Abstract:
The present invention provides LED array systems with improved methods of powering LED in the array by monitoring the relationship between temperature and driving power to predict how much power can be safely applied to the LEDs. The present invention also provides for a control system for LED arrays that allows for display of images or light patterns across and array of luminiairs over a low bandwidth control protocol. The present invention also provides for a LED array luminair with reduced color fringing, light spill reduction and beam angle control.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a system and method for driving LED arrays when used in a light beam producing luminaire. More particularly the invention relates to a system and method for driving an array of such Luminaires to generate images or light patterns. The invention also relates to preventing spill light and controlling the beam angle of an LED array. Additionally, the invention relates to a system and method for maximizing the light output from the LEDs while maintaining them at or below their optimum operating temperature and uniformity across the LED array or a plurality of LED arrays. 
       BACKGROUND OF THE INVENTION 
       [0002]    High power LEDs are commonly used in luminaires for example in the architectural lighting industry—in stores, in offices and businesses; as well as in the entertainment industry—in theatres, television studios, concerts, theme parks, night clubs and other venues. In such applications LED arrays are frequently used to present images to an audience. It is common when projecting large images for the images to be divided into parts and then the parts transmitted to portions of the array. The transmission of these images can require significant bandwidth. In such applications the LED arrays are also frequently used to project a beam of light. 
         [0003]    In these applications it is a common requirement to obtain the maximum light possible out of the LEDs without exceeding their operating temperature. LEDs are highly temperature sensitive and running them at too high a temperature will both reduce their output and shorten their life. In such applications, it is also frequently desirable to have the appearance of the image, light beam or plurality of light beams from a plurality of LED arrays be of consistent luminosity. 
         [0004]    It is well known in the art to include a temperature sensor in an LED system to measure the temperature of the LEDs and use that information to control the operating current and voltage so that the LED always operates within safe operating parameters. However, the critical temperature is that of the LED semiconductor die itself and such temperature probes are often situated to measure the LED package or the heat sink rather than directly measuring the temperature of the die. To compensate for this many manufacturers include a safety band or dead space in the operating parameters to ensure that the temperature never rises too high. This safety band means that the LEDs are never achieving maximum possible brightness. 
         [0005]    It is also known to consider the total power and heat dissipation of a bank of LEDs rather than that for each individual LED. If, for example, the luminaire has Red, Green and Blue LEDs mounted on a single circuit board or heat sink then if only the Red LEDs are illuminated it is possible to run those Red LEDs at a higher power than if all three groups, Red, Green and Blue were illuminated simultaneously. 
         [0006]    These LED array fixtures are also used to project color light beams. For color control it is common to use an array of LEDs of different colors. For example a common configuration is to use a mix of Red, Green and Blue LEDs. This configuration allows the user to create the color they desire by mixing appropriate levels of the three colors. For example illuminating the Red and Green LEDs while leaving the Blue extinguished will result in an output that appears Yellow. Similarly Red and Blue will result in Magenta and Blue and Green will result in Cyan. By judicious control of the LED controls by color the user may achieve any color they desire within the color gamut defined by the LED colors employed in the array. More than three colors may also be used. For example it is well known to add an Amber or White LED to the Red, Green and Blue to enhance the color mixing and improve the gamut of colors available. 
         [0007]    The differently colored LEDs may be arranged in an array in the luminaire where there is physical separation between each LED, and this separation, coupled with differences in die size and placement for each color, may affect the spread of the individual colors and results in objectionable spill light and color fringing of the combined mixed color output beam. It is common to use a lens or other optical device in front of each LED to control the beam shape and angle of the output beam; however these optical devices are commonly permanently attached to the luminaire requiring tools and skilled labor to change and may additionally need to be individually changed for each LED or pixel individually. It would be advantageous to be able to simply and rapidly change such optical devices for the entire array simultaneously without the use of tools. 
         [0008]    There is a need for an inexpensive LED driving system which can maximize the output of connected LEDs in a luminaire while making the luminosity consistent across an array of LED array luminaires. There is also a need for a system and method that allows for the display of images or light patterns across an array of luminairs the display of which is controlled with conventional relatively low bandwidth control protocol. 
         [0009]    There is also a need for a beam control system for an LED array luminaire which can be quickly and easily changed and provide improvements in spill light reduction and beam angle control. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherein: 
           [0011]      FIG. 1  illustrates an LED array multi-parameter automated luminaire; 
           [0012]      FIG. 2  illustrates an exemplar LED array of the multi-parameter automated luminaire embodiment of  FIG. 1 ; 
           [0013]      FIG. 3  illustrates an exemplar graph of temperature versus time for an LED; 
           [0014]      FIG. 4  illustrates an exemplar graph of temperature versus power for an LED; 
           [0015]      FIG. 5  illustrates an embodiment of the invention showing major software components; 
           [0016]      FIG. 6  illustrates an array of automated luminaires each with an array of LEDs where the luminaires are configured in a linear arrangement; 
           [0017]      FIG. 7  illustrates an of automated luminaries in a two-dimensional array configuration where each luminaire includes an LED array in order to display an image(s) or light pattern; 
           [0018]      FIG. 8  illustrates another embodiment of an array of automated LED array luminaires configured in a two-dimensional array; 
           [0019]      FIG. 9  illustrates an other embodiment of the luminaire array of  FIG. 7  wherein the spacing between the luminaries has been increased; 
           [0020]      FIG. 10  illustrates another embodiment of the luminaire array of  FIG. 7  wherein the spacing between the luminaries is not uniform or consistent; 
           [0021]      FIG. 11  illustrates an alternative embodiment of the invention with a beam control system mounted proximate to the LED array; 
           [0022]      FIG. 12  illustrates a view of the beam control system of  FIG. 11  with the beam control system detached from the LED array; 
           [0023]      FIG. 13  illustrates a problem with prior art LED array lighting fixtures; 
           [0024]      FIG. 14  illustrates a single cell of an embodiment of the beam control array of  FIG. 11 ; 
           [0025]      FIG. 15  illustrates an exploded diagram view of the beam control array embodiment of  FIG. 14 ; 
           [0026]      FIG. 16  illustrates an exploded diagram view of the embodiment of the beam control array embodiment of  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Preferred embodiments of the present invention are illustrated in the FIGUREs, like numerals being used to refer to like and corresponding parts of the various drawings. 
         [0028]    The present disclosure generally relates to a method for driving LEDs when used in a light beam producing luminaire, specifically to a method relating to maximizing the light output from the LEDs while maintaining them at or below their optimum operating temperature. In one embodiment the present disclosure utilizes a temperature sensor within an LED array and a predictive algorithm to maximize LED output. 
         [0029]      FIG. 1  illustrates an embodiment of an automated luminaire  10  with an LED array  12  light source. In the embodiment shown the luminaire is mounted to a yoke  14  that is capable of providing motorized pan and tilt movement for the LED array  12  of the luminaire  10 . The yoke in turn is mounted to a top box  16  which may contain movement processing electronics  42 , motor drivers  44  and driving electronics for the LEDs  48  as well as communication systems  40  to allow it to receive data such as from an industry standard DMX512 data stream or some other similar protocol. In further embodiments, the top box  16  may also contain a media server  46  capable of outputting pixel mapped images under command of a DMX512 signal. The media server may be a module that can be easily removed or replaced. The pixel mapped images may control individual LEDs or LED pixels comprising adjacent red, green and blue LEDs in the LED array  12  so that they behave as pixels in an image display. Use of the LEDs in an LED luminaire to display images in this manner is well known in the art. 
         [0030]      FIG. 2  illustrates an embodiment of an LED array  12  of the mltiparameter luminaire  10  of  FIG. 1  with a plurality of LEDs  20  in the LED array  12 . In the embodiment shown the LEDs  20  are mounted on a substrate or circuit board  22 . The LEDs  20  may be of a single color and type or may be, as shown here, of multiple colors. In the example illustrated three colors of LEDs are used; Red (R), Green (G) and Blue (B). The disclosure is not limited by the number or types of LEDs used and is applicable with any layout of any number of any type of LEDs or OLEDs. 
         [0031]    A temperature probe  24  is also mounted on the substrate or circuit board  22 . In alternative embodiments temperature probe  24  may also be mounted in other locations such as on a heat sink (not shown). 
         [0032]      FIG. 3  illustrates an exemplar curve  30  on a temperature versus time graph  32  for an LED, or LED array, run at a fixed power level. When the power is set to a normalized level the temperature will rise over time and tend towards an asymptotic limit  34 . 
         [0033]      FIG. 4  illustrates an exemplar curve  40  on a temperature versus power graph  42  for an LED, or LED array. In this case the temperature rises increasingly with power. As we near the point where the heat sink is incapable of dissipating the heat generated  44  the array may go into a thermal runaway situation where the temperature rises rapidly and the LEDs are permanently damaged. It is important to avoid such a result. In the embodiment illustrated a single probe is used. This probe may consist of a single sensor or it may consist of a temperature sensor with thermal connection to receive temperature signals from one or more sections or locations on the circuit board  22  and or heat sink(s). In other embodiments, the temperature probe may have several sensors located in different sections of the circuit board  22  and/or heat sink(s) (not shown). In other embodiments several individual sensors or probes may be employed to provide temperature information to the LED driver software described below. 
         [0034]      FIG. 5  illustrates an embodiment of the major software components of the embodiment illustrated in  FIG. 1 . User input  50  to a control desk (not shown) is processed  52  on the control desk before transmitting through the data link  54  to the electronics  56  onboard the luminaire (not shown). The data stream is initially processed in the onboard electronics  56  and split into its major components. Luminaire movement data passes to the movement processing section  58  and thence to the motor drivers  60 . Another major component is the image or light pattern data for the desired output of the LED array (not shown). 
         [0035]    One of the routines performed by the LED driver hardware ( 48  from  FIG. 1  and software drivers  66  is as follows:
       a. Set the LED power to a known value;   b. Measure the temperature of the substrate or circuit board using a temperature probe;   c. Measure and establish the rate of rise curve for Temperature with Time as illustrated in  FIG. 3 ;   d. Increase the Power a known amount and repeat (b) and (c) to establish the rate of rise curve for Temperature with Power as illustrated in  FIG. 4 ;   e. Take as many measurements as necessary to complete this data throughout the nominal range of operations.       
 
         [0041]    The curves established may be extrapolated back to allow both the prediction of final steady state die temperature from any desired input power and the time that will be taken to achieve that temperature. 
         [0042]    Now, when it is desired to maximize the output of any particular LED or sub-group of LEDs in the luminaire for continuous operation, we may take the power needed to illuminate that sub-group of LEDs, compare that with the known data for the entire set of LEDs and the known rate of rise curves for Power and Temperature of those LEDs as well as the current temperature returned by the temperature probe and derive a total power possible for the sub-group. For example, if the total power capacity for the entire luminaire is 300 W when all R, G and B LEDs are illuminated and the user wishes to only illuminate the R and G LEDs. Assuming all three groups are equal in nominal consumption and efficiency then the simple solution when running two groups out of three would be to supply ⅔ of the full capacity power or 200 W. However by taking note of the temperature rise and the relationship between Power and Temperature for the luminaire as seen in  FIG. 3  we may increase the power to, for example, 250 W and still maintain acceptable temperatures on all LEDs. 
         [0043]    In a further embodiment we may increase the power supplied to an LED when the use is intermittent, such as when being used as a strobe. In this case we can use our knowledge of the temperature/time relationship as shown in  FIG. 3  as well as the temperature/power relationship as shown in  FIG. 4  to apply power at much increased levels when the LED is on in the knowledge that the LED will then be off for a period of time thus allowing heat to dissipate. 
         [0044]    In a further embodiment we apply compensation to the temperature reported by temperature probe to compensate for any thermal lag that might be present between the LED die and the position of thermal probe. In one embodiment this compensation takes the form of increasing the value of the measured temperature. In a preferred embodiment this compensation increases the value of the measured temperature as a function of the rate of change of temperature based on the known values that the LEDs in the array are being driven. 
         [0045]    In a further embodiment a fan (not shown) may be used to assist with cooling the LEDs. In some entertainment venues such as theatres or opera houses it is important to minimize the noise produced by luminaires and running any fans at as low a speed as possible can assist with this need. The speed of the fan may be controlled to provide the right amount of cooling while keeping the fan speed as low as possible so as to minimize noise produced by the luminaire. The luminaire may optimally control the fan speed to minimize noise using knowledge of (i) the temperature reported by temperature probe  24 , (ii) the power and thus heat load required by the LEDs and, (iii) the current ambient temperature. 
         [0046]    In a single LED array the routine may be used to control the entire array in unison so that the adjustment of the control signals to the LEDs is consistent. In alternative embodiments it may be used only to control a subset of the array, particularly when multiple temperature sensors or temperature probes are used. In the later case, if the fixture is being used to provide light, it might be desirable to maximize light output from each subsection. If the fixture is being used to project an image it might be desirable to maximize the consistence of adjustment across the entire LED array. 
         [0047]      FIG. 6  illustrates another embodiment of the invention. In this embodiment, a series of yoke mounted automated LED luminaires  50 ,  52 , and  54  connected together through a serial daisy chain signal and cable  56 ,  58 , and  60 . In the embodiment employing DMX512, input cable  56  carries the DMX512 signal from a control desk to first luminaire  50  and thence in a daisy chain manner through cable  58  to luminaire  52  and cable  60  to luminaire  54 . Each automated LED luminaire  50 ,  52 , and  54  may be addressed such that it responds to data on the DMX512 signal that is specific to said luminaire. Each LED luminaire  50 ,  52 , and  54  may contain a media server capable of outputting pixel mapped images under command of a DMX512 signal that control the LEDs in its associated LED array. Through the common DMX512 signal such a series of luminaires may behave in a coordinated manner. For example, the luminaires may share there temperature information with each other and the control desk so that if desired, the luminaries may coordinate so that the drivers drive the LEDs so that the correction to color and intensity as a result of the above described routine of the LED drivers is uniform across the array of LED luminairs rather than just for individual LEDs, or individual LED arrays or sub-arrays. Such coordination may also be employed so that a single image may appear across all the LED arrays, portion  1  on luminaire  50 , portion  2  on luminaire  52  and portion  3  on luminaire  54  as is described in greater detail below. When an array of LED luminairs is employed to project a single image, it might be desirable to have the light color and output adjustments be uniform from fixture to fixture. If the array of luminaries is being used to provide light rather than display an image it may be desirable that the total output from each array be consistent across the array of luminaries. 
         [0048]    The image displayed may be a stationary image or a stream of images representing a moving video based image provided by the local store within each LED luminaire  10 . 
         [0049]      FIG. 11  illustrates an embodiment of the invention: an automated luminaire  400  with an array of LEDs fitted with a beam control array  414  may be mounted to the front of the luminaire adjacent to the LEDs  404 . Beam control array  414  is retained on the luminaire  400  by retention clip  412 . Retention clip  412  may be recessed such that the unit is secure against accidental removal of the beam control array  414 . In an alternative embodiment the beam control array  414  may be a fixed feature of the luminaire. However in the preferred embodiment it is removable so that it can be cleaned or replaced or substituted with a differently shaped array the benefits of which will be appreciated below. 
         [0050]      FIG. 12  illustrates an exploded view of the embodiment illustrated in  FIG. 11 . Luminaire  400  contains an array of LEDs  304 . A beam control array  414  may be mounted to the front of the luminaire adjacent to the LEDs  404 . Beam control array  414  is retained on the luminaire  400  by retention clip  412 . And may be easily installed and removed as a single item. 
         [0051]      FIG. 13  illustrates a problem posed by prior art LED array luminaries.  FIG. 13  illustrates two LEDs as may be used in an LED array luminaire causing light spill and or color fringing. LED  422  and LED  424  may be of differing colors and, due to the different optical properties and construction of the LED dies, produce light beams  432  and  434  respectively that differ in beam spread. The differing beam spreads mean that the light beams from LEDs  422  and  424  will impinge on an illuminated object  440  in such a way that areas  444  and  446  of the object are illuminated by a single LED only rather than the desired mix of both. This results in areas  444  and  446  being colored differently from the central mixed area and appearing as colored fringes. Two LEDs only are illustrated here for clarity and simplicity however the same problem exists with systems incorporating more than two colors of LED. 
         [0052]      FIG. 14  illustrates a single cell of the beam control array  414 . The light output from the same LEDs  442  and  424  with differing beam angles as used in the prior art system shown in  FIG. 13  are impinging on object  440 . However, in the disclosed device the light from LEDs  442  and  424  is modified by optical element  450  and louver mask  416  such that the beam angles from each LED are constrained to be very similar and the areas of color fringing  444  and  446  are significantly reduced in size. Optical element  450  is an optional component in the system and may be a lens, lens array, micro-lens array, holographic grating, diffractive grating, diffuser, or other optical device known in the art. It can be seen that changing the height of louver mask  416  will alter the constrained beam angle of the output beam. A taller louver  416  will produce a narrower beam and a shorter louver will produce a wider beam. The louver mask  416  may be of fixed height or may be adjustable. Louver mask  416  may preferably be non-reflective so as to avoid spill light, this may be achieved by painting or coating the louver mask with a matte black paint, anodizing or other coating as known in the art to preferably absorb or scatter rather than reflect light. LEDs  422  and  424  may be of a single color and type or may be, as shown here, of multiple colors. In the example illustrated two colors of LEDs are used. The invention is not limited by the number, colors, or types of LEDs used and is applicable with any layout of any number of any type and any color of LEDs or OLEDs.  FIG. 14  shows both LEDs  422  and  424  within the same louver mask  416  however other embodiments may utilize separate louver masks for each LED. In alternative embodiments rather than increasing the height  419  of the louvers  416  the width  418  of the louver(s) may be increased for a similar result. 
         [0053]      FIG. 15  illustrates an exploded diagram of an embodiment of the beam control array  414 . Beam control array  414  comprises a louver mask array  462  containing multiple cells  420 . Mounted onto the louver mask array  462  are optical element carriers  452  which clip into the cells  420  of the louver array  462 . Each optical element carrier  452  may in turn contain an optical element  450 . Optical elements  450  are here illustrated as micro lens arrays; however, the invention is not so limited and optical elements  450  may be any optical beam control device as known in the art. Each optical element  450  is clipped into an associated optical element carrier  452 . 
         [0054]    In one embodiment of the beam control array, every optical element  450  is identical but in further embodiments the optical elements  450  may differ across the beam control array  414 . For example, alternating optical elements  450  may be of two different beam angles. In a yet further embodiment, the optical elements  450  around the periphery of the beam control array  414  may be of one beam angle that differs from the beam angle of the optical elements  450  in the center of the beam control array  414 . In yet further embodiments, the height of louver mask array  462  may be varied to effect different controlled beam angles for the emitted light. Such combinations of differing optical elements and louver array height may be advantageously chosen so as to allow fine control of the beam shape and quality. Notwithstanding the above and the various combinations of optical elements the entire beam control array  414  may be installed or removed from the luminaire as a single easily replaced item. When installed on the luminaire the beam control array is adjacent to the LEDs  482  mounted on the LED Circuit board  478 , reduces color fringing or halation and controls the beam angle to provide the lighting designer with a well controlled and defined beam of a single homogeneous color. 
         [0055]    In one embodiment optical elements and louver arrays are provides such that symmetrical beams with angles of 12°, 25°, and 45° are available. In further embodiments an asymmetrical optical element may be used that provides an elliptical beam such as one that is 15° in one direction and 45° in an orthogonal direction. The beam angles given here are examples only and the invention is not so limited. Any beam angle or combination of beam angles is possible within a beam control array without departing from the spirit of the invention. 
         [0056]    Beam control array  414  may further provide mechanical protection and dust exclusion for the LEDs  404 . To allow for such protection without the optical element affecting the beam angle an optical element comprising a clear, flat window may be used. Such a window has no effect on the beam while still providing protection and dust exclusion. 
         [0057]    The control array  414  may also be of different shaped cells than those shown. For example the cells be me round or hexagonal or other regular or non-regular shapes. 
         [0058]    The user or rental company may stock a range of different beam control arrays with differing optical elements and louver array heights to facilitate quick and easy customization of a luminaire to provide the beam angle required for the current event or show. 
         [0059]      FIG. 16  illustrates an assembled array of an embodiment of a beam control array  414 .  FIG. 16  is viewed from the reverse direction of  FIG. 15  and shows an assembled beam control array  414 . Louver mask array  462  cells  420  may contain multiple sub-compartments  480  each of which may control the light output for a single LED die. Optical element carrier  452  clips into the louver mask array  462  and, in turn, contains optical element  450 . Optical element  450  is adjacent to the LED dies  482  mounted on LED support  478 . Each LED  482  may comprise a single LED die of a single color or a group of LED dies of the same or differing colors. For example in one embodiment LED  482  comprises one each of a Red, Green, Blue and Amber die. In such systems each LED die may be independently aligned with a sub-compartment  480  of the louver mask cell  420 . 
         [0060]    While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure as disclosed herein. The disclosure has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the disclosure.