GB2255837A

GB2255837A - Lighting optimization.

Info

Publication number: GB2255837A
Application number: GB9126663A
Authority: GB
Inventors: Peng Seng Toh; Wan Sing Ng
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-04-16
Filing date: 1991-12-16
Publication date: 1992-11-18
Also published as: GB9126663D0; GB9108063D0

Abstract

A large combination of lighting arrangements can be tested rapidly to locate an optimum illumination for a particular machine vision task, using multiple light sources 6 supported by a ring 3 and controlled by a host computer which can switch sources on/off, control their brightness, rotate the ring, move the sources radially concurrently or individually and tilt the sources concurrently or individually. This permits simulating a dome-shape lighting, which in principle covers all the possible front-lighting orientations, and accommodating objects of different sizes. A mechanism for maintaining the direction of all the light sources at a fixed focal point during movement is incorporated which can also be used for simulating the dome-shape lighting. <IMAGE>

Description

Techniques for Lighting Optimization This is an apparatus of and a technique for enabling rapid optimization of lighting arrangement, such as the position, direction and the intensity of multiple light sources required to achieve a machine vision task.

Adjustment of lighting arrangement such as the position and illuminant intensity of light sources so as to project a desirable illumination pattern on the object space is critical to the success of a machine vision task. The search for the optimum arrangement is often laborious and ad hoc in nature. Manual adjustment is the current practice as automatic means of searching for the optimum combination has not arrived despite its importance.

The number of different lighting arrangement is very large and can be combinatorial explosive, manual adjustment is therefore inefficient and time consuming. It is also not feasible to find the optimum lighting arrangement by a purely analytical approach such as the ray-tracing method because of the complexity of objects cannot be easily modeled The application of machine vision system in the industry usually has a very specific task. Many a priori knowledge are known in advance which can be used to assist in solving a given problem. A "figure of merit" or a "performance index" can be formulated to indicate how well a vision algorithm accomplishes a particular task. This is also known as task oriented gathering of information in the field of machine or computer vision.If the same algorithm is applied to images of the same scene captured with different lighting arrangements, then the highest performance index can be used to indicate which of these images is optimum which in turn implies which lighting arrangement is. the best for illuminating the object space. Several examples can be cited to explain the concept of using the performance index.

1. The performance index can be equated to the number of edges detected in an image.

2. The performance index can be assigned to indicate the contrast strength of an image.

3. The extent to which specular highlights are reduced in an image can be used as a performance index.

4. The relative ease of thresholding may also be assigned as the performance index.

These examples are merely illustrative and are by no means exhaustive.

It is well known that the surface of a sphere, known as the Gaussian sphere in the field of computer/machine vision, contains all the possible surface orientations.

Considering only front lighting, then only the hemisphere in the form of a dome situated above the object is required. If infinite number of infinitely small lighting elements were to cover the entire surface of the dome with each of their optical axis oriented normal to the surface, then a complete simulation of all the possible front lighting directions is possible.

This is termed herein as the "dome-shaped lighting". In practice, such a dome with infinitely small lighting elements with which each can be individually controlled is difficult to be constructed. The need. to have a variable size dome so as to accommodate object of different sizes further complicates the matter. The essence of the present invention is to be able to simulate systematically the dome-shaped lighting without having to build a dome.

Central to the present invention is to have multiple number of light sources distributed on the circumference of a circle. Each of these light sources can be individually turned on or off as required and points in the direction of the center of the circle at a tilt angle. The tilt angle refers to the angle subtended between the optical axis of the light source and the plane of the circle. Mechanisms are provided such that these light sources can traverse along the radial direction of the circle either concurrently orindividually.

Mechanisms are also provided such that the tilt angle of the light sources can either be simultaneously or individually adjusted as required. While traversing the light sources radially, mechanism is provided such that the change in the tilt angle can be predetermined This mechanism, for example, is useful in enabling all the light sources to converge at a focal point while traversing radially. Features for controlling the intensity, polarization, spectral distribution of the light sources can be added. The structure which houses a number of light sources and the mechanisms for manipulating the light sources is herein collectively known as the ring-of-light or ROL for short.

The applicability of the present invention is in facilitating rapid prototyping of lighting design which save tiine and cost leading to effective production implementation.

There are also cases where this invention may be used in the actual production environment in which complex illumination pattern and rapid changes of patterns are required. For instance, a vision system needs to carry out multiple tasks and each requires different illumination patterns, rapid switching between complex illumination patterns under computer control is therefore essential and can be accomplished by the present invention.

As there is a large combination of switching on/off of the light sources in conjunction with a large possibility of illumination directions, this presents a wide variation of illumination pattern. One of these different combinations of illumination pattern will enable an image of the object to be acquired that best achieve a vision task. This combination provides the optimum illumination pattern.

The preferred embodiment of the present invention is described with the following accompanied drawings: Fig. 1 is the overall block diagram of the invention showing the video source, the host computer, the ROL controller and the ROL.

Fig. 2 illustrates the ROL controller in greater detail and the plan view of the ROL mechanisms indicating the provisions for rotation and radial adjustment.

Fig. 3 is an exemplary schematic design of the lighting controller. Only the essential registers used by the lighting controllers are shown.

Fig. 4 shows a block diagram of the motion controller.

Fig. 5a shows the plan view of the ROL with the necessary mechanisms for activating the radial movement of the light sources.

Fig. 5b shows the cross-sectional view of the ROL taken along A-A indicated in Fig. 5a Fig. 6 illustrates the mechanisms for adjusting the tilt angle, 0, of one of the light sources.

Fig. 7 illustrates the concept of the "dome-shape" lighting.

Fig. 8 illustrates the combine use of the tilt angle adjustment and the radial movement to simulate the dome-shape lighting.

Fig. 9 is the flow chart showing the procedure for automatic lighting optimization using the present invention.

The invention disclosed herein is to be used in conjunction with facilities for mounting a camera or any other form of video source 1 and a platform for object placement 2 as shown in Fig. 1. The present invention consists of the ROL 3 on which houses a number of light sources 6 pointing in the direction of the object space as shown in Fig. 1.

The ROL is controlled by an ROL controller 4. Switching on/off of the light sources and the control of their intensities can be remotely controlled by a computer 5 activated through the ROL controller 4. The host computer also controls and receives input from the video source 1. Other than being able to be controlled by a computer program, the ROL controller 4 can also be used manually.

The number of light sources 6 mounted on the ROL 3 is preferably at least four when the invention is to be used for finding the optimum lighting arrangement. Take for instance, one may use 8 light sources distributed at equal spacing around the ROL 3. In other words, there is a light source at every 45 degrees on the circumference of the ROL 3 as shown in Fig. 2. Note that the number of light source can be less than four if the present invention is used in the actual production environment instead of as a means for designing and prototyping lighting arrangement. The number of lighting combinations possible is as follows:

where m is the total number of light sources 6 mounted on the ROL 3, n is the number of light sources that is to be turned on. In using 8 light sources, the total number of different arrangement is 256.In other words, the combination is also equal to 2m.

In this case, a m-bit representation is sufficient to manipulate all possible arrangements for switching on/off the light sources in which one-bit controls one light source. The number of combinations can be reduced when some a priori knowledge associated with the task is available. For example, in knowing that specular reflections have to be minimized, then only even number of light sources with illumination direction pointing in opposite to one another are allowed to be used. In this example, light sources located in opposite direction will tend to produce minimum specular reflection as directional components will be mutually cancelled.

The functional blocks of the ROL controller 4 is shown in Fig. 2. The ROL controller consists of an input/output unit 7 which interacts with the host computer 5 by serial or parallel communications; the input/output unit 7 is connected to the lighting controller 8 which controls the brightness control unit 9 and a set of switches 10. The controller 8 also receives feedback from the monitoring device 11 which monitors the status of each of the light sources. The ROL controller 4 also accepts manual control other than receiving commands from the host computer for setting the brightness and switching of the light sources. There is a provision for selecting between remote control (by the host computer 5) or manual control (by operator's front-panel input).

The input/output unit 7 is either a serial interface circuit which accepts standard serial communication protocols (eg. RS 232, RS 432, RS433) or parallel communication protocols (eg. IEEE488, Digital input/output), but not limited to the examples mentioned here. The lighting controller 8, whose block diagram is given in Fig. 3, consists of a set of registers for storing the commands and data issued either by the host computer or the manual input. These registers in the lighting controller 8 can either be read or write by the host computer 5. The lighting controller 8 also consists of a status register 15 which retains the status of the light sources provided by the monitoring devices 11. The monitoring devices 11 advocated by the present inventions are photocells located close to each individual light source.Other light-sensitive devices (for examples, photo-diode, photo-transistor) which can serve the same function may be used instead. The brightness control register 18 contains a digital word that causes the brightness control 9 to adjust the level of power supplies to the light sources through the switches 10. For those skilled in the art, the brightness control 9 used in the present invention can be implemented by triacs or other power control circuits dependent upon the type of light sources used. The on/off register 19 is m-bit wide which is used to turn on or off the switches in 10. The switches in 10 typically consist of relays whose control terminals are connected to the on/off register 19 as shown in Fig. 3.

The function of the monitoring device 11 is to monitor the operational status of each of the light sources. The status information required from the monitoring device 11 is the level of illumination and this will also indicate whether the light is on or off. The operational status of the all the light sources employed in the invention can therefore be retained in the status register 20 of the lighting controller 8 and accessed by the host computer through the input/output unit 7. The host computer can then make use of the status register 20 to determine whether the light sources have responded according to its commands and whether there is any defective light source.

The following described the various mechanisms of the ROL 3. The present invention allows the entire ROL 3 to be rotated 16 around the object so that other light source positions can be explored as shown in Fig. 2. This is especially useful when using a small number of light sources to explore a large combination of lighting arrangement. For example, when the light sources are originally spaced at 45 degree around the frame, by rotating the ROL 3 about the center axis of the ring (also the optical axis of the video source 1), the light sources can now occupy many different positions. This will enable the light sources to take up positions previously not occupied.

The motion controller 13 is shown in fig. 4. It has motor control logic 21 and motor driver circuit 22 to control and drive two independent motors. The two motors are responsible for the radial movement 17 and the tilt angle 37 of the light sources respectively.

The structure of the preferred embodiment of the ROL 3 is shown in Fig. Sa and 5b. Fig. Sa shows the plan view of the ROL 3 and Fig. 5b is the cross-sectional view. The main component of the ROL 3 is the ring-frame 23 which has a through hole in the center serving as the see-through hole for the video source 1. A plurality of radial guides 24 with light sources 6 are mounted to the ring-frame 23. A movable radial block 26 is attached to the each of the radial guide 24 serving as a moving platform for the light source. At the outer end of each of the radial guide is a end pulley 27. A corresponding pulley, front pulley 28, is located on the ring-frame 23, which is linked to the end pulley 27 by a radial belt 29. All the front pulleys 28 are driven by a common belt 25 which is in turn driven by the radial motor 14 or a hand crank via the driver pulley 31.When the common belt 25 rotates, it rotates all the front pulleys 28 which causes all the radial belts 29 to move concurrently. The movement of the radial belt 29 further activates the radial block pulley 30 to move and hence resulted in the movement of the radial block 26 and the attached light source 6. All the radial blocks 26 and their associated light sources can be moved concurrently. Any of the radial block pulleys 30 can be disengaged from their respective radial belts 29 and hence the concurrent movement of the radial blocks 26 can be disabled The most direct use of this radial movement 17 facility is in accommodating objects of different sizes without having to change the size of the ROL 3.

Further refer to the cross-sectional view of the ROL 3 shown in Fig 5b, there is a provision for adjusting the tilt angle 37 of all the light sources 6 in synchronism with their radial movements 17. The tilt.angle of all the light sources illuminating the object space can be preset to converged at a fixed point (but not necessarily on the object surface) at any radial position. In other words, mechanisms are provided such that traversing the light sources radially will alter their tilt angle according to a predetermined locus. Alternatively, at any radial position, the tilt angle of all the light sources can be adjusted simultaneously.

An angle plate 32 is placed above the ring-frame 23 and can be moved vertically 38 along the angle plate guide 33. The angle plate 32 similarly has a through-hole in its center for the field of view of the video source 1. The movement of the angle plate is transmitted by the angle links 34 and the link rods 36 to the light sources 6. Each of the angle link 34 is pivoted by a angle post 35 mounted at the outer end of the radial guide 24. The angle plate 32 can be either driven by the angular motor 15 under automatic mode or manually. The mechanism for controlling the tilt angle of the light sources is further illustrated in Fig. 6.

The same light source is shown at two different radial positions of which one is drawn in dotted lines. The position of the angle plate 32 determines the angle a of the angle link 34.

When the radial block 26 is moved, the link rod 36 will be moved either up or down according to the angle of the angle link. The vertical action of link rod 36 causes the light source to swing around its pivot point 39 and achieve in adjusting the tilt angle of the light source. The two different tilt angles are denoted as O1 and O2 respectively. There is yet another provision to alter the length of the pivot link 40 thus changing the initial tilt angle of the light source. Each of the pivot link 40 can be independently adjusted.

According to another aspect of the invention, the ROL 3 is mounted on a vertical fixture that permits vertical movement with respect to the object as shown in Fig. 1. This provides flexibility for accommodating objects of different heights.

One of the use of the present invention is in simulating a dome-shaped lighting. The concept of "dome-shape" lighting is illustrated in Fig. 7. The simulation of the dome-shape lighting is illustrated in Fig. 8. The ROL 3 is first positioned at a preferred height h above the object as shown in Fig 8. The lighting direction (tilt angles - ..... On) can be preset by adjusting the angle plate 32 such that the direction of the optical axes of the light sources will always point toward the object during radial movement. The position of the angle plate alters the angle of the angle link 34 which further causes the link rod 36 to move vertically.

The vertical movement of the link rod pulls the light source either up or down with respect to its pivot point and hence changes the direction of the light source. Although traversing the light sources to different position alters the light source to object distance and would cause changes in the illumination intensity, the intensity control feature provided in the present invention allows the host computer 5 to carry out intensity adjustment to compensate for the change according to the "inverse square law". The present invention uses multiple light sources distributed in a regular manner allows a more efficient simulation of the dome-shaped lighting than a single light source. It is evident that traversing multiple light sources on a plane above the object and controlling their intensity would systematically simulate the dome-shape lighting.

The procedure for finding the optimum lighting arrangement associated with a specific task is given by the flow chart of Fig. 9. It begins with the setting up of an initial lighting arrangement, not necessarily optimum but conforms to the constraints imposed by the given task if there is any. The constraints could include the height of the ROL 3 to the object, the desired field of view of the video source 1 among others. The complete lighting arrangement is determined by the brightness of the light source, the switching combination, the radial position and the tilt angle of the light sources. Once the initial set up has been carried out, an image of the object is captured under the present illumination. The image is processed and analyzed which computes a performance index most relevant to the machine vision task required.The performance index associated with the current lighting arrangement is recorded. The host computer 5 then instructs the ROL controller 4 to set to another possible lighting arrangements by changing either the light sources intensity, or the switching combination, or the light sources position (radial position and tilt angle). The image is then captured and the performance index for this new lighting arrangement is computed and its value is stored for evaluation later on. The loop repeats until all the desired lighting combinations are completed. At the end of all the combinatorial trials, a set of performance index associated with each different lighting arrangement is produced The best performance index is then located and the associated lighting arrangement is selected as the optimum lighting arrangement. Further fine tuning would be possible with the optimum lighting identified by the automatic procedure.

Claims

1. This invention is a technique of and an apparatus for machine vision lighting which is generally used in conjunction with a video source, an image acquisition system and a host computer. Said invention comprises a plurality of light sources distributed on arms projecting from a ring-shape frame; the plurality of light sources provides illumination for the object; the parameters of the light sources including the state, direction, position and intensity of the light sources are manually controllable and/or remotely controllable by the host computer; the state of the light source refers to its on/off state; the direction of the light source refers to the orientation of the optical axis of the light source; the position of the light source refers to its relative position from a reference point in the object space; the intensity of the light source refers to its output radiant energy.

2. The technique of and an apparatus for machine vision lighting as claimed in claim 1 has provision for changing the parameters: state, direction, position and intensity of the light sources either individually or concurrently by either manual means or remotely by the host computer.

3. The technique of and an apparatus for machine vision lighting as claimed in claim 2 provides the video source to acquire images of the objects under different lighting arrangements effected by varying the parameters of the light sources; each said images is processed by the image acquisition system and/or the host computer according to the machine vision task and a performance index computed; the lighting arrangement that gives rise to the maximum performance index for the machine vision task is the optimum arrangement.

4. The technique of and apparatus for machine vision lighting as claimed in claim 3 controls the parameters of the light sources to simulate a dome-shape lighting; said dome-shape lighting has all the possible orientations for front lighting.