IEC 62899-302-2:2018

IEC 62899-302-2 ®
Edition 1.0 2018-05
INTERNATIONAL
STANDARD
Printed electronics –
Part 302-2: Equipment – Inkjet – Imaging-based measurement of droplet volume

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IEC 62899-302-2 ®
Edition 1.0 2018-05
INTERNATIONAL
STANDARD
Printed electronics –
Part 302-2: Equipment – Inkjet – Imaging-based measurement of droplet volume

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 19.080; 37.100.10 ISBN 978-2-8322-5671-8

– 2 – IEC 62899-302-2:2018 © IEC 2018
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Droplet volume measurement . 6
4.1 General . 6
4.1.1 Overview . 6
4.1.2 Volume measurement and droplet shape equalization processes . 6
4.1.3 Imaging optics . 7
4.1.4 Image shape processing . 7
4.1.5 Calibration . 7
4.1.6 Uncertainties . 7
4.2 Processes for measurement of inkjet droplet volume . 8
4.2.1 General . 8
4.2.2 Process for measurement of inkjet droplet volume – Method 1 . 8
4.2.3 Process for measurement of inkjet droplet volume – Method 2 . 8
Annex A (informative) Key considerations for in-flight droplet volume measurement . 10
A.1 Jetted droplet volume in printed electronics . 10
A.1.1 General . 10
A.1.2 Image resolution . 10
A.1.3 Greyscale-to-binary image conversion . 11
A.1.4 Absolute droplet volume . 13
A.2 Formulae for inkjet droplet volume . 14
A.3 Results . 15
Bibliography . 16

Figure 1 – Representation of greyscale drop size 1 (“native drop”) to size 7 . 5
Figure A.1 – Magnified droplet grey image . 10
Figure A.2 – Threshold value influence on binary image: on the left, a threshold of 25;
on the right, a threshold of 75 . 12
Figure A.3 – Apparent image height of objects imaged near the focal plane (FP) using
a conventional lens . 12
Figure A.4 – Example of percentage size distortion in image plane for a conventional lens . 13
Figure A.5 – Shadowgraph of inkjet-printed droplets, ligaments and satellites in-flight . 14

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PRINTED ELECTRONICS –
Part 302-2: Equipment – Inkjet –
Imaging-based measurement of droplet volume

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62899-302-2 has been prepared by IEC technical committee 119:
Printed Electronics.
The text of this International Standard is based on the following documents:
FDIS Report on voting
119/204/FDIS 119/216/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 4 – IEC 62899-302-2:2018 © IEC 2018
A list of all parts in the IEC 62899 series, published under the general title Printed electronics,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

PRINTED ELECTRONICS –
Part 302-2: Equipment – Inkjet –
Imaging-based measurement of droplet volume

1 Scope
This part of IEC 62899 specifies the method for determining accurate inkjet droplet volume
based on images obtained by drop-in-flight measurement systems. It does not apply to
imaging systems using interference fringes, such as holography or phase doppler
anemometry. This document is not limited to drop-on-demand inkjet systems, but might not be
applicable to continuous inkjet or liquid dispensing systems. This document includes a
description of the issues concerning such measurements and consideration of the limits to
measurement accuracy.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
droplet volume
amount of jetted fluid from an inkjet print-head nozzle measured by single event imaging
Note 1 to entry: For a single event drive pulse designed to produce sub-drops that are intended to merge in-flight
to form a larger droplet, for example to form a specific greyscale image value on deposition, droplet volume refers
to the large merged droplet and not to smaller component sub-drops.
3.2
native drop volume
amount of fluid within the smallest sub-drop jetted from a greyscale inkjet print-head used to
create images with droplets formed by multiple sub-drops within a single event
Note 1 to entry: Native drops (or threads or satellite drops) might be too small for accurate measurements by
flash imaging, but satellite drops that have merged in-flight might be large enough for drop analysis systems. (See
Figure 1 for a representation of the relative sizes for greyscale droplets in-flight.)
IEC
Figure 1 – Representation of greyscale drop size 1
(“native drop”) to size 7
– 6 – IEC 62899-302-2:2018 © IEC 2018
4 Droplet volume measurement
4.1 General
4.1.1 Overview
This document concerns accurate determination of inkjet droplet volume from high speed
flash images of inkjet droplets travelling in-flight from inkjet print-head nozzles. Accurate
relative (rather than absolute) droplet volumes are useful for industrial inkjet-printed
electronics applications. Short flash durations avoid significant motion blurring in droplet
images. Two widely used scenarios, for flash imaging as applied to measurement of inkjet
drop speed, are considered in this document because they produce slightly different
information about droplet volumes. Images that contain superposed nominally identical and
similarly placed droplets can provide an average size for volume measurements, whereas the
single event images give measures of the size and variations of the size and centroid location
produced during volume measurements. Clause A.1 gives further information about specific
instrumentation limits.
Shadowgraph imaging can readily determine individual inkjet droplet sizes if the fluid bodies
appear dark against a light background [1 ]. However, droplets should be well-focused, and
the optical images have a suitable pixel resolution and background intensity with low intensity
variations, and image blur due to droplet motion during the flash should have minimal effect
on droplet size determination. For liquid droplets, background intensity level, refraction and
diffraction can alter apparent image size, and if the liquid is not opaque refraction often
produces a bright central spot. As spherical droplet shapes are preferred for the most
accurate and rapid online image analysis and conversion to volume, all in-flight
measurements should be made only where any sub-drops (and satellites) from the same
single event pulse have merged, and also any droplet shape oscillations have fully damped
out. Droplet volume is then inferred from the diameter (pixels) or area (number of pixels
covered) of the dark region by assuming spherical geometry and image symmetry about the
focal plane and linear calibration of the optical system (in µm/pixel). Accurate fitting
algorithms determine droplet size to sub-pixel levels, but also depend on an assumption about
where the droplet boundary is located in an image. This can provide an absolute droplet
volume if calibrated using a suitable traceable method. For example, the measured weight of
a known number of drops of known density gives the average drop volume which may be
compared with that deduced by the drop measurement system [2].
More commonly, reference objects of known dimension(s) placed in the drop measurement
system are imaged and analysed using the same optical conditions as for the inkjet droplets.
Relative volume comparisons can be made between droplets without an absolute calibration.
4.1.2 Volume measurement and droplet shape equalization processes
In principle, once droplets have been ejected from an inkjet print-head nozzle, their volume
does not change if evaporation losses or drop merging are negligible. However the resultant
droplet shapes can alter markedly from their jetted shapes until they relax towards their final
shapes (see Figure A.5). Accurate in-flight measurements always analyse spherical shapes,
avoiding long thin jet shapes [1] (or spinning non-spherical blobs of liquid) that might not
actually lie symmetrically in the (2D) image focal plane and hence might not be convertible
into volume. Some inkjet fluids used in printed electronics and other applications do not
always form fully smooth or spherical droplets under all jetting conditions [3] and in such
cases the drop analysis system provides less accurate (or even misleading) results. Examples
are highly shear-thinning high viscosity fluids, gels and fluids with large particles. Accurate
droplet volume measurement systems using multiple event imaging also require very stable
jetting by the inkjet print-head and avoidance of first droplet and burst printing effects in inkjet
printing [4, 5].
___________
Numbers in square brackets refer to the Bibliography.

4.1.3 Imaging optics
Drop-on-demand inkjet drops move at 5 m/s to 10 m/s and typically need sub-microsecond
high power flash illumination and also high resolution digital cameras with 10X magnification
(or more) for in-flight measurement of droplet volume using shadowgraph imaging.
Tele-centric optical designs and high power LED flash can deliver (background) illumination
and imaging conditions with a depth of focus sufficient for accurate inkjet droplet volume
measurements using drop analysis systems. Appropriate flash delay times can locate droplets
near the centre of the optical field of view for accurate droplet volume determination. Optical
field of view is determined by the magnification and camera sensor area and spans typically a
few hundred micrometers. Multiple-event imaging increases the background image level
where the single event flash intensity is limited, at the cost of achievable droplet volume
accuracy. Background intensity levels, expressed as a greyscale intensity level, are ideally
specified for the drop analysis system, as is apparent from recent studies of image analysis
errors [6]. Proper focusing of drop analysis systems can reduce unwanted blurring of droplet
images, which can otherwise cause inaccurate analysis of the droplet shape, size and
volume.
4.1.4 Image shape processing
One or more regions of interest, within the total field of view of the drop imaging system, may
be set (by a user or automatically) to contain the particular droplet images for analysis. This
assists automatic identification of droplets and speeds up the analysis and presentation of
results. Either axial or spherical symmetry of droplet images should be assumed by the drop
analysis system so that droplet volumes can be computed from individual shadowgraph
images. The image shape processing used by the drop analysis system may involve
thresholding, edge detection, boundary location, circles, ellipses, equivalent circular
diameters, maximum lengths and widths, area, sliced drums or cones, or even evolving
shapes [6, 7, 8, 9]. Accurate determination of droplet volume requires sub-pixel techniques,
as shown in Clause A.1.
4.1.5 Calibration
The linear calibration of the optical field of view should be established with grids, lines or
objects of suitable known size and spacing under similar light conditions and flash duration;
the threshold value used for such calibrations should equal that used for drop measurements.
Calibration factors of 1,00 µm/pixel or less are typical for drop analysis systems accurately
measuring drop-on-demand inkjet droplet volumes, using camera pixel sizes of 10 µm or less.
An approximate (~1 %) calibration factor can be conveniently found for some measurements
by imaging several inkjet nozzles or emerging jets within the same field of view
(i.e. > 100 µm) and comparing the (nominal) nozzle pitch with the apparent pixel separation.
However this is often not feasible for industrial inkjet print-heads because they have a
shielded nozzle plane.
4.1.6 Uncertainties
Imaging-based measurements of droplet volume depend on the cube of the linear calibration
factor, so that droplet volume uncertainty is three times that of the calibration factor
uncertainty. Thus the minimum uncertainty in accurate droplet volume measurements is ± 3 %
for a linear calibration factor known to ± 1 %. As a typical example, the calibration factor of
1,00 µm/pixel in 4.1.5 has no error shown, so that the minimum assumed linear uncertainty is
± 0,01 µm, i.e. ≥ ± 1 %, and therefore the minimum assumed absolute volume uncertainty is
≥ ±3 %. Traceable standards should normally have an absolute uncertainty more than ten
times lower than this. Uncertainties in droplet volumes appropriate to relative comparisons of
images recorded with a drop analysis system are given in Clause A.2.

– 8 – IEC 62899-302-2:2018 © IEC 2018
4.2 Processes for me
...