In
bright
light level conditions the image is virtually free of "photon
granularity". The optical resolution in linepairs /mm is constant and
does not depend on the
illumination intensity. These working conditions are called resolution
limited regime and the limiting resolution is described by means of the
modulation
transfer function MTF and the contrast
transfer function CTF.
The contrast C of an optical system is a function of
the spatial
modulation frequency of the input signal. The input must be 100 %
modulated and may be an image of parallel black and white linepairs.
The output will then be an image of darker gray and lighter gray
linepairs. The contrast of the output is
then defined by
C = (max. brightness - min. brightness) / (max. brightness + min.
brightness)
with the brightnesses taken from the output image. This contrast
depends on the spatial frequency: the more linepairs per millimeter,
i.e. the smaller the images details, the lower the contrast of the
output.
For example:
Input signal contrast:
Cin = (100 - 0) / (100
+ 0) = 1,0 for all spatial frequencies.
The corresponding output signals contrasts may then be:
10 lp/mm:
Cout = (99 - 1) / (99 + 1) = 98%
20 lp/mm:
Cout = (90 - 10) / (90 + 10) = 80%
60 lp/mm:
Cout = (60 - 40) / (60 + 40) = 20%
as shown in the diagram. |
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The
difference between the modulation transfer function and the
contrast transfer function is only that for the measurement of the MTF
a sinusoidal input signal is applied, whereas a square wave function is
used for the CTF measurement.
The human eye is able to notice a contrast down to 3 %, which therefore
is defined as Limiting Resolution RL.
In the following we give the limiting resolutions of the single ICCD
components and the resulting limiting resolution of the whole ICCD
camera.
1. Photocathode
The limiting resolution of the photocathode depends
mainly on the
acceleration voltage between photocathode and micro channel plate. It
amounts to about 140 lp/mm.
2. Micro channel plate
With a mean distance between two micro channels of 6 microns the
limiting resolution of a single stage microchannel plate results in
85
lp/mm.
3. Phosphor screen
The limiting resolution of the phosphor screen is
given by the voltage
between micro channel plate and phosphor screen and by the thickness of
the phosphor layer. It typically amounts to 120 lp/mm.
4. Coupling lens
The STANFORD COMPUTER OPTICS coupling
lenses achieve a limiting
resolution of more than 260 lp/mm as can be seen here.
Manufacturer of image intensifiers often state a limiting resolution of
up to 62lp/mm or even more for intensifiers with a 18mm aperture.
However, our experience from working 20 years in the field of
ICCD cameras suggest a more realistic and reliable value of about 44.
Of course, this does not sound as good, but it`s simply reality today.
But, combined with the reproduction factor of our coupling lenses of
2.25 this
gives a limiting resolution of the image at the CCD chip's surface of
100lp/mm.
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