Coupling Lens
The phosphor
screen of the image intensifier emits the intensified object
image. This light must be coupled to the CCD sensor to be detected. In
principal, there are two kinds of coupling techniques:
- lens coupling and
- fiber taper coupling
Both
techniques have their own advantages and disadvantages:
STANFORD
COMPUTER OPTICS
lens coupling:
Advantages:
|
+
|
No
additional honeycomb
structures |
| + |
Optical
resolution is fully maintained |
| + |
Absolutely
distortion-free imaging |
| + |
Absolutely
no optical
vignetting
|
Disadvantage:
|
-
|
trong
laterally
emitted light from the phosphor gets lost, because of the mounting
distance between phosphor screen and coupling lens |
Fiber taper
coupling:
Advantage:
|
+
|
Allmost
all of the
emitted light is collected and transfered to the CCD sensor |
Disadvantages:
|
-
|
Fiber
bundle
adds strong honeycomb structures to the image |
| - |
Clearly
visible distortions due to the tapering of the fiber bundle |
| - |
Strong
additional degradation of the optical
resolution |
We
use only coupling lenses because
of the
prevailing disadvantages of the optical fiber taper
coupling. All our ICCD image intensified cameras are
equipped with in-house developed high
performance lenses.
Modern CCD sensors are featured with microlenses for each pixel.
Light-insensitive areas as transfer registers, located
between the active pixels, normally considerably reduce the
light-sensitive surface area of a CCD sensor. The microlenses also
cover
the light-insensitive areas and also focus the light therefrom to the
according active pixels. In order to obtain most from the
microlens technology we only use highly sophisticated
telecentric lens design to assure perpendicular incidence of the
light on to the
CCD sensors microlenses.
The
following table gives the reproduction scales of the used coupling
lenses for 18mm and 25mm image intensifier cross sections. The diagonal
of a 1/2" CCD sensors surface is 8mm.
Diameter
of the image
intensifier
|
18mm
|
25mm
|
Reproduction
scale of the
coupling lens
|
8mm
/ 18mm =
1 / 2.250
|
8mm
/ 25mm =
1 / 3,125
|
|
|
Modulation
Transfer Function of ICCD Camera
The
following diagrams show the modulation
transfer function for our coupling
lenses. The first
diagram
gives the MTF as a function of the spatial frequency. The solid curves
show the MTF at several radial positions of the images cross section,
ranging from the centre at r = 0mm to the brink at r = R, with R the
radius of the image. It can easily be seen that the MTF is almost
independent of the radial position on the image and even at a spatial
frequency of 200 linepairs/mm the MTF still amounts to 30% and thereby
lies far above the optical
resolution limit of 3%.
For comparison, the first
diagram also includes dashed MTF curves for a
commercially available aspherical photo lens with a focal length
of 90mm. As known from photo lenses, the MTF strongly decreases with
both the number of linepairs and also with the image radius although
the lens
was stopped down to f8.
Because the image
intensifier is the most serious resolution limiting
component of an ICCD camera it is essential that the coupling optics
keeps the image intensifiers optical resolution. However, a photo lens
coupling, and also a tapered fiber bundle, would significantly degrade
the attainable image resolution
The hatched area
above the diffraction limit represents the physical
limit of the MTF due to the finite diameter of the lenses. To keep clearness in the diagrams, only
averaged curves including both
sagittal and tangential image structures are plotted.
The second diagram plots
the MTF against the radial image position for
several numbers of linepairs per mm. One can easily see the outstanding
uniformity of the image quality ranging over the images whole cross
section to the border.
Our coupling lenses
guarantee perfect image quality without any
additional honeycomb structures and
absolutely without vignetting and
distortions. Just maximum image quality!
Please
note: Optical
vignetting must not be confused with natural
vignetting, which is also called natural illumination fall-off.
Whereas
optical vignetting arises from poor lens design natural vignetting is a
lens-independent consequence of the geometrical
"cosine fourth law".
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