolution of which
shall be at least 0,5 mm, focussing optics, light sources, a defined lighting geometry, a panel support and a
light-tight enclosure. The resolution of the optical system shall be higher than the resolution of the image
acquisition sensor. The image information is processed digitally. The characteristic line of the image
acquisition system, i.e. the correlation between the brightness of a pixel and its greyscale value, significantly
influences the result of the subsequent image analysis and shall therefore be stated in the test report. All
changes to the original greyscale values or colour information shall be clearly documented.
In order to ensure the repeatability and reproducibility of the digital image processing, calibration shall be
performed at regular intervals. This can be done using reference panels, for example.
4.1.6 Image processing
The digitized images are then stored. If a compressed file format is used, this shall be free from losses.
For different types of defect, a selection of sequential image processing methods is available. Image
processing covers numerous aspects, such as brightness, contrast, shading correction, edge detection and
gamma correction.
Different types of software can use different algorithms for the same task, and the images processed using
different image processing algorithms will subsequently not be identical. A calibration process, e.g. using
reference panels, shall therefore be used to ensure that results are comparable.
In order to ensure reproducibility within a series of measurements related to the same calibration, all
images shall be processed in identical fashion.
4.1.7 Image analysis
Different methods can be used for image analysis, e.g. binary (black and white), greyscale value or colour
analysis.
As in image processing, automatic image analysis requires a number of sequential steps for the analysis of
different types of surface defect. Algorithms shall be selected carefully for each specific application. Modern
image analysis systems use software that can be adapted to different applications.
4.1.8 Image evaluation
The result of the image analysis is a set of values that are typical for the relevant application. This set of
values is assigned to one or more characteristic values or categories with the aid of a classification or
rating scheme. In order to ensure comparability with conventional methods using visual assessment, it is
important that image processing and analysis produces characteristic values similar to those obtained using
the existing classification scales.
NOTE Nevertheless, the user is free to extract from the set of data analysis values more accurate characteristic
values for a more detailed analysis.

ISO/DIS 21227-1:2025(en)
4.2 Optical imaging methods
4.2.1 Introduction
An image of a test panel with e.g. corrosion or stone-chip damage is acquired by detecting the light reflected
from the surface with the aid of a light-sensitive sensor and converting this information into digital image
points (pixels). The sensor is installed in a digital camera, which transmits the pixels via an interface directly
to the memory of a computer for further processing. Digital cameras can take the form of both an area scan
camera or a line scan camera. An area scan camera (matrix camera) immediately supplies an image of the
object under testing every time an image is acquired. By contrast, a line scan camera needs to perform a
scanning motion before it can generate a two-dimensional image of the surface of the object under testing.
Alongside the sensitivity of the sensor, the type of lighting used also has a major impact on image quality.
For the visualization of smallest structures on the coated surface of the sample, for example delamination
due to filiform corrosion, coaxial reflection illumination has proved successful, in which semi-transparent
half mirrors are used to match the optical axis of light coming in from the side to the optical axis of the
beam to the sensor. Depending on the angle of incidence of the light, a distinction is made between bright-
field illumination and dark-field illumination. With the matrix camera application, a distinction is made
between coaxial bright-field illumination and diffuse white light illumination. Further strengths of this type
of lighting are the relatively reflection-free imaging of objects with glossy coatings and the visualization of
smallest structures regardless of the colour of the coating. Other types of lighting are also permitted.
Optical imaging methods enable the evaluation of damage to coatings, such as blisters or inclusions, but also
delamination, corrosion, flaking or damage due to a cross-cut test. Coating damage over a wide area, such as
stone-chip damage, can also be assessed.
Scratches and soiling make the analysis more difficult. With the digital image processing methods, the
scratching-off of coatings, application of markings and lettering, as well fingerprints and other defects
should therefore be avoided within the relevant regions. On very matt surfaces diffuse reflection will result
in very low contrast in the image.
4.2.2 System for acquiring images using an area scan camera
4.2.2.1 Description of the method
The system, consisting of image acquisition hardware and image analysis software, allows digital images
of surface defects on sample panels to be acquired. It can classify images based on pre-defined categories
or user-defined parameters. The system has two main operating modes: operation in a directional bright
field is used to quantitatively characterize surface regions with varyingly strong reflective properties. If
there are rough or raised regions on reflective surfaces, the bright-field illumination creates a contrast
between these rough/raised regions and the smooth regions. It is important that the sample panels not only
have a preferably glossy surface but that, in addition, they are not curved or deformed and are instead as
flat as possible. The second operating mode uses soft (diffused) white light. This operating mode is used to
quantitatively characterize surface regions with varying brightness or different colours.
4.2.2.2 Technical prerequisites for image acquisition
For the image acquisition it is necessary that various parameters can be adjusted and varied in order to
generate reproducible results. Accordingly, such a system requires various types of lighting (e.g. directional
lighting, diffused lighting, bright-field or dark-field illumination), which can also be combined if necessary,
so that different surface qualities and different types of coating damage can be recorded and presented.
a) type of camera
Image acquisition with, for example, the aid of a CCD or CMOS camera (e.g. area scan camera or line scan
camera).
Example camera: 5,03 megapixels; resolution = 2 592 × 1 944

ISO/DIS 21227-1:2025(en)
Objective: Focal length f = 20 mm; 15,67 pixels/mm or 0,053 mm/pixel
b) image acquisition parameters
Further parameters such as image sharpness, illuminance, exposure time and the position and size of the
sample panel (e.g. 160 mm × 120 mm) shall also be taken into account during the image acquisition. In all
cases uniform illumination of the surface shall be ensured. Details about the individual parameters shall be
defined with regard to the individual tests.
In order to achieve adequate quality, there are minimum requirements in terms of the number of pixels
(e.g. 15,67 pixels/mm) and greyscale depth (at least 8 bit) that shall be met during the image acquisition. All
system components shall be adjustable in order to ensure reproducible images.
Subsequent image processing is possible and may in some cases improve image quality. It shall be noted that
all adjustments made during image processing shall be documented. In the case of a series of measurements,
the image processing parameters shall be kept constant for all sample panels.
c) substrates
The tests described in this document do not require special preparation of the substrate. The sample panel
shall display general cleanliness. Visible defects caused by soiling in the analysis zone are to be avoided, as
they can influence the test or analysis result. Proper conditioning, depending on the relevant test standard,
has no influence on data collection and assessment. It is not necessary to scratch off raised delamination.
4.2.2.3 Performance of the measurement
To start with, an image of the sample panel is acquired using the suitable hardware. The parameters used
here shall be documented. If the images satisfy all criteria for sufficient quantification then the analysis is
performed in the next step.
Software interprets the analysis image with the aid of defined scanning parameters in the sense of the
criteria from the relevant standard(s). In order to improve the quality of the image it can be adjusted with
the aid of certain parameters, e.g. edge smoothing, colour tolerances, brightness thresholds, granularity and
joining together of connected areas or removal of irrelevant defects.
In connection with the analysis there is also the step of quantification. The analysis of the image generates
a necessary set of values. This data is converted to characteristic values via the software syntax. These
characteristic values correspond to the quantification of the sample panel and are dependent upon the
criteria from the relevant standard(s).
Figure 1 shows an example of a system for the acquisition of optical images by means of an area scan camera.
Figure 2 shows an example of an analysis using a directional bright field.
Figure 3 shows an example of an analysis of stone-chipping sample panels.

ISO/DIS 21227-1:2025(en)
Key
1 full mirror
2 fresnel lens
3 half mirror
4 directional light
5 reflector
6 diffuse light
7 sample panel
8 camera
9 enclosure
Figure 1 — Example of a system for the acquisition of optical images by means of an area scan
camera
Figure 2 — Example of an analysis using a directional bright field

ISO/DIS 21227-1:2025(en)
Figure 3 — Example of an analysis of stone-chipping sample panels
4.2.3 System for acquiring images using a line scan camera
4.2.3.1 Line scan camera with coaxial line lighting
Line scan cameras are semiconductor cameras with only one light-sensitive line. A two-dimensional image is
produced as a result of a scanning motion of the object under testing or of the camera. During the movement,
the individual line signals are pieced together bit-by-bit into a 2D image in the computer.
In order for the image to be in correct proportion, the transport speed of the object under testing and the
camera mounting shall be accurately synchronized.
The lighting is performed by means of collinear matching of the optical axis of a line light to the optical axis
of the line sensor. The combination of both components forms the image acquisition system of the line scan
camera with coaxial line lighting.
For optimum and high-contrast imaging, the image acquisition system can be arranged perpendicular or at
a certain angle to the sample panel.
Figure 4 shows an example of a line scan camera with coaxial line lighting.

ISO/DIS 21227-1:2025(en)
a) bright-field illumination b) dark-field illumination, angle of inclination 8°
Key
1 line scan camera
2 line sensor
3 half mirror
4 LED line light
5 collimator lens
6 sample panel
7 transport unit
Figure 4 — Line scan camera with coaxial line lighting
The bright-field illumination sends the focused light perpendicular to the sample panel. Horizontal surfaces
reflect most of the light to the camera and appear bright. Steep edges and structures scatter the incident
light in other directions and appear dark. Finest surface structures are imaged with very high contrast.
Figure 5 shows the beam path in bright-field illumination.
Figure 5 — Beam path in bright-field illumination
Figure 6 shows filiform corrosion, imaging with bright-field illumination.

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Figure 6 — Filiform corrosion, imaging with bright-field illumination
Dark-field illumination delivers very good imaging results for manifestations of corrosion over a wide area
with ramp-shaped edges. These appear bright, and flat parts of the sample panel are shown dark in the
image. The relief-like imaging of the structures is typical for dark-field illumination. Figure 7 shows the
beam path in dark-field illumination.
Figure 7 — Beam path in dark-field illumination
Figure 8 shows delamination due to corrosion over a wide area recorded with dark-field illumination.

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Figure 8 — Delamination due to corrosion over a wide area recorded with dark-field illumination
The optical resolution of the line scan camera system should be at least 25 pixels/mm or 635 dpi.
4.2.3.2 Examples of analyses with a line scan camera with coaxial line lighting
Figure 9 shows an example of the analysis of filiform corrosion with marking of the delamination area (red)
and the longest filaments on the left and right-hand sides (green). The ends of the scribe traces and the
central area are excluded with an adjustable size (blue).
Figure 9 — Analysis of filiform corrosion

ISO/DIS 21227-1:2025(en)
Figure 10 shows an example of a test of the stone-chip resistance of coatings with marking of the impact
depths.
a) stone-chipping sample panel b) display of damaged layers with false colours
c) enlargement of yellow marking d) impact depth marking in a)
Red = substrate penetration
Figure 10 — Test of the stone-chip resistance of coatings with marking of the impact depths
Figure 11 shows a determination of the area of corrosion (red rust) and delamination along the scribe trace.
a) determination of the area of corrosion (red rust)

ISO/DIS 21227-1:2025(en)
b) delamination along the scribe trace
Figure 11 — Determination of the area of corrosion (red rust) and delamination along the scribe
trace
4.2.4 Limits of digital image processing methods
Measurement results from digital image processing that are obtained from curved, bent or deformed
samples or real parts (e.g. three-dimensional test specimens) can be incorrect and may need to be checked
using alternative measurement methods.
A high-contrast image is required for digital image processing from optical imaging methods. This requires
the sample to be reflective. If the colour or brightness values of a sample are too similar due to an excessively
matt surface, incorrect measurement results can be expected.
4.3 Chromatic focus line sensor
4.3.1 Chromatic focus line sensor (CFL) for contactless surface measurement of defects (chromatic
...