Achromatic and Apochromatic Objective Lenses in Hyperspectral Imaging
By Slater Kirk, Optical Engineer - March 7, 2024
Chromatic aberration occurs when different
colors of light form different images in a lens-based imaging system. It is one
of the most often encountered aberrations in imaging systems and is more likely
to be apparent in systems that cover a wide spectral range as well as in
systems with a wide field of view.
Lenses work because light bends as it
transitions between materials with different indexes of refraction. A classic
example is the transition from air to glass in a prism, which leads to
separated colors of light, called dispersion. Similarly, in a simple lens
designed to focus light at a single point, dispersion will cause the lens to
focus different wavelengths of light closer to or further away from this single
point. This effect is called chromatic aberration.
Note, different colors of light have different
wavelengths. We use the term “wavelength” instead of “color” in this post to
treat the effect in a more general manner.
Two Main Types of Chromatic Aberration
Lateral Chromatic Aberration
Figure 1: Off-axis imaging showing the source of lateral color aberration.
In the example shown in Figure 1, red light
bends and focuses differently than blue, creating an image that is red along
the outside edge and blue along the inside. This aberration is often called color
fringing. It can be seen at the edges of standard RGB images, as shown in Figure
2.
Figure 2: Photo of a bird with zoomed-in section focusing on sharp edge with color fringing. Photo courtesy of Daniel López Sánchez, Ph.D.
Chromatic Change of Focus
Figure 3: Example showing the source of chromatic change of focus.
A second manifestation of this wavelength
dependance in imaging systems is that the ideal image plane location is
different for every wavelength. This is known as chromatic change of focus, or
axial aberration. The location of the perfect focus for blue (as demonstrated
by the point of the blue triangle in Figure 3) is different than for the green and
red wavelengths.
This aberration can be minimized by refocusing
for systems with narrow wavelength ranges. In hyperspectral cameras, though,
the wavelength range is typically large and every wavelength needs to have as
close to the same focal location as possible to provide accurate data,
especially for small objects in a scene.
Chromatic Aberrations in Hyperspectral Camera Objective Lenses
The chromatic aberrations described above
occur to some degree in all lens-based imaging systems. When light is spread
out using a grating, as is done in a hyperspectral camera, these aberrations
can become even more pronounced.
Most hyperspectral cameras utilize an
objective lens to generate an initial image at the input of the spectrometer
optics. Regardless of how well-corrected the imaging spectrometer is, the final
image will only be as good as the image formed by the objective lens. Any
amount of defocus or distortion caused by the objective lens will be carried through
to the final image.
Achromatic and Apochromatic Lenses
Objective lens designers attempt to minimize
the impact of chromatic aberration in their lenses. Achromatic
lenses correct for chromatic aberrations at two wavelengths while Apochromatic
lenses go further and provide additional chromatic correction.
To demonstrate the effect of lateral color on
an image, two scans were taken at Resonon using an achromatic lens and an
apochromatic lens on our Pika XC2
hyperspectral camera. As a rule, the shorter the focal length of a lens, the
harder it becomes to correct chromatic aberration. To best demonstrate the
aberration, we used relatively short focal-length 8 mm objective lenses.
In Figure 4, below, the achromatic lens-generated
image is on the left-hand side and the apochromatic lens-generated image is on
the right-hand side. Both images use the 420 nm, 600 nm, and 850 nm channels
captured by the Pika XC2 for the Red, Green, and Blue pixel values,
respectively.
Figure 4: Images from datacubes collected with a Pika XC2 and an achromatic 8 mm objective lens (left) and an apochromatic 8 mm objective lens (right).
These images look similar when considering the
whole frame but zooming in on the edge of the image (the top and bottom, in
this orientation), the effects are quite noticeable (Figure 5).
Figure 5: Zoomed in sections of Figure 4. The achromatic objective lens image is on the top and apochromatic objective lens image is on the bottom. Note the purple edges of the branches (top) recorded with the achromatic objective.
The color fringing seen
in the achromatic objective image on the top of Figure 5 is a result of the red,
green, and blue channels not imaging at the same pixel location, as was diagrammed
in
Figure 1. The
apochromatic objective image on the bottom of Figure 5 has much sharper branch
details, as that objective lens has better chromatic aberration correction.
Looking on axis (towards the center of each
wide-angle view) at single-channel grayscale images, we can see the change in
focus as we move through various wavebands.
Figure 6: 420 nm (left), 600 nm (middle), and 850 nm (right) channels using the achromatic lens (top row) and the apochromatic lens (bottom row). Note the lack of sharpness against the tree trunks and snow boundary in the 420 nm and 850 nm channels for the achromatic lens vs. the relative consistency in sharpness of the same details for the apochromatic lens.
Achromatic or Apochromatic Objective Lens
As demonstrated, the apochromatic objective
lenses are clearly superior. Not surprisingly, they are also more expensive.
Achromatic objective lenses often meet the
needs of our customers, especially if the application does not involve small
objects or the need to distinguish between very subtle spectral changes.
If you are uncertain about which type of
objective lens you need for your application, a Resonon Sales team member can talk through your application and provide guidance.
