LINLOG™

LINLOG™ Technology the key to programmable linear, logarithmic, or combined linear and logarithmic response without image lag or distortion.

The LINLOG™ Technology was developed at the Swiss Federal Institute of electronics and Microtechnology, CSEM in Zurich to overcome drawbacks of known High Contrast imaging principles. The success of the CMOS sensors developed at CSEM lead to the spin off company Photonfocus Ltd. who further developed the LINLOG™ Technology and holds all relevant intellectual property rights.

With the award winning LINLOG™ Technology the drawbacks of pure logarithmic image sensors can be overcome. The LINLOG™ technology uses a linear response at low illumination levels and logarithmic compression at high intensities. See figure 1 for the principle of the LINLOG™ technology.

Figure 1  Principle of the LINLOG™ Response

Figure 2  LINLOG™ Image of a 100W light bulb and a fan rotating at ~ 600rpm. The image is acquired at 100ms exposure time and with aperture N = 1.4 of a 16mm focal lens. The picture presents raw, uncorrected data.

The transition between linear and logarithmic response can be programmed to the sensor. It stays though the same for the whole sensor, which prevents artifacts and discontinuities in image processing algorithms. Special care has further been used to guarantee a steady and smooth transition between linear and logarithmic response. The LINLOG™ pixel comprises global shutter to prevent motion artifacts and guarantees image lag free acquisition even at high frame rates. The use of the linear response for low light levels further increases the signal swing at the output of the pixel and thus increases the sensitivity. It further reduces effects of fixed pattern offset and gain noise known from logarithmic sensors. The performance of the LINLOG™ pixel can be seen in figure 2.

At an 8 bit digitalization, and using a full well capacity of about 200 000 electrons a sensitivity of 825pJ/cm2 has been measured. This high sensitivity compared to most Logarithmic sensors can be achieved because under low illumination conditions the optical signal is integrated over the exposure time. A measurement of the first LINLOG™ sensor with a pixel of 10.6 x 10.6 mm and an optical fill factor of 35% can be seen in figure 3.

Figure 3  Sensor response in the linear range.

Figure 4  Histogram of the sensor output at 8-bit digitalization and with homogenous illumination. The FPN is of only 2.5 gray values RMS.

LINLOG™ 2 (Programmable compression)

The LINLOG™ technology brings major advantages to high dynamic imaging. Image quality could be significantly improved, both, regarding temporal and fixed pattern noise. The use of integration at low light intensities increases sensitivity. An over all in scene Dynamic range of 180dBA has been measured. The global shutter operation of the LINLOG™ Technology permits imaging of fast moving objects without distortion and the use of pulsed illumination.

In further implementations of the LINLOG™ technology not only the transition between linear and logarithmic response, but also the degree of logarithmic compression will be programmable. See figure 1 for the principle of this further development and figure 2 for example images.

Figure 1 Principle of the extended LINLOG™ Response, permitting programmable logarithmic compression.

Figure 2  Images of a laser triangulation application in welding inspection . a) Linear image: the laser line is completely saturated b) LINLOG™ image: the laser line is more clearly visible, though on the welding it vanishes due to the strong logarithmic compression. c) advanced LINLOG™ image: due to the reduced compression the laser line is clearly distinguishable over the whole scene by simple thresholding. The background is still clearly visible for texture recognition.

Linlog Products

Currently there are available the following products with LINLOG™ Technology:
Photonfocus camera series MV-D1024, MV-D752, MV-D1024x128

Disadvantage of classical logarithmic sensors

In a classical logarithmic photo detector the pixel converts the photocurrent directly in a voltage logarithmically dependent on the impinging intensity. Due to this mechanism, no actual exposure time is needed. This brings the advantage that the sensor can be read out completely randomly, though it brings some major disadvantages when imaging moving objects since the logarithmic sensor behaves like a rolling shutter sensor due to the sequential addressing. One of the major disadvantages of rolling shutter exposure control is the distortion it creates when imaging fast moving objects, or when used with pulsed illumination. Figure 1 describes the effect of the rolling shutter when imaging an object moving in line direction. Suppose a sensor is used to acquire an image of a fast moving bar. When reading out the line x where the impinging light is instantly converted to a voltage signal, the bar enters the field of view. As the readout pointer scans through the sensor the bar moves orthogonal to the scan direction, its position in column direction at the readout time depends on the line number. The result is a severe distortion, as it can be seen in the images of a fast moving van, shown in Figure 2.

Figure 1  Effect of the sequential readout when imaging moving objects.

Figure 2a Image of the fan at 0 rpm with a traditional logarithmic response sensor.
Figure 2b Image of the fan at ~ 1600 rpm with logarithmic response. The fan is strongly distorted due to the implicit rolling shutter mechanism.

Besides motion artifacts the rolling shutter behavior prohibits the use of a pulsed light source for illumination, since the light pulse cannot be synchronized to an integration time.

A further disadvantage of pure logarithmic sensors is the important image lag. Besides blurry images this can lead to reduced frame rate even if a sensor permits fast readout. The actual frame rate can be limited to less than 1 frame per second, depending on illumination conditions, since at higher rates, the same scene information is read out various times.

To understand the response time of a logarithmic detector, the conductance of the loading MOS transistor is calculated by formula 1.

Formula 1 and 2

Thus at a given light intensity, the response time to a small change of the photocurrent is given by formula 2. This response time becomes excessively long for very small photocurrent values.

References

Awards
Vision Award (October 2000): CSEM from which Photonfocus spun off wins the innovation Award of the major European Machine vision trade show for its invention in the CMOS image sensing field, and particularly the LINLOG Technology.
Photonics Circle of Excellence Award (January 2002): Photonfocus wins for its MV-D1024k CMOS Camera implementing for the first time a sensor with LINLOG response this award distributed by Lauring Publishing at Photonics West 2002, the major SPIE conference and trade show in San Jose USA.

Expert Opinions
Don Braggins works as independent scientific journalist. He reports for the last 20 years frequently about the Image Sensor Developments and Applications around the World.
Don Braggins about Photonfocus: "I understand that Photonfocus is a spin off from CSEM Zurich, formerly the Paul Scherrer Institute Zurich. I have followed developments there since 1980 when I was impressed by a paper by Dr. Peter Seitz, at a conference I chaired in the Hague, and I have visited the labs in Zurich serveral times. I am a member of the jury for the annual innovaton prize awarded each October at the Stuttgart “Vision” exhibition, and in 2000 I voted the Linlog sensor submitted by CSEM as top entry, and I am pleased to say that other members for the jury agreed with my vote.
Going back to my days as a practitioner of image analysis in the 1970s, it was always necessary to choose between the vidicon (thermionic tube) camera with a near logarithmic response but poor sensitivity, or one using the much more expensive (proprietary) Plumicon with a linear response and better sensitivity but limited dynamic range which meant it would “bloom” very easily if exposed to bright light. Solid state cameras have now very largely replaced thermionic ones, but until the advent of cameras based on the lin-log sensor now made by Photonfocus, it was similarly necessary to choose between solid state camera types, mostly having as linear response with very few offering anything like the vidicon’s ability to deal with a very wide dynamic range.
Reporting the 2000 Vision event for Advanced Imaging, I said
“One of those non-German exhibitors was CSEM Zurich which won the hotly contested competition for the best innovation with is very high dynamic range LinLog photosensing principle. This enables CSEM to claim high dynamic range, high-speed image sensing with the worlds best low light level performance. The CMOS camera combines the advantages of linear response for low light levels and logarithmic compression at higher illumination. The changeover point is controlled by software with a simple slider control in the user interface. Peter Seitz, until very recently head of the image sensors group at CSEM Zurich, now heads the entire operation there, and indicated that the CSEM Mountain View range of cameras is likely to be the subject of a spin-off into a separate company soon, following the example of other recent developments from the organisation.”
I talked at some length with Photonfocus staff at Vision 2001, and I am satisfied that they will continue the highly innovative tradition inherited from CSEM Zurich, with the great advantage that the company’s products will be available in commercial quantities, which CSEM was never equipped to offer."