CN105009565B - Method for generating images and line sensor camera - Google Patents

Abstract

The invention relates to a linear sensor camera, intended to be viewed by scanningA scene is observed, which is swept across the camera. A sensor (IMS) provides, for each image point observed, an output level (Ss) representative of the illuminance of this point for a possible varying effective accumulation duration Ti, said camera comprising: a synchronization input means for receiving relative scan rate information items; for establishing an image line reading cycle period T_LThe image line reading cycle period T_LInversely proportional to the relative scan rate; and receiving a desired cumulative duration value Ti_DThe apparatus of (1). The method is characterized in that in at least a first operating mode, in which the camera provides an illuminance measurement level (Sc) for each image point, the accumulated duration Ti that is active is equal to the reading period T_LThe illuminance measurement level (Sc) is multiplied by the desired integration duration Ti_DAnd row read cycle period T_LThe output level of the sensor of the ratio (c) is proportional. The application comprises the following steps: and (5) industrial control.

Description

Method for generating images and line sensor camera

technical field

The present invention relates to a line sensor camera, which is intended to observe a scene by scanning, which is in relative motion with reference to the camera. These cameras are especially intended for inspecting objects, for example luggage on conveyor belts, or inspecting objects for controlling industrial production.

Background technique

The sensor can comprise a single row of pixels or multiple rows of pixels, and in the latter case it can be operated in a so-called TDI ("Time Delay Integration") mode to increase the signal-to-noise ratio, in TDI mode , the pixels of various rows continuously see the same image row, and the charges light-generated by the various rows are superimposed at the instant they see the same image row.

It will be appreciated that in the case of a TDI sensor it is very important that the speed of the motion is preferably synchronized with the frequency of capture of the image lines. If this is not the case, signals that do not see the same image line will also be superimposed. This is why a camera is provided for processing that receives an input of an item of information about the velocity of the relative movement, and this item of information is used to define the periodicity with which the reading is performed for rows of pixels.

Even if the sensor is not operating in TDI mode, and even if it has only one row of pixels, it is important to synchronize the capture period of the image rows with reference to the speed of the relative motion. If this is not the case, the reconstructed image will be flattened or stretched in the direction of motion. Furthermore, velocity fluctuations during image capture will cause uncontrollable deformations of the reconstructed image. In this case, the camera receives an item of information about the velocity of the relative movement, which item of information defines the periodicity of the line readout.

Furthermore, for a given lighting condition, the signal level produced by the sensor is substantially dependent on the accumulation time during which the photogenerated charges are accumulated by the pixel prior to being read out. If the charge is read periodically with a read period _TL , the accumulation time Ti is still limited by the value of _TL ; in fact, the charge can be accumulated for the duration of the period _TL , but at the end of this period , they need to be read and the pixels reset for a new read. In the case of a CCD sensor, resetting is done by transferring charge to a readout register (or other row of pixels for a TDI sensor); in the case of a sensor with active pixels, by storing the charge of the pixel Nodes are cleared for re-provisioning.

If the speed of motion is faster, the read period _TL will be shorter, and thus the accumulation time will be shorter. In this case, it is preferable that charges will be accumulated throughout the read period _TL . Conversely, if the speed of motion is slow, it is preferred that the camera includes means for adjusting the accumulation time so that charge is only accumulated during part of the readout period, otherwise there is a risk of saturating the pixels.

In the case where the means for controlling the accumulation duration is utilized, even if the speed changes, it is possible to achieve a peak in the stable brightness of the image because it is sufficient to provide a fixed accumulation duration independently of the reading period, fixed The duration of accumulation is determined by the speed of motion of the image past the sensor. But in case the light is accumulated during the row readout period _TL , the brightness level of the electronic image varies proportionally to this duration _TL . Thus, the brightness level of an electronic image varies according to fluctuations in speed from one image to another, even during the imaging process. This is particularly inconvenient when the speed changes a lot during image capture.

Variations in movement speed may result from uncontrolled fluctuations in the movement system. They can also result from deliberate changes, for example starting from almost zero velocity of motion, followed by acceleration, stabilization, and then braking in the event that an object is desired to be inspected. In the latter case, it may need to work with a constant accumulation duration, so as not to suffer from global brightness variations of such electronic images produced. Therefore, this needs to work with a camera with cumulative time control. But cameras controlled with accumulation time are less efficient in terms of fast time sensitivity, specifically because they do not use the entire readout period time to accumulate the photogenerated charge. In fact, in this case, for a fixed row read period (T _L ), the cumulative time may not be greater than T _{L −TM} , where _TM is the dead time as will be explained further.

Contents of the invention

In order to facilitate the acquisition of good images even under variable conditions of illumination and motion, especially during the capture of the image, it is proposed according to the invention to generate an image by means of a camera comprising a linear an image sensor, said linear image sensor moving relative to the image to be observed, said sensor providing, for each observed image point, an output representing the illuminance at that point for an effective accumulation duration Ti (which may vary) level, said camera comprising: synchronization input means for receiving an item of information about the velocity of the relative motion; means for establishing an image line readout cycle period _{T L} which is related to The speed of relative motion is inversely proportional; used to receive the value of the desired cumulative duration Ti _D. The method is characterized in that it comprises two separate operating modes.

- a first mode of operation in which the effective accumulation duration Ti is equal to the read cycle period _TL and in which the camera provides for each image point an illuminance measurement level multiplied by the desired accumulation The duration Ti _D is proportional to the output level of the sensor to the ratio of the row read period _TL .

- and a second mode of operation which has an accumulation duration which is less than the read cycle period _TL and in which the camera provides for each row an illuminance measurement level which is not related to the desired accumulation duration Ti _D is proportional to the output level of the sensor weighted by the ratio of the row read period _TL .

The selection between the first mode and the second mode is made according to the speed of the relative movement, the first mode being utilized as long as the read cycle period is below a threshold and the second mode being utilized in case this threshold is exceeded.

Thus, assuming that the read period T _L bounded by the speed of motion changes during the snapshot and becomes different from the desired accumulation duration Ti _D for certain rows, the camera will provide a weighted ratio Ti _D /T _L for these rows Correction signal (signal rectifié); correction level to compensate, in fact, the actual accumulation duration is no longer Ti _D , but T _L . For example, T _L becomes smaller than Ti _D , and the correction level provided by the camera becomes higher than the actual level provided by the sensor; thus, the camera uses the duration Ti _D to emulate a snapshot, although this duration is not actually used for these lines. Conversely, if, for example, it is assumed that the desired accumulation duration _Ti is equal to the minimum reading period allowed by the environment (the environment corresponds to the maximum speed of motion in the envisaged application), and the speed of motion is assumed to slow down, the actual accumulation duration will be Increased beyond the desired duration; the camera will then provide a correction level that weights the ratio Ti _D /T _L and is lower than the actual value provided by the sensor.

This involves the case where the accumulation duration is equal to the period of the read cycle, so that the overall brightness level becomes consistent by compensating for level changes due to changes in motion speed. The value of the desired accumulation duration Ti _D will not be used to make an effective accumulation duration adjustment, since the accumulation duration is equal to the duration of the read cycle, but it is only used to establish the multiplicative factor Ti _D /T _L , which compensates for Velocity fluctuates and provides consistency to the electronic signal representing the image.

The invention is then applicable to cameras in which the sensor operates in TDI mode, that is to say summing the signals in two or more pixels seeing the same image point during a movement synchronized with the reading cycle.

In a second mode of operation, with an accumulation duration smaller than the read cycle period _TL , the sequencing device establishes an effective accumulation duration equal to the desired accumulation duration Ti _D. Thus, there is a first mode without effective control of the accumulation duration (which is equal to the cycle period), and a second mode with active control of the accumulation duration (which is equal to the desired duration).

In a sensor with CMOS active pixels, where each pixel includes a photodiode, a charge storage node, a transistor for resetting the storage node, a charge transfer transistor, and a transistor for reading the potential of the storage node, the read The period _TL is defined by the periodicity of the transfer pulse, which ensures the transfer of charge from the photodiode to the storage node before it is read. If effective accumulation duration adjustments are made, this is achieved by establishing an additional charge transfer pulse between two periodic transfer pulses, which is established at the storage node in such a way that this additional pulse empties the charge from the storage node The potential at the moment is re-preset by re-presetting the transistor. In pixels with an additional transistor for resetting the potential of the photodiode, this diode is controlled by a pulse for resetting the potential of the photodiode, so that an adjustment of the effective accumulation duration, effective accumulation duration Defined by the time separating the end of this reset pulse from the end of the following transmit pulse.

For a camera with these two modes of operation it is preferably provided that the camera operates in the first mode as long as the read cycle period is below a threshold and in the second mode when this threshold is exceeded. This threshold depends on the speed of motion (which is for a given desired accumulation duration).

The motion speed threshold for switching from a mode without cumulative duration control to a mode with cumulative duration control is the threshold for the sensor read cycle period because motion speed and cycle duration are related. The read cycle duration threshold T _LS is chosen to be equal to the sum of the desired accumulation duration Ti _D and the value T _M , where T _M is the duration of the estimated dead time during which the sensor is in the accumulation time control mode charge cannot accumulate in the pixel row under working conditions. The duration of this estimated dead time is dependent on the configuration of the sensor.

In a specific embodiment, the camera can operate in TDI mode without cumulative duration control, but cannot operate in TDI mode with cumulative duration control. In this case, a distinction needs to be made between the accumulation duration in TDI mode and the accumulation duration in non-TDI mode, because they do not have the same meaning, due to the fact that TDI mode with N rows gives higher than the same accumulation duration N times the signal level. As will be further explained, the threshold for switching from a mode without cumulative duration control (with TDI) to with cumulative duration control (without TDI) is the value T _LS =Ti _D +T _M , but with respect to Such a condition that the desired accumulation duration Ti _D is considered as the duration in the mode with accumulation duration control, rather than the desired accumulation duration T' _iD in the mode without control but with TDI. With Ti _D =NxT'i _D , and the read cycle duration threshold T _LS is NxT'i _D +T _M .

Preferably, in both operating modes, an output level of the sensor is provided which is further multiplied by a safety factor Ks greater than 1, aimed at avoiding the risk of saturation of the digital output of the sensor; such saturation may occur especially in Change the threshold of the control mode in the neighborhood of the control mode that does not have the cumulative duration. Preferably, the coefficient Ks is at least equal to (T _Lmin +T _M )/T _Lmin , where T _Lmin is the minimum reading period below which the sensor cannot operate, and T _M is the dead time of the sensor in cumulative time control mode duration. The meaning and choice of this coefficient Ks will be explained further. The factor Ks is multiplied by 2 for a sensor operating as a TDI charge-summing sensor with two lines, and by N for a TDI sensor with N lines, which in mode without accumulation duration control utilizes TDI summing sum to work, and not with TDI summation with cumulative duration control. If the mode with cumulative duration control is the mode with TDI summation, the coefficient Ks is at least equal to (NxT _Lmin +T _M )/T _Lmin .

In addition to the method for generating the images, which has already been explained in general terms, the subject of the invention is also the camera itself, which makes it possible to obtain these images. Therefore, the present invention proposes a scanning camera comprising a linear image sensor with at least one row of pixels capable of producing an output voltage of the sensor representing the illuminance of an image point within an effective accumulation duration Ti (which may vary). Flat, said camera comprises: synchronous input means, it is used to receive the information item about the speed of relative motion; Be used for establishing the means of image line reading cycle period _TL (it is inversely proportional to the speed of relative motion); Means for receiving the value of the desired accumulation duration Ti _D ; sequencing means capable of establishing at least one first mode of operation in which the effective accumulation duration Ti is equal to the reading cycle period _TL ; said camera is characterized in that: The sequencing device is designed to establish a second mode of operation with an accumulation duration equal to the desired accumulation duration and less than the reading period _TL ; characterized in that said camera comprises:

- multiplication means for providing, in the first mode, for each row of the image the illuminance measurement level (which is multiplied by the output of the sensor multiplied by the ratio of the desired accumulation duration Ti _D to the row read period T _L proportional to the level), and in the second mode, provides the illuminance measurement level unweighted by this ratio.

- and means for automatically switching the camera from the first mode to the second mode, and from the second mode to the first mode, depending on the speed of the relative movement.

Description of drawings

Other features and advantages of the present invention will become apparent by reading the following detailed description given with reference to the accompanying drawings in which:

- Figure 1 represents a row pixel image sensor;

- Figure 2 represents the circuit for digitally processing the signal provided by the sensor following the circuit for reading the pixels;

- Figure 3 represents a timing diagram of the operation of the image sensor of Figure 1 without the control of the accumulation duration;

- Figure 4 represents a timing diagram of the operation of the image sensor of Figure 1 with accumulation duration control;

- Figure 5 represents another timing diagram of the operation of the image sensor of Figure 1 with accumulation duration control;

Figure 6 summarizes the two modes of operation of the camera and the digital output signals derived from each mode;

- Figure 7 represents the dead time _TM preventing the effective accumulation duration from being equal to the duration of the reading period;

- Figure 8 represents a block diagram of a camera showing that the input values Ti _D and _TL are simultaneously controlled in one mode of operation of the camera, with the result being a weighted or unweighted sensor output signal.

Detailed ways

The invention will mainly be described in the context of sensors with a single row, since the invention is applicable to cameras utilizing sensors with or without TDI summing, and even to sensors with a single row of pixels.

The invention applies to charge-transfer passive pixel sensors of CCD technology or MOS technology, and also to active pixel sensors. The active pixel sensor will be described in detail.

The camera according to the invention comprises input means for receiving an item of information about the relative speed of motion of the image with reference to the sensor. This item of information is used to employ the readout period _TL of the row of pixels in synchronization with the movement. This is an instantaneous velocity; the velocity may actually vary during the observation of the image. Even though the duration of the read period varies from one row to another, the expression "read period" is used for the sake of simplicity.

With reference to the relative movement of the camera with respect to the object, information on the speed of movement can be transmitted to the camera, for example via a code wheel driven by a mechanical system.

The camera also comprises means for receiving an item of information about the desired accumulation duration Ti _D . This is the expected duration for viewing image lines. In case the user uses the camera, it adjusts this duration according to the illumination and motion speed conditions. For example, for industrial control applications, the nominal speed of motion can be imposed by a conveyor belt. If fast motion is desired, the light should be as strong as possible, otherwise there would be too weak a signal per cycle; but at the same time cost would need to be limited both in terms of consumption and cost of high power lamps, and this might limit the speed of motion .

If the camera is controlled by a computer, the desired accumulation duration Ti _D , which is selected according to these conditions, is applied to the camera, for example through a software interface. The duration is adjusted for the entire observed image. This duration is constant for each row.

In one example, the user learns that the sensor cannot function with a read period time less than a certain value (eg, 10 microseconds). The user fixes the nominal cycle time to a value of 10 microseconds. And the user fixes the desired accumulation duration Ti _D to this same value. If the sensor works in TDI mode with N lines, the user can fix the accumulation duration T'i _D to 10 microseconds, knowing that in practice this gives an accumulation duration equivalent to its accumulation duration Ti _D = N.T' A signal of i _D (ie N times 10 microseconds), and the latter value Ti _D will be considered as the "desired accumulation duration".

Figure 1 shows the basic structure of an active pixel row with four MOS transistors in which the invention can be implemented. Optionally, the fifth transistor of the pixel is indicated by a dashed line.

Each pixel comprises a photodiode PH for converting photons into charges and a capacitive charge storage node FD for temporarily storing the generated charges so that they can be read later.

In addition, the pixel includes a transfer transistor T1 that connects the photodiode to the storage node and can allow charges accumulated through the photodiode to be transferred to the storage node. The transfer transistor T1 is controlled by a conductor TG1 which is common to all pixels in a row. The transistor T2 for resetting the storage node may reset the potential of the storage node to the reference potential VREFP. This transistor is controlled via a control conductor RST which is common to all pixels in a row. The read transistor T3 has a gate connected to the storage node, and it is installed as a voltage follower so that a potential corresponding to the potential of the storage node can be transferred onto its source. Its drain is connected to a supply potential, which may be the potential VREFP, or a different potential (preferably greater than VREFP). The active transistor T4 may connect the source of the follower transistor T3 to a conductor of the so-called "column conductor" COL associated with the pixel. For each pixel in a row, there is a corresponding column conductor. The active transistor T4 is controlled by a conductor SEL which is common to all pixels in a row. This transistor T4 and its control conductor are optional for sensors with only one row of pixels. It is used to select rows when there are multiple rows that should be read independently of each other. Finally, the fifth transistor T5 can optionally be set to reset the potential of the photodiode.

The column conductor is preferably connected to a constant current source, which, with transistor T4 turned on, causes transistor T3 to operate as a voltage follower.

The charge collected by the photodiode and transferred to the storage node FD modifies the potential of this node and is read out on the column conductor COL associated with the pixel. Sampling and analog-to-digital conversion circuitry CL is connected below the column conductors. The various readout circuits are connected to a multiplexer, which sequentially transmits the digital signal Ss issued at each cycle from the individual pixels of the sensor to a digital output device.

In a simplified embodiment, the readout circuit can be constituted as in FIG. 2, two sampling capacitors Cr, Cs, two respective circuit breakers KR, KS, and an analog-to-digital converter ADC, the analog-to-digital converter ADC Used to convert the difference in potential levels sampled in the two capacitors. The capacitor Cr is used to sample the reset level of the storage node before the charge generated by the photodiode is transferred; the capacitor Cs is used to sample the useful potential level of the storage node after the charge transfer. The digital signal Ss sent from the sensor can be processed by the digital processing circuit DSP. It will be further seen that the present invention proposes to include such a digital processing circuit in which the signal of the sensor is multiplied by the value Ti _D /T _L , and optionally by a safety factor Ks, so that it is equal to Ss × Ti _D /T _L or The digital output signal Sc of the camera, which is proportional to Ss×Ti _D / _TL , peaks.

The timing diagram of Fig. 3 represents the duty cycle of the sensor in the mode without adjustment of the accumulation time, in which mode the charge is accumulated for the duration of the read period _TL . The timing diagram represents the control signals applied to the respective conductors TG1, RST, and the signals SHR and SHS applied to the circuit breakers KR and KS, respectively.

All signals are built up periodically with a period _TL defined by the speed of motion. The read periods are visible in Figure 3 and are labeled CYC1, CYC2 and CYC3. Each cycle sequentially includes the following operations: A cycle start that can be done arbitrarily at any time, and in this example is considered to be defined by the start of the storage node reset signal RST:

- Issue a high logic level RST for resetting the potential of the storage node and thus emptying the charge present in this node; then returning to zero of the signal RST;

- then a sampling pulse SHR is issued which closes the circuit breaker KR and charges the capacitor Cr to the re-preset potential level present on the column conductor;

- then a transfer pulse TG1 is issued which turns on the transfer transistor T1; the charge accumulated by the photodiode during the previous cycle (due to the end of the pulse TG1 of the previous cycle) is transferred to the storage node;

- The sampling pulse SHS is then issued, which closes the circuit breaker KS and, after transmission TG1 , charges the capacitor Cs to a potential representing the potential of the storage node.

Note: Row SEL, if present, is active (transistor T4 is on) at least during pulses SHR and SHS.

The analog-to-digital conversion CONV1 occurs at the end of the cycle, after sampling the SHS, and mainly during the next cycle because it takes a certain amount of time. This peaks the sensor signal levels Ss1 , Ss2 and Ss3 for cycles CYC1 , CYC2 and CYC3 respectively.

According to the invention, for example in a digital multiplexer MLT ( FIG. 2 ) forming part of a digital processor DSP, the digital signal Ss emanating from the sensor is multiplied by the desired ratio of the accumulation time to the duration of the reading period Ti _D /T _L .

Therefore, the camera comprises multiplication means, which are arranged at each period and provide for each pixel an illuminance measurement level Sc multiplied by the desired accumulation duration Ti _D and the row readout period T _L is proportional to the ratio of the sensor output signal level Ss.

The level Sc provided compensates for the duration _TL (which may vary from period to period) so that under the same lighting conditions the user does not see any difference in brightness even if there are speed fluctuations.

In the practical example shown before, with a nominal speed of motion corresponding to a read period T _Lmin of 10 microseconds, and with a chosen desired accumulation duration equal to the same value, the camera signal is multiplied by T _Lmin /T _L , therefore, if the actual speed decreases with the ratio T _L /T _Lmin , the compensation is made in the direction of decrease.

This principle can be used not only for cameras with active pixels, but also for charge-transfer cameras with passive pixels using CCD technology (with double gate level) or MOS technology (single gate level).

In the previous embodiment, the camera utilizes a single mode of operation, wherein the effective accumulation duration Ti is independent of the desired accumulation duration Ti _D since it is equal to the period duration T _L . Actually, the photodiode accumulates charges from the end of the transfer pulse TG1 until the end of the next transfer pulse. Afterwards, all of the photogenerated charge is in the storage node, and the photodiode resumes accumulating charge corresponding to the next cycle.

In a variant embodiment, the camera can be operated not only according to a first mode (control without cumulative duration) but also according to a second mode (control with cumulative duration). Then, the mode without control is intended primarily for faster movement speeds, since it is desired to maximize charge generation; the mode with duration control is intended especially for slower movement speeds, since it is desired to avoid passing excessive Prolonged illuminance without the risk of saturation at the output of the sensor (saturation could be pixel saturation, analog read chain saturation, or analog-to-digital converter saturation).

In the pixel of Figure 1, the accumulation duration can be performed by simultaneously transferring the transfer potential (positive level on conductor TG1) to transistor T1 and the reset potential (positive level on conductor RST) to transistor T2 control. Then, the charge in the photodiode is drained to the drain through the transistor T1 and the transistor T2, and the photodiode is reset. The effective accumulation duration starts only at the end of this ejection, not at the moment of the previous transmission pulse.

Figure 4 shows the corresponding timing diagram. It is similar to the timing diagram of Figure 3, but the pulse on conductor TG1 is added between the pulses actually used to transfer useful charge to the storage node; this additional pulse is marked by hatching to distinguish it from the other pulses; the The extra pulses are during the high period of RST, and the other pulses are outside the reset instant of the storage node. The effective accumulation period Ti is defined between the end of the hatched pulse and the end of the following non-hatched pulse.

Alternatively, if a fifth transistor T5 is present, the accumulation duration is adjusted by applying a positive potential level to its gate, which empties the charge already accumulated in the photodiode and prevents a new accumulation of charge as long as it exists . The falling edge of this level is located between the period _TL of the two transfer pulses, and the accumulation of charges continues during the time Ti (from the falling edge of the level applied to the gate to the falling edge of the transfer pulse TG1) .

Figure 5 shows the corresponding timing diagram.

In this example, the camera can thus operate according to the mode of FIG. 3 or the mode of FIG. 4 . It therefore has means for generating an additional pulse TG1 in the case of FIG. 4 , or a control signal for transistor T5 in the case of FIG. 5 . These control means receive an item of information about the desired accumulation duration Ti _D and generate suitable control slots within the time period T _L which define the effective accumulation time Ti=Ti _D .

When the camera is operating in the second mode, the sensor output signal SS is no longer multiplied by the ratio of Ti _D to _TL because the accumulation of light does occur within the desired accumulation time. The signal provided by the camera is the signal Ss provided by the sensor unweighted by the ratio Ti _D /T _L .

Figure 6 symbolically summarizes how it works in the two modes:

- in the upper part of the diagram, the accumulation is carried out over the duration of the reading and the output signal Sc of the camera is weighted by the ratio Ti _D /T _L ;

- In the lower part of the figure, the accumulation takes place over a duration Ti smaller than the line duration, and the output signal Sc of the camera is not weighted by the ratio Ti _D /T _L .

The camera includes a program for sequencing the signals in the timing diagram of the preceding figure, and this program automatically switches from one mode to another depending on the value of the read period _TL , in this way the user cannot see the difference between the modes Variety.

Below a determined threshold T _LS the camera automatically operates in the first mode, above this threshold T _LS the camera operates in the second mode.

The value of the threshold depends on the technical characteristics of the sensor and the desired accumulation duration which has been adjusted elsewhere. In fact, in the second mode with accumulation time control, there is a dead time _TM , which is used to preserve the reading of the potential of the sampling and storage _nodes , during which the charges may be accumulated. In the case of the timing diagram of FIG. 4 , this dead time comprises at least the duration from the start of the pulse for transferring the useful charge to the storage node until the end of the sampling pulse SHS.

Figure 7 illustrates the presence of this dead time _TM : the reset pulse (hatched) cannot start before the end of the pulse SHS, and accumulation starts only at the end of the reset pulse, which determines the accumulated The start of duration Ti. Thus, the dead time T _M is at least from the end of the transmit pulse (not hatched) until the end of the reset pulse (hatched). In Fig. 7, the maximum cumulative duration Ti _max is indicated as a specified duration _TL : it is equal to _TL - _TM .

If the desired accumulation duration provided to the camera is Ti _D , the camera determines a threshold for the reading period, or for the same motion speed threshold the amount for which the camera has to change the mode of operation. The threshold T _LS is equal to Ti _D +T _M .

For T _L less than T _LS =Ti _D +T _M , even if T _L is not equal to Ti _D , the camera operates with an accumulation time equal to T _L without accumulation time control, and will be equal to Ti _D /T _L Compensation weighting is applied to the sensor signal. For T _L greater than or equal to the threshold T _LS , the camera operates in the second mode and does not apply any compensation weighting.

If the sensor is one that operates in the first mode (below the threshold T _LS ), but not in the second mode (above the threshold), with TDI summation of N rows, the threshold will also be equal to Ti _D + T _M , where Ti _D is the estimated expected accumulation duration in non-TDI mode; Ti _D is equal to N.T'i _D , where T'i _D is the expected accumulation duration in TDI mode.

The switchover between the two modes is automatic, the camera has the information needed for the threshold calculation (T _L , Ti _D , T _M ) and is designed to change the mode at this threshold, the change of the mode includes on the one hand changing the ordering Timing diagrams, on the other hand, include weighted or unweighted ratios Ti _D /T _L .

However, there may be a risk that the user adjusts the sensor digital or analog gain (which with respect to the desired accumulation duration) to the value provided by the sensor, and for some pixels the digital signal Ss is somewhat too close to the maximum value Ssmax that the sensor can provide.

If in the mode with accumulation duration control the digital signal Ss is too close to the maximum value Ssmax, switching to the mode without control will cause the accumulation duration to increase from Ti _D to Ti _D +T _M , so for these The still strong signal of the pixel exceeds the upper limit Ssmax of the sensor output signal.

In fact, instead of the accumulation duration Ti _D , there is a readout duration T _LS =Ti _D +T _M in the case where the camera operates immediately when the effective accumulation duration equal to Ti _D exceeds the threshold T _LS .

But in case the camera is on immediately below the threshold, still having a read duration T _LS , there is now an effective cumulative duration equal to the read duration and thus T _LS and thus Ti _D +T _M .

Therefore, when switching from the second mode to the first mode, the accumulation duration jumps sharply by the factor (Ti _D +T _M )/Ti _D. This transition saturates the digital output level of the sensor if already close to the maximum level Ssmax in the second mode.

In order to prevent this risk, and to do so in a manner invisible to the user, the following enhancement is proposed: it consists in introducing a safety factor Ks into the signal processing, the digital processing is based on the digital level provided in the first mode or in the second mode Sc to generate the camera's digital output signal.

This coefficient Ks is intended to force the camera to provide an overestimated signal relative to the actual value (which should provide the illuminance considered), in order to encourage the user not to adjust the setting of the accumulation duration with the risk of peaking when the signal Ss is saturated.

The signal Sc provided by the sensor will be multiplied by a factor _Ks greater than 1 and the camera will provide _digital Output level SN=Sc×Ks.

The user will adjust the desired integration time, lighting conditions and camera gain to have an unsaturated output signal SN, although this multiplication factor appears to be greater than 1, thus ensuring that the upstream digital processing (which consists of There is no saturation when switching from the second mode to the first mode).

The choice of the safety factor Ks is directly related to the value of the above-mentioned jump in the accumulation duration and, therefore, to the ratio (Ti _D +T _M )/Ti _D.

At present, the ratio Ti _D /(Ti _D + _TM ) is the most unfavorable in case the reading time has the lowest possible value that allows the sensor to work. In practice, the dead time T _M is an almost fixed value for the sensor, independent of the cycle time. Therefore, the ratio of accumulation time to cycle time is expected to be highest when the cycle time is shortest, and the coefficient Ks needs to be selected to adjust for this most unfavorable case.

If T _Lmin is the minimum reading period below which the sensor may no longer work, the coefficient Ks will preferably be chosen to be equal to or greater than (T _Lmin +T _M )/T _Lmin .

By way of example, if the minimum read duration T _Lmin is 10 microseconds, and the dead time is 4 microseconds, then Ks would be chosen equal to 1.4 or greater than 1.4 (if further increased safety margin is desired).

The digital processor DSP will therefore comprise multiplication means for multiplying the digital signal Sc emanating from the sensor by this coefficient Ks.

FIG. 8 summarizes the structure of a camera according to the invention in the case that the camera according to the invention can operate according to a mode without and with accumulation duration control. The clock CLK (which manages the read cycle) receives the information on the speed of movement (which is marked by the input _TL ). The sensor sequencing means SEQ receives an item of information defined by the user regarding the desired accumulation duration Ti _D . These means control the IMS linear image sensor and operate according to one mode or another depending on whether the period _TL is below or above a threshold _TLS . The sensor readout circuit CL provides a digital signal Ss representative of the illuminance of the pixel during motion. The signal Ss is multiplied or not multiplied by the ratio Ti _D /T _L depending on whether the sequencing device applies the first mode or the second mode. In cameras, multiplication is usually envisaged to take place in the processor used for the digital processing of the signal. Thus, the digital signal Sc resulting from this multiplication is either Ss or Ss×Ti _D /T _L . This signal Sc is itself multiplied by a safety factor Ks to give the digital output signal of the camera, SN=Ks*Sc.

The invention may notably be implemented with a TDI sensor having two rows of pixels, such as described in patent publication FR2971621.

Claims (8)

1. a kind of method for generating image by camera, the camera includes linear imaging sensor, the line Property imaging sensor be in relative motion with reference to the image to be observed, the sensor is provided for the picture point each observed Indicate this may variation effective cumulative duration Ti in illumination output level (Ss), the camera bag Synchronous input unit is included, is used to receive the item of information about speed of related movement；For establishing the image line read cycle period T_LDevice, described image row read cycle period T_LIt is inversely proportional with speed of related movement；And it is held for receiving desired accumulation The value Ti of continuous time_D, the method is characterized in that the method includes two kinds of operating modes, respectively：

- the first operating mode, wherein effective cumulative duration Ti is equal to read cycle period T_L, and wherein, eedle of taking a picture To each picture point, illumination photometry level (SN) is provided, the illumination photometry level (SN) is held with desired accumulation has been multiplied by Continuous time Ti_DWith row read cycle period T_LRatio Ti_D/T_LSensor output level it is directly proportional,

- the second operating mode has and is less than read cycle period T_LCumulative duration, wherein collator establishes In the effective cumulative duration of desired cumulative duration, and wherein, the camera is each for often going, being directed to Picture point, provides illumination photometry level, the illumination photometry level with not to desired cumulative duration Ti_DIt is read with row Period T_LRatio Ti_D/T_LThe output level for the sensor being weighted is directly proportional,

The selection between first mode and second mode is carried out according to the speed of relative motion, is less than in the read cycle period First mode is utilized when threshold value, and second mode is utilized when more than this threshold value.

2. according to the method described in claim 1, it is characterized in that, according to the speed of relative motion come execute from first mode to The switching of second mode and from second mode to the switching of first mode, and the switching is happened at read cycle period threshold T_LS=Ti_D+T_MIt is interior, wherein T_MDuration for the dead time estimated in the second operation mode, during dead time, Imaging sensor cannot accumulate any charge.

3. according to the method described in claim 2, it is characterized in that, under two kinds of operating modes, the output level of sensor is another Outer be multiplied by is equal to or more than (T_Lmin+T_M)/T_LminSafety coefficient (Ks), wherein T_LminFor the minimum read cycle period, it is less than This period sensor cannot work.

4. a kind of smear camera comprising have the linear imaging sensor of at least one-row pixels, the scanography function The output level (Ss) of sensor is enough generated, the output level indicates in the effective accumulation period time Ti that may change Picture point illumination, the camera includes collator；Synchronous input unit, is used to receive about speed of related movement Item of information；For establishing image line read cycle period T_LDevice, described image row read cycle period T_LWith relative motion Speed be inversely proportional；And the value Ti for receiving desired cumulative duration_DDevice, the collator can establish First operating mode, wherein effective cumulative duration Ti is equal to read cycle period T_L, which is characterized in that collator quilt It is designed as establishing the second operating mode, the cumulative duration having is equal to desired cumulative duration, and less than reading Period T_L, and it is characterized in that, the camera includes：

Multiplier (MLT) is used in first mode rather than under second mode, and illumination survey is provided for the often row of image Level (SN) is measured, the illumination photometry level (SN) is directed to each picture point, and has been multiplied by desired cumulative duration Ti_DWith Row reads period T_LRatio sensor output level it is directly proportional, and

For according to the speed of relative motion come by camera automatically from first mode switch to second mode and automatically from Second mode switches to the device of first mode.

5. camera according to claim 4, which is characterized in that for camera to be switched to the second mould from first mode The device of formula is designed such that the camera utilizes first mode when read cycle period TL is less than threshold value, more than the threshold Second mode is utilized when value.

6. camera according to claim 5, which is characterized in that be directed to read cycle period threshold T_LS=Ti_D+T_MAnd it builds The vertical switching from a pattern to another pattern, wherein T_MFor continuing for dead time for estimating in the second operation mode Time, during dead time, imaging sensor cannot accumulate any charge.

7. the camera according to one of claim 4 to 6, which is characterized in that the camera includes further being used for Under two kinds of operating modes, the output level of sensor is multiplied by or is more than (T_Lmin+T_M)/T_LminSafety coefficient (Ks) Device, wherein T_LminFor the minimum read cycle period, cannot work less than this period sensor, and T_MFor in the second work The duration for the dead time estimated under operation mode, imaging sensor cannot tire out any charge during dead time period Product.

8. camera according to claim 5, which is characterized in that the sensor includes N row pixels, and N is more than 1, and The sensor can be worked with time delay integration (TDI) pattern in the first operation mode.