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Effects of pixel size in line scan cameras.
Jari Löytömäki
JAI Finland
13 November 2015
JAIIs a camera manufacturer with global presence and
operates via own sites and distribution in more
than 40 countries
San Jose Germany MiyazakiYokohamaShanghaiHelsinkiCopenhagen (HQ)
What is JAI doing ?
A unique blend of technologies & competencies
• Area scan and line scan camera technology
• Software engineering
• Image pre-processing techniques
• High-speed interfaces
• Optical knowledge
• Imager and multi-imager technology
CCD/CMOS
2CCD 3CCD 3CMOS 4CCD 4CMOS
4
EFFECT OF PIXEL AREA
• Change of pixel size is compensated either by changing the
viewing distance or the lens or a combination of both.
• How do I then get the advantage of larger pixels and the expected
better responsivity ?
• How much do I gain ?
Principle of imaging: one pixel on the target equals one pixel
on the sensor regardless of the pixel sizes.
5
EXAMPLE: ANOTHER LENS TO VIEW THE SAME
• When in focus, the only source
of light for each pixel is the
counterpart pixel on the target.
• That pixel reflects light into all
directions.
• The size of the lens aperture
(pupil) determines how much of
all that light is captured into the
camera and focused onto that
single pixel on the sensor.
• 4x increase of pixel size results
in increase of collected light
from the same target by the
same ratio.
Pixel size:
20 x 20 µmPixel size:
10 x 10 µm
lens: 28 mm
#F4
Ø = 28/4 = 7 mm
Aperture ≈ 40 mm2
lens: 56 mm
#F4
Ø = 56/4 = 14 mm
Aperture ≈160 mm2
6
EXAMPLE: 10 OR 20 µm SQUARE PIXELS FOR THE SAME TASKWhat happens with the lenses ?
1 2 2047 2048
10
µm
1 2 2047 2048
28 mm lens – F4
Aperture size = 39 mm2
1 meter 1 meter
WD =
0.7 meter
WD =
1.4 meter
28 mm lens – F4
Aperture size = 39 mm2
1 2 2047 2048
56 mm lens – F4
Aperture size = 156 mm2
1 meter
sweep
At half the distance, the
intensity of light is 4x.
Same lens and aperture now
collects 4 times the light.
Keeping the distance, a
longer focal length lens can
now be used.
With the same F-number,
the real size of the lens
pupil is now 4x and thus
each pixel collects 4x
the light.
20
µm
20
µm
WD =
1.4 meter
7
LOWER REQUIREMENTS TO LENS QUALITYObtain better image sharpness when low-cost lenses are used
Target
Standard quality lens
(low LP/mm rating) Lines focussed on imager
10 µm pixels
20 µm pixels
Lines spread across
multiple pixels
causing soft
gray edges.
Lines fit on
individual pixels
for better image
sharpness.
sweep
LARGER PIXELS NEED FEWER LINE PAIRS PER
MILLIMETER (LP/MM) TO RESOLVE DETAILS.AVOIDS THE NEED FOR HIGHER QUALITY
LENSES, KEEPING COSTS DOWN.
8
MAIN BENEFITS OF LARGER PIXEL SIZES
1. LESS LIGHT, LOWER COSTS
2. LESS NOISE
3. SHARPER IMAGES
4. HIGHER MAXIMUM PRODUCTION THROUGHPUT
A simplified list:
20 µm
20 µm
20 µ
m
9
THE MODELS WITH 20 µm SQUARE PIXELS
2K LINE SENSOR
3CMOS 4CMOSMONOCHROME
Sweep SW-2000M-CL
Sweep SW-2000M-CXP
Sweep+ SW-2000T-CL
Sweep+ SW-2000T-CXP2Sweep+ SW-2000Q-CL
Sweep+ SW-2000Q-CXP2
SWEEP
10
EVEN LARGER PIXEL SIZE FOR DUAL SWIRInGaAs imaging technology
Soon to be released new technology:
- 2-channel beam splitter
- 1024 pixels each
- 25 µm square pixels
- 39 kHz
900 nm to… 1600 nm
Short Wave InfraRed (SWIR)
(900nm – 1700nm)
Near InfraRed (NIR)
(750nm – 1100nm)
Visible Light
(400nm – 750nm)
WAVE
SERIESWA-1000D-CL
NEW !
11
GOING FROM 2K to 8K RESOLUTION – WHAT HAPPENS ?What is the difference between pixel sizes on target and light requirements
1 2048
8192
Line 1
Line 2
Line 3
l/4l/4l/4
16x the light is needed due to smaller pixel size
to get the same response per pixel.
1
l
l/4
Line 1
Line 4
Line rate:
80 kHzsweep
Required line rate:
320 kHz (?)
Field of view / pixel size on moving target
2K
8KLine 1
81921
Line rate:
80 kHzl
20 µm
5 µm
4x the light is needed
due to faster scan rate.
8K5 µm
Pixel size on target is
streching as same
distance is imaged.
In practice the quantum
well is smaller with
smaller pixels and the
line rate can not be that
high.
12
HIGHER RESOLUTION AVAILABLE SOON
Model : Mono Colour / Prism Colour / Trilinear
Examples 4K 8K
Pixel size [µm] 7.5 x 10.5 3.75 x 5.78
Line rate, max. 200 kHz 100 kHz
Non-square pixels in first models.
13
NON-SQUARE PIXELS
Example: 10 µm square vs. 10 µm x 15 µm rectangular pixel (same width).
Steady.
1:1 imaging.
One exposure time.
= center of pixel
10 µm 10 µm
20 µm 25 µm
Direction of the movement (belt).
14
NON-SQUARE PIXELS
The original 50% difference in length causes less than 20% more overlapping and only in one dimension.
1:1 imaging.
One exposure time.
Some optical
inaccuracy added.
10 µm 10 µm
20 µm 25 µm
35 µm30 µm
15
BINNING IMPROVES OPTICAL ACCURACY
The optical overlap between the pixels to be binned will all be covered by the new binned pixel.
BINNED
16
SUMMARY (1 of 3).
1. The principles of line scanning in terms of horizontal and vertical resolution do not depend on the
size or shapes of the pixels.
2. The triggering, timings, resolution and dimensions can be set based on the center point of each pixel
regardless of the shape, size or binning of the pixels. The area and shape of the pixel around it can
be thought of as the collecting area of light (image) for that center point.
3. The horizontal resolution (along the line) is the distance between these center points.
4. The vertical resolution is the distance the center point travels during one line scan.
17
SUMMARY (2 of 3).
5. Overlapping of pixels occurs in the vertical direction due to movement during each scan and in both
dimensions due to optical limitations.
6. Rectangular pixels increase vertical overlapping, but clearly less than what the dimensions would
indicate.
7. Binning helps equally well with both square and non-square pixels by hiding the common overlapping.
18
SUMMARY (3 of 3).
8. Larger pixels bring a benefit proportional to the pixel area. This can be used to increase response or
to decrease noise.
9. Use of higher scan rates does not respectively increase the vertical resolution as the scans will start
to overlap each other.
10. Use of low scan rates minimises vertical overlapping with the cost of lower resolution.
