advanced camera and image sensor technology...rolling shutter cameras can offer a offer a flash...
TRANSCRIPT
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Advanced Camera and Image Sensor
Technology Steve Kinney
Imaging Professional Camera Link Chairman
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Content
• Physical model of a camera • Definition of various parameters for EMVA1288 • EMVA1288 and image quality • CCD and CMOS sensors, image artifacts • Noise in cameras • Spectral response and penetration depth • Area scan and line scan • Spatial trigger • Monochrome and color • Single-, dual-, tri-linear sensors, TDI • Mechanical and optical pixel size limitations • Camera mounting standards
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Physical Model of a Camera
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• When light is emitted from a light source, then passes through the optics and hits the Silicon, electrons will be generated.
• The probability of how many electrons will be generated per 100 photons is called Quantum Efficiency (QE).
QE = number of electrons / number of photons
• Typical values are 30 to 60 %.
What are the Important Things? Quantum Efficiency
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• Each pixel has a maximum capacity to collect electrons.
• But we will limit that to a lower maximum, because non-linear effects occur as the maximum capacity is reached.
• A pixel may have a saturation capactity of 50000 electrons.
• A glass of wheat beer contains approx. 50000 drops of beer.
• A glass of wine or champagne might have 10000 drops.
What are the Important Things?
Full Well Capacity
Saturation Capacity
Full Well and Saturation Capacity
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• When refilling the pixel to the same previous value, there is a slight jitter in the amount of electrons created. This “noise“ is equivalent to the square root of the number of electrons.
noise = 𝑒− • So the maximum Signal to Noise Ratio (𝑆𝑆𝑆𝑚𝑚𝑚)
is:
𝑆𝑆𝑆𝑚𝑚𝑚 = 𝑒𝑚𝑚𝑚−
𝑒𝑚𝑚𝑚− = 𝑠𝑚𝑠.𝑐𝑚𝑐.
𝑠𝑚𝑠.𝑐𝑚𝑐. = 𝑠𝑠𝑠. 𝑐𝑠𝑐.
• 5000𝑒− = approx. 224 • Refilled beer glasses do not have the same
content.
What are the Important Things?
Refilled Level
Signal to Noise Ratio
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• There is always some residual noise, even when no light hits the sensor.
• Depending on the sensor, this is between 8 to 110 electrons.
• Compare this to the remaining drops of beer in an “empty“ glass.
• Heat is a contributor.
What are the Important Things?
Full Well Capacity
Dark Noise
Saturation Capacity
Dark noise
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• If the signal is the same as the dark noise, we call this detection limit.
• CCD sensors usually have a detection limit of 8 to 25 electrons. This corresponds to 15 - 70 photons.
• CMOS sensors will start at 10 to 110 electrons, or 24 -100's of photons.
What are the Important Things?
Dark Noise
Detection Limit
Detection Limit
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• Dynamic range is the ratio between a full glass of beer and an empty glass. So, a pixel's dynamic range is:
𝐷𝐷𝑆 = 𝑠𝑚𝑠.𝑐𝑚𝑐.𝑑𝑚𝑑𝑑 𝑛𝑛𝑛𝑠𝑒
= 50000𝑒−
25𝑒− = 200
• A glass of beer will always
have dynamics!
What are the Important Things?
Dark Noise
Saturation Capacity
Dynamic Range
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The Diagram
1 10 100 1000 10000 100000 1000000
1
10
100
0
1
2
3
4
5
6
7
8
Number of Photons
Signal to Noise Ratio
bits
Typical CCD
Typical CMOS
dB 50
40
30
20
10
0
Dynamic Range
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• We use a logarithmic scale to display the large range of values we are concerned with.
• A linear scale will not represent this properly. example: Imagine a scale that can display both kilometers and millimeters at the same time with the same precision.
How to Understand the Diagram?
1 10 100 1000 10000 100000 1000000
1 10 100 1000 10000 100000 1000000
Number of Photons
Number of Electrons
Set Quantum Efficiency to 50%, 100 photons will generate 50 electrons.
The x-Axis
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Actual SNR is also displayed on a logarithmic scale.
A linear scale always adds the same unit.
A logarithmic scale always multiplies by the same unit.
Bits (2𝑛) and decibel (𝑑𝑑) are logarithmic scales.
Multiplying SNR by 2 = increasing by 1 bit = increasing by 6 𝑑𝑑.
How to Understand the Diagram?
1
10
100
0
1
2
3
4
5
6
7
8 bits dB
50
40
30
20
10
0
20
2 3 4 6
30 40
200 300
60
Signal to Noise Ratio
1 bit = 6.02 dB
The y-Axis
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• An excellent image is SNR = 40 or better.
• A good image quality is SNR = 10.
How to Understand the Diagram?
1
10
100
0
1
2
3
4
5
6
7
8 bits dB
50
40
30
20
10
0
20
2 3 4 6
30 40
200 300
60
Signal to Noise Ratio
Image Quality
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Saturation capacity: 18000 electrons Dark noise: 9 electrons QE @ 545 nm: 56 % (electrons per photons)
Saturation reached with:
180000.56⁄ ≈ 32140 photons
Detection limit: 9 0.56⁄ ≈ 16 photons
Max SNR: 18000 ≈ 134 ≈ 7.1 bits ≈ 43 dB (134 ≈128 = 27)
Dynamic range:
saturation cap./dark noise = 18000 / 9 = 2000 ≈ 11 bits ≈ 66 dB (2000 ≈
Examples: Sony ICX285
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Saturation capacity: 8000 electrons Dark noise: 8 electrons QE @ 545 nm: 50 % (electrons per photons)
Saturation with 8000
0.50⁄ ≈ 16000 photons Detection limit 8
0.50⁄ ≈ 16 photons
Max SNR: 8000 ≈ 89 ≈ 6.5 bits ≈ 39 dB
(89 is between 64 = 26 and 128 = 27 ) Dynamic range: saturation cap./dark noise = 16000/16 = 1000 ≈ 10 bits ≈ 60 dB (1000 ≈
1024= 210)
Examples: Sony ICX274
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Image quality depends on the signal to noise ratio (SNR). Total noise consists of:
– Temporal Noise (~ √number of photons) – Dark Noise (~ number of photons)
Limitations in the Diagram
1 10 100 1000 10000 100000 1000000
100
1
10
Number of Photons
0
SNR ~#Photons
~√#Photons
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• Today's CMOS sensors are able to achieve the same low read noise as CCD sensors (about 7 to 10 electrons).
• In sensors with a rolling shutter, it can be even less, due to less transistors being used.
Today's CMOS Sensors
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Progressive Scan vs. Interlaced Scan
Why is progressive scan important?
• Progressive scan gives you a full image for all lines (or pixels).
• Interlaced scan uses the odd & even lines separately.
• This can result in a “fringe effect“ like you see in a TV's news ticker.
• Progressive Scan:
•Interlaced Scan:
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Standard Resolution, Progressive Scan
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Standard Resolution, Interlaced
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Interlaced Artifacts
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Global Shutter vs. Rolling Shutter
Why is global shutter important?
• Global shutter has the start and stop of exposure time for all pixels at the same time.
• A rolling shutter starts and stops exposure one line after the other (sequentially). This is like a flatbed scanner.
• Global Shutter:
• Rolling Shutter:
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Flash Window and Hybrid Mode
Rolling shutter cameras can offer a offer a flash window. When the last line starts exposure, the flash window opens until the first line stops exposure. Using a strobe light during this time can freeze the image, effectively becoming a global shutter.
• Flash Window:
• Some "hybrid" sensors exist that start exposure like a global shutter, and end exposure like a rolling shutter. Downside is uneven exposure times.
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Standard Resolution, Global Shutter
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Standard Resolution, Rolling Shutter
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Global Shutter Artifacts
Source: Wikipedia
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Resolution
What is the best or required resolution?
• Most people think, the higher the resolution, the better.
• This is not true for machine vision, because all data have to be calculated and displayed.
• Often the higher sensor resolution results in a smaller pixel size.
• Smaller pixels are often below optical resolution and have a lower saturation capacity. A lower saturation capacity results in a worse signal-to-noise ratio and a lower dynamic range.
Refilled Level
Dark Noise
Saturation Capacity
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Standard Resolution
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High Resolution
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Column Noise
• Usually a CMOS sensor has one ADC (analog to digital converter) per column, a CCD has only one (per tap), meaning typically 1 to 4 ADCs for a CCD.
• In the case of a 4 Mpixel CMOS camera, there are 2048 ADCs.
• All of them will be slightly different.
• A correction done in the dark and without power to the sensor can calibrate all ADCs.
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FPN or DSNU
• Fixed pattern noise (FPN) or Dark Signal Non-Uniformity (DSNU) occurs, when all voltages are applied to the sensor, but no light is collected.
• This can be corrected by adding a certain offset per pixel.
• CMOS‘s FPN is up to 10x worse than a CCD‘s, but latest designs are rather equal!!
CMOS CCD
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PRNU
• Photo Response Non-Uniformity (PRNU) describes the different gain per pixel.
• With an illuminated sensor, the different (conversion-) gains can be corrected to create an homogeneous image unter bright light conditions.
• CMOS and CCD behave the same.
Raw data Through a low pass shading corrected
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Spectral Response
0%
10%
20%
30%
40%
50%
60%
400 450 500 550 600 650 700 750 800 850 900 950 1000
Wavelength [nm]
Tota
l Qua
ntum
Eff
icie
ncy
[%]
The spectral response depends on wavelength and sensor type. CMOS sensors often have a wavy curve due to interferometric issues.
CMOS sensors
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Cameras cover a spectral range of the visible (VIS) and near infrared (NIR), wavelengths from 400 to 1100 nm.
Penetration Depth
λ Wavelength [nm]
Penetration depth: 1/10000 of a human
hair
400 500 600 700 800 900 1000 1100
Wavelength : 1/100 of a human hair
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Area Scan and Line Scan
• Scan types can be separated by area scan and line scan.
• Area scan is analogous to a digital still camera. – 1 shot and the 2D image is taken. – As an example: resolution 1300 x 1000 pixels. After an exposure
time of 10 ms, everything is captured. All pixels have an exposure time of 10 ms.
• Line scan is analogous to a Xerox machine. – One line after the other is taken to get the total image. – As an example: resolution 1000 pixels, 1300 lines. With a total
exposure time of 10 ms the image is taken, BUT: every single line (or each pixel) has only an exposure time of 7.7 µs!
– This is a very short exposure time You need much more light!
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The Need of Spatial Trigger
• Camera speed independent of object speed can distort. (e.g. acceleration after a traffic light).
• Only a spatial trigger gives the right information.
• It synchronizes the camera with the object (e.g., an encoder).
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Color: Area Scan
• Color on area scan can be taken either with a: – 3 CCD setup: a beamsplitter separates the colors to three
different CCDs. – Advantage: Every pixel has the full color information. – Disadvantages: Expensive, special lenses, alignment, color
shades.
– In most cameras a Bayer pattern is used.
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Color: Line Scan I
Single Line Dual Line
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Color: Line Scan II
Triple Line 3 CCD Line
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How to Match RGB to One Image I
• The object on the conveyer belt is moved beneath the camera.
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How to Match RGB to One Image II
• Take 1, 8, and 15, but there is a color shade within one group RGB.
Lines 1, 8, 15: 100% 100% 100%
Lines 2, 9, 16: 100% 80% 20%
Lines 3, 10, 17: 0% 0% 10%
Lines 4, 11, 18: 60% 100% 100%
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What is Possible with the Camera?
• Raw Image Raw Corrected
• Corrected Image
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TDI Line Scan Sensor
• TDI stands for Time Delay and Integration. • The object is exposed several times across many
lines, charges are accumulated and shifted simultaneously with the trigger.
• Signal is taken N times, Noise reduces by 𝑵. t1 t1 t2 t3 t4 tN Read Out
Time
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Depth of Focus (DOF)
The depth of focus (DOF) depends on the pixel size, the diameter of the iris and the focal length of the lens.
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Depth of Focus (DOF)
The depth of focus (DOF) depends on the pixel size, the diameter of the iris and the focal length of the lens.
DOF = 2 * Pixelsize * f / d = 2 * Pixelsize * F/#
•f: Focal Length
d:
Dia
met
er Depth of Focus
Pixelsize
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Tilt of the Sensor
The sensor has to be aligned perpendicular to the optical axis.
Given max Depth of Focus
Angle
The whole sensor has to be taken into account
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Examples of Geometrical DOF
Real numbers: Pixel size = 5 µm F/# = 4
Depth of Focus: ±20 µm (40µm) Real numbers:
Pixel size = 4 µm F/# = 2
Depth of Focus: ±8 µm (16 µm)
f: Focal Length
d: D
iam
eter
Depth of Focus
Pixelsize
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Diffraction
Due to the physical structure of light as an electromagnetic wave, the rays are blurred by diffraction. Diffraction depends on the F-number and wavelength λ. ØAiry = 2.44 * λ * F/# As a rule of thumb, the diameter of the Airy disc is F/# in microns, like F/# is 4, the diameter of the Airy disc is approx. 4 µm. Barely resolved
No longer resolved
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Real Spot-size and DOF
The real spot-size and DOF is the geometrics folded with the diffraction. To make life a little easier we will treat the diameter as independent errors (deviations): Øtotal = √Øgeom
2 + Ødiff2
Total DOF
Geometric DOF
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Errors of Higher Order
In case of an 8k line scan sensor with 10 µm pitch we might have a further issue:
The sensor might be bent by Δz = 50 to 80 µm (acceptable)
The lens (e.g. 5.6/90mm) will cause a field curvature of Δz = 50 to 100 µm, whereas the astigmatism might occur and the meridional and sagittal focal plane might run the opposite direction.
Optical Axis
Sensor
Focal Plane
Δz = 50 to 80 µm
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Field Curvature and Astigmatism
In case of an 8k line scan sensor with 10 µm pitch we might have a difference of about 100 µm between both focal planes (meridional and sagittal).
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Pixel Size
• Pixels are mainly between 10 to 3.5 µm. • Trend is toward smaller pixels, because they allow higher
resolution with less silicon More sensors from a single wafer lower cost.
• "Reasonable" limit: 5 µm for monochrome, 2.5 µm for color. • Full-Well capacity (saturation capacity, resp.) is lower for
smaller pixels. Therefore the max SNR is not as good as on a larger pixel.
Optics
Sensor
System
10 5 3.5 µm
$
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Image Format
1” format 1/2” format 2/3” format 1/3” format
12.8 mm 8.8 mm 6.4 mm 4.4 mm
16 mm 11 mm 8 mm 6 mm
1” Tube
Deflection yoke
The image size in inches relates back to the tube camera. The image format of a tube which could be placed in a 1” deflection coil was called 1” format. The active image size is 16 mm in diagonal. It is the same as for a 16 mm film format.
BIPM (Bureau International des Poids & Mesures) recommend to use metric designation. Below is the most common image formats shown.
Diagonal:
Type 1 Type 1/2 Type 2/3 Type 1/3
Image size
Sony:
CCD and CMOS Sensors
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Optical Sizes • In case of C-mount lenses are usually restricted to
2/3“ optical format imagers. Longer focal length or higher f-stop lenses may work up to 1“ format
• The CMOSIS sensor 2MP is 2048 x 1088 pixels with a 12.75 mm in diagonal and fits 2/3“ optical format. The 4MP sensor is 2048 x 2048 pixels, with a diagonal of 15.93 mm requiring a 1“ format lens.
• The 4 MP sensor must use proper lenses! Otherwise there is a shading or vignetteing seen in the image.
CCD
1“ = 25.4 mm ^
CMOS CMOS
16 mm sensor with 1” format 16 mm sensor with 2/3” format will cause vignetting
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Recommended Mechanical Interfaces (Mounts)
Class Min. Image Size [mm]
Max. Image Size [mm]
Optical Size [type]
Mechanical Interface (1st Choice)
I 0 4 ≈ ¼ C-, CS-, NF-, S-Mount
II 4 16 ≈ 1 C-, CS-, NF-, S-Mount
III 16 31.5 ≈ 2 F-Mount, 48 mm Ring, M42 x 1, M48 x 0.75
IV 31.5 50 ≈ 3 M58 x 0.75 (and F-Mount if possible)
V 50 63 ≈ 4 M72 x 0.75 VI 63 80 ≈ 5 M95 x 1 VII 80 100 ≈ 6 M105 x 1
Please check with JIIA LER-004-2010 for more details.
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Questions?
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Contact Information
Steve Kinney Imaging Professional AIA Camera Link Chairman
Phone: +1 (408) 910-4476
Email:[email protected]