illumination cameras microscopy / objectives filters / accessories liquid lens / specialty telecentric fixed focal length resource guide
H
m = FOV 1.1
ξ 2.6 Object Space = ξImage Space × m
ξObject Space μm = ×
2 × ξObject Space
lp
mm
s
ξ m Object Space μm =
1000μm
1mm
1
1000μm
1mm
ξ 2 × 3.45μm Image Space = × ≈ 145
1mm
1000μm
1mm
1000μm
1
8.45mm
100mm
lp
mm
12 +44(0) 1904 788600 | Edmund Optics® targets Contrast is how well black can be distinguished from white at a given
resolution. For an image to appear well defined, black details need to
appear black and the white details must appear white (See Figure 2.3).
The more the black and white information trend into the intermediate
greys, the lower the contrast at that frequency. The greater the difference
in intensity between a light and dark line, the better the contrast.
While this may appear obvious, it is critically important.
The contrast at a given frequency can be calculated in Equation 2.9,
where IMAX is the maximum intensity (usually in pixel greyscale values,
if a camera is being used) and IMIN is the minimum intensity:
The lens, sensor, and illumination all play key roles in determining
the resulting image contrast. Each one can detract from the overall
system contrast at a given resolution if not applied correctly
and concertedly.
RESOLUTION AND MAGNIFICATION CALCULATION
EXAMPLES USING A SONY ICX 625 SENSOR
Known Parameters:
Pixel Size: 3.45 × 3.45μm
Number of Pixels: 2448 × 2050
Desired FOV (Horizontal): 100mm
Limiting Sensor Resolution:
Sensor Dimensions:
Magnification:
Resolution:
1
ξ 2 × s Image Space = ×
1000μm
1mm
HHorizontal = 3.45μm × 2448 × = 8.45mm
HVertical = 3.45μm × 2050 × = 7.07mm
m = = 0.0845X
lp
mm
lp
mm
2.7
2.8
ξObject Space μm = 145 × 0.0845 = 12.25 ≈ 41μm
Section 2.3: Contrast
IMAX – IMIN
IMAX + IMIN
% Contrast = × 100% 2.9
Imaging Pixels
White
Square Wave Contrast
Black
Imin
Imax
Figure 2.3: Transition from black to white is high contrast while intermediate
greys indicate lower contrast.
Figure 2.3
CONTRAST EXAMPLE
The ability to resolve detail is directly related to both a lens’s
ability to reproduce contrast and the number of pixels utilized.
The images below are of the same test target, but taken by two
different lenses using the same number of pixels on the sensor.
Both images are cropped from the center of the sensor. Each
lens’s ability to reproduce contrast is the determining factor in
the performance of the system.
Lens 1 yields a contrast level of 22.6%, while Lens 2 produces
a contrast level of 12.7%. This is a 78% difference in performance
between the two lenses, even though the images look
somewhat equivalent to the human eye.
It must also be understood that a lens will not necessarily produce
the same contrast at the same frequency across the entire
FOV. Additionally, contrast levels will change as a lens’s f/# is
adjusted. More detail on this can be found in Sections 2.1 and 2.4.
Lens 1
Lens 2
System magnification scales the image space resolution up to the object
space resolution (ξObject Space).
When developing an application, a system’s object space resolution is
typically specified as a length dimension, rather than in lp/mm. There
are two ways to make this conversion:
While it is easy to jump to the limiting resolution on the object by using
the last formula, it is very useful to determine the imaging space
resolution and magnification to simplify lens selection. It is also important
to note that there are many additional factors involved, and
this limitation is often much lower than what can be easily calculated
using the equations. Learn more about contrast limitations and lens
selection in Sections 2.3 and 6.