Omron V440-F User Manual page 208

8 Advanced Settings
 Illumination Distribution
Evenly-distributed illumination over the inspection area is a key component for accurate and repeatable
grading. In general, ISO 15416 grading will be the most sensitive of the grading methods provided with
regard to variation in illumination. This is because 1D symbols generally span a larger range in the field
of view and many of the grading measurements that rely on absolute measurements can be impacted
noticeably over that range. It is important to take care in achieving as flat a response as possible of the
inspection area. Uneven illumination will not only impact the grade of a given symbol, but can also have
a very large impact on repeatability of measurements. For example, a given symbol may achieve a
different result in the center of the field of the view then it does near the edges or corner. Illumination
providing a perfectly flat response will remove locational variance but this is not always possible. Care
in symbol presentation, location, and consistency will have a significant impact on grading consistency.
 Optical Distortion
Optical distortion of the symbol under test can also impact results. Optical distortion is introduced
through a few different sources. The optics themselves will introduce non-linear distortions. These
distortions will have a more significant impact on the symbol as the working distance is shortened.
Lengthening the working distance reduces the variation in angle at which light traveling from the symbol
enters the lensing system. Moving the symbol farther away also reduces the pixel per element
resolution and changes illumination profiles, so care must be taken to optimize these parameters
together. The symbol should be presented as close to 90 degrees to the imager as possible. Off-axis
imaging will introduce distortion. Optical distortion mostly impacts 2D grading, particularly Axial and
Grid Non-Uniformity, but will have cascading effects on other metrics.
 Digital Resolution
A minimum digital resolution in the image is required to allow the software to have enough data to
accurately measure the grading parameters. For ISO 15416 grading, 4 pixels per nominal symbol
dimension are required. For ISO 15415 and ISO 29158 grading, 6 pixels per nominal symbol element
are required. Under the highest-quality imaging, good performance can be seen down to 3 pixels for 1D
and 5 pixels for 2D. This is system-dependent and requires optimal illumination. Not having enough
resolution will impact repeatability and introduce overall degradation in dependent parameters.
8
 Optical Resolution
Optical resolution is the ability of the optics to resolve detail in the image. High digital resolution of the
symbol in the image does not equate to a high optical resolution. The system needs to resolve the
detail of the image accurately. This requires making sure your lens is at optimal focus and that other
considerations such as working distance and color of illumination have been evaluated to ensure they
are not negatively impacting optical resolution in the system. When designing an optical setup you
should evaluate and optimize to achieve the best possible modulation transfer function MTF for the
imager. Poor optical resolution typically results in a degraded modulation score can have a cascading
effect on other metrics.
General Grading Functionality
 The Grading Process
The three grading specifications described in this topic are independent of each other as an inspection
process but are all dependent on the X-Mode decoder being properly set up to read the symbol type
provided. The grading evaluation does not take place until a symbol has first been identified by the X-
Mode decoder. Once a symbol supported by an enabled grading specification has been decoded by the
decoder a separate grading evaluation will commence. Note this evaluation can potentially add
significant processing time to the overall process and each specification requires independent
evaluation. Furthermore, enabling any single parameter within a given specification requires the
complete grading evaluation.
8-25
V440-F C-Mount Code Reader User Manual