Stephane Laveau (DxO) : “Colour shading : root causes and review of solutions”.
This presentation started with showing images of the effect in existing (high-end) mobile phone cameras. Where does colour shading come from ? Main causes can be found in :
- IR filter (IR cut is depending on the angle of incidence, thus spectrum varies with light angle),
- microlenses (creating optical crosstalk),
- colour filters,
- optical stack.
Factors that make colour shading more complicated to model and to correct :
- light sources, they come in various flavours,
- manufacturing tolerances resulting in different IR cut off wavelengths.
Correction methods :
- calibration per unit, but is very expensive,
- in the camera by means of white balance adjustment, but this has a limited application in the case of fluorescent or mixed lighting.
Adaptive correction : statistics will be obtained from a single image by looking for changes in hue. Examples of this adaptive correction were shown to illustrate the improvements of the method.
Yang Ni (New Imaging Technologies) : “MAGIC : Logarithmic Image Sensing using photodiode in solar cell mode”.
A classical method of doing wide dynamic range is the use of logarithmic pixels based on a transistor operating in weak inversion. This solution is giving quite a bit of FPN, is suffering from a low sensitivity and shows a large image lag. NIT has changed the concept of the logarithmic pixel : the photodiode is used in the solar cell mode, and the voltage across the PD is a logarithmic function of the current generated by light input. Advantages : physically exact dark reference (no light input means 0 V across the diode), on-chip FPN correction, high sensitivity, no image lag. The noise in this logarithmic mode pixel is equivalent to the noise of a 3T pixel, being the classical kTC noise (around 500 uV). This noise level is constant in the logarithmic mode. It does limit the dark performance of the sensor, but does not limit the dynamic range of the sensor.
A 768 x 576 is developed with 5.6 um pixel design, in 0.18 um CMOS process, fill factor is larger than 70 % (no micro-lenses), and the light detection threshold is smaller than 7 mlux @ 25 fps. Good colour rendition can be maintained over 120 dB variation in light input, no white balance is needed, neither any tone mapping is needed. Impressive videos were shown of the performance of the devices in a lab environment.
Sandro Tedde (Siemens) : ”Organic and hybrid photodiodes for active matrix imaging”.
Fabrication of the devices is based on a spray-coating technique on top of glass or even on metal foils or polymer foils. QE of 85 % @ 550 nm is reported. An attractive feature of this technology is the option to change the organic material and in this way the sensor can be optimized for a particular wavelength region.
Applications for this technology :
- near-infrared imagers with polymers, by changing the bandgap of the light absorber (changing material) from 1.9 eV to 1.46 eV,
- flat panel X-ray imaging, a prototype of 256 x 256 pixels is shown including videos to demonstrate the capabilities of the image sensor,
- organic position-sensitive devices, based on a device with only 4 pixels each about 1 cm2,
- thin and optic free light barrier based on these organic photodiodes,
- low bandgap material by doping of PdS quantum dots, the diameter of the quantum dots can be used to optimize the sensitivity of the devices for a particular wavelength,
- integration of organic photodiodes on CMOS backplanes, a feasibility study on a 1.2 Mpixel with 4.8 um pixel size was performed. The first results were shown.
Renato Turchetta : “Large area CMOS image sensors for X-ray and charged particle detection”
The presentation started with an overview of some theory about detection of particles in silicon, and the difference between direct and indirect detection of incoming particles. Next Renato introduced their INMAPS process, being a CMOS process based on p-substrates with the nMOS transistors in a p-well and the pMOS transistors on an n-well. Underneath the n-well an extra deep p-well is introduced to protect the n-well of acting as an electron drain. An example of a sensor designed in this technology is a ToF Mass Spectrometer, 72 x 72 pixel array, 70 um x 70 um pixel size, time resolution < 100 ns, equivalent to more than 10 Mfps.
For integrating sensors, 2 examples were shown :
- medical imaging full field mammo : about A4 size, and chest imaging : about A3 size. The devices are processed on 200 mm wafers, and are made 3-side buttable, photodiode size is determining the noise level to 30 e-, design is just completed : 145 mm x 120 mm, 50 um pixles, 40 fps analog out, 2x and 4x binning,
- TEM sensor in production : 4k x 4k sensor, 4 sensors/wafer, direct detection of electrons, showing good MTF and DQE, having a good radiation resistance, 0.35 um CMOS, 61 x 63 mm2, ROI and pixel binning.