Lighting control method having a light output ramping function

"LIGHTING CONTROL METHOD WITH A GRADUAL LIGHT OUTPUT CHANGE FUNCTION"

Related Orders

This application is a Continuation-in-Part (CIP) of US Application No.^the11/875,083, filed on October 19, 2007, and incorporated herein in its entirety for reference purposes.

Field of Invention

The present invention relates generally to lighting control, and more particularly to a method of lighting control with a function of gradually changing light output to provide a substantially uniform light output during a warm-up period.

Background of the Invention

Many disadvantages have been identified in existing lighting control systems that can lead to a lighting device performing below expectations. These disadvantages include, but are not limited to, voltage variations between LED lighting modules, resulting in non-uniform light output. These voltage variations can arise from a lack of uniformity in the manufacturing of the LEDs used in a lighting device.

Another disadvantage of existing lighting control systems is the inability of the lighting circuitry system to compensate for the effects of temperature changes on the LED forward voltages, such as the changes required in the drive voltage caused by an increase in temperature. In this regard, existing lighting control systems do not compensate for the inherent forward voltage variations, as evidenced by an output driver across the entire operating temperature range of the lighting fixture.

Furthermore, as is well known to those skilled in the art, the light output of an LED is inversely proportional to the junction temperature of the LED. Therefore, when the LED is first activated (i.e., cold start), the junction temperature is low and the light output is high. As the junction temperature increases during a warm-up period (which lasts approximately 30 minutes), the light output of the LED will decrease until the LED reaches a steady-state condition. Once the steady-state condition is reached, the operating temperature of the LED junction will generally remain constant, and, as a consequence, the light output will generally remain constant during the period of continuous use.

The decay in LED light output during the warm-up period can reach 20%. Therefore, the LED light output during the warm-up period is generally not uniform, and the steady-state LED light output may be considerably lower than the expected nominal steady-state output. It is also possible that the LED light output during cold start-up may exceed an upper limit of light output.

The reported disadvantages are particularly aggravated in cases where the lighting device is a surgical lamp that requires a substantially constant light output or lux readings.

The present invention addresses these and other disadvantages to offer an improved lighting control method for a lighting device.

Summary of the Invention

According to the present invention, a lighting control method is proposed for controlling an LED light source during an LED heating mode. The method comprises: (a) retrieving a pre-stored setpoint duty cycle corresponding to a light intensity level; (b) operating the LED light source during an initial time interval at an operating duty cycle equal to the setpoint duty cycle minus a pre-stored duty cycle offset value; (c) setting a gradual duty cycle change value equal to a pre-stored duty cycle increment value; (d) operate the LED light source for a time interval in an operating duty cycle equal to the setpoint duty cycle less the duty cycle offset value plus the gradual increase duty cycle value; and determine whether (1) a gradual increase time has elapsed, (2) the operating duty cycle is equal to the setpoint duty cycle, or (3) a new light intensity level has been selected. The LED heating mode is terminated if any of (1), (2), or (3) has occurred. If (1), (2), and (3) have not occurred, then the gradual duty cycle change value is increased by the duty cycle increment value, and the method returns to step (d).

According to another aspect of the present invention, a lighting control method is proposed for controlling an LED light source during an LED warm-up mode, the method comprising: (a) establishing a light intensity level; (b) retrieving a pre-stored setpoint duty cycle corresponding to the light intensity level; (c) retrieving a pre-stored duty cycle offset value; (d) operate the LED light source during an initial time interval in an operating duty cycle equal to the setpoint duty cycle minus the duty cycle offset value; (e) retrieve a pre-stored gradual increase time; (f) retrieve a pre-stored duty cycle increment value; (g) set a duty cycle gradual change value equal to the pre-stored duty cycle increment value; (h) operate the LED light source for a time interval in an operating duty cycle equal to the setpoint duty cycle minus the duty cycle offset value plus the gradual change in duty cycle value; (i) determine whether a predetermined condition has been met; (j) if the predetermined condition has been met, then increase the gradual activity cycle change value by the activity cycle increment value and return to step (h); and (k) if the predetermined condition has not been met, then terminate the warm-up mode.

One advantage of the present invention is obtaining a method of lighting control that offers improved uniformity in the light output of an LED light source during an LED warm-up period.

Another advantage of the present invention is obtaining a method of lighting control that prevents the light output of an LED light source from exceeding an upper limit value for light output.

These and other advantages will become evident based on the following description, considered in conjunction with the accompanying drawings and the appended claims.

Brief Description of the Drawings

The invention may take physical form in certain parts and arrangements of parts, of which an embodiment will be described in detail in the descriptive report and illustrated in the accompanying drawings which constitute a part of the invention, and in which:

FIG. 1 is a general block diagram of a lighting control system for a lighting device, according to an embodiment of the present invention;

FIG. 2 is a schematic view of a drive output circuit, according to an embodiment of the present invention;

FIG. 3 is a schematic view of a first LED module including a temperature compensation circuit, according to an embodiment of the present invention;

FIG. 4 is a schematic view of a second LED module including a compensation circuit, according to an embodiment of the present invention; and

Figures 5A and 6B show a flowchart illustrating a method for controlling lighting during a warm-up period, according to an embodiment of the present invention.

Detailed Description of the Invention

Referring now to the drawings, whose illustrations serve only to illustrate an embodiment of the invention and not to limit it, FIG. 1 shows a block diagram of the lighting control system 10 for a lighting device, such as a surgical lamp, according to an embodiment of the present invention. The lighting control system 10 is generally composed of a primary controller 20, a drive circuit system 30 composed of at least one drive controller 32 and at least one drive output 34, one or more first LED modules 50 (module A), and one or more second LED modules 80 (module B). In the illustrated embodiment, the primary controller 20 and the drive circuit system 30 are located on a first printed circuit board PCB1. Each of the first and second LED modules, 50 and 80, is located respectively on the second and third printed circuit boards PCB2 and PCB3. The printed circuit boards PCB1, PCB2, and PCB3 may be located together within a housing (not shown) for the lighting device. It should be noted that, in the alternative embodiment, the components of the 50 and 80 LED modules residing separately on printed circuit boards PCB2 and PCB3 may be located together on a single substrate (i.e., the printed circuit board).

In the illustrated embodiment, the primary controller 20 is a microcontroller. For example, the primary controller 20 can take the form of an ARM-based processor with a variety of peripherals integrated on the chip, including, but not limited to, internal FLASH memory for program storage, RAM memory for data storage, UARTs, timer/counters, a bus interface, 15 a serial interface, an SPI interface, a programmable watchdog timer, programmable I/O lines, an A/D converter and PWM outputs. The primary controller 20 sends commands to the drive controllers 32 and reads condition information from each drive controller 32.

It should be understood that the primary controller 20 can also communicate 20 with other electronic devices not illustrated in FIG. 1, including, but not excluding, a user interface (e.g., front panel display with numeric keypad, control keys or buttons), a communications interface, a video input connector and a camera module. The user interface allows the user to turn the lighting device on/off and select an intensity level for the lighting device. It can also allow the user to turn on/off other accessories configured with the lighting system.

The primary controller 20 communicates with the drive controllers 32 via a bus 22. In the illustrated embodiment, the bus 22 is a serial bus (e.g., IC). The primary controller 20 also provides a constant clock signal to the drive controllers 32 via a synchronization line 24, as will be explained in more detail later.

In the illustrated embodiment, the drive controller 32 is a microcontroller. For example, each drive controller 32 can take the form of an ARM microcontroller with a variety of peripherals integrated on the chip, including, but not limited to, 35 internal FLASH memory for program storage, RAM memory for data storage, UARTs, timer/counters, a serial interface, an A/D converter, a programmable watchdog timer, and lines of

Programmable I/O. In the illustrated embodiment, each drive controller 32 has a unique identification number that allows the primary controller 20 to individually address each drive controller 32.

Referring now to FIG. 2, each drive output 34 is a general circuit5 comprising a comparator 42 (e.g., LMV7235 from National Semiconductor), a voltage regulator, a diode 45, a setpoint potentiometer (POT) 46, a power field-effect transistor (FET) 48, and a feedback resistor (Rs) 47. The drive outputs 34 are driven (i.e., activated) at a fixed frequency (i.e., the fixed-frequency activation signal provided by line 43). In the illustrated embodiment, the drive outputs 34 are activated with an activation signal with a fixed frequency of 300 Hz.

Voltage regulator 44 provides a precise fixed output voltage (e.g., 5V) when activated. The output voltage (Vout) of voltage regulator 44 is electrically connected to power FET 48. FET 48 is used to manage the current required by the LED modules 50, 80. Sensing resistor (Rs) 47 provides current sensing. The setpoint potentiometer 46 is used to adjust the output voltage of the voltage regulator 44 until the detected current associated with Rs 47 is within a target current range.

Comparator 42 monitors the output voltage of a drive output 34.20. In this respect, comparator 42 receives a reference voltage (V)._REF) as a first input and receives a detected voltage (V_s) as a second entry via line 49. Comparator 42 compares V_REF with V_s to determine if the detected current (l_s) associated with V_s exceeds a threshold current (for example, approximately 1.26 A). If the threshold current has been exceeded, then comparator 42 generates a signal to disable voltage regulator 44, thus disabling V._THEout of voltage regulator 44. The drive controller 32 can also disable voltage regulator 44 under certain conditions (e.g., detection of a short-circuit or open-circuit fault).

Figures 3 and 4 illustrate, respectively, schematic views of the 50 LED module (module A) and the 80 LED module (module B). The 50 and 80 LED modules are LED light sources. In the illustrated embodiment, the 50 and 80 LED modules are electrically connected in series by a wiring harness connected between connector J2 of the 50 LED module and connector J4 of the 80 LED module. Thus, each pair of LED modules connected in series 50, 80 collectively provides a set of six (6) LEDs connected in series. A first pair of LED modules connected in series 50, 80 can be connected in parallel 35 with a pair of LED modules connected in series 50, 80. The first and second pairs of LED modules connected in series 50, 80 are driven from a single drive output 34 (i.e., drive output channel). Each LED module 50 is electrically connected to a drive output 34 by means of a wiring harness assembly (not shown) connected to connector J1. In the illustrated embodiment, two pairs of LED modules 50, 80 are electrically connected to drive output A and the two pairs of LED modules 50, 80 are electrically connected to drive output B.

Referring now to FIG. 3, the LED module 50 includes a plurality of LEDs 52, a temperature compensation circuit 60 and an optional remote temperature sensor circuit 70. In the illustrated embodiment, the LED module 50 includes three (3) LEDs connected in series 52 (e.g., high-brightness LEDs). The temperature compensation circuit 60 compensates for changes in forward voltage required to drive the LEDs 10 due to increased temperatures. As LED temperatures increase, the forward voltage must be reduced in order to maintain a constant drive current to the LEDs. The temperature compensation circuit 60 includes a field-effect transistor (FET) Q2, a thermistor 62, and a resistor network 64 composed of resistors R1 and R2. Power is supplied to the temperature compensation circuit 60 via connector J1. The thermistor 62 is a resistive temperature-sensing device. The FET Q2 balances (that is, equalizes) the network of resistors 63 by activating itself more (or less) to regulate the current.

The remote temperature sensor circuit 70 includes a temperature sensor 72 (for example, the Analog Devices TMP35 low voltage temperature sensor) to 20 provide the primary controller 20 with temperature data to monitor the temperature in the vicinity of the printed circuit board PCB2. The temperature sensor 72 provides an output voltage that is linearly proportional to the detected temperature. The temperature sensor circuit 70 is electrically connected to the primary controller 20 via connector J3 and line 26. The primary controller 20 receives the output from the temperature sensor circuit 70. The primary controller 20 can read a limited number of temperature sensor inputs from the printed circuit boards PCB2. In the illustrated embodiment, only two temperature sensor circuits 70 in the LED modules 50 are selected or connected to the primary controller 20.

Referring now to FIG. 4, the LED module 80 includes a plurality of LEDs 82 and a compensation circuit 90. In the illustrated embodiment, the LED module 80 includes three (3) LEDs connected in series 82 (e.g., high-brightness LEDs).

The 90° compensation circuit compensates for differences in forward voltage values ​​between LEDs due to non-uniformity in LED manufacturing. In this regard, the compensation circuit 90 balances the voltage drop differences between the series-connected LEDs 52, 82 to ensure that the appropriate voltage is applied across the series-connected LEDs 52, 82 to set the desired forward current value and make all the LED modules 50, 80 appear identical (i.e., uniform illumination). The compensation circuit includes an adjustable FET Q1 controlled by an amplifier (comparator) 96 (e.g., the Analog Devices AD9220 JFET input instrumentation amplifier) ​​which provides a means by which the paired LED modules 50, 80 can be calibrated (i.e., “cut off”) to a fixed voltage drop across the pair of modules, as described below. A digital potentiometer (POT) 92 (e.g., MAX 5417, A digital potentiometer (from Maxim Integrated Products) is used to set the gate voltage for FET Q1. A micropower voltage regulator 94 (e.g., the LM4040 voltage reference from Maxim Integrated Products) is used to power amplifier 96 and digital potentiometer 92. Voltage regulator 94 supplies 5V to digital potentiometer, amplifier 96, and the bias circuits (not shown). The input for voltage regulator 94 uses a blocking diode D1 and two capacitors (not shown). The combination of diode D1 and the two capacitors provides a small capacitive storage between pulses to maintain a constant voltage under the minimum duty cycle at the normal operating frequency (e.g., 25% at 300 Hz). Voltage regulator 94 is always powered once voltage is applied to LEDs 52 and 82.

The operation of the lighting control system 10 will now be described in detail. The primary controller 20 is programmed to provide overall control of the lighting control system 10. In this respect, the primary controller 20 communicates with drive controllers 32, as well as with other system components, such as a user interface and a video camera.

In the illustrated embodiment, the primary controller 20 generates a 30 kHz clock signal composed of clock pulses of fixed duration. The clock signal is supplied to each drive controller 32 by the synchronization line 24. The clock signal is used to maintain synchronization between the drive controllers 32 and to provide each drive controller 32 with a fixed time base used to drive the respective LED modules 50, 80. In this respect, the clock signal directly triggers two internal timers within each drive controller 32. The first internal timer of each drive controller 32 is associated with a first drive output 34 (drive output A), and the second internal timer of each drive controller 32 is associated with a second drive output 34 (drive output B). The internal timers allow the two drive outputs 34 (i.e., drive output A and drive output B) to provide drive output signals that are out of phase with each other, thus preventing large fluctuations in current consumption when the lighting device is activated. According to a preferred embodiment of the present invention, the phase is different for each drive output 34 of all drive controllers 32. Thus, the drive output A of drive controller 1, the drive output B of drive controller 1, the drive output A of drive controller 2, and the drive output B of drive controller 2 provide drive output signals that are out of phase with each other.

The drive output signals associated with the 34 of 5 drive outputs have a fixed frequency of 300 Hz. The frequency of 300 Hz is selected because it is a multiple of 50 Hz (the scan rate of PAL video cameras) and 60 Hz (the scan rate of NTSC video cameras). When using an optional video camera with the lighting device associated with the present invention, the camera will detect a perceptible flicker in the light if the output frequency of LEDs 52, 82 is not a multiple of 10 of the camera's scanning rate.

The primary controller 20 sends multiple commands to each drive controller 32 in order to “activate” the LED modules 50, 80 (that is, to activate the LEDs 52, 82). The commands include a command that indicates a selected operating duty cycle (also called the “target duty cycle”) for the drive output signals of the 15 drive outputs 34, a command that indicates the “phase deviation” for each drive output 34, and a command that indicates the activation of the LED modules 50, 80, called the “start” command. The operating duty cycle is indicated by a number of pulses from the clock signal output by the primary controller 20. As will be explained in more detail later, the number of pulses of the primary controller clock signal 20 will establish the “on period” for each period of the drive output signal produced by the drive outputs 34.

The operational duty cycle is the ratio of the ON time of the drive output signal to the period of the drive output signal for the 34 drive outputs. As indicated above, each drive output signal preferably has a fixed frequency of 300 Hz, and therefore has a period of 3.33 msec. In the illustrated embodiment, the LED modules 50, 80 are activated (i.e., illuminated) during the ON time of the drive output signals. The internal timers of each drive controller 32 count a predetermined number of clock signal pulses provided by the primary controller 20 to establish the on-time for each period of the 30 drive output signals. Thus, the predetermined number of pulses counted corresponds to the duty cycle selected for the drive output signals. For example, in a selected operating duty cycle of 40%, 40 clock signal pulses are counted to establish the on-time for the period of the drive output signals.

Furthermore, a phase shift of 10 units is generated in the clock signal emitted by the primary controller 20. The start command indicates to the drive controllers 32 that the associated LED modules 40, 80 are about to be activated (i.e., to turn on the lights).

(LED). The drive controllers 32 use the start command to initialize their respective internal timers and prepare for the start of the clock signal generated by the primary controller 20. The primary controller 20 can also send a “stop” command to the drive controllers 32 in order to inform the drive controllers 32 to disable the associated drive outputs 34 and interrupt their respective internal timers.

As discussed above, the clock signal from the primary controller 20 triggers the two internal timers within each drive controller 32, thus allowing the drive controllers 32 to control the associated LED modules 50, 80 in the 10th operating activity cycle, via the drive output signals from the drive outputs 34. The values ​​for the various operating duty cycles provided by the primary controller 20 are set to correspond to a plurality of predetermined and user-selectable LED intensity levels. The duty cycle values ​​associated with each intensity level can be pre-stored in a lookup table 15 in the primary controller 20's memory. By way of example, and not limitation, the illustrated implementation may include the nine intensity levels presented in TABLE 1:

TABLE 1

<td>Intensity level</td><td>Activity Cycle</td>

<td> 1</td><td> 40%</td>

<td> 2</td><td> 50%</td>

<td> 3</td><td> 60%</td>

<td> 4</td><td> 70%</td>

<td> 5</td><td> 80%</td>

<td> 6</td><td> 90%</td>

<td> 7</td><td> 100%</td>

<td>Maintenance</td><td> 25%</td>

<td>Calibration</td><td> 100%</td>

The duty cycle value for the maintenance intensity level provides a low duty cycle of 20 in order to obtain low light intensity to facilitate the inspection of 50 faulty LED modules with less visual discomfort. The duty cycle value for the calibration intensity level provides a maximum duty cycle that allows convenient adjustment of the power supplies until the lowest drive current output is at the target drive current, thus distributing sufficient drive output current to all 50, 80 LED modules.

As will be described in more detail below, the primary controller 20 is also programmed to operate the LED modules 50, 80 during a warm-up mode, so that the light output provided by the LED modules 50, 80 does not exceed a predetermined maximum light output level and so that a substantially uniform light output is maintained.

The operation of LED module 50 (module A) will now be described in detail with reference to FIG. 3. The temperature compensation circuit 60 adjusts the total voltage drop across the LED module pairs 50, 80, since the forward voltage characteristics of LEDs 52, 82 change with LED temperature. As LEDs 52, 82 heat up, their forward voltage drops. Reductions in forward voltage lead to an increase in the current flowing through LEDs 52, 82. The total voltage drop across the six LEDs connected in series 52, 82 of the 50, 80 LED modules is high enough to require some form of temperature compensation that keeps the LED drive current at the target drive current and prevents the 50, 80 LED modules from shutting down due to overcurrent.

The temperature compensation circuit of the 60 LED module (LED module A, for example) includes a FET Q2 that is biased so that when the 50 and 80 LED modules are cold, the FET Q2 is fully activated. This causes the forward resistance of the FET Q2 to be very low, therefore there is a relatively small amount of voltage drop across the FET Q2 when cold. As the 50 and 80 LED modules begin to heat up, the thermistor 62 acts to reduce the gate voltage on FET Q2 and increase its forward resistance. This action effectively absorbs the reduction in forward voltage as the 52 and 82 LEDs heat up. As the 52 and 82 LEDs begin to heat up, the thermistor 62 in the bias network of FET Q2 acts to reduce the gate voltage on FET Q2 and increase its forward resistance. This action effectively absorbs the reduction in forward voltage as LEDs 52 and 82 heat up. As the resistance of thermistor 62 becomes increasingly smaller, the gate voltage to FET Q2 becomes sufficiently low, causing the resistance of FET Q2 to be much greater than that of the pair of low-value parallel power resistors R1 and R2. At this point, virtually all the current flowing through the temperature compensation circuit 60 passes through the parallel resistors, R1 and R2, effectively switching off FET Q2. Switching off FET Q2 and switching on the fixed resistors R1 and R2 allows FET Q2 to be smaller and less expensive, since FET Q2 does not need to have its nominal value adjusted to manage the total current at higher temperatures. The temperature compensation circuit 60 is an independent circuit that does not provide feedback to the drive controller 32 or the primary controller 20.

As indicated above, the temperature sensor circuit 70 provides data to the primary controller 20 for display only and indicates the operating temperature in the vicinity of the LED module 50.

The operation of LED module 80 (module B) will now be described in detail with reference to FIG. 4. The compensation circuit 90 of LED module 80 offers the ability to insert an adjustable fixed voltage drop in series with the six LEDs 52 and 82 to calibrate the pair of LED modules 50 and 80 to a fixed input voltage used to power all LED modules 50 and 80 in the lighting device. An adjustable voltage drop in series with LEDs 52 and 82 allows the electrical voltage of each pair of modules 50 and 80 to be adjusted to a common electrical voltage at a specified electrical current. This feature allows the pairs of modules 50 and 80 to be driven in parallel.

Each drive output 34 drives two pairs of 50 and 80 LED modules electrically connected in parallel. If the two parallel pairs of 50 and 80 LED modules do not have substantially similar forward voltage drops, the currents through the two pairs of 50 and 80 LED modules will not be equivalent, and thus the light output of the two parallel pairs of 50 and 80 LED modules will not vary accordingly. Amplifier 96 of compensation circuit 90 generates the gate voltage of FET Q1 based on the difference between the positive input of the FET drain and the negative input, which is defined using digital potentiometer 92. When digital potentiometer 92 is being adjusted to an appropriate resistance value, FET Q1 acts as a resistor fixed in series with LEDs 52 and 82. Adjusting the forward resistance of FET Q1 effectively cancels out the forward voltage variations of the LED modules 50 and 80 caused by the different forward voltages of LEDs 52 and 82.

The POT 92 is adjusted and programmed as part of the LED module production process by connecting connector J5 to a programming tool (e.g., a test and calibration instrument) that records a setpoint value for the POT 92. The adjustment of the POT 92 is performed during a test and production process when the 25 LED 50 and 80 modules are electrically connected together. During the production process of LED modules 50 and 80, approximately 24V is applied by a test and calibration instrument to LED module 50 via connector J1. POT 92 is then adjusted so that the drive current through LEDs 52 and 82 is a predetermined target drive current value. Compensation circuit 90 is an independent circuit and does not have feedback to drive controller 32 or primary controller 20.

It should be noted that the 50 and 80 LED modules may be overdriven to account for optical losses during the lighting fixture assembly. In this respect, the LED drive current control target is defined as a predetermined fixed deviation above the nominal direct drive current of the LED. Consequently, the manufacturing personnel will be able to increase the intensity of LEDs 52 and 82 by adjusting the drive current to a value within the permissible range of the LED manufacturer, thereby achieving a desired lux reading from the lighting device.

A calibration function is provided by the primary controller 20 to allow further adjustment to be made to “tune” the drive current so that it is closer to the target drive current. The 24VDC adjustable output power supplies to be powered to luminaires that include 50 and 80 LED modules can have their outputs adjusted up or down in order to increase or decrease the drive current readings.

The drive controller 32 is programmed to sample the LED drive current and determine if the LED drive current is within the target drive current value plus/minus a predefined tolerance to provide fault messages to the display medium. If the LED drive current is outside the permitted tolerance, an audible or visual alarm can be used to indicate to the user that the power supplies need to be adjusted, or that the 50 and 80 LED modules (or the associated wiring harness) need to be replaced.

Primary controller 20 is programmed to monitor the LED drive current from drive outputs 34 to determine if one or both of the associated pairs of LED modules 50 and 80 have failed (i.e., open circuit) in order to provide a fault message to the display medium. If one LED module 50 and 80 of the pair of LED modules has failed due to an open circuit, the drive current will be approximately 50% of a target drive current setting. If both pairs of LED modules have failed, the drive current reading will be approximately 0 mA. Fault conditions are detected by the primary controller 20 and indicator alarms are generated on the user interfaces. A portion of each drive output 34 determines whether an LED module 50 and 80 has failed due to a short circuit. In this respect, drive output 34 detects the presence of a short circuit and generates an excessive current indication for the associated drive controller 32. This drive controller 32 then switches off drive output 34 associated with the short-circuited LED module 50, 80, and prevents drive output 34 from being switched on until the short-circuit fault condition has been eliminated. A fault message may also be displayed to the user.

As discussed above, the primary controller 20 is programmed to operate according to a heating mode, as will be explained in detail with reference to FIGS. 5A and 5B. FIGS. 5A and 5B show a flowchart of a lighting control method for operating LEDs during a heating mode. The warm-up mode is initiated when the 50, 80 LED modules are initially powered on at startup, provided that the ambient temperature of the electronic components for the LED light source is below a predetermined setpoint temperature (T_SETpoint), for example, of

38°C. The ambient temperature is checked in step 105. The primary controller 20 retrieves the pre-determined setpoint duty cycle (D_Sr) corresponding to a user-selected light intensity level (Step 110). For example, intensity levels 1 to 7 may respectively correspond to setpoint activity cycles (D_SP) of 40%, 50%, 60%, 70%, 80%, 90% and 100%. After that, the primary controller 20 retrieves a pre-stored duty cycle offset value (D_TR|_M) associated with the selected light intensity level (Step 120). Primary controller 20 is also programmed to determine if a gradual change duty cycle function has been activated (Step 120). The gradual change duty cycle function is described in detail below.

In step 130, the operational activity cycle (D) is established in the setpoint activity cycle (D_Sr, corresponding to the selected intensity level, as compensated by D_TThus, during an initial time interval (e.g., 15 minutes), the 50 and 80 LED modules are operated at an operating duty cycle (D) equal to the setpoint duty cycle (D)._SP) reduced by D_TR(Step 140). As discussed above, the operating activity cycle (D) refers to the activity cycle of the drive output signals of the drive outputs 34. After the first time interval has elapsed, the primary controller 20 determines whether the gradual change activity cycle function has been activated (Step 145). If the gradual change activity cycle function has not been activated, then the heating mode is terminated. Alternatively, if the gradual change activity cycle has been activated, then the gradual change activity cycle function is initiated (Step 150).

In step 160, the primary controller 20 retrieves pre-stored values ​​for the gradual increase time (T)._R) and the activity cycle increment value (D_Step)· The activity cycle increment value (D_Step) is a percentage value (e.g., 2%). The gradual change in activity cycle time (Dramr) value is initially set equal to D._Step (Step 170). In Step 180, the 50 and 80 LED modules are operated for a time interval (e.g., 5 minutes) in an operating duty cycle (D) defined as D_Sp - D_TR|_M + Dramr. At the end of the time interval, the primary controller 20 determines whether the gradual increase time (T_R) has already elapsed, if the operating activity cycle (D) has reached the setpoint activity cycle D_Sp for the selected light intensity level or if a new light intensity level has been manually selected by the user, thus resulting in a new operating activity cycle (Step 185). If either of these conditions has been met, then the heating mode is terminated. If neither of these conditions has been met, then the primary controller 20 increases the current Dramr by D_STep (Step 190). The primary controller 20 then operates the LED modules 50, 80 for the subsequent time interval in the newly established operational duty cycle (D), which is equal to D_Sp - D_Trim + Dramp (Step 180). Since Dramp was increased in Step 190, the operating activity cycle (D) will be increased in Step 180. This gradual step increase in the operating activity cycle (D) will continue for subsequent time intervals, until one of the conditions in step 185 is met, thus ending the warm-up mode.

Other modifications and alterations will be conceived after reading and understanding the descriptive report. It should be understood that the present invention is contemplated to have many alternative configurations. For example, in one configuration, 28 LED modules are grouped into 14 pairs of LED modules. Thus, four 10-drive controllers are connected to the primary controller. In another configuration, 56 modules

LEDs are grouped into 28 pairs of LED modules. Seven drive controllers are then connected to the primary controller. Furthermore, it is contemplated that LEDs of varying colors may be used in place of the single-color LEDs of the illustrated embodiment. It is intended that all such modifications and alterations be included to the extent that they fall within the scope of the invention as claimed or its equivalents.

Having described the invention, the following is claimed.