WO1991016786A1

WO1991016786A1 - Illumination device

Info

Publication number: WO1991016786A1
Application number: PCT/DE1991/000305
Authority: WO
Inventors: Ernst Schmidberger; Peter Berner
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1990-04-12
Filing date: 1991-04-12
Publication date: 1991-10-31
Also published as: DE4011842A1; DE4011842C2

Abstract

Described is a device for illuminating objects which are to be photographed by a camera, comprising at least a charge-coupled-device image recorder, the image signals from the camera being fed to an image-processing unit with a light source which controls a control unit for pulse operation. The invention is characterized in that the light source comprises an LED array, the diodes of which are selectively addressed by the control unit with at least one pulse of variable width during each image period.

Description

Lighting device

Description

Technical field

The invention relates to a lighting device for objects that are recorded by a camera that has at least one CCD image sensor, the image signal of which is applied to an image processing device, with a light source that controls a control unit in pulse mode.

State of the art

In digital image processing for industrial purposes, for example for controlling a processing center, an important influencing variable is the quality of the recorded image signal. The type of lighting device is an important factor here:

Current light sources such as incandescent lamps, halogen lamps or light sources with light guides have disadvantages in a number of applications:

For example, in some cases it is necessary to selectively illuminate an object in order to avoid reflections or to appropriately display surface structures. With the light sources mentioned above, this can generally not be achieved, or at least not satisfactorily, at least with reasonable effort.

Another disadvantage of these light sources occurs when using CCD cameras when fast moving objects objects to be recorded. Due to the structure of the CCD sensor, such cameras deliver images with blurring in the moving areas.

Although it is possible to eliminate such blurring with electronic locking devices, the use of such electronic locks has the disadvantage that the lighting has to be illuminated more appropriately. Working with a wider aperture to avoid lighting problems is rarely possible because a wider aperture means a smaller depth of field; however, a shallow depth of field is often not desired in industrial image processing. For this reason, when using electronic shutters, light sources of larger dimensions are generally required. Under certain circumstances, it may even be necessary to dissipate the resulting power loss of the lighting device using cooling devices.

An alternative to permanent lighting in video cameras with a holding capacity for the charge image, as is the case with CCD cameras, are lighting devices which have flash or stroboscopic light sources. These devices provide a high radiation power periodically for a short time, namely in the blanking interval between the images of the video image. The electrical power required on average is thus considerably lower than the electrical power that would be expended in a CCD camera with an electronic shutter device, in which illumination is carried out with continuous light. Known flash or stroboscopic lighting devices, which work with xenon flash lamps, for example, are power-efficient and eliminate the aforementioned disadvantages in the case of moving image recording. There are, however, work-physiological reasons that speak against the use of such stroboscopes: Xenon flash lamps, for example, light up in the visible range. Therefore, working in their environment is perceived as unpleasant.

Furthermore, from EP-A-0 181 412 a lighting device for objects that are recorded by a camera that has at least one CCD image sensor is known according to the preamble of patent claim 1. This known generic lighting device has a pulsed infrared radiator as the light source, which can be, for example, a light-emitting diode or a laser diode.

The known lighting devices for objects that work with flash lamps or a pulsed diode only allow the integral lighting of the object to be recorded; Furthermore, an adjustment of the brightness with which the object is illuminated is not provided in the lighting device known from EP-AO 181 412.

Illumination devices that enable selective illumination would, as a rule, mean an unacceptable effort due to the use of a number of flash lamps or a corresponding number of fast closures in the beam path. But even if several biltz lamps were used, there would be no possibility of adjusting the brightness, or it would be possible to implement them only with great effort. Presentation of the invention

The invention is based on the object of providing an illumination device for objects which are picked up by a camera and which has at least one CCD image sensor, the image signal of which is applied to an image processing device which, in the case of rapidly moving objects, which is selective and with adjustable brightness should be illuminated, an image recording without motion blurring with a light source allows comparatively low power consumption.

This object is achieved in that the light source has a light-emitting diode array, the diodes of which are selectively acted upon by the control unit with at least one pulse of variable width during each image period.

According to the invention, selective lighting is achieved in that the light source - in a manner known per se - from many small, individually or in groups (claim 4) by means of an internal control unit, a process computer (claim 9) or manually (claim 8) controllable individual light sources consists.

In order to keep the cost as low as possible, light emitting diodes are used as individual light sources, since light emitting diodes combined in arrays can be comparatively inexpensively produced and can also be controlled by a control unit, which can also be constructed comparatively inexpensively (claims 12 and 13). Above all, however, light-emitting diodes without major problems are suitable for pulse operation, by means of which a strobe effect is achieved. Incandescent or halogen lamps are not suitable for this application as a light source since they are not suitable for pulse operation. They have a very low switch-on input resistance, which in the short term leads to very high temperatures in the filament and thus to premature destruction. Xenon flash lamps, on the other hand, have the disadvantages mentioned above.

The control unit provides synchronous pulses with the video signal, the width of which can be selected within certain limits. In typical applications, the width of the pulses is between 4 and 400 μsec. The width of the pulses enables the light energy per pulse and thus the average brightness to be set.

In the case of an image processing device which can process fields (field integration mode), a light pulse is emitted every 20 msec, while in the case of an image processing device which processes frames, i.e. With a camera operating in frame integration mode, a light pulse is only generated every 40 msec. It is particularly preferred if the lighting device according to the invention can be switched between the two modes, so that it can be used with any image processing devices and cameras (claim 14).

In any case, the design according to the invention provides an illumination device which has all the advantages of stroboscopes without having their disadvantages or the disadvantages of conventional illuminations, and which moreover permits selective illumination of a scene with adjustable brightness.

Further developments of the invention are specified in the subclaims: According to claim 2, the diodes emit light in the near infrared range between 850 and 1000 nm. Since the emitted light is thus outside the visible spectral range, but within the sensitive spectral range of most CCD cameras, a work-physiologically undesirable disturbance of the Avoid surroundings during pulse operation.

Claim 3 states that the diodes are GaAs light-emitting diodes or GaAlAs light-emitting diodes. These light-emitting diodes have small geometric dimensions and are outstandingly suitable for pulse operation. During pulse operation, these light-emitting diodes can be loaded with a much higher current than would be possible with continuous operation, provided the ratio of the duty cycle to the period does not exceed a diode-specific maximum amount. There is an almost linear relationship between the electrical current flow and the emitted radiation, so that high beam powers can be achieved in this operating mode. Since the emitted spectrum is in the near infrared (850-1000 nm), the pulsed brightness fluctuations are not visible to humans. In addition, LEDs generally have a very long life expectancy. It is on average in the range of many years, so that the reliability is very high.

In claim 4 is characterized in that diodes of the array are combined into groups or segments, and that the groups or segments can be controlled individually. A separate power level for each segment drives the individual light-emitting diodes of a segment. The light-emitting diodes can be arranged in any arrangement, for example in a matrix (claim 5) and combined to form smaller groups or segments. For example, the diodes can be grouped together in one row.

Furthermore, it is possible for the diodes to be arranged concentrically in several rings for illuminating rotationally symmetrical objects (claim 6). Each ring can consist of several, individually controllable segments (claim 7).

In order to better illuminate the scene to be illuminated than is possible with a generally flat light-emitting diode array, it is preferred if, according to claim 15, the light of the light-emitting diodes is coupled into light guides which add the light from the individual diodes of the light-emitting diode array of the scene to be illuminated.

The selection of the diodes or groups of diodes selectively controlled by the control unit and the respective brightness, i.e. The duration of the impulses affecting a group can be done manually (claim 8) or by means of a process computer (claim 9), for example via a V24 interface (claim 10).

In any case, the design according to the invention provides an extremely flexible lighting device which has a number of advantages over conventional devices:

Full-area, homogeneous lighting and, moreover, targeted selective lighting is possible. Due to the pulse operation, no blurring occurs with moving objects, even without a locking mechanism being used.

The pulse-shaped lighting is easy to synchronize with the video signal, so that the lighting occurs only in the blanking interval between the images of the video image.

The periodic fluctuations in brightness are invisible in the preferred embodiment according to claim 2, since the emitted spectrum is in the near infrared. Of course, it is also possible to use light emitting diodes that emit light in the visible range.

When using a filter that is only permeable to IR, in this embodiment the illumination is independent of the ambient light.

The geometrical arrangement of the light-emitting diodes can be freely selected, so that the lighting device according to the invention can also be used in places that are difficult to access — optionally with additional light guides.

By control using a suitable computer, adaptive lighting, i.e. brightness control possible.

The light emitting diodes used have a very long service life. Brief description of the drawing

The invention is described below by way of example without limitation of the general inventive concept on the basis of exemplary embodiments with reference to the drawing, to which reference is expressly made with regard to the disclosure of all details according to the invention not explained in detail in the text. Show it:

1 shows a block diagram of the control unit, FIG. 2 shows a block diagram of the power driver, and FIGS. 3 to 5 light-emitting diode arrangements used according to the invention.

Representation of exemplary embodiments

The control unit shown in FIG. 1 is intended to deliver one pulse of variable width per video field, with which light-emitting diodes (not shown in FIG. 1) are actuated. If light-emitting diodes, as are typically used for a lighting device designed according to the invention, are to be loaded with a current of, for example, 1.1 A, the operating time for such diodes may e.g. amount to a maximum of 1/50 of the period.

With a video field duration of 20 ms, the maximum duty cycle is therefore 400 μs. A pulse duration of 400 μs is referred to (in the exemplary embodiment shown) as a 100% pulse width.

For programming shorter pulse durations, a two-stage programmable BCD down counter ZI is provided, which counts in the 8-4-2-1 code. The counter ZI is clocked at 250 kHz via a gate circuit TO and an oscillator 01. With this design, the pulse duration can be reduced in steps of 1%, ie from 4 μs to zero become. A counting step therefore corresponds exactly to the time of 4 μs.

A counting cycle is started by the vertical video synchronization pulse. For this purpose, the BAS video signal is applied to a decoder D1, which is followed by a monoflop Ml, which shortens the pulse to approximately 100 ns. This signal loads a number in the BCD code, which can be set from the outside in a manner which will be explained in the following, and which is between 0 and 99 in the exemplary embodiment shown, as the beginning of the count in the counter ZI and at the same time sets an RS flip-flop FF1, that starts the counter oscillator 01.

According to the preprogrammed number, a multiple of 4 μs passes until the counter value reaches zero. The counter ZI indicates the zero crossing by means of a transmission pulse. This transmission pulse serves to reset the flip-flop FF1 and thus to stop the counter oscillator 01. This state remains stable until the next vertical sync pulse occurs. The entire cycle described above is then repeated periodically in synchronism with the video signal. The desired signal thus results at the output connection of the flip-flop FF1. By programming the counters ZI, the pulse width can be entered directly in percent with respect to the maximum permitted pulse width.

The counter can switch in manual mode using BCD coding ^co _e * L _ner ^{un <**} - ^co _ze omputer ^{ut> he} inverting driver ^¬ gradually to both counters of the BCD counter ZI be set. It is also possible to use a process computer (not shown), for example via a V24 interface and a downstream serial Parallel converter (UART) on an 8-bit data bus. The counter reading is not read out from the coding switches, but from registers R in parallel. A control unit assigns the parameters to the individual registers. If a separate control unit is provided for each individual segment, simultaneous different power control of the segments or groups of light-emitting diodes is also possible.

Driver stages and registers are provided with tri-state outputs so that they do not influence each other. In order to enable constant monitoring, the counter reading and thus the brightness are shown on a seven-segment display A.

The gate circuit TO connected downstream of the oscillator 01 enables the entire lighting to be switched on and off asynchronously by hand or by the controlling computer.

The training of the power drivers will be explained below with reference to FIG. 2:

The power drivers have to switch very high currents for a short time without putting too much strain on the control unit. In addition, it must be possible to explicitly select the individual segments.

For this purpose, AND gates "&" are provided on the input connections of the driver stages. These pass on the input signal to the drivers explained in more detail below only if they either by DIP switches and inverting drivers (manual operation) or by the corresponding bit in the segment register TS, which is affected by an 8 bit acted upon by the V24 interface Data bus (computer drive) has been selected. The AND gates & are equipped with open collector outputs so that higher currents can also flow.

The driver stages themselves have transistors T1 and T2 in the so-called Darlington circuit, so that the required current amplification can be achieved. Transistor T1 is a transistor of the small signal type, while transistor T2 is a power transistor. In the collector circuit of T2 there is a light-emitting diode segment with e.g. ten diodes Dl ... Dn. A resistor R3 limits the current through this LED segment. The entire power stage is fed by a correspondingly high voltage, the individual drivers being connected in parallel in terms of power.

The power supply unit consists of a regulated power supply unit which has two separate regulations for the voltage supply of the control and the power section. A generously dimensioned capacitor at the output helps to bridge short-term power withdrawals.

3 to 5 show possible arrangements of the light-emitting diodes D. The arrangement according to FIG. 3, in which the individual diodes are arranged in matrix form, is particularly suitable for bright and dark field incident light or transmitted light. The arrangements according to FIGS. 4 and 5 are particularly suitable for rotationally symmetrical bright-field incident light. Both the arrangement and the number of segments and the number of light-emitting diodes per segment are variable within wide limits. For example, in the arrangement shown in FIG. 4, the light-emitting diodes D of a "line" can each be combined to form a group which is controlled jointly.

Claims

Patent claims

1. Illumination device for objects that are recorded by a camera that has at least one CCD image sensor, the image signal of which is applied to an image processing device, with a light source that controls a control unit in pulse mode, characterized in that that the light source has a light-emitting diode array, the diodes of which the control unit selectively applies at least one pulse of variable width during each image period.

2. Device according to claim 1, characterized in that the diodes emit light in the near infrared range between 850 and 1000 nm.

3. Device according to claim 1 or 2, characterized in that the diodes are GaAs light-emitting diodes or GaAlAs light-emitting diodes.

4. Device according to one of claims 1 to 3, characterized in that diodes of the array are combined into groups, and that the groups are individually controllable bar.

5. Device according to claim 4, characterized in that the diodes are arranged in the form of a matrix, and that the diodes are combined in one row to form a group.

6. Device according to claim 4, characterized in that for illuminating rotationally symmetrical objects, the diodes are arranged concentrically in a plurality of rings, of which at least the diodes combined in different rings can be controlled separately.

7. Device according to claim 6, characterized in that each ring consists of several, individually controllable segments.

8. Device according to one of claims 1 to 7, characterized in that the selection of the selectively controlled by the control unit diodes or groups of diodes and the respective brightness is done manually.

9. Device according to one of claims 1 to 7, characterized in that the selection of the selectively controlled by the control unit diodes or groups of diodes and the control of the brightness of the individual diodes or groups of diodes is carried out by a process computer.

10. Device according to claim 9, characterized in that the process computer for controlling the brightness varies the width of the pulses as a function of an output signal of the image processing device.

11. The device according to claim 9 or 10, characterized in that the control unit can be controlled by the process computer via a standard serial interface.

12. Device according to one of claims 1 to 11, characterized in that the control unit has a Rück¬ down counter, the counter reading can be set to adjust the pulse width.

13. The device according to claim 12, characterized in that a gate circuit is provided which enables asynchronous switching on and off of the device.

14. Device according to one of claims 1 to 13, characterized in that every second pulse can be suppressed for full-screen image processing devices.

15. Device according to one of claims 1 to 14, characterized in that light guides are provided which guide the light of the individual diodes of the light-emitting diode array to the scene to be illuminated.