20 +44 (0)1904 788600 • EDMUND OPTICS® imaging lenses filters microscopy cameras illumination targets imaging resource guide
Different wavelengths bend at different angles as light passes through
a medium (glass, water, air, etc.). This is commonly observed when
sunlight passes through a prism and creates a rainbow effect; shorter
wavelengths are bent more than longer ones. This same effect creates
problems when trying to resolve details and gain information in
imaging systems. To avoid this issue, imaging and machine vision systems
commonly use monochromatic illumination, which involves only
single wavelengths or narrow bands of the spectrum. Monochromatic
illumination, e.g. from a 660 nm LED, practically eliminates what are
known as chromatic aberrations in an imaging system.
Chromatic aberrations exist in two fundamental forms: lateral color
shift (Figure 3.6) and chromatic focal shift (Figure 3.7).
Lateral color shift, Figure 3.6, can be seen as you move from the
center of the image towards the edge of the image. In the center,
the spots for different wavelengths of light are concentric. Moving
towards the corner of the image, wavelengths tend to separate and
produce a rainbow effect. As a result of this color separation, a given
point on the object is imaged over a larger area, resulting in reduced
contrast. On sensors with smaller pixels, this result is even more pronounced,
as the blurring spreads over more pixels. In depth details on
lateral color can be found in Section 6.2 on aberrations.
MTF: f/2, 200nm WD, 35mm FL
MTF: f/2, 450nm WD, 35mm FL
Chromatic focal shift, Figure 3.7, relates to the ability of a lens to
focus all wavelengths at the same distance away from the lens. Different
wavelengths will have different planes of best focus. This shift in
focus with respect to wavelength results in reduced contrast, since the
different wavelengths create different spot sizes at the image plane
where the camera sensor is located. In the image plane of Figure 3.7 a
small spot size in the red wavelengths, a larger spot size in green, and
the largest spot size in blue is shown. Not all colors will be in focus all
at once. Advanced details can be found in Section 6.2 on Aberrations.
Choosing the Optimal Wavelength
Monochromatic illumination enhances contrast by eliminating both
chromatic focal shift and lateral chromatic aberration. It is readily
available in the form of LED illumination, lasers, and through the use
of filters. However, different wavelengths can have different MTF effects
in a system. The diffraction limit defines the smallest theoretical
spot which can be created by a perfect lens, as defined by the Airy
Disk diameter, which has a wavelength (λ) dependence. See Section
2.5 for more details on the Airy Disk and diffraction limit. Using the
Equation 3.2, one can analyze the change in spot size for both different
wavelengths and different f/#s.
Minimum spot size (Airy Disk diameter) in μm = 2,44 x λ (μm) x f/#
Figure 3.6: Lateral Color Shift.
Figure 3.7: Chromatic Focal Shift.
(Continued from page 19)
Comparing Lens Designs and Configurations
u Example 4: The Effect of Changing Working
Distance on MTF
For Figure 3.5, working distances of 200 mm (a) and 450 mm (b) are
examined for the same 35 mm lens design at f/2. A large performance
difference can be seen, which is directly related to the ability to balance
aberrational content in lens design over a range of working distances.
Changing working distance, even with refocusing, will lead to variations
or reductions in performance as the lens moves away from its designed
range. These effects are most profound at lower f/#s. More details on
these effects can be found in the section Designing to Balance Aberrations
and the section on Aberrations in Section 6.1, page 40 and Section
6.2, page 41 respectively.
Figure 3.5: MTF curves for a 35 mm focal length lens at f/2 with different
Spatial Frequency in Cycles per mm
Spatial Frequency in Cycles per mm
0 75 150
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for all TECHSPEC® imaging lenses which
feature these MTF curves.
Section 3.3: Wavelength Effects on Performance