Patent Publication Number: US-2023161213-A1

Title: Controlling transitions in optically switchable devices

Description:
INCORPORATION BY REFERENCE 
     An Application Data Sheet is filed concurrently with this specification as part of the present application. Each application that the present application claims benefit of or priority to as identified in the concurrently filed Application Data Sheet is incorporated by reference herein in its entirety and for all purposes. 
     BACKGROUND 
     Electrochromic (EC) devices are typically multilayer stacks including (a) at least one layer of electrochromic material, that changes its optical properties in response to the application of an electrical potential, (b) an ion conductor (IC) layer that allows ions, such as lithium ions, to move through it, into and out from the electrochromic material to cause the optical property change, while preventing electrical shorting, and (c) transparent conductor layers, such as transparent conducting oxides or TCOs, over which an electrical potential is applied to the electrochromic layer. In some cases, the electric potential is applied from opposing edges of an electrochromic device and across the viewable area of the device. The transparent conductor layers are designed to have relatively high electronic conductances. Electrochromic devices may have more than the above-described layers such as ion storage or counter electrode layers that optionally change optical states. 
     Due to the physics of the device operation, proper function of the electrochromic device depends upon many factors such as ion movement through the material layers, the electrical potential required to move the ions, the sheet resistance of the transparent conductor layers, and other factors. The size of the electrochromic device plays an important role in the transition of the device from a starting optical state to an ending optical state (e.g., from tinted to clear or clear to tinted). The conditions applied to drive such transitions can have quite different requirements for different sized devices. 
     What are needed are improved methods for driving optical transitions in electrochromic devices. 
     SUMMARY 
     Aspects of this disclosure concern controllers and control methods for applying a drive voltage to bus bars of optically switchable devices such as electrochromic devices. Such devices are often provided on windows such as architectural glass. In certain embodiments, the applied drive voltage is controlled in a manner that efficiently drives an optical transition over the entire surface of the optically switchable device. The drive voltage is controlled to account for differences in effective voltage experienced in regions between the bus bars and regions proximate the bus bars. Regions near the bus bars experience the highest effective voltage. 
     Certain aspects of the disclosure may concern an apparatus for controlling a transition of an optically switchable device from a starting optical state to an ending optical state. The apparatus may include a processor designed or configured to apply a voltage to a first bus bar of the optically switchable device. At one or more times, the processor may determine an open circuit voltage between the first bus bar and a second bus bar of the optically switchable device and, at one or more times, determine a charge supplied to the optically switchable device. The processor may additionally determine whether the open circuit voltage has a magnitude greater than or equal to a magnitude of a target open circuit voltage, and may determine whether the supplied charge gives rise to a total delivered charge density greater than or equal to a threshold charge density. In an embodiment, if the magnitude of the determined open circuit voltage is greater than or equal to the magnitude of the target open circuit voltage, and if the magnitude of the total delivered charge density is determined to be greater than or equal to the magnitude of the threshold charge density, the processor may apply a hold voltage to the first bus bar of the optically switchable device. 
     In some embodiments, the open circuit voltage is determined at a defined time after applying the voltage to the first bus bar of the optically switchable device. The defined time may be between about 15-90 seconds, for example about 30 seconds in some cases. In other examples, the defined time is longer, for example up to about 120 minutes in some cases. The target open circuit voltage may have a magnitude that is between about 0-1V greater than the magnitude of the hold voltage, for example between about 0-0.4V greater than the magnitude of the hold voltage. In various cases, the magnitude of the target open circuit voltage is at least about 0.025V greater than the magnitude of the hold voltage. The magnitude of the threshold charge density may be between about 1×10 −5  C/cm 2  and about 5 C/cm 2  in certain embodiments. In some cases, for example, the magnitude of the threshold charge density is between about 0.01-0.04 C/cm 2 . 
     In an implementation, after determining whether the open circuit voltage has a magnitude greater than or equal to a magnitude of a target circuit voltage, but before determining whether the supplied charge gives rise to a total delivered charge density greater than or equal to a threshold charge density, the processor may increase the magnitude of voltage applied as a result of determining that either the magnitude of the open circuit voltage is less than the magnitude of the target open circuit voltage, or that the magnitude of the total delivered charge density is less than the magnitude of the threshold charge density. The processor may further operate to repeat, at a frequency of between about 5 seconds and 5 minutes, the determination of whether the open circuit voltage has a magnitude greater than or equal to the magnitude of the target open circuit voltage and to determine whether the supplied charge density gives rise to a total delivered charge density greater than or equal to a threshold charge density. 
     In various embodiments, the optically switchable device is an electrochromic device. The bus bars may be separated from one another by at least about 10 inches in some cases. 
     Other aspects of the disclosure concern an apparatus for controlling a transition of an optically switchable device from a starting optical state to an ending optical state. Such apparatus may be characterized by the following elements: a processor designed or configured to apply a voltage to a first bus bar of the optically switchable device. At one or more times during the transition, reduce the magnitude of the voltage applied to the first bus bar of the optically switchable device to a probe voltage and detect a current response, and periodically determine a charge supplied to the optically switchable device and to determine whether the detected current response reaches a target current, and determine whether a magnitude of the supplied charge is greater than or equal to a threshold charge. If it is determined the detected current response reaches the target current, and that the magnitude of the supplied charge is greater than or equal to the magnitude of the threshold charge density, apply a hold voltage. 
     The processor may be further designed or configured to determine the open circuit voltage and total delivered charge density in at a defined time after applying the voltage to the first bus bar of the optically switchable device. The defined time may be between about 15-90 seconds, for example about 30 seconds in some cases. In other examples, the defined time is longer, for example up to about 120 minutes in some cases. 
     The target open circuit voltage may have a magnitude that is between about 0-1V greater than the magnitude of the hold voltage, for example between about 0-0.4V greater than the magnitude of the hold voltage. In various cases, the magnitude of the target open circuit voltage is at least about 0.025V greater than the magnitude of the hold voltage. The magnitude of the threshold charge density may be between about 1×10 −5  C/cm 2  and about 5 C/cm 2  in certain embodiments. In some cases, for example, the magnitude of the threshold charge density is between about 0.01-0.04 C/cm 2 . 
     In certain embodiments, the processor is further designed or configured to, after determining whether the open circuit voltage has a magnitude greater than or equal to a magnitude of a target open circuit voltage and before determining whether the supplied charge gives rise to a total delivered charge density greater than or equal to a threshold charge density, increase the magnitude of voltage applied to the bus bars as a result of determining that either the magnitude of the determined open circuit voltage is less than the magnitude of the target open circuit voltage, or that the magnitude of the determined total delivered charge density is less than the magnitude of the threshold charge density. The processor may repeat, such as at a frequency of between about 5 seconds and about 5 minutes, the determining of whether the open circuit voltage has a magnitude greater than or equal to a magnitude of a target open circuit voltage and whether the supplied charge gives rise to a total delivered charge density that is greater than or equal to a threshold charge density. The optically switchable device may be an electrochromic device. The bus bars may be separated from one another by at least about 10 inches in some cases. 
     In a further aspect of the disclosed embodiments, another method of controlling a transition of an optically switchable device is provided. The method involves transitioning from a starting optical state to an ending optical state by applying a voltage to a first bus bar of the optically switchable device; determining, at one or more times during the transition, whether an open circuit voltage between the first bus bar and a second bus bar of the optically switchable device comprises a magnitude greater than or equal to a magnitude of a target open circuit voltage, and determining whether a charge supplied to the optically switchable device gives rise to a total delivered charge density greater than or equal to a threshold charge density. The method may continue with determining if the magnitude of the open circuit voltage is greater than or equal to the magnitude of the target open circuit voltage, and if the magnitude of the total delivered charge density is determined to be greater than or equal to the magnitude of the threshold charge density, applying a hold voltage to the first bus bar of the optically switchable device. 
     The target current may be about 0 Amps in some cases. The magnitude of the threshold charge density may be between about 1×10 −5  C/cm 2  and about 5 C/cm 2  in certain embodiments. In some cases, for example, the magnitude of the threshold charge density is between about 0.01-0.04 C/cm 2 . In some embodiments, the probe voltage may have a magnitude that is between about 0-1V greater than the magnitude of the hold voltage, for example between about 0-0.4V greater than the magnitude of the hold voltage. 
     These and other features will be described in further detail below with reference to the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  schematically depicts a planar bus bar arrangement. 
         FIG.  1 B  presents a simplified plot of the local voltage value on each transparent conductive layer as a function of position on the layer 
         FIG.  1 C  presents a simplified plot of V eff  as a function of position across the device 
         FIG.  2    is a graph depicting voltage and current profiles associated with driving an electrochromic device from clear to tinted and from tinted to clear. 
         FIG.  3    is a graph depicting certain voltage and current profiles associated with driving an electrochromic device from clear to tinted. 
         FIG.  4 A  is a graph depicting an optical transition in which a drop in applied voltage from V drive  to V hold  results in a net current flow establishing that the optical transition has proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. 
         FIG.  4 B  is a graph depicting an optical transition in which an initial drop in applied voltage from V drive  to V hold  results in a net current flow indicating that the optical transition has not yet proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. Therefore the applied voltage is returned to V drive  for a further period of time before again dropping again to V hold  at which point the resulting current establishes that the optical transition has proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. 
         FIG.  5 A  is a flow chart depicting a process for probing the progress of an optical transition and determining when the transition is complete. 
         FIG.  5 B  is a flow chart depicting a process for probing the progress of an optical transition and speeding the transition if it is not progressing sufficiently fast. 
         FIGS.  5 C- 5 F  are flow charts depicting alternative processes for probing the progress of an optical transitioning and determining when the transition is complete. 
         FIGS.  6 A and  6 B  show graphs depicting the total charge delivered over time and the voltage applied over time during an electrochromic transition when using the method of  FIG.  5 E  to probe and monitor the progress of the transition, at room temperature ( FIG.  6 A ) and at a reduced temperature ( FIG.  6 B ). 
         FIG.  6 C  illustrates an electrochromic window having a pair of voltage sensors on the transparent conductive oxide layers according to an embodiment. 
         FIGS.  7 A and  7 B  present cross-sectional views of an example electrochromic device in operation. 
         FIGS.  8  and  9    are representations of window controllers and associated components. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     An “optically switchable device” is a thin device that changes optical state in response to electrical input. It reversibly cycles between two or more optical states. Switching between these states is controlled by applying predefined current and/or voltage to the device. The device typically includes two thin conductive sheets that straddle at least one optically active layer. The electrical input driving the change in optical state is applied to the thin conductive sheets. In certain implementations, the input is provided by bus bars in electrical communication with the conductive sheets. 
     While the disclosure emphasizes electrochromic devices as examples of optically switchable devices, the disclosure is not so limited. Examples of other types of optically switchable device include certain electrophoretic devices, liquid crystal devices, and the like. Optically switchable devices may be provided on various optically switchable products, such as optically switchable windows. However, the embodiments disclosed herein are not limited to switchable windows. Examples of other types of optically switchable products include mirrors, displays, and the like. In the context of this disclosure, these products are typically provided in a non-pixelated format. 
     An “optical transition” is a change in any one or more optical properties of an optically switchable device. The optical property that changes may be, for example, tint, reflectivity, refractive index, color, etc. In certain embodiments, the optical transition will have a defined starting optical state and a defined ending optical state. For example the starting optical state may be 80% transmissivity and the ending optical state may be 50% transmissivity. The optical transition is typically driven by applying an appropriate electric potential across the two thin conductive sheets of the optically switchable device. 
     A “starting optical state” is the optical state of an optically switchable device immediately prior to the beginning of an optical transition. The starting optical state is typically defined as the magnitude of an optical state which may be tint, reflectivity, refractive index, color, etc. The starting optical state may be a maximum or minimum optical state for the optically switchable device; e.g., 90% or 4% transmissivity. Alternatively, the starting optical state may be an intermediate optical state having a value somewhere between the maximum and minimum optical states for the optically switchable device; e.g., 50% transmissivity. 
     An “ending optical state” is the optical state of an optically switchable device immediately after the complete optical transition from a starting optical state. The complete transition occurs when optical state changes in a manner understood to be complete for a particular application. For example, a complete tinting might be deemed a transition from 75% optical transmissivity to 10% transmissivity. The ending optical state may be a maximum or minimum optical state for the optically switchable device; e.g., 90% or 4% transmissivity. Alternatively, the ending optical state may be an intermediate optical state having a value somewhere between the maximum and minimum optical states for the optically switchable device; e.g., 50% transmissivity. 
     “Bus bar” refers to an electrically conductive strip attached to a conductive layer such as a transparent conductive electrode spanning the area of an optically switchable device. The bus bar delivers electrical potential and current from an external lead to the conductive layer. An optically switchable device includes two or more bus bars, each connected to a single conductive layer of the device. In various embodiments, a bus bar forms a long thin line that spans most of the length of the length or width of a device. Often, a bus bar is located near the edge of the device. 
     “Applied Voltage” or V app  refers the difference in potential applied to two bus bars of opposite polarity on the electrochromic device. Each bus bar is electronically connected to a separate transparent conductive layer. The applied voltage may different magnitudes or functions such as driving an optical transition or holding an optical state. Between the transparent conductive layers are sandwiched the optically switchable device materials such as electrochromic materials. Each of the transparent conductive layers experiences a potential drop between the position where a bus bar is connected to it and a location remote from the bus bar. Generally, the greater the distance from the bus bar, the greater the potential drop in a transparent conducting layer. The local potential of the transparent conductive layers is often referred to herein as the VTOL. Bus bars of opposite polarity may be laterally separated from one another across the face of an optically switchable device. 
     “Effective Voltage” or V eff  refers to the potential between the positive and negative transparent conducting layers at any particular location on the optically switchable device. In Cartesian space, the effective voltage is defined for a particular x,y coordinate on the device. At the point where V eff  is measured, the two transparent conducting layers are separated in the z-direction (by the device materials), but share the same x,y coordinate. 
     “Hold Voltage” refers to the applied voltage necessary to indefinitely maintain the device in an ending optical state. In some cases, without application of a hold voltage, electrochromic windows return to their natural tint state. In other words, maintenance of a desired tint state requires application of a hold voltage. 
     “Drive Voltage” refers to the applied voltage provided during at least a portion of the optical transition. The drive voltage may be viewed as “driving” at least a portion of the optical transition. Its magnitude is different from that of the applied voltage immediately prior to the start of the optical transition. In certain embodiments, the magnitude of the drive voltage is greater than the magnitude of the hold voltage. An example application of drive and hold voltages is depicted in  FIG.  3     
     Context and Overview 
     The disclosed embodiments make use of electrical probing and monitoring to determine when an optical transition between a first optical state and a second optical state of an optically switchable device has proceeded to a sufficient extent that the application of a drive voltage can be terminated. For example, electrical probing allows for application of drive voltages for less time than previously thought possible, as a particular device is driven based on electrical probing of its actual optical transition progression in real time. Further, real time monitoring can help ensure that an optical transition progresses to a desired state. In various embodiments, terminating the drive voltage is accomplished by dropping the applied voltage to a hold voltage. This approach takes advantage of an aspect of optical transitions that is typically considered undesirable—the propensity of thin optically switchable devices to transition between optical states non-uniformly. In particular, many optically switchable devices initially transition at locations close to the bus bars and only later at regions far from the bus bars (e.g., near the center of the device). Surprisingly, this non-uniformity can be harnessed to probe the optical transition. By allowing the transition to be probed in the manner described herein, optically switchable devices avoid the need for custom characterization and associated preprogramming of device control algorithms specifying the length of time a drive voltage is applied as well as obviating “one size fits all” fixed time period drive parameters that account for variations in temperature, device structure variability, and the like across many devices. Before describing probing and monitoring techniques in more detail, some context on optical transitions in electrochromic devices will be provided. 
     Driving a transition in a typical electrochromic device is accomplished by applying a defined voltage to two separated bus bars on the device. In such a device, it is convenient to position bus bars perpendicular to the smaller dimension of a rectangular window (see  FIG.  1 A ). This is because the transparent conducting layers used to deliver an applied voltage over the face of the thin film device have an associated sheet resistance, and the bus bar arrangement allows for the shortest span over which current must travel to cover the entire area of the device, thus lowering the time it takes for the conductor layers to be fully charged across their respective areas, and thus lowering the time to transition the device. 
     While an applied voltage, V app , is supplied across the bus bars, essentially all areas of the device see a lower local effective voltage (V eff ) due to the sheet resistance of the transparent conducting layers and the current draw of the device. The center of the device (the position midway between the two bus bars) frequently has the lowest value of V eff . This may result in an unacceptably small optical switching range and/or an unacceptably slow switching time in the center of the device. These problems may not exist at the edges of the device, nearer the bus bars. This is explained in more detail below with reference to  FIGS.  1 B and  1 C . 
       FIG.  1 A  shows a top-down view of an electrochromic lite  100  including bus bars having a planar configuration. Electrochromic lite  100  includes a first bus bar  105  disposed on a first conductive layer  110  and a second bus bar  115  disposed on a second conductive layer,  120 . An electrochromic stack (not shown) is sandwiched between first conductive layer  110  and second conductive layer  120 . As shown, first bus bar  105  may extend substantially across one side of first conductive layer  110 . Second bus bar  115  may extend substantially across one side of second conductive layer  120  opposite the side of electrochromic lite  100  on which first bus bar  105  is disposed. Some devices may have extra bus bars, e.g. on all four edges, but this complicates fabrication. A further discussion of bus bar configurations, including planar configured bus bars, is found in U.S. patent application Ser. No. 13/452,032 filed Apr. 20, 2012, which is incorporated herein by reference in its entirety. 
       FIG.  1 B  is a graph showing a plot of the local voltage in first transparent conductive layer  110  and the voltage in second transparent conductive layer  120  that drives the transition of electrochromic lite  100  from a clear state to a tinted state, for example. Plot  125  shows the local values of the voltage V TCL  in first transparent conductive layer  110 . As shown, the voltage drops from the left hand side (e.g., where first bus bar  105  is disposed on first conductive layer  110  and where the voltage is applied) to the right hand side of first conductive layer  110  due to the sheet resistance and current passing through first conductive layer  110 . Plot  130  also shows the local voltage V TCL  in second conductive layer  120 . As shown, the voltage increases (decreases in magnitude) from the right hand side (e.g., where second bus bar  115  is disposed on second conductive layer  120  and where the voltage is applied) to the left hand side of second conductive layer  120  due to the sheet resistance of second conductive layer  120 . The value of the applied voltage, V app , in this example is the difference in voltage between the right end of potential plot  130  and the left end of potential plot  125 . The value of the effective voltage, V eff , at any location between the bus bars is the difference in values of curves  130  and  125  at the position on the x-axis corresponding to the location of interest. 
       FIG.  1 C  is a graph showing a plot of V eff  across the electrochromic device between first and second conductive layers  110  and  120  of electrochromic lite  100 . As explained, the effective voltage is the local voltage difference between the first conductive layer  110  and the second conductive layer  120 . Regions of an electrochromic device subjected to higher effective voltages transition between optical states faster than regions subjected to lower effective voltages. As shown, the effective voltage is the lowest at the center of electrochromic lite  100  and highest at the edges of electrochromic lite  100 . The voltage drop across the device is due to ohmic losses as current passes through the device. The voltage drop across large electrochromic windows can be alleviated by configuring additional bus bars within the viewing area of the window, in effect dividing one large optical window into multiple smaller electrochromic windows which can be driven in series or parallel. However, this approach may not be aesthetically appealing due to the contrast between the viewable area and the bus bar(s) in the viewable area. That is, it may be much more pleasing to the eye to have a monolithic electrochromic device without any distracting bus bars in the viewable area. 
     As described above, as the window size increases, the electronic resistance to current flowing across the thin face of the TC layers also increases. This resistance may be measured between the points closest to the bus bar (referred to as edge of the device in following description) and in the points furthest away from the bus bars (referred to as the center of the device in following description). When current passes through a TCL, the voltage drops across the TCL face and this reduces the effective voltage at the center of the device. This effect is exacerbated by the fact that typically as window area increases, the leakage current density for the window stays constant but the total leakage current increases due to the increased area. Thus with both of these effects the effective voltage at the center of the electrochromic window falls substantially, and poor performance may be observed for electrochromic windows which are larger than, for example, about 30 inches across. This issue can be addressed by using a higher V app  such that the center of the device reaches a suitable effective voltage. 
     Typically the range of safe operation for solid state electrochromic devices is between about 0.5V and 4V, or more typically between about 0.8V and about 3V, e.g. between 0.9V and 1.8V. These are local values of V eff . In one embodiment, an electrochromic device controller or control algorithm provides a driving profile where V eff  is always below 3V, in another embodiment, the controller controls V eff  so that it is always below 2.5V, in another embodiment, the controller controls V eff  so that it is always below 1.8V. The recited voltage values refer to the time averaged voltage (where the averaging time is of the order of time required for small optical response, e.g. few seconds to few minutes). 
     An added complexity of electrochromic windows is that the current drawn through the window is not fixed over the duration of the optical transition. Instead, during the initial part of the transition, the current through the device is substantially larger (up to 100× larger) than in the end state when the optical transition is complete or nearly complete. The problem of poor coloration in center of the device is further exacerbated during this initial transition period, as the value V eff  at the center is significantly lower than what it will be at the end of the transition period. 
     In the case of an electrochromic device with a planar bus bar, it can be shown that the V eff  across a device with planar bus bars is generally given by: 
       Δ V (0)= V   app   −RJL   2 /2
 
       Δ V ( L )= V   app   −RJL   2 /2
 
       Δ V ( L/ 2)= V   app −3 RJL   2 /4  Equation 1
 
     where:
 
V app  is the voltage difference applied to the bus bars to drive the electrochromic window;
 
ΔV(0) is V eff  at the bus bar connected to the first transparent conducting layer (in the example below, TEC type TCO);
 
ΔV(L) is V eff  at the bus bar connected to the second transparent conducting layer (in the example below, ITO type TCO);
 
ΔV(L/2) is V eff  at the center of the device, midway between the two planar bus bars;
 
R=transparent conducting layer sheet resistance;
 
J=instantaneous average current density; and
 
L=distance between the bus bars of the electrochromic device.
 
     The transparent conducting layers are assumed to have substantially similar, if not the same, sheet resistance for the calculation. However those of ordinary skill in the art will appreciate that the applicable physics of the ohmic voltage drop and local effective voltage still apply even if the transparent conducting layers have dissimilar sheet resistances. 
     As noted, certain embodiments pertain to controllers and control algorithms for driving optical transitions in devices having planar bus bars. In such devices, substantially linear bus bars of opposite polarity are disposed at opposite sides of a rectangular or other polygonally shaped electrochromic device. In some embodiments, devices with non-planar bus bars may be employed. Such devices may employ, for example, angled bus bars disposed at vertices of the device. In such devices, the bus bar effective separation distance, L, is determined based on the geometry of the device and bus bars. A discussion of bus bar geometries and separation distances may be found in U.S. patent application Ser. No. 13/452,032, entitled “Angled Bus Bar”, and filed Apr. 20, 2012, which is incorporated herein by reference in its entirety. 
     As R, J or L increase, V eff  across the device decreases, thereby slowing or reducing the device coloration during transition and even in the final optical state. Referring to Equation 1, the V eff  across the window is at least RJL 2 /2 lower than V app . It has been found that as the resistive voltage drop increases (due to increase in the window size, current draw etc.) some of the loss can be negated by increasing V app  but doing so only to a value that keeps V eff  at the edges of the device below the threshold where reliability degradation would occur. 
     In summary, it has been recognized that both transparent conducting layers experience ohmic drop, and that drop increases with distance from the associated bus bar, and therefore V TCL  decreases with distance from the bus bar for both transparent conductive layers. As a consequence V eff  decreases in locations removed from both bus bars. 
     To speed along optical transitions, the applied voltage is initially provided at a magnitude greater than that required to hold the device at a particular optical state in equilibrium. This approach is illustrated in  FIGS.  2  and  3   . 
       FIG.  2    shows a complete current profile and voltage profile for an electrochromic device employing a simple voltage control algorithm to cause an optical state transition cycle (tinting followed by clearing) of an electrochromic device. In the graph, total current density (I) is represented as a function of time. As mentioned, the total current density is a combination of the ionic current density associated with an electrochromic transition and electronic leakage current between the electrochemically active electrodes. Many different types of electrochomic devices will have the depicted current profile. In one example, a cathodic electrochromic material such as tungsten oxide is used in conjunction with an anodic electrochromic material such as nickel tungsten oxide in counter electrode. In such devices, negative currents indicate coloration/tinting of the device. In one example, lithium ions flow from a nickel tungsten oxide anodically coloring electrochromic electrode into a tungsten oxide cathodically coloring electrochromic electrode. Correspondingly, electrons flow into the tungsten oxide electrode to compensate for the positively charged incoming lithium ions. Therefore, the voltage and current are shown to have a negative value. 
     The depicted profile results from ramping up the voltage to a set level and then holding the voltage to maintain the optical state. The current peaks  201  are associated with changes in optical state, i.e., tinting and clearing. Specifically, the current peaks represent delivery of the ionic charge needed to tint or clear the device. Mathematically, the shaded area under the peak represents the total charge required to tint or clear the device. The portions of the curve after the initial current spikes (portions  203 ) represent electronic leakage current while the device is in the new optical state. 
     In the figure, a voltage profile  205  is superimposed on the current curve. The voltage profile follows the sequence: negative ramp ( 207 ), negative hold ( 209 ), positive ramp ( 211 ), and positive hold ( 213 ). Note that the voltage remains constant after reaching its maximum magnitude and during the length of time that the device remains in its defined optical state. Voltage ramp  207  drives the device to its new the tinted state and voltage hold  209  maintains the device in the tinted state until voltage ramp  211  in the opposite direction drives the transition from tinted to clear states. In some switching algorithms, a current cap is imposed. That is, the current is not permitted to exceed a defined level in order to prevent damaging the device (e.g. driving ion movement through the material layers too quickly can physically damage the material layers). The coloration speed is a function of not only the applied voltage, but also the temperature and the voltage ramping rate. 
       FIG.  3    illustrates a voltage control profile in accordance with certain embodiments. In the depicted embodiment, a voltage control profile is employed to drive the transition from a clear state to a tinted state (or to an intermediate state). To drive an electrochromic device in the reverse direction, from a tinted state to a clear state (or from a more tinted to less tinted state), a similar but inverted profile is used. In some embodiments, the voltage control profile for going from tinted to clear is a mirror image of the one depicted in  FIG.  3   . 
     The voltage values depicted in  FIG.  3    represent the applied voltage (V app ) values. The applied voltage profile is shown by the dashed line. For contrast, the current density in the device is shown by the solid line. In the depicted profile, V app  includes four components: a ramp to drive component  303 , which initiates the transition, a V drive  component  313 , which continues to drive the transition, a ramp to hold component  315 , and a V hold  component  317 . The ramp components are implemented as variations in V app  and the V drive  and V hold  components provide constant or substantially constant V app  magnitudes. 
     The ramp to drive component is characterized by a ramp rate (increasing magnitude) and a magnitude of V drive . When the magnitude of the applied voltage reaches V drive , the ramp to drive component is completed. The V drive  component is characterized by the value of V drive  as well as the duration of V drive . The magnitude of V drive  may be chosen to maintain V eff  with a safe but effective range over the entire face of the electrochromic device as described above. 
     The ramp to hold component is characterized by a voltage ramp rate (decreasing magnitude) and the value of V hold  (or optionally the difference between V drive  and V hold ). V app  drops according to the ramp rate until the value of V hold  is reached. The V hold  component is characterized by the magnitude of V hold  and the duration of V hold . Actually, the duration of Wow is typically governed by the length of time that the device is held in the tinted state (or conversely in the clear state). Unlike the ramp to drive, V drive , and ramp to hold components, the V hold  component has an arbitrary length, which is independent of the physics of the optical transition of the device. 
     Each type of electrochromic device will have its own characteristic components of the voltage profile for driving the optical transition. For example, a relatively large device and/or one with a more resistive conductive layer will require a higher value of V drive  and possibly a higher ramp rate in the ramp to drive component. U.S. patent application Ser. No. 13/449,251, filed Apr. 17, 2012, and incorporated herein by reference, discloses controllers and associated algorithms for driving optical transitions over a wide range of conditions. As explained therein, each of the components of an applied voltage profile (ramp to drive, V drive , ramp to hold, and V hold , herein) may be independently controlled to address real-time conditions such as current temperature, current level of transmissivity, etc. In some embodiments, the values of each component of the applied voltage profile is set for a particular electrochromic device (having its own bus bar separation, resistivity, etc.) and does not vary based on current conditions. In other words, in such embodiments, the voltage profile does not take into account feedback such as temperature, current density, and the like. 
     As indicated, all voltage values shown in the voltage transition profile of  FIG.  3    correspond to the V app  values described above. They do not correspond to the V eff  values described above. In other words, the voltage values depicted in  FIG.  3    are representative of the voltage difference between the bus bars of opposite polarity on the electrochromic device. 
     In certain embodiments, the ramp to drive component of the voltage profile is chosen to safely but rapidly induce ionic current to flow between the electrochromic and counter electrodes. As shown in  FIG.  3   , the current in the device follows the profile of the ramp to drive voltage component until the ramp to drive portion of the profile ends and the V drive  portion begins. See current component  301  in  FIG.  3   . Safe levels of current and voltage can be determined empirically or based on other feedback. U.S. Pat. No. 8,254,013, filed Mar. 16, 2011, issued Aug. 28, 2012 and incorporated herein by reference, presents examples of algorithms for maintaining safe current levels during electrochromic device transitions. 
     In certain embodiments, the value of V drive  is chosen based on the considerations described above. Particularly, it is chosen so that the value of V eff  over the entire surface of the electrochromic device remains within a range that effectively and safely transitions large electrochromic devices. The duration of V drive  can be chosen based on various considerations. One of these ensures that the drive potential is held for a period sufficient to cause the substantial coloration of the device. For this purpose, the duration of V drive  may be determined empirically, by monitoring the optical density of the device as a function of the length of time that V drive  remains in place. In some embodiments, the duration of V drive  is set to a specified time period. In another embodiment, the duration of V drive  is set to correspond to a desired amount of ionic and/or electronic charge being passed. As shown, the current ramps down during V drive . See current segment  307 . 
     Another consideration is the reduction in current density in the device as the ionic current decays as a consequence of the available lithium ions completing their journey from the anodic coloring electrode to the cathodic coloring electrode (or counter electrode) during the optical transition. When the transition is complete, the only current flowing across device is leakage current through the ion conducting layer. As a consequence, the ohmic drop in potential across the face of the device decreases and the local values of V eff  increase. These increased values of V eff  can damage or degrade the device if the applied voltage is not reduced. Thus, another consideration in determining the duration of V drive  is the goal of reducing the level of V eff  associated with leakage current. By dropping the applied voltage from V drive  to V hold , not only is V eff  reduced on the face of the device but leakage current decreases as well. As shown in  FIG.  3   , the device current transitions in a segment  305  during the ramp to hold component. The current settles to a stable leakage current  309  during V hold . 
     Controlling V drive  Using Feedback from the Optical Transition 
     A challenge arises because it can be difficult to predict how long the applied drive voltage should be applied before transitioning to the hold voltage. Devices of different sizes, and more particularly devices having bus bars separated by particular distances, require different lengths of time for applying the drive voltage. Further, the processes employed to fabricate optically switchable devices such as electrochromic devices may vary subtly from one batch to another or one process revision to another. The subtle process variations translate into potentially different requirements for the length of time that the drive voltage must be applied to the devices used in operation. Still further, environmental conditions, and particularly temperature, can influence the length of time that the applied voltage should be applied to drive the transition. 
     To account for all these variables, current technology may define many distinct control algorithms with distinct periods of time for applying a defined drive voltage for each of many different window sizes or device features. A rationale for doing this is to ensure that the drive voltage is applied for a sufficient period, regardless of device size and type, to ensure that the optical transition is complete. Currently many different sized electrochromic windows are manufactured. While it is possible to pre-determine the appropriate drive voltage time for each and every different type of window, this can be a tedious, expensive, and time-consuming process. An improved approach, described here, is to determine on-the-fly the length of time that the drive voltage should be applied. 
     Further, it may be desirable to cause the transition between two defined optical states to occur within a defined duration, regardless of the size of the optically switchable device, the process under which the device is fabricated, and the environmental conditions in which the device is operating at the time of the transition. This goal can be realized by monitoring the course of the transition and adjusting the drive voltage as necessary to ensure that the transition completes in the defined time. Adjusting the magnitude of the drive voltage is one way of accomplishing this. 
     Certain disclosed embodiments apply a probing technique to assess the progress of an optical transition while the device is in transition. As illustrated in  FIG.  3   , there are typically distinct ramp to drive and the drive voltage maintenance stages of the optical transition. The probe technique can be applied during either of these. In many embodiments, it is applied during the drive voltage maintenance portion of the algorithm. 
     In certain embodiments, the probing technique involves pulsing the current or voltage applied to drive the transition and then monitoring the current or voltage response to detect an overdrive condition in the vicinity of the bus bars. An overdrive condition occurs when the local effective voltage is greater than needed to cause a local optical transition. For example, if an optical transition to a clear state is deemed complete when V eff  reaches 2V, and the local value of V eff  near a bus bar is 2.2V, the position near the bus bar may be characterized as in an overdrive condition. 
     One example of a probing technique involves pulsing the applied drive voltage by dropping it to the level of the hold voltage (or the hold voltage modified by an appropriate offset) and monitoring the current response to determine the direction of the current response. In this example, when the current response reaches a defined threshold, the device control system determines that it is now time to transition from the drive voltage to the hold voltage. 
       FIG.  4 A  is a graph depicting an optical transition in which a drop in applied voltage from V drive  to V hold  results in a net current flow establishing that the optical transition has proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. This is illustrated by a voltage drop  411  in V app  from V drive  to V hold . Voltage drop  411  is performed during a period when the V app  might otherwise be constrained to remain in the drive phase shown in  FIG.  3   . The current flowing between the bus bars began dropping (becoming less negative), as illustrated by current segment  307 , when the applied voltage initially stopped increasing (becoming more negative) and plateaued at Vane. However, when the applied voltage now dropped at  411 , the current began decreasing more readily as illustrated by current segment  415 . In accordance with some embodiments, the level of current is measured after a defined period of time passes following the voltage drop  411 . If the current is below a certain threshold, the optical transition is deemed complete, and the applied voltage may remain at V hold  (or move to V hold  if it is at some other level below V drive ). In the particular example of  FIG.  4 A , the current threshold is exceeded as illustrated. Therefore, the V app  remains at V hold  for the duration of the ending optical state. V hold  may be selected for the ending optical state it provides. Such ending optical state may be a maximum, minimum, or intermediate optical state for the optical device undergoing the transition. 
     In situations where the current does not reach the threshold when measured, it may be appropriate to return V app  to V drive .  FIG.  4 B  illustrates this situation.  FIG.  4 B  is a graph depicting an optical transition in which an initial drop in applied voltage from V drive  to V hold  (see  411 ) results in a net current flow indicating that the optical transition has not yet proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. Note that current segment  415 , which has a trajectory resulting from voltage drop  411 , does not reach the threshold when probed at  419 . Therefore the applied voltage is returned to V drive  for a further period of time—while the current recovers at  417 —before again dropping again to Wow. ( 421 ) at which point the resulting current ( 423 ) establishes that the optical transition has proceeded far enough to permit the applied voltage to remain at V hold  for the duration of the ending optical state. As explained, the ending optical state may be a maximum, minimum, or intermediate optical state for the optical device undergoing the transition. 
     As explained, the hold voltage is a voltage that will maintain the optical device in equilibrium at a particular optical density or other optical condition. It produces a steady-state result by generating a current that offsets the leakage current in the ending optical state. The drive voltage is applied to speed the transition to a point where applying the hold voltage will result in a time invariant desired optical state. 
     The probing technique described herein may be understood in terms of the physical mechanisms associated with an optical transition driven from bus bars at the edges of a device. Basically, the technique relies on differential values of the effective voltage experienced in the optically switchable device across the face of the device, and particularly the variation in V eff  from the center of the device to the edge of the device. The local variation in potential on the transparent conductive layers results in different values of V eff  across the face of the device. The value of V eff  experienced by the optically switchable device near the bus bars is far greater the value of V eff  in the center of the device. As a consequence, the local charge buildup in the region next to the bus bars is significantly greater than the charge buildup in the center the device. 
     At some point during the optical transition, the value of V eff  at the edge of the device near the bus bars is sufficient to exceed the ending optical state desired for the optical transition whereas in the center of the device, the value of V eff  is insufficient to reach that ending state. The ending state may be an optical density value associated with the endpoint in the optical transition. While in this intermediate stage of the optical transition, if the drive voltage is dropped to the hold voltage, the portion of the electrochromic device close to the bus bars will effectively try to transition back toward the state from which it started. However, as the device state in the center of the device has not yet reached the end state of the optical transition, when a hold voltage is applied, the center portion of the device will continue transitioning in the direction desired for the optical transition. 
     When the device in this intermediate stage of transition experiences the change in applied voltage from the drive voltage to the hold voltage (or some other suitably lower magnitude voltage), the portions of the device located near the bus bars—where the device is effectively overdriven—generate current flowing in the direction opposite that required to drive the transition. In contrast, the regions of the device in the center, which have not yet fully transitioned to the final state, continue to promote current flow in a direction required to drive the transition. 
     Over the course of the optical transition, and while the device is experiencing the applied drive voltage, there is a gradual increase in the driving force for causing current to flow in the reverse direction when the device is subject to a sudden drop in applied voltage. By monitoring the flow of current in response to perturbations away from drive voltage, one can determine a point at which the transition from the first state to the second state is sufficiently far along that a transition from drive voltage to hold voltage is appropriate. By “appropriate,” it is meant that the optical transition is sufficiently complete from the edge of the device to the center of the device. Such transition can be defined in many ways depending upon the specifications of the product and its application. In one embodiment, it assumes that the transition from the first state to the second state is at least about 80% of complete or at least about 95% of complete. Complete reflecting the change in optical density from the first state to the second state. The desired level of completeness may correspond to a threshold current level as depicted in the examples of  FIGS.  4 A and  4 B . 
     Many possible variations to the probing protocol exist. Such variations may include certain pulse protocols defined in terms of the length of time from the initiation of the transition to the first pulse, the duration of the pulses, the size of the pulses, and the frequency of the pulses. 
     In one embodiment, the pulse sequence is begun immediately upon the application of a drive voltage or a ramp to drive voltage that initiates the transition between the first optical state and second optical state. In other words, there would be no lag time between the initiation of the transition and the application of pulsing. In some implementations, the probe duration is sufficiently short (e.g., about 1 second or less) that probing back and forth between V drive  and V hold  for the entire transition is not significantly detrimental to switching time. However, in some embodiments, it is unnecessary to start probing right away. In some cases, switching is initiated after about 50% of an expected or nominal switching period is complete, or about 75% of such period is complete. Often, the distance between bus bars is known or can be read using an appropriately configured controller. With the distance known, a conservative lower limit for initiating probing may be implemented based on approximate known switching time. As an example, the controller may be configured to initiate probing after about 50-75% of expected switching duration is complete. 
     In some embodiments, the probing begins after about 30 seconds from initiating the optical transition. Relatively earlier probing may be especially helpful in cases where an interrupt command is received. An interrupt command is one that instructs the device to switch to a third optical transmission state when the device is already in the process of changing from a first to a second optical transmission state). In this case, early probing can help determine the direction of the transition (i.e, whether the interrupt command requires the window to become lighter or darker than when the command is received). In some embodiments, the probing begins about 120 minutes (e.g., about 30 minutes, about 60 minutes, or about 90 minutes) after initiating the optical transition. Relatively later probing may be more useful where larger windows are used, and where the transition occurs from an equilibrium state. For architectural glass, probing may begin about 30 seconds to 30 minutes after initiating the optical transition, in some cases between about 1-5 minutes, for example between about 1-3 minutes, or between about 10-30 minutes, or between about 20-30 minutes. In some embodiments, the probing begins about 1-5 minutes (e.g., about 1-3 minutes, about 2 minutes in a particular example) after initiating an optical transition through an interrupt command, while the probing begins about 10-30 minutes (e.g., about 20-30 minutes) after initiating an optical transition from an initial command given when the electrochromic device is in an equilibrium state. 
     In the examples of  FIGS.  4 A and  4 B , the size of the pulses is between the drive voltage value and the hold voltage value. This may be done for convenience. Other pulse magnitudes are possible. For example, the pulse may a magnitude of about +/−about 500 mV of the hold voltage, or about +/−200 mV of the hold voltage. For context, an electrochromic device on a window, such as an architectural window, may have a drive voltage of about 0 volts to +/−20 volts (e.g., about +/−2 volts to +/−10 volts) and a hold voltage of about 0 volts to +/−4 volts (e.g., about +/−1 volt to +/−2 volts). 
     In various embodiments, the controller determines when during the optical transition the polarity of the probe current opposes the polarity of the bias due to transition proceeding to a significant extent. In other words, the current to the bus bars flows in a direction opposite of what would be expected if the optical transition was still proceeding. 
     Probing by dropping the applied voltage magnitude from V drive  to V hold  provides a convenient, and broadly applicable, mechanism for monitoring the transition to determine when the probe current first reverses polarity. Probing by dropping the voltage to a magnitude other than that of V hold  may involve characterization of window performance. It appears that even very large windows (e.g., about 60″) essentially complete their optical transition when the current first opposes the transition upon probing from V drive  to V hold . 
     In certain cases, probing occurs by dropping the applied voltage magnitude from V drive  to V probe , where V probe  is a probe voltage other than the hold voltage. For example, V probe  may be V hold  as modified by an offset. Although many windows are able to essentially complete their optical transitions when the current first opposes the transition after probing from V drive  to Wow, certain windows may benefit from pulsing to a voltage slightly offset from the hold voltage. In general, the offset becomes increasingly beneficial as the size of the window increases, and as the temperature of the window drops. In certain cases, the offset is between about 0-1V, and the magnitude of V probe  is between about 0-1V higher than the magnitude of V hold . For example, the offset may be between about 0-0.4V. In these or other embodiments, the offset may be at least about 0.025V, or at least about 0.05V, or at least about 0.1V. The offset may result in the transition having a longer duration than it otherwise would. The longer duration helps ensure that the optical transition is able to fully complete. Techniques for selecting an appropriate offset from the hold voltage are discussed further below in the context of a target open circuit voltage. 
     In some embodiments, the controller notifies a user or the window network master controller of how far (by, e.g., percentage) the optical transition has progressed. This may be an indication of what transmission level the center of the window is currently at. Feedback regarding transition may be provided to user interface in a mobile device or other computational apparatus. See e.g., PCT Patent Application No. US2013/036456 filed Apr. 12, 2013, which is incorporated herein by reference in its entirety. 
     The frequency of the probe pulsing may be between about 10 seconds and 500 seconds. As used in this context, the “frequency” means the separation time between the midpoints of adjacent pulses in a sequence of two or more pulses. Typically, the frequency of the pulsing is between about 10 seconds and 120 seconds. In certain embodiments, the frequency the pulsing is between about 20 seconds and 30 seconds. In certain embodiments, the probe frequency is influenced by the size of the electrochromic device or the separation between bus bars in the device. In certain embodiments, the probe frequency is chosen as a function the expected duration of the optical transition. For example, the frequency may be set to be about ⅕ th  to about 1/50 th  (or about 1/10 th  to about 1/30 th ) of the expected duration of the transition time. Note that transition time may correspond to the expected duration of V app =V drive . Note also that the expected duration of the transition may be a function of the size of the electrochromic device (or separation of bus bars). In one example, the duration for 14″ windows is ˜2.5 minutes, while the duration for 60″ windows is ˜40 minutes. In one example, the probe frequency is every 6.5 seconds for a 14″ window and every 2 minutes for a 60″ window. 
     In various implementations, the duration of each pulse is between about 1×10 −5  and 20 seconds. In some embodiments, the duration of the pulses is between about 0.1 and 20 seconds, for example between about 0.5 seconds and 5 seconds. 
     As indicated, in certain embodiments, an advantage of the probing techniques disclosed herein is that only very little information need be pre-set with the controller that is responsible for controlling a window transition. Typically, such information includes only the hold voltage (and voltage offset, if applicable) associated for each optical end state. Additionally, the controller may specify a difference in voltage between the hold voltage and a drive voltage, or alternatively, the value of V drive  itself. Therefore, for any chosen ending optical state, the controller would know the magnitudes of V hold , Voffset and V drive . The duration of the drive voltage is determined using the probing algorithm described here. In other words, the controller determines how to appropriately apply the drive voltage as a consequence of actively probing the extent of the transition in real time. 
       FIG.  5 A  presents a flowchart  501  for a process of monitoring and controlling an optical transition in accordance with certain disclosed embodiments. As depicted, the process begins with an operation denoted by reference number  503 , where a controller or other control logic receives instructions to direct the optical transition. As explained, the optical transition may be an optical transition between a tinted state and a more clear state of electrochromic device. The instructions for directing the optical transition may be provided to the controller based upon a preprogrammed schedule, an algorithm reacting to external conditions, manual input from a user, etc. Regardless of how the instructions originate, the controller acts on them by applying a drive voltage to the bus bars of the optically switchable device. See the operation denoted by reference number  505 . 
     As explained above, in conventional embodiments, the drive voltage is applied to the bus bars for a defined period of time after which it is presumed that the optical transition is sufficiently complete that the applied voltage can be dropped to a hold voltage. In such embodiments, the hold voltage is then maintained for the duration of the pending optical state. In contrast, in accordance with embodiments disclosed herein, the transition from a starting optical state to an ending optical state is controlled by probing the condition of the optically switchable device one or more times during the transition. This procedure is reflected in operations  507 , et seq. of  FIG.  5 A . 
     In operation  507 , the magnitude of the applied voltage is dropped after allowing the optical transition to proceed for an incremental period of time. The duration of this incremental transition is significantly less than the total duration required to fully complete the optical transition. Upon dropping the magnitude of the applied voltage, the controller measures the response of the current flowing to the bus bars. See operation  509 . The relevant controller logic may then determine whether the current response indicates that the optical transition is nearly complete. See decision  511 . As explained above, the determination of whether an optical transition is nearly complete can be accomplished in various ways. For example, it may be determined by the current reaching a particular threshold. Assuming that the current response does not indicate that the optical transition is nearly complete, process control is directed to an operation denoted by reference number  513 . In this operation, the applied voltage is returned to the magnitude of the drive voltage. Process controls then loops back to operation  507  where the optical transition is allowed to proceed by a further increment before again dropping the magnitude of the applied voltage to the bus bars. 
     At some point in the procedure  501 , decision operation  511  determines that the current response indicates that the optical transition is in fact nearly complete. At this point, process control proceeds to an operation indicated by reference number  515 , where the applied voltage is transitioned to or maintained at the hold voltage for the duration of the ending optical state. At this point, the process is complete. Separately, in some implementations, the method or controller may specify a total duration of the transition. In such implementations, the controller may be programmed to use a modified probing algorithm to monitor the progress of the transition from the starting state to the end state. The progress can be monitored by periodically reading a current value in response to a drop in the applied voltage magnitude such as with the probing technique described above. The probing technique may also be implemented using a drop in applied current (e.g., measuring the open circuit voltage) as explained below. The current or voltage response indicates how close to completion the optical transition has come. In some cases, the response is compared to a threshold current or voltage for a particular time (e.g., the time that has elapsed since the optical transition was initiated). In some embodiments, the comparison is made for a progression of the current or voltage responses using sequential pulses or checks. The steepness of the progression may indicate when the end state is likely to be reached. A linear extension to this threshold current may be used to predict when the transition will be complete, or more precisely when it will be sufficiently complete that it is appropriate to drop the drive voltage to the hold voltage. 
     With regard to algorithms for ensuring that the optical transition from first state to the second state occurs within a defined timeframe, the controller may be configured or designed to increase the drive voltage as appropriate to speed up the transition when the interpretation of the pulse responses suggests that the transition is not progressing fast enough to meet the desired speed of transition. In certain embodiments, when it is determined that the transition is not progressing sufficiently fast, the transition switches to a mode where it is driven by an applied current. The current is sufficiently great to increase the speed of the transition but is not so great that it degrades or damages the electrochromic device. In some implementations, the maximum suitably safe current may be referred to as I safe . Examples of I safe  may range between about 5 and 250 μA/cm 2 . In current controlled drive mode, the applied voltage is allowed to float during the optical transition. Then, during this current controlled drive step, could the controller periodically probes by, e.g., dropping to the hold voltage and checking for completeness of transition in the same way as when using a constant drive voltage. 
     In general, the probing technique may determine whether the optical transition is progressing as expected. If the technique determines that the optical transition is proceeding too slowly, it can take steps to speed the transition. For example, it can increase the drive voltage. Similarly, the technique may determine that the optical transition is proceeding too quickly and risks damaging the device. When such determination is made, the probing technique may take steps to slow the transition. As an example, the controller may reduce the drive voltage. 
     In some applications, groups of windows are set to matching transition rates by adjusting the voltage and/or driving current based on the feedback obtained during the probing (by pulse or open circuit measurements). In embodiments where the transition is controlled by monitoring the current response, the magnitude of the current response may be compared from controller to controller (for each of the group of windows) to determine how to scale the driving potential or driving current for each window in the group. The rate of change of open circuit voltage could be used in the same manner. 
       FIG.  5 B  presents a flowchart  521  depicting an example process for ensuring that the optical transition occurs sufficiently fast, e.g., within a defined time period. The first four depicted operations in flowchart  521  correspond to the first four operations in flowchart  501 . In other words, operation  523 ,  525 ,  527 , and  529  of flowchart  521  correspond to operations  503 ,  505 ,  507 , and  509  of flowchart  501 . Briefly, in operation  523 , the controller or other appropriate logic receives instructions to undergo an optical transition. Then, at operation  525 , the controller applies a drive voltage to the bus bars. After allowing the optical transition to proceed incrementally, the controller drops the magnitude of the applied voltage to the bus bars. See operation  527 . The magnitude of the lower voltage is typically, though not necessarily, the hold voltage. As mentioned, the lower voltage may also be the hold voltage as modified by an offset (the offset often falling between about 0-1V, for example between about 0-0.4V in many cases). Next, the controller measures the current response to the applied voltage drop. See operation  529 . 
     The controller next determines whether the current response indicates that the optical transition is proceeding too slowly. See decision  531 . As explained, the current response may be analyzed in various ways determine whether the transition is proceeding with sufficient speed. For example, the magnitude of the current response may be considered or the progression of multiple current responses to multiple voltage pulses may be analyzed to make this determination. 
     Assuming that operation  531  establishes that the optical transition is proceeding rapidly enough, the controller then increases the applied voltage back to the drive voltage. See operation  533 . Thereafter, the controller then determines whether the optical transition is sufficiently complete that further progress checks are unnecessary. See operation  535 . In certain embodiments, the determination in operation  535  is made by considering the magnitude of the current response as discussed in the context of  FIG.  5 A . Assuming that the optical transition is not yet sufficiently complete, process control returns to operation  527 , where the controller allows the optical transition to progress incrementally further before again dropping the magnitude of the applied voltage. 
     Assuming that execution of operation  531  indicates that the optical transition is proceeding too slowly, process control is directed to an operation  537  where the controller increases the magnitude of the applied voltage to a level that is greater than the drive voltage. This over drives the transition and hopefully speeds it along to a level that meets specifications. After increasing the applied voltage to this level, process control is directed to operation  527  where the optical transition continues for a further increment before the magnitude of the applied voltage is dropped. The overall process then continues through operation  529 ,  531 , etc. as described above. At some point, decision  535  is answered in the affirmative and the process is complete. In other words, no further progress checks are required. The optical transition then completes as illustrated in, for example, flowchart  501 . 
     Another application of the probing techniques disclosed herein involves on-the-fly modification of the optical transition to a different end state. In some cases, it will be necessary to change the end state after a transition begins. Examples of reasons for such modification include a user&#39;s manual override a previously specified end tint state and a wide spread electrical power shortage or disruption. In such situations, the initially set end state might be transmissivity=40% and the modified end state might be transmissivity=5%. 
     Where an end state modification occurs during an optical transition, the probing techniques disclosed herein can adapt and move directly to the new end state, rather than first completing the transition to the initial end state. 
     In some implementations, the transition controller/method detects the current state of the window using a voltage/current sense as disclosed herein and then moves to a new drive voltage immediately. The new drive voltage may be determined based on the new end state and optionally the time allotted to complete the transition. If necessary, the drive voltage is increased significantly to speed the transition or drive a greater transition in optical state. The appropriate modification is accomplished without waiting for the initially defined transition to complete. The probing techniques disclosed herein provide a way to detect where in the transition the device is and make adjustments from there. 
     It should be understood that the probing techniques presented herein need not be limited to measuring the magnitude of the device&#39;s current in response to a voltage drop (pulse). There are various alternatives to measuring the magnitude of the current response to a voltage pulse as an indicator of how far as the optical transition has progressed. In one example, the profile of a current transient provides useful information. In another example, measuring the open circuit voltage of the device may provide the requisite information. In such embodiments, the pulse involves simply applying no voltage to device and then measuring the voltage that the open circuit device applies. Further, it should be understood that current and voltage based algorithms are equivalent. In a current based algorithm, the probe is implemented by dropping the applied current and monitoring the device response. The response may be a measured change in voltage. For example, the device may be held in an open circuit condition to measure the voltage between bus bars. 
       FIG.  5 C  presents a flowchart  501  for a process of monitoring and controlling an optical transition in accordance with certain disclosed embodiments. In this case, the process condition probed is the open circuit voltage, as described in the previous paragraph. The first two depicted operations in flowchart  541  correspond to the first two operations in flowcharts  501  and  521 . In other words, operations  543  and  545  of flowchart  541  correspond to operations  503  and  505  of flowchart  501 . Briefly, in operation  543 , the controller or other appropriate logic receives instructions to undergo an optical transition. Then, at operation  545 , the controller applies a drive voltage to the bus bars. After allowing the optical transition to proceed incrementally, the controller applies open circuit conditions to the electrochromic device at operation  547 . Next, the controller measures the open circuit voltage response at operation  549 . 
     As is the case above, the controller may measure the electronic response (in this case the open circuit voltage) after a defined period has passed since applying the open circuit conditions. Upon application of open circuit conditions, the voltage typically experiences an initial drop relating to the ohmic losses in external components connected to the electrochromic device. Such external components may be, for example, conductors and connections to the device. After this initial drop, the voltage experiences a first relaxation and settles at a first plateau voltage. The first relaxation relates to internal ohmic losses, for example over the electrode/electrolyte interfaces within the electrochromic devices. The voltage at the first plateau corresponds to the voltage of the cell, with both the equilibrium voltage and the overvoltages of each electrode. After the first voltage plateau, the voltage experiences a second relaxation to an equilibrium voltage. This second relaxation is much slower, for example on the order of hours. In some cases it is desirable to measure the open circuit voltage during the first plateau, when the voltage is relatively constant for a short period of time. This technique may be beneficial in providing especially reliable open circuit voltage readings. In other cases, the open circuit voltage is measured at some point during the second relaxation. This technique may be beneficial in providing sufficiently reliable open circuit readings while using less expensive and quick-operating power/control equipment. 
     In some embodiments, the open circuit voltage is measured after a set period of time after the open circuit conditions are applied. The optimal time period for measuring the open circuit voltage is dependent upon the distance between the bus bars. The set period of time may relate to a time at which the voltage of a typical or particular device is within the first plateau region described above. In such embodiments, the set period of time may be on the order of milliseconds (e.g., a few milliseconds in some examples). In other cases, the set period of time may relate to a time at which the voltage of a typical or particular device is experiencing the second relaxation described above. Here, the set period of time may be on the order of about 1 second to several seconds, in some cases. Shorter times may also be used depending on the available power supply and controller. As noted above, the longer times (e.g., where the open circuit voltage is measured during the second relaxation) may be beneficial in that they still provide useful open circuit voltage information without the need for high end equipment capable of operating precisely at very short timeframes. 
     In certain implementations, the open circuit voltage is measured/recorded after a timeframe that is dependent upon the behavior of the open circuit voltage. In other words, the open circuit voltage may be measured over time after open circuit conditions are applied, and the voltage chosen for analysis may be selected based on the voltage vs. time behavior. As described above, after application of open circuit conditions, the voltage goes through an initial drop, followed by a first relaxation, a first plateau, and a second relaxation. Each of these periods may be identified on a voltage vs. time plot based on the slope of curve. For example, the first plateau region will relate to a portion of the plot where the magnitude of dVoc/dt is relatively low. This may correspond to conditions in which the ionic current has stopped (or nearly stopped) decaying. As such, in certain embodiments, the open circuit voltage used in the feedback/analysis is the voltage measured at a time when the magnitude of dVoc/dt drops below a certain threshold. 
     Returning to  FIG.  5 C , after the open circuit voltage response is measured, it can be compared to a target open circuit voltage at operation  551 . The target open circuit voltage may correspond to the hold voltage. In certain cases, discussed further below, the target open circuit voltage corresponds to the hold voltage as modified by an offset. Techniques for choosing an appropriate offset from the hold voltage are discussed further below. Where the open circuit voltage response indicates that the optical transition is not yet nearly complete (i.e., where the open circuit voltage has not yet reached the target open circuit voltage), the method continues at operation  553 , where the applied voltage is increased to the drive voltage for an additional period of time. After the additional period of time has elapsed, the method can repeat from operation  547 , where the open circuit conditions are again applied to the device. At some point in the method  541 , it will be determined in operation  551  that the open circuit voltage response indicates that the optical transition is nearly complete (i.e., where the open circuit voltage response has reached the target open circuit voltage). When this is the case, the method continues at operation  555 , where the applied voltage is maintained at the hold voltage for the duration of the ending optical state. 
     The method  541  of  FIG.  5 C  is very similar to the method  501  of  FIG.  5 A . The main difference is that in  FIG.  5 C , the relevant variable measured is the open circuit voltage, while in  FIG.  5 A , the relevant variable measured is the current response when a reduced voltage is applied. In another embodiment, the method  521  of  FIG.  5 B  is modified in the same way. In other words, the method  521  may be altered such that probing occurs by placing the device in open circuit conditions and measuring the open circuit voltage rather than a current response. 
     In another embodiment, the process for monitoring and controlling an optical transition takes into account the total amount of charge delivered to the electrochromic device during the transition, per unit area of the device. This quantity may be referred to as the delivered charge density or total delivered charge density. As such, an additional criterion such as the total charge density delivered may be used to ensure that the device fully transitions under all conditions. 
     The total delivered charge density may be compared to a threshold charge density (also referred to as a target charge density) to determine whether the optical transition is nearly complete. The threshold charge density may be chosen based on the minimum charge density required to fully complete or nearly complete the optical transition under the likely operating conditions. In various cases, the threshold charge density may be chosen/estimated based on the charge density required to fully complete or nearly complete the optical transition at a defined temperature (e.g., at about −40° C., at about −30° C., at about −20° C., at about −10° C., at about 0° C., at about 10° C., at about 20° C., at about 25° C., at about 30° C., at about 40° C., at about 60° C., etc.). 
     The optimum threshold charge density may also be affected by the leakage current of the electrochromic device. Devices that have higher leakage currents should have higher threshold charge densities. In some embodiments, an appropriate threshold charge density may be determined empirically for an individual window or window design. In other cases, an appropriate threshold may be calculated/selected based on the characteristics of the window such as the size, bus bar separation distance, leakage current, starting and ending optical states, etc. Example threshold charge densities range between about 1×10 −5  C/cm 2  and about 5 C/cm 2 , for example between about 1×10 −4  and about 0.5 C/cm 2 , or between about 0.005-0.05 C/cm 2 , or between about 0.01-0.04 C/cm 2 , or between about 0.01-0.02 in many cases. Smaller threshold charge densities may be used for partial transitions (e.g., fully clear to 25% tinted) and larger threshold charge densities may be used for full transitions. A first threshold charge density may be used for bleaching/clearing transitions, and a second threshold charge density may be used for coloring/tinting transitions. In certain embodiments, the threshold charge density is higher for tinting transitions than for clearing transitions. In a particular example, the threshold charge density for tinting is between about 0.013-0.017 C/cm 2 , and the threshold charge density for clearing is between about 0.016-0.020 C/cm 2 . Additional threshold charge densities may be appropriate where the window is capable of transitioning between more than two states. For instance, if the device switches between four different optical states: A, B, C, and D, a different threshold charge density may be used for each transition (e.g., A to B, A to C, A to D, B to A, etc.). 
     In some embodiments, the threshold charge density is determined empirically. For instance, the amount of charge required to accomplish a particular transition between desired end states may be characterized for devices of different sizes. A curve may be fit for each transition to correlate the bus bar separation distance with the required charge density. Such information may be used to determine the minimum threshold charge density required for a particular transition on a given window. In some cases, the information gathered in such empirical determinations is used to calculate an amount of charge density that corresponds to a certain level of change (increase or decrease) in optical density. 
       FIG.  5 D  presents a flow chart for a method  561  for monitoring and controlling an optical transition in an electrochromic device. The method starts at operations  563  and  565 , which correspond to operations  503  and  505  of  FIG.  5 A . At  563 , the controller or other appropriate logic receives instructions to undergo an optical transition. Then, at operation  565 , the controller applies a drive voltage to the bus bars. After allowing the optical transition to proceed incrementally, the magnitude of the voltage applied to the bus bars is reduced to a probe voltage (which in some cases is the hold voltage, and in other cases is the hold voltage modified by an offset) at operation  567 . Next at operation  569 , the current response to the reduced applied voltage is measured. 
     Thus far, the method  561  of  FIG.  5 D  is identical to the method  501  of  FIG.  5 A . However, the two methods diverge at this point in the process, with method  561  continuing at operation  570 , where the total delivered charge density is determined. The total delivered charge density may be calculated based on the current delivered to the device during the optical transition, integrated over time. At operation  571 , the relevant controller logic may determine whether the current response and total delivered charge density each indicate that the optical transition is nearly complete. As explained above, the determination of whether an optical transition is nearly complete can be accomplished in various ways. For example, it may be determined by the current reaching a particular threshold, and by the delivered charge density reaching a particular threshold. Both the current response and the total delivered charge density must indicate that the transition is nearly complete before the method can continue on at operation  575 , where the applied voltage is transitioned to or maintained at the hold voltage for the duration of the ending optical state. Assuming at least one of the current response and total delivered charge density indicate that the optical transition is not yet nearly complete at operation  571 , process control is directed to an operation denoted by reference number  573 . In this operation, the applied voltage is returned to the magnitude of the drive voltage. Process control then loops back to operation  567  where the optical transition is allowed to proceed by a further increment before again dropping the magnitude of the applied voltage to the bus bars. 
       FIG.  5 E  presents an alternative method for monitoring and controlling an optical transition in an electrochromic device. The method starts at operations  583  and  585 , which correspond to operations  503  and  505  of  FIG.  5 A . At  583 , the controller or other appropriate logic receives instructions to undergo an optical transition. Then, at operation  585 , the controller applies a drive voltage to the bus bars. After allowing the optical transition to proceed incrementally, open circuit conditions are applied to the device at operation  587 . Next at operation  589 , the open circuit voltage of the device is measured. 
     Thus far, the method  581  of  FIG.  5 E  is identical to the method  541  of  FIG.  5 C . However, the two methods diverge at this point in the process, with method  581  continuing at operation  590 , where the total delivered charge density is determined. The total delivered charge density may be calculated based on the current delivered to the device during the optical transition, integrated over time. At operation  591 , the relevant controller logic may determine whether the open circuit voltage and total delivered charge density each indicate that the optical transition is nearly complete. Both the open circuit voltage response and the total delivered charge density must indicate that the transition is nearly complete before the method can continue on at operation  595 , where the applied voltage is transitioned to or maintained at the hold voltage for the duration of the ending optical state. Assuming at least one of the open circuit voltage response and total delivered charge density indicate that the optical transition is not yet nearly complete at operation  591 , process control is directed to an operation denoted by reference number  593 . In this operation, the applied voltage is returned to the magnitude of the drive voltage. Process control then loops back to operation  587  where the optical transition is allowed to proceed by a further increment before again applying open circuit conditions to the device. The method  581  of  FIG.  5 E  is very similar to the method  561  of  FIG.  5 D . The principal difference between the two embodiments is that in  FIG.  5 D , the applied voltage drops and a current response is measured, whereas in  FIG.  5 E , open circuit conditions are applied and an open circuit voltage is measured. 
       FIG.  5 F  illustrates a flowchart for a related method  508  for controlling an optical transition in an electrochromic device. The method  508  of  FIG.  5 F  is similar to the method  581  of  FIG.  5 E . The method  508  begins at operation  510  where the controller is turned on. Next, at operation  512 , the open circuit voltage (Voc) is read and the device waits for an initial command. An initial command is received at operation  514 , the command indicating that the window should switch to a different optical state. After the command is received, open circuit conditions are applied and the open circuit voltage is measured at operation  516 . The amount of charge delivered (Q) may also be read at block  516 . These parameters determine the direction of the transition (whether the window is supposed to get more tinted or more clear), and impact the optimal drive parameter. An appropriate drive parameter (e.g., drive voltage) is selected at operation  516 . This operation may also involve revising the target charge count and target open circuit voltage, particularly in cases where an interrupt command is received, as discussed further below. 
     After the open circuit voltage is read at operation  516 , the electrochromic device is driven for a period of time. The drive duration may be based on the busbar separation distance in some cases. In other cases, a fixed drive duration may be used, for example about 30 seconds. This driving operation may involve applying a drive voltage or current to the device. Operation  518  may also involve modifying a drive parameter based on the sensed open circuit voltage and/or charge count. Next, at operation  520 , it is determined whether the total time of the transition (thus far) is less than a threshold time. The threshold time indicated in  FIG.  5 F  is 2 hours, though other time periods may be used as appropriate. If it is determined that the total time of transition is not less than the threshold time (e.g., where the transition has taken at least 2 hours and is not yet complete), the controller may indicate that it is in a fault state at operation  530 . This may indicate that something has caused an error in the transition process. Otherwise, where the total time of transition is determined to be less than the threshold time, the method continues at operation  522 . Here, open circuit conditions are again applied, and the open circuit voltage is measured. At operation  524 , it is determined whether the measured open circuit voltage is greater than or equal to the target voltage (in terms of magnitude). If so, the method continues at operation  526 , where it is determined whether the charge count (Q) is greater than or equal to the target charge count. If the answer in either of operations  524  or  526  is no, the method returns to block  518  where the electrochromic device transition is driven for an additional drive duration. Where the answer in both of operations  524  and  526  is yes, the method continues at operation  528 , where a hold voltage is applied to maintain the electrochromic device in the desired tint state. Typically, the hold voltage continues to be applied until a new command is received, or until a timeout is experienced. 
     When a new command is received after the transition is complete, the method may return to operation  516 . Another event that can cause the method to return to operation  516  is receiving an interrupt command, as indicated in operation  532 . An interrupt command may be received at any point in the method after an initial command is received at operation  514  and before the transition is essentially complete at operation  528 . The controller should be capable of receiving multiple interrupt commands over a transition. One example interrupt command involves a user directing a window to change from a first tint state (e.g., fully clear) to a second tint state (e.g., fully tinted), then interrupting the transition before the second tint state is reached to direct the window to change to a third tint state (e.g., half tinted) instead of the second tint state. After receiving a new command or an interrupt command, the method returns to block  516  as indicated above. Here, open circuit conditions are applied and the open circuit voltage and charge count are read. Based on the open circuit voltage and charge count readings, as well as the desired third/final tint state, the controller is able to determine appropriate drive conditions (e.g., drive voltage, target voltage, target charge count, etc.) for reaching the third tint state. For instance, the open circuit voltage/charge count may be used to indicate in which direction the transition should occur. The charge count and charge target may also be reset after receiving a new command or an interrupt command. The updated charge count may relate to the charge delivered to move from the tint state when the new/interrupt command is received to the desired third tint state. Because the new command/interrupt command will change the starting and ending points of the transition, the target open circuit voltage and target charge count may need to be revised. This is indicated as an optional part of operation  516 , and is particularly relevant where a new or interrupt command is received. 
     In certain implementations, the method involves using a static offset to the hold voltage. This offset hold voltage may be used to probe the device and elicit a current response, as described in relation to  FIGS.  5 A,  5 B, and  5 D . The offset hold voltage may also be used as a target open circuit voltage, as described in relation to  FIGS.  5 C and  5 E . In certain cases, particularly for windows with a large separation between the bus bars (e.g., at least about 25″), the offset can be beneficial in ensuring that the optical transition proceeds to completion across the entire window. 
     In many cases, an appropriate offset is between about 0-0.5V (e.g., about 0.1-0.4V, or between about 0.1-0.2V). Typically, the magnitude of an appropriate offset increases with the size of the window. An offset of about 0.2V may be appropriate for a window of about 14 inches, and an offset of about 0.4V may be appropriate for a window of about 60 inches. These values are merely examples and are not intended to be limiting. In some embodiments, a window controller is programmed to use a static offset to V hold . The magnitude and in some cases direction of the static offset may be based on the device characteristics such as the size of the device and the distance between the bus bars, the driving voltage used for a particular transition, the leakage current of the device, the peak current density, capacitance of the device, etc. In various embodiments, the static offset is determined empirically. In some designs, it is calculated dynamically, when the device is installed or while it is installed and operating, from monitored electrical and/or optical parameters or other feedback. 
     In other embodiments, a window controller may be programmed to dynamically calculate the offset to V hold . In one implementation, the window controller dynamically calculates the offset to V hold  based on one or more of the device&#39;s current optical state (OD), the current delivered to the device (I), the rate of change of current delivered to the device (dI/dt), the open circuit voltage of the device (V oc ), and the rate of change of the open circuit voltage of the device (dV oc /dt). This embodiment is particularly useful because it does not require any additional sensors for controlling the transition. Instead, all of the feedback is generated by pulsing the electronic conditions and measuring the electronic response of the device. The feedback, along with the device characteristics mentioned above, may be used to calculate the optimal offset for the particular transition occurring at that time. In other embodiments, the window controller may dynamically calculate the offset to V hold  based on certain additional parameters. These additional parameters may include the temperature of the device, ambient temperature, and signals gathered by photoptic sensors on the window. These additional parameters may be helpful in achieving uniform optical transitions at different conditions. However, use of these additional parameters also increases the cost of manufacture due to the additional sensors required. 
     The offset may be beneficial in various cases due to the non-uniform quality of the effective voltage, V eff , applied across the device. The non-uniform V eff  is shown in  FIG.  2   , for example, described above. Because of this non-uniformity, the optical transition does not occur in a uniform manner. In particular, areas near the bus bars experience the greatest V eff  and transition quickly, while areas removed from the bus bars (e.g., the center of the window) experience the lowest V eff  and transition more slowly. The offset can help ensure that the optical transition proceeds to completion at the center of the device where the change is slowest. 
       FIGS.  6 A and  6 B  show graphs depicting the total charge delivered over time and the applied voltage over time during two different electrochromic tinting transitions. The window in each case measured about 24×24 inches. The total charge delivered is referred to as the Tint Charge Count, and is measured in coulombs (C). The total charge delivered is presented on the left hand y-axis of each graph, and the applied voltage is presented on the right hand y-axis of each graph. In each figure, line  602  corresponds to the total charge delivered and line  604  corresponds to the applied voltage. Further, line  606  in each graph corresponds to a threshold charge (the threshold charge density multiplied by the area of the window), and line  608  corresponds to a target open circuit voltage. The threshold charge and target open circuit voltage are used in the method shown in  FIG.  5 E  to monitor/control the optical transition. 
     The voltage curves  604  in  FIGS.  6 A and  6 B  each start out with a ramp to drive component, where the magnitude of the voltage ramps up to the drive voltage of about −2.5V. After an initial period of applying the drive voltage, the voltage begins to spike upwards at regular intervals. These voltage spikes occur when the electrochromic device is being probed. As described in  FIG.  5 E , the probing occurs by applying open circuit conditions to the device. The open circuit conditions result in an open circuit voltage, which correspond to the voltage spikes seen in the graphs. Between each probe/open circuit voltage, there is an additional period where the applied voltage is the drive voltage. In other words, the electrochromic device is driving the transition and periodically probing the device to test the open circuit voltage and thereby monitor the transition. The target open circuit voltage, represented by line  608 , was selected to be about −1.4V for each case. The hold voltage in each case was about −1.2V. Thus, the target open circuit voltage was offset from the hold voltage by about 0.2V. 
     In the transition of  FIG.  6 A , the magnitude of the open circuit voltage exceeds the magnitude of the target open circuit voltage at about 1500 seconds. Because the relevant voltages in this example are negative, this is shown in the graph as the point at which the open circuit voltage spikes first fall below the target open circuit voltage. In the transition of  FIG.  6 B , the magnitude of the open circuit voltage exceeds the magnitude of the target open circuit voltage sooner than in  FIG.  6 A , around 1250 seconds. 
     The total delivered charge count curves  602  in  FIGS.  6 A and  6 B  each start at  0  and rise monotonically. In the transition of  FIG.  6 A , the delivered charge reaches the threshold charge at around 1500 seconds, which was very close to the time at which the target open circuit voltage was met. Once both conditions were met, the voltage switched from the drive voltage to the hold voltage, around 1500 seconds. In the transition of  FIG.  6 B , the total delivered charge took about 2100 seconds to reach the charge threshold, which is about 14 minutes longer than it took the voltage to reach the target voltage for this transition. After both the target voltage and threshold charge are met, the voltage is switched to the hold voltage. The additional requirement of the total charge delivered results in the  FIG.  6 B  case driving the transition at the drive voltage for a longer time than might otherwise be used. This helps ensure full and uniform transitions across many window designs at various environmental conditions. 
     In another embodiment, the optical transition is monitored through voltage sensing pads positioned directly on the transparent conductive layers (TCLs). This allows for a direct measurement of the V eff  at the center of the device, between the bus bars where V eff  is at a minimum. In this case, the controller indicates that the optical transition is complete when the measured V eff  at the center of the device reaches a target voltage such as the hold voltage. In various embodiments, the use of sensors may reduce or eliminate the benefit from using a target voltage that is offset from the hold voltage. In other words, the offset may not be needed and the target voltage may equal the hold voltage when the sensors are present. Where voltage sensors are used, there should be at least one sensor on each TCL. The voltage sensors may be placed at a distance mid-way between the bus bars, typically off to a side of the device (near an edge) so that they do not affect (or minimally affect) the viewing area. The voltage sensors may be hidden from view in some cases by placing them proximate a spacer/separator and/or frame that obscures the view of the sensor. 
       FIG.  6 C  presents an embodiment of an EC window  690  that utilizes sensors to directly measure the effective voltage at the center of the device. The EC window  690  includes top bus bar  691  and bottom bus bar  692 , which are connected by wires  693  to a controller (not shown). Voltage sensor  696  is placed on the top TCL, and voltage sensor  697  is placed on the bottom TCL. The sensors  696  and  697  are placed at a distance mid-way between the bus bars  691  and  692 , though they are off to the side of the device. In some cases the voltage sensors may be positioned such that they reside within a frame of the window. This placement helps hide the sensors and promote optimal viewing conditions. The voltage sensors  696  and  697  are connected to the controller through wires  698 . The wires  693  and  698  may pass under or through a spacer/separator placed and sealed in between the panes of the window. The window  690  shown in  FIG.  6 C  may utilize any of the methods described herein for controlling an optical transition. 
     In some implementations, the voltage sensing pads may be conductive tape pads. The pads may be as small as about 1 mm 2  in some embodiments. In these or other cases, the pads may be about 10 mm 2  or less. A four wire system may be used in embodiments utilizing such voltage sensing pads. 
     Electrochromic Devices and Controllers—Examples 
     Examples of electrochromic device structure and fabrication will now be presented.  FIGS.  7 A and  7 B  are schematic cross-sections of an electrochromic device,  700 , showing a common structural motif for such devices. Electrochromic device  700  includes a substrate  702 , a conductive layer (CL)  704 , an electrochromic layer (EC)  706 , an optional ion conducting (electronically resistive) layer (IC)  708 , a counter electrode layer (CE)  710 , and another conductive layer (CL)  712 . Elements  704 ,  706 ,  708 ,  710 , and  712  are collectively referred to as an electrochromic stack,  714 . In numerous embodiments, the stack does not contain ion conducting layer  708 , or at least not as a discrete or separately fabricated layer. A voltage source,  716 , operable to apply an electric potential across electrochromic stack  712  effects the transition of the electrochromic device from, e.g., a clear state (refer to  FIG.  7 A ) to a tinted state (refer to  FIG.  7 B ). 
     The order of layers may be reversed with respect to the substrate. That is, the layers may be in the following order: substrate, conductive layer, counter electrode layer, ion conducting layer, electrochromic material layer, and conductive layer. The counter electrode layer may include a material that is electrochromic or not. If both the electrochromic layer and the counter electrode layer employ electrochromic materials, one of them should be a cathodically coloring material and the other should be an anodically coloring material. For example, the electrochromic layer may employ a cathodically coloring material and the counter electrode layer may employ an anodically coloring material. This is the case when the electrochromic layer is a tungsten oxide and the counter electrode layer is a nickel tungsten oxide. 
     The conductive layers commonly comprise transparent conductive materials, such as metal oxides, alloy oxides, and doped versions thereof, and are commonly referred to as “TCO” layers because they are made from transparent conducting oxides. In general, however, the transparent layers can be made of any transparent, electronically conductive material that is compatible with the device stack. Some glass substrates are provided with a thin transparent conductive oxide layer such as fluorinated tin oxide, sometimes referred to as “FTO.” 
     Device  700  is meant for illustrative purposes, in order to understand the context of embodiments described herein. Methods and apparatus described herein are used to identify and reduce defects in electrochromic devices, regardless of the structural arrangement of the electrochromic device. 
     During normal operation, an electrochromic device such as device  700  reversibly cycles between a clear state and a tinted state. As depicted in  FIG.  7 A , in the clear state, a potential is applied across the electrodes (transparent conductor layers  704  and  712 ) of electrochromic stack  714  to cause available ions (e.g. lithium ions) in the stack to reside primarily in the counter electrode  710 . If electrochromic layer  706  contains a cathodically coloring material, the device is in a clear state. In certain electrochromic devices, when loaded with the available ions, counter electrode layer  710  can be thought of as an ion storage layer. 
     Referring to  FIG.  7 B , when the potential on the electrochromic stack is reversed, the ions are transported across ion conducting layer  708  to electrochromic layer  706  and cause the material to enter the tinted state. Again, this assumes that the optically reversible material in the electrochromic device is a cathodically coloring electrochromic material. In certain embodiments, the depletion of ions from the counter electrode material causes it to color also as depicted. In other words, the counter electrode material is anodically coloring electrochromic material. Thus, layers  706  and  710  combine to reduce the amount of light transmitted through the stack. When a reverse voltage is applied to device  700 , ions travel from electrochromic layer  706 , through the ion conducting layer  708 , and back into counter electrode layer  710 . As a result, the device clears. 
     Some pertinent examples of electrochromic devices are presented in the following US patent applications, each incorporated by reference in its entirety: U.S. patent application Ser. No. 12/645,111, filed Dec. 22, 2009; U.S. patent application Ser. No. 12/772,055, filed Apr. 30, 2010; U.S. patent application Ser. No. 12/645,159, filed Dec. 22, 2009; U.S. patent application Ser. No. 12/814,279, filed Jun. 11, 2010; U.S. patent application Ser. No. 13/462,725, filed May 2, 2012 and U.S. patent application Ser. No. 13/763,505, filed Feb. 8, 2013. 
     Electrochromic devices such as those described in relation to  FIGS.  7 A and  7 B  are used in, for example, electrochromic windows. For example, substrate  702  may be architectural glass upon which electrochromic devices are fabricated. Architectural glass is glass that is used as a building material. Architectural glass is typically used in commercial buildings, but may also be used in residential buildings, and typically, though not necessarily, separates an indoor environment from an outdoor environment. In certain embodiments, architectural glass is at least 20 inches by 20 inches, and can be much larger, e.g., as large as about 72 inches by 120 inches. 
     In some embodiments, electrochromic glass is integrated into an insulating glass unit (IGU). An insulating glass unit includes multiple glass panes assembled into a unit, generally with the intention of maximizing the thermal insulating properties of a gas contained in the space formed by the unit while at the same time providing clear vision through the unit. Insulating glass units incorporating electrochromic glass are similar to insulating glass units currently known in the art, except for electrical terminals for connecting the electrochromic glass to voltage source. 
     The optical transition driving logic can be implemented in many different controller configurations and coupled with other control logic. Various examples of suitable controller design and operation are provided in the following patent applications, each incorporated herein by reference in its entirety: U.S. patent application Ser. No. 13/049,623, filed Mar. 16, 2011; U.S. patent application Ser. No. 13/049,756, filed Mar. 16, 2011; U.S. Pat. No. 8,213,074, filed Mar. 16, 2011; U.S. patent application Ser. No. 13/449,235, filed Apr. 17, 2012; U.S. patent application Ser. No. 13/449,248, filed Apr. 17, 2012; U.S. patent application Ser. No. 13/449,251, filed Apr. 17, 2012; U.S. patent application Ser. No. 13/326,168, filed Dec. 14, 2011; U.S. patent application Ser. No. 13/682,618, filed Nov. 20, 2012; and U.S. patent application Ser. No. 13/772,969, filed Feb. 21, 2013. The following description and associated figures,  FIGS.  8  and  9   , present certain non-limiting controller design options suitable for implementing the drive profiles described herein. 
       FIG.  8    shows a cross-sectional axonometric view of an embodiment of an IGU  102  that includes two window panes or lites  216  and a controller  250 . In various embodiments, IGU  102  can include one, two, or more substantially transparent (e.g., at no applied voltage) lites  216  as well as a frame,  218 , that supports the lites  216 . For example, the IGU  102  shown in  FIG.  9    is configured as a double-pane window. One or more of the lites  216  can itself be a laminate structure of two, three, or more layers or lites (e.g., shatter-resistant glass similar to automotive windshield glass). In IGU  102 , at least one of the lites  216  includes an electrochromic device or stack,  220 , disposed on at least one of its inner surface,  222 , or outer surface,  224 : for example, the inner surface  222  of the outer lite  216 . 
     In multi-pane configurations, each adjacent set of lites  216  can have an interior volume,  226 , disposed between them. Generally, each of the lites  216  and the IGU  102  as a whole are rectangular and form a rectangular solid. However, in other embodiments other shapes (e.g., circular, elliptical, triangular, curvilinear, convex, concave) may be desired. In some embodiments, the volume  226  between the lites  116  is evacuated of air. In some embodiments, the IGU  102  is hermetically-sealed. Additionally, the volume  226  can be filled (to an appropriate pressure) with one or more gases, such as argon (Ar), krypton (Kr), or xenon (Xn), for example. Filling the volume  226  with a gas such as Ar, Kr, or Xn can reduce conductive heat transfer through the IGU  102  because of the low thermal conductivity of these gases. The latter two gases also can impart improved acoustic insulation due to their increased weight. 
     In some embodiments, frame  218  is constructed of one or more pieces. For example, frame  218  can be constructed of one or more materials such as vinyl, PVC, aluminum (Al), steel, or fiberglass. The frame  218  may also include or hold one or more foam or other material pieces that work in conjunction with frame  218  to separate the lites  216  and to hermetically seal the volume  226  between the lites  216 . For example, in a typical IGU implementation, a spacer lies between adjacent lites  216  and forms a hermetic seal with the panes in conjunction with an adhesive sealant that can be deposited between them. This is termed the primary seal, around which can be fabricated a secondary seal, typically of an additional adhesive sealant. In some such embodiments, frame  218  can be a separate structure that supports the IGU construct. 
     Each lite  216  includes a substantially transparent or translucent substrate,  228 . Generally, substrate  228  has a first (e.g., inner) surface  222  and a second (e.g., outer) surface  224  opposite the first surface  222 . In some embodiments, substrate  228  can be a glass substrate. For example, substrate  228  can be a conventional silicon oxide (SO x )-based glass substrate such as soda-lime glass or float glass, composed of, for example, approximately 75% silica (SiO 2 ) plus Na 2 O, CaO, and several minor additives. However, any material having suitable optical, electrical, thermal, and mechanical properties may be used as substrate  228 . Such substrates also can include, for example, other glass materials, plastics and thermoplastics (e.g., poly(methyl methacrylate), polystyrene, polycarbonate, allyl diglycol carbonate, SAN (styrene acrylonitrile copolymer), poly(4-methyl-1-pentene), polyester, polyamide), or mirror materials. If the substrate is formed from, for example, glass, then substrate  228  can be strengthened, e.g., by tempering, heating, or chemically strengthening. In other implementations, the substrate  228  is not further strengthened, e.g., the substrate is untempered. 
     In some embodiments, substrate  228  is a glass pane sized for residential or commercial window applications. The size of such a glass pane can vary widely depending on the specific needs of the residence or commercial enterprise. In some embodiments, substrate  228  can be formed of architectural glass. Architectural glass is typically used in commercial buildings, but also can be used in residential buildings, and typically, though not necessarily, separates an indoor environment from an outdoor environment. In certain embodiments, a suitable architectural glass substrate can be at least approximately 20 inches by approximately 20 inches, and can be much larger, for example, approximately 80 inches by approximately 120 inches, or larger. Architectural glass is typically at least about 2 millimeters (mm) thick and may be as thick as 6 mm or more. Of course, electrochromic devices  220  can be scalable to substrates  228  smaller or larger than architectural glass, including in any or all of the respective length, width, or thickness dimensions. In some embodiments, substrate  228  has a thickness in the range of approximately 1 mm to approximately 10 mm. In some embodiments, substrate  228  may be very thin and flexible, such as Gorilla Glass® or Willow™ Glass, each commercially available from Corning, Inc. of Corning, N.Y., these glasses may be less than 1 mm thick, as thin as 0.3 mm thick. 
     Electrochromic device  220  is disposed over, for example, the inner surface  222  of substrate  228  of the outer pane  216  (the pane adjacent the outside environment). In some other embodiments, such as in cooler climates or applications in which the IGUs  102  receive greater amounts of direct sunlight (e.g., perpendicular to the surface of electrochromic device  220 ), it may be advantageous for electrochromic device  220  to be disposed over, for example, the inner surface (the surface bordering the volume  226 ) of the inner pane adjacent the interior environment. In some embodiments, electrochromic device  220  includes a first conductive layer (CL)  230  (often transparent), an electrochromic layer (EC)  232 , an ion conducting layer (IC)  234 , a counter electrode layer (CE)  236 , and a second conductive layer (CL)  238  (often transparent). Again, layers  230 ,  232 ,  234 ,  236 , and  238  are also collectively referred to as electrochromic stack  220 . 
     A power source  240  operable to apply an electric potential (V app ) to the device and produce V eff  across a thickness of electrochromic stack  220  and drive the transition of the electrochromic device  220  from, for example, a clear or lighter state (e.g., a transparent, semitransparent, or translucent state) to a tinted or darker state (e.g., a tinted, less transparent or less translucent state). In some other embodiments, the order of layers  230 ,  232 ,  234 ,  236 , and  238  can be reversed or otherwise reordered or rearranged with respect to substrate  238 . 
     In some embodiments, one or both of first conductive layer  230  and second conductive layer  238  is formed from an inorganic and solid material. For example, first conductive layer  230 , as well as second conductive layer  238 , can be made from a number of different materials, including conductive oxides, thin metallic coatings, conductive metal nitrides, and composite conductors, among other suitable materials. In some embodiments, conductive layers  230  and  238  are substantially transparent at least in the range of wavelengths where electrochromism is exhibited by the electrochromic layer  232 . Transparent conductive oxides include metal oxides and metal oxides doped with one or more metals. For example, metal oxides and doped metal oxides suitable for use as first or second conductive layers  230  and  238  can include indium oxide, indium tin oxide (ITO), doped indium oxide, tin oxide, doped tin oxide, zinc oxide, aluminum zinc oxide, doped zinc oxide, ruthenium oxide, doped ruthenium oxide, among others. As indicated above, first and second conductive layers  230  and  238  are sometimes referred to as “transparent conductive oxide” (TCO) layers. 
     In some embodiments, commercially available substrates, such as glass substrates, already contain a transparent conductive layer coating when purchased. In some embodiments, such a product can be used for both substrate  238  and conductive layer  230  collectively. Examples of such glass substrates include conductive layer-coated glasses sold under the trademark TEC Glass™ by Pilkington, of Toledo, Ohio and SUNGATE™  300  and SUNGATE™  500  by PPG Industries of Pittsburgh, Pa. Specifically, TEC Glass™ is, for example, a glass coated with a fluorinated tin oxide conductive layer. 
     In some embodiments, first or second conductive layers  230  and  238  can each be deposited by physical vapor deposition processes including, for example, sputtering. In some embodiments, first and second conductive layers  230  and  238  can each have a thickness in the range of approximately 0.01 μm to approximately 1 μm. In some embodiments, it may be generally desirable for the thicknesses of the first and second conductive layers  230  and  238  as well as the thicknesses of any or all of the other layers described below to be individually uniform with respect to the given layer; that is, that the thickness of a given layer is uniform and the surfaces of the layer are smooth and substantially free of defects or other ion traps. 
     A primary function of the first and second conductive layers  230  and  238  is to spread an electric potential provided by a power source  240 , such as a voltage or current source, over surfaces of the electrochromic stack  220  from outer surface regions of the stack to inner surface regions of the stack. As mentioned, the voltage applied to the electrochromic device experiences some Ohmic potential drop from the outer regions to the inner regions as a result of a sheet resistance of the first and second conductive layers  230  and  238 . In the depicted embodiment, bus bars  242  and  244  are provided with bus bar  242  in contact with conductive layer  230  and bus bar  244  in contact with conductive layer  238  to provide electric connection between the voltage or current source  240  and the conductive layers  230  and  238 . For example, bus bar  242  can be electrically coupled with a first (e.g., positive) terminal  246  of power source  240  while bus bar  244  can be electrically coupled with a second (e.g., negative) terminal  248  of power source  240 . 
     In some embodiments, IGU  102  includes a plug-in component  250 . In some embodiments, plug-in component  250  includes a first electrical input  252  (e.g., a pin, socket, or other electrical connector or conductor) that is electrically coupled with power source terminal  246  via, for example, one or more wires or other electrical connections, components, or devices. Similarly, plug-in component  250  can include a second electrical input  254  that is electrically coupled with power source terminal  248  via, for example, one or more wires or other electrical connections, components, or devices. In some embodiments, first electrical input  252  can be electrically coupled with bus bar  242 , and from there with first conductive layer  230 , while second electrical input  254  can be coupled with bus bar  244 , and from there with second conductive layer  238 . The conductive layers  230  and  238  also can be connected to power source  240  with other conventional means as well as according to other means described below with respect to a window controller. For example, as described below with reference to  FIG.  9   , first electrical input  252  can be connected to a first power line while second electrical input  254  can be connected to a second power line. Additionally, in some embodiments, third electrical input  256  can be coupled to a device, system, or building ground. Furthermore, in some embodiments, fourth and fifth electrical inputs/outputs  258  and  260 , respectively, can be used for communication between, for example, a window controller or microcontroller and a network controller. 
     In some embodiments, electrical input  252  and electrical input  254  receive, carry, or transmit complementary power signals. In some embodiments, electrical input  252  and its complement electrical input  254  can be directly connected to the bus bars  242  and  244 , respectively, and on the other side, to an external power source that provides a variable DC voltage (e.g., sign and magnitude). The external power source can be a window controller (see element  114  of  FIG.  9   ) itself, or power from a building transmitted to a window controller or otherwise coupled to electrical inputs  252  and  254 . In such an embodiment, the electrical signals transmitted through electrical inputs/outputs  258  and  260  can be directly connected to a memory device to allow communication between the window controller and the memory device. Furthermore, in such an embodiment, the electrical signal input to electrical input  256  can be internally connected or coupled (within IGU  102 ) to either electrical input  252  or  254  or to the bus bars  242  or  244  in such a way as to enable the electrical potential of one or more of those elements to be remotely measured (sensed). This can allow the window controller to compensate for a voltage drop on the connecting wires from the window controller to the electrochromic device  220 . 
     In some embodiments, the window controller can be immediately attached (e.g., external to the IGU  102  but inseparable by the user) or integrated within the IGU  102 . For example, U.S. patent application Ser. No. 13/049,750 (Attorney Docket No. SLDMP008) naming Brown et al. as inventors, titled ONBOARD CONTROLLER FOR MULTISTATE WINDOWS and filed 16 Mar. 2011, incorporated by reference herein, describes in detail various embodiments of an “onboard” controller. In such an embodiment, electrical input  252  can be connected to the positive output of an external DC power source. Similarly, electrical input  254  can be connected to the negative output of the DC power source. As described below, however, electrical inputs  252  and  254  can, alternately, be connected to the outputs of an external low voltage AC power source (e.g., a typical 24 V AC transformer common to the HVAC industry). In such an embodiment, electrical inputs/outputs  258  and  260  can be connected to the communication bus between the window controller and a network controller. In this embodiment, electrical input/output  256  can be eventually (e.g., at the power source) connected with the earth ground (e.g., Protective Earth, or PE in Europe) terminal of the system. 
     Although the applied voltages may be provided as DC voltages, in some embodiments, the voltages actually supplied by the external power source are AC voltage signals. In some other embodiments, the supplied voltage signals are converted to pulse-width modulated voltage signals. However, the voltages actually “seen” or applied to the bus bars  242  and  244  are effectively DC voltages. Typically, the voltage oscillations applied at terminals  246  and  248  are in the range of approximately 1 Hz to 1 MHz, and in particular embodiments, approximately 100 kHz. In various embodiments, the oscillations have asymmetric residence times for the darkening (e.g., tinting) and lightening (e.g., clearing) portions of a period. For example, in some embodiments, transitioning from a first less transparent state to a second more transparent state requires more time than the reverse; that is, transitioning from the more transparent second state to the less transparent first state. As will be described below, a controller can be designed or configured to apply a driving voltage meeting these requirements. 
     The oscillatory applied voltage control allows the electrochromic device  220  to operate in, and transition to and from, one or more states without any necessary modification to the electrochromic device stack  220  or to the transitioning time. Rather, the window controller can be configured or designed to provide an oscillating drive voltage of appropriate wave profile, taking into account such factors as frequency, duty cycle, mean voltage, amplitude, among other possible suitable or appropriate factors. Additionally, such a level of control permits the transitioning to any state over the full range of optical states between the two end states. For example, an appropriately configured controller can provide a continuous range of transmissivity (% T) which can be tuned to any value between end states (e.g., opaque and clear end states). 
     To drive the device to an intermediate state using the oscillatory driving voltage, a controller could simply apply the appropriate intermediate voltage. However, there can be more efficient ways to reach the intermediate optical state. This is partly because high driving voltages can be applied to reach the end states but are traditionally not applied to reach an intermediate state. One technique for increasing the rate at which the electrochromic device  220  reaches a desired intermediate state is to first apply a high voltage pulse suitable for full transition (to an end state) and then back off to the voltage of the oscillating intermediate state (just described). Stated another way, an initial low frequency single pulse (low in comparison to the frequency employed to maintain the intermediate state) of magnitude and duration chosen for the intended final state can be employed to speed the transition. After this initial pulse, a higher frequency voltage oscillation can be employed to sustain the intermediate state for as long as desired. 
     In some embodiments, each IGU  102  includes a component  250  that is “pluggable” or readily-removable from IGU  102  (e.g., for ease of maintenance, manufacture, or replacement). In some particular embodiments, each plug-in component  250  itself includes a window controller. That is, in some such embodiments, each electrochromic device  220  is controlled by its own respective local window controller located within plug-in component  250 . In some other embodiments, the window controller is integrated with another portion of frame  218 , between the glass panes in the secondary seal area, or within volume  226 . In some other embodiments, the window controller can be located external to IGU  102 . In various embodiments, each window controller can communicate with the IGUs  102  it controls and drives, as well as communicate to other window controllers, the network controller, BMS, or other servers, systems, or devices (e.g., sensors), via one or more wired (e.g., Ethernet) networks or wireless (e.g., WiFi) networks, for example, via wired (e.g., Ethernet) interface  263  or wireless (WiFi) interface  265 . See  FIG.  9   . Embodiments having Ethernet or Wifi capabilities are also well-suited for use in residential homes and other smaller-scale non-commercial applications. Additionally, the communication can be direct or indirect, e.g., via an intermediate node between a master controller such as network controller  112  and the IGU  102 . 
       FIG.  9    depicts a window controller  114 , which may be deployed as, for example, component  250 . In some embodiments, window controller  114  communicates with a network controller over a communication bus  262 . For example, communication bus  262  can be designed according to the Controller Area Network (CAN) vehicle bus standard. In such embodiments, first electrical input  252  can be connected to a first power line  264  while second electrical input  254  can be connected to a second power line  266 . In some embodiments, as described above, the power signals sent over power lines  264  and  266  are complementary; that is, collectively they represent a differential signal (e.g., a differential voltage signal). In some embodiments, line  268  is coupled to a system or building ground (e.g., an Earth Ground). In such embodiments, communication over CAN bus  262  (e.g., between microcontroller  274  and network controller  112 ) may proceed along first and second communication lines  270  and  272  transmitted through electrical inputs/outputs  258  and  260 , respectively, according to the CANopen communication protocol or other suitable open, proprietary, or overlying communication protocol. In some embodiments, the communication signals sent over communication lines  270  and  272  are complementary; that is, collectively they represent a differential signal (e.g., a differential voltage signal). 
     In some embodiments, component  250  couples CAN communication bus  262  into window controller  114 , and in particular embodiments, into microcontroller  274 . In some such embodiments, microcontroller  274  is also configured to implement the CANopen communication protocol. Microcontroller  274  is also designed or configured (e.g., programmed) to implement one or more drive control algorithms in conjunction with pulse-width modulated amplifier or pulse-width modulator (PWM)  276 , smart logic  278 , and signal conditioner  280 . In some embodiments, microcontroller  274  is configured to generate a command signal V COMMAND , e.g., in the form of a voltage signal, that is then transmitted to PWM  276 . PWM  276 , in turn, generates a pulse-width modulated power signal, including first (e.g., positive) component V PW1  and second (e.g., negative) component V PW2 , based on V COMMAND . Power signals V PW1  and V PW2  are then transmitted over, for example, interface  288 , to IGU  102 , or more particularly, to bus bars  242  and  244  in order to cause the desired optical transitions in electrochromic device  220 . In some embodiments, PWM  276  is configured to modify the duty cycle of the pulse-width modulated signals such that the durations of the pulses in signals V PW1  and V PW2  are not equal: for example, PWM  276  pulses V PW1  with a first 60% duty cycle and pulses V PW2  for a second 40% duty cycle. The duration of the first duty cycle and the duration of the second duty cycle collectively represent the duration, t PWM  of each power cycle. In some embodiments, PWM  276  can additionally or alternatively modify the magnitudes of the signal pulses V PW1  and V PW2 . 
     In some embodiments, microcontroller  274  is configured to generate V COMMAND  based on one or more factors or signals such as, for example, any of the signals received over CAN bus  262  as well as voltage or current feedback signals, V FB  and I FB  respectively, generated by PWM  276 . In some embodiments, microcontroller  274  determines current or voltage levels in the electrochromic device  220  based on feedback signals I FB  or V FB , respectively, and adjusts V COMMAND  according to one or more rules or algorithms to effect a change in the relative pulse durations (e.g., the relative durations of the first and second duty cycles) or amplitudes of power signals V PW1  and V PW2  to produce voltage profiles as described above. Additionally or alternatively, microcontroller  274  can also adjust V COMMAND  in response to signals received from smart logic  278  or signal conditioner  280 . For example, a conditioning signal VCON can be generated by signal conditioner  280  in response to feedback from one or more networked or non-networked devices or sensors, such as, for example, an exterior photosensor or photodetector  282 , an interior photosensor or photodetector  284 , a thermal or temperature sensor  286 , or a tint command signal V TC . For example, additional embodiments of signal conditioner  280  and VCON are also described in U.S. patent application Ser. No. 13/449,235, filed 17 Apr. 2012, and previously incorporated by reference. 
     In certain embodiments, V TC  can be an analog voltage signal between 0 V and 10 V that can be used or adjusted by users (such as residents or workers) to dynamically adjust the tint of an IGU  102  (for example, a user can use a control in a room or zone of building  104  similarly to a thermostat to finely adjust or modify a tint of the IGUs  102  in the room or zone) thereby introducing a dynamic user input into the logic within microcontroller  274  that determines V COMMAND . For example, when set in the 0 to 2.5 V range, V TC  can be used to cause a transition to a 5% T state, while when set in the 2.51 to 5 V range, V TC  can be used to cause a transition to a 20% T state, and similarly for other ranges such as 5.1 to 7.5 V and 7.51 to 10 V, among other range and voltage examples. In some embodiments, signal conditioner  280  receives the aforementioned signals or other signals over a communication bus or interface  290 . In some embodiments, PWM  276  also generates V COMMAND  based on a signal V SMART  received from smart logic  278 . In some embodiments, smart logic  278  transmits V SMART  over a communication bus such as, for example, an Inter-Integrated Circuit (I 2 C) multi-master serial single-ended computer bus. In some other embodiments, smart logic  278  communicates with memory device  292  over a 1-WIRE device communications bus system protocol (by Dallas Semiconductor Corp., of Dallas, Tex.). 
     In some embodiments, microcontroller  274  includes a processor, chip, card, or board, or a combination of these, which includes logic for performing one or more control functions. Power and communication functions of microcontroller  274  may be combined in a single chip, for example, a programmable logic device (PLD) chip or field programmable gate array (FPGA), or similar logic. Such integrated circuits can combine logic, control and power functions in a single programmable chip. In one embodiment, where one pane  216  has two electrochromic devices  220  (e.g., on opposite surfaces) or where IGU  102  includes two or more panes  216  that each include an electrochromic device  220 , the logic can be configured to control each of the two electrochromic devices  220  independently from the other. However, in one embodiment, the function of each of the two electrochromic devices  220  is controlled in a synergistic fashion, for example, such that each device is controlled in order to complement the other. For example, the desired level of light transmission, thermal insulative effect, or other property can be controlled via a combination of states for each of the individual electrochromic devices  220 . For example, one electrochromic device may be placed in a tinted state while the other is used for resistive heating, for example, via a transparent electrode of the device. In another example, the optical states of the two electrochromic devices are controlled so that the combined transmissivity is a desired outcome. 
     In general, the logic used to control electrochromic device transitions can be designed or configured in hardware and/or software. In other words, the instructions for controlling the drive circuitry may be hard coded or provided as software. In may be said that the instructions are provided by “programming”. Such programming is understood to include logic of any form including hard coded logic in digital signal processors and other devices which have specific algorithms implemented as hardware. Programming is also understood to include software or firmware instructions that may be executed on a general purpose processor. In some embodiments, instructions for controlling application of voltage to the bus bars are stored on a memory device associated with the controller or are provided over a network. Examples of suitable memory devices include semiconductor memory, magnetic memory, optical memory, and the like. The computer program code for controlling the applied voltage can be written in any conventional computer readable programming language such as assembly language, C, C++, Pascal, Fortran, and the like. Compiled object code or script is executed by the processor to perform the tasks identified in the program. 
     As described above, in some embodiments, microcontroller  274 , or window controller  114  generally, also can have wireless capabilities, such as wireless control and powering capabilities. For example, wireless control signals, such as radio-frequency (RF) signals or infra-red (IR) signals can be used, as well as wireless communication protocols such as WiFi (mentioned above), Bluetooth, Zigbee, EnOcean, among others, to send instructions to the microcontroller  274  and for microcontroller  274  to send data out to, for example, other window controllers, a network controller  112 , or directly to a BMS  110 . In various embodiments, wireless communication can be used for at least one of programming or operating the electrochromic device  220 , collecting data or receiving input from the electrochromic device  220  or the IGU  102  generally, collecting data or receiving input from sensors, as well as using the window controller  114  as a relay point for other wireless communications. Data collected from IGU  102  also can include count data, such as a number of times an electrochromic device  220  has been activated (cycled), an efficiency of the electrochromic device  220  over time, among other useful data or performance metrics. 
     The window controller  114  also can have wireless power capability. For example, window controller can have one or more wireless power receivers that receive transmissions from one or more wireless power transmitters as well as one or more wireless power transmitters that transmit power transmissions enabling window controller  114  to receive power wirelessly and to distribute power wirelessly to electrochromic device  220 . Wireless power transmission includes, for example, induction, resonance induction, RF power transfer, microwave power transfer, and laser power transfer. For example, U.S. patent application Ser. No. 12/971,576 [SLDMP003] naming Rozbicki as inventor, titled WIRELESS POWERED ELECTROCHROMIC WINDOWS and filed 17 Dec. 2010, incorporated by reference herein, describes in detail various embodiments of wireless power capabilities. 
     In order to achieve a desired optical transition, the pulse-width modulated power signal is generated such that the positive component V PW1  is supplied to, for example, bus bar  244  during the first portion of the power cycle, while the negative component V PW2  is supplied to, for example, bus bar  242  during the second portion of the power cycle. 
     In some cases, depending on the frequency (or inversely the duration) of the pulse-width modulated signals, this can result in bus bar  244  floating at substantially the fraction of the magnitude of V PW1  that is given by the ratio of the duration of the first duty cycle to the total duration t PWM  of the power cycle. Similarly, this can result in bus bar  242  floating at substantially the fraction of the magnitude of V PW2  that is given by the ratio of the duration of the second duty cycle to the total duration t PWM  of the power cycle. In this way, in some embodiments, the difference between the magnitudes of the pulse-width modulated signal components V PW1  and V PW2  is twice the effective DC voltage across terminals  246  and  248 , and consequently, across electrochromic device  220 . Said another way, in some embodiments, the difference between the fraction (determined by the relative duration of the first duty cycle) of V PW1  applied to bus bar  244  and the fraction (determined by the relative duration of the second duty cycle) of V PW2  applied to bus bar  242  is the effective DC voltage V EFF  applied to electrochromic device  220 . The current IEFF through the load—electromagnetic device  220 —is roughly equal to the effective voltage VEFF divided by the effective resistance (represented by resistor  316 ) or impedance of the load. 
     Those of ordinary skill in the art will also understand that this description is applicable to various types of drive mechanism including fixed voltage (fixed DC), fixed polarity (time varying DC) or a reversing polarity (AC, MF, RF power etc. with a DC bias). 
     The controller may be configured to monitor voltage and/or current from the optically switchable device. In some embodiments, the controller is configured to calculate current by measuring voltage across a known resistor in the driving circuit. Other modes of measuring or calculating current may be employed. These modes may be digital or analog. 
     OTHER EMBODIMENTS 
     Although the foregoing embodiments have been described in some detail to facilitate understanding, the described embodiments are to be considered illustrative and not limiting. It will be apparent to one of ordinary skill in the art that certain changes and modifications can be practiced within the scope of the appended claims. For example, while the drive profiles have been described with reference to electrochromic devices having planar bus bars, they apply to any bus bar orientation in which bus bars of opposite polarity are separated by distances great enough to cause a significant ohmic voltage drop in a transparent conductor layer from one bus bar to another. Further, while the drive profiles have been described with reference to electrochromic devices, they can be applied to other devices in which bus bars of opposite polarity are disposed at opposite sides of the devices.