Click to Enlarge Figure 1: The HPLS series light sources are calibrated to give a linear ratio between the intensity setting and the transmitted optical power. The red curve is an example of the typical error that exists between the calculated and measured output powers. For additional details, please see the text.
Output Intensity Control: Rotate the Light Intensity Control Knob on the Front Panel to continuously vary the intensity of light transmitted through the output. Output intensity control operates independently of the shutter, and control of the intensity setting using the tuning knob and software may be performed regardless of whether the shutter is open or closed.
When a host PC is interfaced with the HPLS series light sources, the operator can also use the software to specify an intensity value as a percentage of the maximum possible optical output power. While operating the light source under External control, the intensity can be changed by sending the appropriate voltage signals to BNC ports on the instrument's back panel. When External control is enabled, the intensity tuning knob on the front panel is disabled. More information about External control and software control is included in Sections 5.5 and Chapter 7, respectively, of the manual.
Adjusting the output intensity does not affect the amount of optical power emitted by the bulb; the current driving the bulb is not affected by the intensity adjustment. Instead, the intensity adjustment controls the rotation angle of an attenuation disk placed between the bulb and the output. The disk has an aperture that varies with rotation angle, and adjustments to intensity cause the disk to rotate so that the desired amount of optical power is transmitted through the disk.
Using an attenuation disk to adjust the transmitted intensity, rather than performing this function by changing the current levels driving the bulb module, maintains the bulb in a stable operating state while also enabling the intensity to be quickly tuned; less than 3 seconds are required to adjust the output intensity between 90% and 10%.
The output powers of these high-power plasma light sources are calibrated so that the output power is a linear function of the intensity setting, as is illustrated by the blue curve in Figure 1. The intensity setting is reported as a percentage in the upper left corner of the display screen on the front panel, and its value is independent of the power emitted by the bulb. The red curve in Figure 1 is an example of the typical error between the measured power after the output and the predicted value of the output power calculated from the intensity setting and the calibration data. Each point on the error curve represents an average of 100 measurements. For optimum stability, allow these light sources to warm up for between 45 minutes and an hour before use.
Shutter Control: The shutter state can be toggled between open and closed by pressing the rectangular button beside the Light Intensity Control Knob, by sending the appropriate command to the light source when it is being controlled by a host PC, or by sending a 5 V CMOS signal to the Shutter Control female BNC port on the back panel when external control is enabled. The shutter is closed by default, and the state of the shutter (Open/Closed) is displayed in the lower left corner of the front panel's display screen. When the shutter is open, the LED indicator to the right of the shutter control button is illuminated. The shutter button is always operational, but pressing it while the light source is operating under external control will disable external control of the light source.
The shutter may be repetitively toggled between states with a repetition rate of up to 1 Hz for an indefinite duration, and its state may be toggled at rates up to 10 Hz during bursts of a few minutes at most. If the shutter state is toggled at rates in excess of 1 Hz for longer than a few minutes, the heat generated by this activity can cause the shutter to fail.
For HPLS343 and HPLS345 only: closing the shutter during periods when the output emission of the light source is not needed will extend the lifetime of the LLG. Exposure to the UV portion of the spectrum produced by these high-power light sources causes the transmissive properties of LLGs to gradually degrade, and closing the shutter blocks the coupling of the Luxim® LEP™ bulb's light into the LLG.
Software Control via a Host PC
For greater control of the light source and more configuration options than are offered by the front panel interface, the HPLS series can be controlled by a host PC. A software package is available for download that includes drivers and installs a GUI. Control of the light source may be performed using the GUI or by running custom user-written programs. Please see the Software tab for more information. Prior to running a custom program via the command-line interface, install the drivers included in the software download, power on the light source, and connect a USB cable between the PC and the light source. The GUI and command line language, listed and described in Chapter 7 of the manuals, allow the user to:
Obtain Various Status and Instrument Identification Information
Turn the Bulb On and Off
Set the Output Intensity Optical Power
Set the Shutter State
Set the Operation Mode: Open Loop, Closed Loop, and Closed Loop Eco Modes
Toggle Between External (Voltage Signals Sent to Back Panel Connectors) and Local (Front Panel and PC) Operation
The Liquid Crystal Display (LCD) screen features bright text on a dark background, and the text remains visible from across the room and at oblique angles. The dark background emits less light into the room than do displays featuring dark text on a bright background, which makes it less disruptive under dim lighting conditions. The display, an example of which is shown in Figure 2, has five different fields:
Upper Left Corner: The transmitted light intensity, which is controlled by tuning the attenuator positioned between the bulb and the output, is shown.
Upper Right Corner: MANUAL indicates front panel and PC control, and EXTERNAL indicates control via back panel connectors is enabled.
Lower Right Corner: The current state of the device is shown.
Lower Left Corner: The shutter state, which is either Open or Close, is displayed.
Center: When operating in Open Loop Mode, nothing is displayed in the center (see the image at the right). The letter "C" is displayed when operating in Closed Loop Mode, and the letter "E" is displayed when operating in Eco Mode. Information about the different operating modes and examples of the Closed Loop and Eco display screens are shown in Figures 4 and 5, respectively.
Operation Modes: Open Loop, Closed Loop, and Closed Loop Eco
The HPLS series light sources feature three different operation modes: Open Loop, Closed Loop, and Closed Loop Eco. When the operating mode is changed, the driving current sent to the bulb is affected. This is in contrast to using the front panel knob, software, or back panel connection controls to adjust the intensity, which changes the coupling ratio between the bulb and the liquid light guide but does not affect the current driving the bulb.
The default operation mode, which is set at the factory, is Open Loop mode. When a PC is interfaced with the light source, the operation mode can be changed using the GUI or command-line interface. It is not possible to change the operation mode via interaction with the front panel or when external control is enabled. Changing the operation mode updates the operation mode setting stored in non-volatile memory. When the light source is powered on, the active operation mode is the one that was active when the source was last powered down.
Open Loop Mode: This is the default operation mode, which is set at the factory and produces the highest optical output power from the bulb. In this mode, the current driving the lamp is held constant near the current limit. As the optical output intensity of the bulb is not stabilized, this intensity can be expected to slowly vary during short-term operation. Over longer durations, the output intensity of the bulb will gradually decrease as the bulb ages. Figure 2 shows an example of the display screen corresponding to operation in Open Loop Mode.
Closed Loop Mode: In Closed Loop mode, the optical output power of the bulb is stabilized at a target level using a feedback mechanism. An initialization process that occurs at start-up determines the target intensity. During initialization, the bulb is driven at a constant current level of approximately 90% of the current limit. A photodiode located near the bulb measures the optical output intensity, and then the target optical output power level for this mode is set to be 80% of this measured value. During operation, this optical output power is maintained by using a feedback mechanism to adjust the bulb's driving current as appropriate. Closed Loop mode operation provides a more stable output intensity than Open Loop mode, and it also compensates for the gradual decrease in the lamp's optical output power that occurs as the bulb ages. The letter "C" in the top-center of the LCD display shown in Figure 3 indicates the light source is operating in Closed Loop mode.
Eco Mode: This mode operates similarly to Closed Loop mode, with the difference being that Eco mode stabilizes the optical output power at a lower intensity. Rather than stabilize the optical output power of the bulb at 80% of the Open Loop mode power, as is done in Closed Loop mode, Eco mode stabilizes the optical output power of the bulb at 50% of the Open Loop mode power. A benefit of operating at this lower power level is that the bulb suffers less heat stress than it does when operating in the standard Closed Loop mode, and this is expected to extend the lifetime of the bulb. As is true when operating in the standard Closed Loop mode, Eco mode compensates for both transient variations in the optical output power of the bulb and the gradual decrease in the lamp's optical output power. The letter "E" in the top-center of the LCD display shown in Figure 4 indicates the light source is operating in Eco mode.
Click on the More [+] link below to view plots illustrating the effects of the operating mode and liquid light guide (HPLS343 and HPLS345 only) on the intensity of the transmitted light. Data were acquired over a continuous 48 hour period, and measurements were recorded both just before the input and at the output of the LLG (HPLS343 and HPLS345), or after the free-space output (HPLS301). Results are plotted for Open Loop and Closed Loop modes, with the Closed Loop mode data also applying to Eco mode.
The measurements taken at the input of the LLG and for the free-space version describe time-dependent bulb intensity independent of other effects. The measurements taken at the output of the LLG include the optical transmission characteristics of the LLG. As is discussed in the LLGs tab, LLGs act to homogenize the light transmitted through them, which smooths the time-dependent intensity data curves. In addition, the light transmission levels through LLGs decrease with time in response to exposure to UV radiation. While operation in Closed Loop or Eco modes maintains the optical output power of the bulb at the target level, these modes do not compensate for the attenuation of the transmitted light through the LLG that increases with time as a result of the aging of the liquid in the guide.
Graphs of Optical Intensity, Open and Closed Loop Operation, 48 Hour Duration
Click to Enlarge Output power intensity monitored after the free-space output. Open Loop mode drives the bulb at constant current and does not maintain constant output power, which results in variations of the output power and a gradual decrease in optical intensity as the bulb ages.
Click to Enlarge Output power intensity monitored after the free-space output. Closed Loop mode uses a feedback loop to stabilize the output power of the bulb. This mitigates variations in optical output power and compenstates for the gradual decrease in output intensity as the bulb ages.
Click to Enlarge Output power intensity monitored immediately before the input to the LLG is plotted with respect to time. Open Loop mode drives the bulb at constant current and does not maintain constant output power, which results in variations of the output power and a gradual decrease in optical intensity as the bulb ages.
Click to Enlarge Output power intensity monitored immediately before the input to the LLG is plotted with respect to time. Closed Loop mode uses a feedback loop to stabilize the output power of the bulb. This mitigates variations in optical output power and compenstates for the gradual decrease in output intensity as the bulb ages.
Click to Enlarge Output power intensity monitored at the output of the LLG is plotted with respect to time. The intensity measured at the output of the LLG includes the transmission characteristics of the LLG. The LLG homogenizes the transmitted light, and its attenuation levels gradually increase as the liquid is exposed to ultraviolet light. The transmitted intensity measured at the output of the LLG decreases gradually with time due to the progressive darkening of the LLG as well as the gradual decrease in bulb intensity with time, which is due to the aging of the bulb and is not compensated for in Open Loop mode.
Click to Enlarge Output power intensity monitored at the output of the LLG is plotted with respect to time. The intensity measured at the output of the LLG includes the transmission characteristics of the LLG. The LLG homogenizes the transmitted light, and its attenuation levels gradually increase as the liquid is exposed to ultraviolet light. The transmitted intensity measured at the output of the LLG decreases gradually with time due to the progressive darkening of the LLG, but Closed Loop mode compensates for the gradual decrease in bulb intensity with time due to the aging of the bulb.
External Control via Back Panel BNC and SMA Connections
External control of the light source is enabled by pressing the button labeled "External," which is located above the LCD display on the front panel of the instrument. After the button is pressed, it illuminates and stays illuminated while External mode is active. Enabling External control disables both software (PC) and front panel control, with the exception of the shutter. Both the shutter button on the front panel and software control of the shutter remain active for safety reasons; however, using one of these methods to toggle the shutter will automatically disable External control and return the instrument to Local control, in which the front panel and PC control are active.
External control of the light sources is performed by sending signals to a trio of female BNC connectors of the back panel of the instrument (Shutter Control, Analog IN and Trigger IN). The back panel also includes a pair of female SMA output connectors that provide access to status information (Analog OUT and Trigger OUT). Please see the Front & Back Panels tab for information about the Back Panel connectors.