Lumenera Lt-C1610, User Manual

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118 • Additional Reference Information Teledyne Lumenera Lt Series
7.1.10 Additional Lens Parameters (application specific)
There are other lens parameters that are chosen to meet the needs of the vision application. These
parameters are independent of the Lt Model (assuming that the Lens Mount and Lens Sensor Size
parameters are correct, as previously covered in this section). A vision system integrator or lens
specialist should be consulted when choosing lenses since there is a trade-off between the best
lenses and cost. An abridged list of lens parameters follows – all of which need to be matched to
the application.
•
Focal Length: Defines the focus point of light from infinity. This parameter is related to the
Lt Mount (C, CS, M42 mount). See Back Focal Variance when using any Filter.
•
Field of View: A lens is designed to image objects at some limited distance range, at some
positive or negative magnification. This defines the field of view.
•
F-Number (aperture): The lens aperture defines the amount of light that can pass. Lenses
may have fixed or variable apertures. Additionally, the lens aperture affects Depth of Field
which defines the distance range which is in focus when the lens is focus at some specific
distance.
•
Image Resolution and Distortion: A general definition of image quality. A lens with poor
resolution seems to never be in focus when used to image fine details.
•
Aberrations (defect, chromatic, spherical): Aberrations are specific types of lens faults
affecting resolution and distortion. Lens surface defects or glass faults distort all light or
specific colors. Aberrations are typically more visible when imaging fine details.
•
Spatial Distortions: Describes non-linear lens distortions across the field of view. Such
distortion limits the accuracy of measurements made with that lens.
7.2 Optical Considerations
This section provides an overview to illumination, light sources, filters, lens modeling and lens
magnification. Each of these components contributes to the successful design of an imaging
solution.
7.2.1 Illumination
The amount and wavelengths of light required to capture useful images depend on the application.
Factors include the nature, speed and spectral characteristics of objects being imaged, exposure
times, light source characteristics, environmental and acquisition system specifics, and more. The
Teledyne DALSA Web site provides an introduction to this potentially complicated issue, along with
many other application notes and guides on machine vision. Start with this Knowledge Center
article: http://www.teledynedalsa.com/en/learn/knowledge-center/machine-vision-101-an-
introduction/
It is often more important to consider exposure than illumination. The total amount of energy
(which is related to the total number of photons reaching the sensor) is more important than the
rate at which it arrives. For example, 5
µ
J/cm 2 can be achieved by exposing 5 mW/cm 2 for 1 ms
just the same as exposing 5 W/cm
2
for 1
µ
s.