LIQUID CRYSTAL LENS CIRCUIT

Abstract: In a pair of spectacles using liquid crystal lenses, an electrical circuit, for controlling the optical transmission of the liquid crystal lenses, controls the amplitude and/or duty cycle of the voltage driving the lens so as to operate the liquid crystal lens in the transition region, between maximum and minimum optical densities.

Description: LIQUID CRYSTAL LENS CIRCUIT The present invention relates generally to variable optical density lenses and in particular to liquid crystal lenses in which the optical density is varied as a function of ambient light intensity. One type of liquid crystal spectacle lens with a rapid response to changes in ambient light is described in US-A—4 279 474 and another is disclosed in our International patent application, PCT/GB 93/00119, in which we describe an electrical circuit for controlling the duty cycle of a waveform for driving a liquid crystal lens in dependence upon ambient light levels, in order to adjust the amount of light transmitted through the lens, the circuit including means for adjusting the response time of the circuit to changes in ambient light. That invention consists of a means for varying the rate at which the duty cycle and hence the optical density changes in response to changes in ambient light, and, in addition, comprises an arrangement for setting the threshold at which darkening starts and a means for obtaining a desired darkening characteristic above the threshold. However, practical difficulties arise in implementing the method proposed in US-A 4279474 for two interrelated reasons, namely user comfort and the unavailability of suitable liquid crystal cells. The operating principle outlined in US-A-4279474 involves operating the liquid crystal cell in one of two states, a state of maximum transmission and a state of minimum transmission. The average value of the transmission depends on the proportions of time spent in the two states. It was argued in US-A-4279474 that provided the switching frequency of the liquid crystal cell exceeded a certain value, known as the critical flicker fusion frequency, the wearer would perceive the average value of transmitted light. The critical flicker fusion frequency varies with light level according to the Ferry-Porter law and varies from 10 to 45 Hz over the range of light levels of interest in US-A-4 279 474. Human sensitivity to flicker has been studied in detail (see Lighting and Research Technology 6 , (1974) 127, G Brundrett) . Brundrett reports that the critical flicker frequency also depends on the amplitude of the flicker compared to the mean light level. Sensitivity to flicker above the critical flicker fusion frequency was noted and led to headaches, dizziness and other side effects. The sensitivity of humans to flicker, based on electroretinogram data, has also been measured (see Abstracts of the Annual Meeting of the Association for Research in Vision and Opthalmology , 1988, Vol.29 DS Greenhouse, SM Berman, IL Bailey, & T Raasch) . Although the study indicates that there was still a measurable retinal response at 200 Hz, it was less than one thousandth of the response at the critical flicker fusion frequency. Therefore, to avoid undesirable side-effects, when using the method employed in US-A-4279474, the switching frequency should be at least 200 Hz. To implement this approach with a switching frequency of 200 Hz, the switching time of the cell needs to be short compared to the times in the two transmission states - a switching time of 20% of the minimum time in a state could be taken as a guide. The minimum time in a state will be the time in the state of minimum transmission when the liquid crystal cell is giving its highest average transmission, or the time in the state of maximum transmission when the liquid crystal cell is giving its lowest average transmission. A ratio of average values of transmission of 4:1 could be achieved by varying the proportions of time in the state of maximum and minimum transmissions from 1:4 to 4:1 and would give a minimum transmission similar to that found in conventional sunglasses of fixed transmission. At a switching frequency of 200 Hz that corresponds to a minimum time in a state of l s. Therefore a cell switching time of less than 200μs is needed. However, nematic cells of the type proposed in US-A-4279474 have minimum switching times of about 10ms and could not therefore be used. An alternative is the use of ferroelectric cells, which exhibit switching times of 100 μs or less but have practical disadvantages. Ferroelectric cells are, at present, not sufficiently robust for use in sunglasses and suffer irreversible damage if squeezed. The thickness of the liquid crystal is usually less than that in nematic cells, and needs to be more precisely controlled, making the manufacture of ferro¬ electric cells more difficult. In addition, a higher drive voltage is needed, up to 15V compared to the 5V used for nematic cells and to achieve effective operation a complex drive waveform is often employed. The generation of this kind of waveform is unattractive in solar powered eyewear, where it is desirable to use the minimum power at a low voltage to minimize the size of the solar cell array. Accordingly, there is a need for another method of operating liquid crystal lenses, particularly of the nematic type, which alleviates the problems of flicker. According to the present invention there is provided an electrical circuit, for controlling the optical transmission of a liquid crystal lens, having means for controlling the amplitude and/or duty cycle of the voltage driving the lens so as to operate the liquid crystal lens in the transition region, between maximum and minimum optical densities. Preferably, the RMS value of the pulse voltage is controlled so as to be arranged to remain between the threshold values of the liquid crystal lens. The liquid crystal lens is preferably driven by a pulse voltage signal, the duty cycle of which is adjusted by said means, in accordance with the level of ambient light. Advantages of this method of operation are that a cell with slow response characteristics can be used and that flicker in the transmitted light at the driving frequency is very small. This thus avoids problems inherent in driving cells between states of maximum and minimum light transmission. The transmission of a nematic liquid crystal cell depends on the root mean square (RMS) of the applied voltage over a wide range of voltage waveforms. In the case of a rectangular waveform (which is convenient to generate) , the RMS and hence the transmission, will depend on the amplitude of the waveform and the square root of the duty cycle, at least for duty cycles of a few percent or greater. The approach of varying the RMS by changing the amplitude, but maintaining an essentially constant duty cycle, has been exploited in sunglasses using LC cells as lenses (see H.Seki, Y.Masuda and Y. Itoh, Proc .Soc . for Information Display 32-3(1991) ppl91-195) . A serious drawback with this approach is that nematic LC cells show a rapid change in transmission over a small range of RMS voltage, demanding sensitive and stable control of the amplitude of the applied waveform, on which the RMS depends directly. In consequence, where the amplitude is determined by a supply voltage susceptible to change, as in solar powered spectacles, the resulting darkening characteristic is unlikely to be near the optimum. Although improvement would be possible by including circuitry, typically an analogue amplifier, to vary precisely the amplitude of the drive waveform, it is not practical to include analogue circuits in the type of low power consumption application specific integrated circuits (ASICs) that are desirable for solar powered sunglasses. There are several advantages in the design and operation of the drive circuitry for LC lenses when the duty cycle of the drive waveform is varied. The RMS of the waveform is a function of the square root of the duty cycle, permitting a more gradual variation of RMS value and hence cell transmission. Avoiding the need to vary the amplitude of the drive waveform allows the design of a simpler drive circuit, using standard digital CMOS techniques. Such circuits can be readily incorporated into low power ASICs, which are advantageous in minimizing the power requirements of the system to enable the specification of a small and unobtrusive solar cell as the power source. To avoid unwanted changes in the RMS value of the drive waveform where the supply voltage varies, as it would in solar powered sunglasses, a simple voltage regulator can be employed. Alternatively, the amplitude could be allowed to change with supply voltage and the duty cycle compensated to maintain the RMS vale of the waveform constant. The control of flicker in the transmitted light is important to avoid unpleasant visual effects for the wearer and in order to minimize the risk of headaches and other effects. The amplitude and frequency components of the waveform of the flicker in transmitted light depend on the time-response characteristic of the LC cell, the nature of the drive waveform and the local gradient of the transmission against RMS voltage characteristic. All other things being equal, the slower the response of the LC cell, the smaller the flicker in transmitted light. For a given LC cell, the most favourable condition with a rectangular drive waveform is a 50% duty cycle, but this will not be maintained in a system where the duty cycle is varied to alter the transmission of the LC cell. For a waveform of constant period, the flicker will grow as the time between the high to low and low to high transition increases with reducing duty cycle. An improvement is to divide the time for which the drive is high into several shorter pulses, whilst maintaining the same RMS value for the waveform. An alternative solution is to reduce the period of the drive waveform, i.e. increase its frequency, as the duty cycle falls. If the time for which the drive waveform is low does not change with duty cycle, the flicker will be reduced. A penalty with both techniques is that the number of transitions per second would increase, requiring more power to charge and discharge the capacitance of the LC cell. It is presently proposed to employ conventional nematic cells with the circuit of the invention. Examples of devices according to the present invention will now be described with reference to the accompanying drawings, in which: Figure IA is a simplified block diagram of a circuit for controlling the optical density of a lens; Figure IB shows associated waveforms; Figs. 2(a) to 2(c) are diagrams showing waveforms used in operation of the lens. Figures 3A to 3E a circuit diagrams of a further control circuit for implementing the technique of the invention; Figure 4 is a graph showing LCD transmission percentage vs. RMS voltage across the LCD; and Figure 5 is a simplified block diagram of a third circuit for implementing the present invention. The circuit shown in figure l is a simplified block diagram of a circuit for controlling the optical density of a lens in response to changes in ambient light. The circuit can be implemented using analogue or digital circuit techniques, or a mixture of these. Preferably, the whole circuit will be provided on one application specific integrated circuit (ASIC) in order to keep manufacturing costs to appropriate levels, for size requirements and to minimize overall power consumption. A largely digital implementation is likely to be preferred. Referring to Figure 1, the circuit includes a detector 1 which may be a photodiode or other photosensitive device. The particular sensor chosen will depend on ease of fitting in the spectacle frames and cost, as well as its response characteristics. The output A from the detector 1 is fed to a signal conditioning stage 2. In a digital implementation, analogue-to-digital conversion would be performed at this stage; the signal would be pre-processed before conversion, for example by taking its logarithm to produce a convenient output as a function of light level, taking into account the properties of the light detector and the desired response characteristic of the sunglasses. Figure 5 illustrates such a digital circuit (see further below) . In an analogue version, similar functions would be performed. However, a time constant setting circuit and circuits to allow adjustments to the threshold of darkening and the form of darkening characteristic would be included. In the digital implementation (see Figure 5) a pulse- width modulated signal C (ie one with a variable duty cycle) would be obtained from a digitally controlled pulse generator. An analogue-to-digital converter 2b converts the signal from the amplifier/signal conditioner stage 2a. Time constant control is possible by placing a digital filter 2c in the input to the pulse generator and arbitrary response characteristics can be obtained by using a look-up table (ROM) 2d to translate the output of the digital filter into the appropriate control data for a pulse width modulator 3. In the analogue version, the voltage controlled oscillator taking its input from the signal conditioning stage 2, produces a pulse-width modulated waveform C. The driver stage 4 produces an output signal at D comprising alternate pulses of equal amplitude, but opposite polarity, of the same duration as at C. Providing pulses of opposite polarity avoids the generation of a DC component across cell 5. Some liquid crystal cell types darken with increasing voltages and others clear with increasing applied voltage. In the present invention either can be used. The period of the waveform may change with duty cycle, dependent on what type of waveform it is most convenient to generate in a given circumstance. The control of flicker will be an important factor in the choice of waveform. Figures 2(a) - (c) show appropriate waveforms D(a) - D(c) of variable duty cycle but constant period. Figure 4 illustrates a typical relationship of percentage of maximum light transmission vs RMS voltage for a nematic LC cell, and shows a range of RMS voltage of 2.0V to 2.8V over which the cell can be operated in accordance with this invention in order to provide an operating characteristic which lies in the transition region of the LCD in between maximum transmission and maximum opacity, in the present case between 20% and 92% maximum light transmission. In the drive and control circuit shown in Figures 3A to 3E a supply voltage V s is provided on line 104. The light detection and signal conditioning circuit comprises a photodiode 100, operated in the reverse bias mode, a capacitor 101, a diode 102, two transistors 105 and 106 and a resistor 103 which define a current I x which is proportional to the logarithm of the light level falling on the photodiode. This allows a wide range of light levels to be accommodated and the variable resistance 103 adjusts the sensitivity of the circuit, varying the threshold at which the LC lens starts to change transmission and the rate of change of transmission above the threshold. The analogue signal is converted into digital form by a circuit block 110. Capacitor 101 charges at a rate determined by the current I x from an initially discharged state. During conversion a counter 150 counts clock pulses from the master clock 111. When the capacitor voltage reaches the threshold of the Schmitt input inverter 107 the count reached at that point is loaded into latches 114 and 115, but the counter 150 continues to count. At a count of 255 the conversion cycle is stopped and capacitor 101 is discharged by a transistor 130, driven by a pulse formed by resistor capacitor network 131, on a positive going edge on the Schmitt inverter 132. This approach ensures that the conversion rate is constant, even though the voltage on the capacitor 101 reaches the threshold value at different times depending on the light level. The data stored on latches 114 and 115 is transferred to latches 116 and 117 at the end of conversion under the control of a timing pulse from circuit block 110. This ensures that the binary number representing light level only changes at the end of the conversion cycle and not at the point at which the count representing the light level is loaded into the data latches. The output from the latches 116 and 117 is sent to a circuit block 151 which sets the time response of the system. The output from this circuit block is a binary number which controls a pulse width modulator comprising magnitude comparators 123 and 124. The "current" value output to the pulse width modulator is stored on the two up/down counters 120 and 121. This "current" value of output is compared with the binary number, from latches 116 and 117 representing light level, by magnitude comparators 118 and 119. If the "current" value of output is less than the most recent value of input, the up/down counters 120 and 121 will be set to count up and vice versa. The rate of change in output depends on the number of pulses allowed through the gating circuit 133. The number of pulses varies with the "Any bit" line 128 selected, from 256 per conversion cycle (bit 0) to one every eight cycles (bit 11), in multiples of 2. The greater the number of pulses admitted per cycle, the faster the change in output towards the input and the time constant of the circuit would thereby be shorter. The pulse width modulation is achieved by magnitude comparators 123 and 124. The desired pulse width is loaded into the "A" inputs, and the output from the counter 150 goes to the "B" inputs. The output stays low until the output of the counter 150 exceeds the value at the "A" input. A bipolar output signal for the LC lenses is obtained with circuit block 125, comprising a J-K flip-flop 126 and NAND gates 127. Since, for a given LC cell it is known where the threshold voltage is above which the cell begins to darken (see figure 4), the adjustment resistor 103 is set appropriately so that the logarithmically scaled current I χ is generated by the signal conditioning circuit at an appropriate value such that the RMS voltage produced by the control circuit is above the threshold. Because the current I x is generated in accordance with the logarithm of the incident light, the system can be adjusted to provide an acceptable response, so that any normally available light level will not cause an RMS voltage value to be generated which is beyond the upper limit point at which the LC cell begins to stop changing transmissivity again, ie the cell fails to darken further substantially. Using a look-up table the desired response characteristic, in so far as both threshold response and darkening characteristic are concerned, can be obtained. This could be implemented, as shown in figure 5, by inserting a ROM (read-only-memory) 2d between the digital filter 2c and the pulse wave modulator 3. The look-up table would take a binary number representing light level to give the desired binary output control signal to the pulse wave modulator in order to provide the required cell transmission. As the LC cells responds to the RMS value of the drive waveform, which depends on the amplitude as well as the duty cycle of that waveform, it is necessary to provide some means of avoiding unwanted changes in RMS as a result of fluctuations in amplitude. The most important cause of fluctuations is changes in supply voltage, and these will occur in a solar powered device. The shunt regulator 153 shown in Figure 3E is used to stabilize the supply voltages to the circuit of Figures 3A to 3D. The operation of the circuit is known to those familiar with voltage regulators. A divided fraction of the supply voltageV s , from resistors 160 and 161, is compared to a reference voltage across diode 162. If the supply voltage rises, the currents through transistors 163, 164 and 165 increase. The increase in current drawn by the circuit will be enough to pull down the supply voltage if it is derived from a source of appreciable internal impedance, such as an array of solar cells.

Claims: CLAIMS 1. An electrical circuit, for controlling the optical transmission of a liquid crystal lens, having means for controlling the amplitude and/or duty cycle of the voltage driving the lens so as to operate the liquid crystal lens in the transition region, between maximum and minimum optical densities. 2. An electrical circuit according to claim 1, wherein the RMS value of the pulse voltage is controlled so as to be arranged to remain between the threshold values of the liquid crystal lens. 3. An electrical circuit according to claim 2 , wherein the liquid crystal lens is driven by a pulse voltage signal, the duty cycle of which is adjusted by said means, in accordance with the level of ambient light. 4. An electrical circuit according to claim 3, wherein a look-up table is provided in order to enable a binary representation of the ambient light, provided as an input, to cause a binary output to control the duty cycle of the pulse voltage. 5. An electrical circuit according to any of the preceding claims, wherein the ambient light level is detected by a light sensor to produce a signal, the signal being compressed logarithmically to produce a second signal logarithmically proportional to the ambient light level. 6. An electrical circuit according to claim 5, wherein the light sensor signal is an analogue signal which is converted to a digital signal, the value of which is held in a latch to provide an input to a pulse width modulator which provides an output signal to control the lens. 7. An electrical circuit according to claim 3 or any claim dependent thereon, for use with a solar cell power supply, which includes a voltage regulator for regulating the supply voltage to a substantially constant value. 8. An electrical circuit according to any of claims 1 to 3, including means for pre-processing a signal representative of the ambient light level in order to adjust the range of the duty cycle of the voltage driving the lens. 9. A pair of sunglasses having liquid crystal lenses and an electrical control circuit, for controlling the lenses, according to any of the preceding claims.

Inventors: MCMAHON RICHARD ANTHONY, MCMAHON, RICHARD, ANTHONY
Applicants: CRYSTALENS LTD, MCMAHON RICHARD ANTHONY, CRYSTALENS LIMITED
Application year: 1993
Main classification: G02F1
IPC codes: G02C7, G02F1