FR2530039A1 - Safety (protective) glasses having automatically adjustable transmission using liquid crystals - Google Patents

Abstract

Adjustable-transmission liquid-crystal safety glasses. According to the invention, the glasses comprise at least one liquid-crystal cell 10 and control means comprising a photodetector 12 capable of receiving the light flux transmitted by the cell and an adjustable-voltage generator 18 controlled by the photodetector and connected to the cell. The photodetector/generator/cell assembly constitutes a feedback loop capable of automatically keeping the transmitted light flux at a regulated value.

Description

 The present invention relates to protective goggles with adjustable transmission using liquid crystals. It finds an application in the production of sunglasses or protective glasses against intense radiation which can come from artificial sources such as electric arcs, lasers, ovens, etc.

 Variable transmission sunglasses are already known. They use photochromic lenses whose transparency decreases under ultraviolet excitation. The absorption of solar rays is therefore all the greater as the intensity of these is high since the solar radiation comprises a part of ultraviolet.

 Such a device has many drawbacks. First of all, it is based on an action of ultraviolet rays, which supposes that they are present in the radiation from which we want to protect ourselves. However, this is not always the case. Indeed, the share of ultraviolet may very well be reduced in an intense luminous flux. This is the case, for example, when the observer is behind a window which partially absorbs ultraviolet light. This is also the case for artificial monochromatic sources. Another drawback of this type of device is that the transmitted light intensity is not constant. Finally, a last drawback is that the response time of the glasses is long which prevents effective protection in the event of a sudden change in brightness.

 The inventlon overcomes these drawbacks in that it makes it possible to obtain a substantially constant transmitted light flux, regardless of the presence or absence of ultraviolet rays, the response time being very short and of the order 1/10 of a second.

These results are obtained through the use of liquid crystals instead of photochromic lenses. It is known that liquid crystals lend themselves well to the production of display cells or modulation devices. Furthermore, the idea of using such products in the production of protective glasses is already known. This is how American patent 3,873,804 issued to Mr. Gordon on 25
March 1975 and titled flwelding Helmet with Eye Piece
Control "describes an arc welder's helmet, which is provided with a liquid crystal window controlled so that it is transparent in the absence of an arc and opaque in the presence of the latter. transparency or opacity is effected by means of a voltage taken from the arc supply means. Such a device therefore operates according to the "all or nothing" mode
Furthermore, American patent 4,152,846 issued to FA Witt on May 8, 1979 and entitled Flight
Training Method and Apparatus "describes piloting glasses for airplane pilots; these glasses use liquid crystals still functioning by all or nothing, in the sense that they are transparent if the pilot looks at the on-board instruments and opaque if lepilste look outside the camera.

 Such systems are of very limited application and cannot in any case be suitable for sunglasses where the transparency must be able to be continuously adjusted.

 Liquid crystals have already been used in the production of optical walls such as panes, with adjustable transparency. Such a device is described in the document FR-A-2 373 808. It comprises a source of direct voltage and means for adjusting this source in order to ensure the desired transmission to the wall. Such a device has a double inc # disadvantage : first of all, the use of a voltage. continuous causes parasitic effects in the liquid crystal which reduce its lifespan; then the luminous flux transmitted by the window is not regulated, that is to say maintained at an appropriate value, when the conditions of use change, even if one uses, as the document foresees, a photodetector capable of measure ambient light, this photodetector does not take into account any changes in the optical wall.

 The subject of the present invention is precisely glasses which avoid these drawbacks. To this end, they are provided with means capable of ensuring that the light flux transmits a regulated intensity.

 Specifically, the subject of the invention is protective glasses comprising a liquid crystal cell with adjustable transmission and a control means for this cell, these glasses being characterized in that this control means comprises at least one photodetector capable of receive the light flux transmitted by the cell and an adjustable voltage generator controlled by the photodetector and connected to the cell, the photodetector-generator-cell assembly constituting a control loop capable of automatically maintaining the light flux transmitted to a regulated intensity.

 By "at least one cell" is meant that the device of the invention can comprise either a single cell to constitute a screen of a single piece placed between the eyes of the observer and any light source, or two cells depending on the usual configuration of sunglasses, a cell being assigned to each eye. In the latter case, a photodetector can be deposited behind only one of the two cells or behind both. If two photodetectors are used, the two # control loops can be independent or partially confused. In the latter case an average signal can be formed from the two signals delivered by the two photodetectors, and a single command generator can control the two cells from the average signal. In some cases, when the eyes of the observer present asymmetry, the transmission values of the two cells can be set to different values, which may be of interest in ophthalmological treatments.

 Although the voltage generator used to control the cell can take a large number of variants, preferably it is an alternating voltage generator (that is to say with zero mean value) having a duty cycle adjustable, the photodetector acting on this generator to vary this duty cycle. The form of the tension-can be any: sinusoidal, rectangular, etc ...

 With regard to the liquid crystals used, several types are possible: #liquid crystals involving the phenomenon of dynamic diffusion (in English terminology "Dynamic Scattering Mode" or in DSM aggregation); but also liquid crystals with molecular orientation. In this latter family, helical nematics seem to be advantageous.

 In all cases, the cell consists of a film of liquid crystals interposed between two transparent electrodes deposited on a transparent insulating substrate #, for example made of glass or plastic.

In the case of helical nematics, the cell also comprises an analyzer and a polarizer which are crossed or parallel. A cell of the latter type is described in French patent FR-A 2 117 358 entitled "Optical cell".

The characteristics of the invention will appear better after the description which follows, of exemplary embodiments given by way of explanation and in no way limitative. This description refers to appended drawings in which - Figure 1 is a block diagram of the device
of the invention; - Figure 2 shows a first embodiment
of a control loop working in modula
amplitude, - Figure 3 shows a second embodiment
tion of a servo loop working in mo
frequency modulation, - Figure 4 shows a third embodiment
tion of a control loop acting on the
duty cycle, - Figure 5 shows an example of a circuit allows
as long as a variation in the duty cycle is obtained, - - Figure 6 represents a curve of the variations in
transmission coefficient of a crystal cell
liquids as a function of duty cycle - Figure 7 shows an example of the arrangement
adopted in the invention for the different means
- Figure 8 illustrates a variant with two photodetects
teurs, - Figure 9 illustrates the corresponding control circuit
corresponding to the previous variantg - Figure 10 illustrates a variant with integrated solar cells
rigged with glasses, - Figure 11 illustrates an example where the cells pre
feel several areas.

 In Figure 1 is shown the block diagram of the regulating means used according to the invention. The device shown comprises a liquid crystal cell 10, a photodetector 12 placed behind this cell with respect to a light source 14, a differential amplifier 16, a control generator 18 whose output is connected to the cell and means 20 delivering a setpoint voltage.

 Such a system forms a control loop, in which the signal delivered by the amplifier 16 constitutes an error (or counter-reaction) signal which translates the difference between the transmitted light intensity and the desired intensity defined by the setpoint voltage. At equilibrium, this error signal is zero, the voltage delivered by the photodetector being equal to this setpoint voltage.

 The intensity of the transmitted light flux is therefore subject to a given value. This intensity is generally constant, except that if the incident flux disappears completely, the transmitted flux obviously becomes zero. Furthermore, if you use helical nematics with a parallel polar polarizer-analyzer, the absorption cannot exceed 50%, so that if the indicative light is very intense and exceeds twice the value of setpoint, the intensity transmitted cannot be lowered to this value.But these are exceptional cases which are rarely encountered in practice.If in a particular application, we wanted to protect against the last drawback, it would be enough to cross the couple polarizer-analyzer to obtain an absorption which varies between 50% at rest and 100% at maximum of excitation.

 It will be observed that the fact of placing the photodetector behind the cell (and not in front as in the case of the document FR-A 2 373 808 cited above), results in the regulation taking place whatever the cause causing the variation of the transmitted flux, whether it is the incident flux which varies, or whether the cell properties change, for example under the effect of temperature, or whether these two causes combine. In all cases, the observer is therefore guaranteed to receive a constant light intensity.

 The observer can adjust the received light intensity as desired by acting on the set value delivered by the circuit 20. This action can be done by a slider, a knurled stick, a touch etc ... glasses and arranged for example on one of the branches.

 Figures 2, 3 and 4 show diagrams illustrating three alternative embodiments of the generator 18. In Figure 2 the generator comprises a square signal generator 30, an integrator 32 and a multiplier 34. The latter is not an example of the control type by the voltage (in English terminology "voltage controlled amplifier" or VCA). I1 has two inputs, one connected to the generator 30 and the other to the integrator 32.

 The operation of this circuit is as follows. The voltage delivered by the integrator is used to modulate the amplitude of the square signal delivered by the generator 30, the shape and the frequency of this signal remaining unchanged. The direction of variation of the voltage applied to the electrodes of the cell depends on the structure used for it. With a crossed polarizer-analyzer couple, in the absence of voltage applied to the electrodes, the cell transmission is zero. It can be increased to 50% by applying a voltage capable of rotating the plane of polarization of the light of 900. With a parallel polarizer-analyzer couple, the transmission is equal to 50% in the absence of voltage and drops to 0% with a voltage rotating the polarization plane of 900. If this latter arrangement is used, the The amplitude of the signal delivered # by the multiplier 34 must therefore increase when the transmitted intensity exceeds the setpoint and vice versa. With the first provision, it is the opposite.

 The diagram in FIG. 3 represents a variant with frequency modulation. The generator 18 also includes an integrator 32 and also an oscillator 36 with adjustable frequency. This oscillator is for example of the type with voltage control ("voltage controlled oscillator" or VCO). It has an input 37 connected to the integrator 18 and an output 38 delivering a square voltage of constant amplitude but of variable frequency.

 Finally, FIG. 4 represents a variant with modulation of the duty cycle which is a particularly advantageous variant. This ratio is the quotient of the duration of presence of the voltage by the period of the signal. In this case, the generator 18 comprises, in addition to the integrator 32, a sawtooth voltage generator 40 and a comparator 42, which has two inputs, one connected to the generator 40 and the other to the integrator 32.

 The operation of this circuit is as follows. The output voltage of the amplifier 42 has the shape of a rectangular pulse whose front edge is synchronized with the vertical front of the sawtooth and whose rear front is triggered at the moment when the sawtooth reaches the error voltage delivered by the integrator 32. The width of the output pulse is therefore adjustable but the period of the signal remains fixed since it is that of the sawtooth signal.

 FIG. 5 illustrates a particular embodiment of this variant. The generator 40 comprises an operational amplifier 44 counter-reacted by a resistor 43 and by a diode 45. It has two inputs, one negative, 46, connected to a positive voltage source 47 through a resistor 48 itself connected to a capacitor 49 and the other positive, 50, connected to ground through a diode 51 and a resistor 52. The input 46 is also connected to the inverting input of the comparator 42. The positive input of the latter is connected to a potentiometer 55 inserted between two resistors 56, 57 connected to two sources of negative voltage 58 and positive 59.

 Circuit 42 compares signal A from the sawtooth oscillator with the fixed reference (or variable in the case of automatic brightness control) given by the resistor network and the potentiometer connected to the input positive.

The different components used can be as follows
diodes 45 and 51: type k4154
capacitor 49: 47 n
resistance 43: 10 kt
resistance 48: B MJQ
resistance 52: 4.7 k3t
source 47: 15 V
amplifier 44. TL 084 from Texas Instruments,
potentiometer 55: 100 k #,
resistors 56, 57: 220 kn,
source voltages 58 and 59 s 15 V
With these # components, the sawtooth voltage delivered by circuit 40 goes from -4v to + 4V.

After the comparator 42 the rectangular voltage B goes from -12V to + 12V. The signal period can be a few milliseconds.

 FIG. 6 represents the variations of the transmission T of a nematic crystal cell in a helix with parallel polarizer and analyzer as a function of the cyclic ratio RC expressed in percent.

 FIG. 7 represents glasses implementing the principles which have just been described.

As shown in these glasses, there are two cells 61, 62, a photodetector 64 and a control electronics 66. On one of the branches 68 is fixed an adjustment system 70 connected to the control electronics 66. The light flux incident 72, of variable intensity, is partially absorbed at the crossing of each of the cells, so that the emerging light flux 74 has a regulated intensity, generally constant.

 Naturally, in practice, the control electronics 66 are integrated into the frames of the glasses, either in one of the branches, or in both, or on the front part. As for the photodetector 64, it is located on the periphery of one of the cells, so as not to interfere with vision. This photodetector is generally placed at the rear of the cell but it could possibly be placed at the front of the cell, provided that a deflection mirror is placed at the back of the latter so that the photodetector receives the light having crossed the cell (in this case the light would have made two crossings).

 FIG. 8 illustrates a variant in which an additional photodetector 80 is placed on the front face of the spectacles, the photodetector 64 being always arranged on the rear face. The diagram in FIG. 9 shows the two loops associated with these two photodetectors with their amplifiers 65, 81 and their integrators 67, 82 both connected to the positive input of the comparator 42. The role of the photodetector 80 is to take into account ambient light to quickly change the control of the cell.

 FIG. 10 illustrates a variant in which solar cells 90 are integrated into the frame of the glasses. These solar cells supply the control circuits, which can be arranged in a housing 92 disposed at the upper part of the frame.

 Finally, FIG. 11 illustrates a variant in which each cell has several independent zones 101, 102, 103 defining different fields of vision. These zones are each associated with a photodetector and their transparency is controlled as indicated above. Each zone can be controlled to present its own transparency. clean: the upper zone 101, which corresponds substantially to a field of vision directed towards the sky, can be made more absorbent than the central zone 102.

In the case of use by skiers, the lower zone 103, which corresponds to the field of vision directed towards the ground, can also be made more absorbent than the middle part.

 In the case of glasses intended for drivers of motor cars or pilots of airplanes, the glasses may. present two zones, one, upper, with weak transmission and the other, lower, with strong transmission, both controlled as indicated above.

 In other applications, it may be advantageous to use circular zones, for example in the form of iris.

 In certain applications, it may be useful to guard against certain radiations, of very specific wavelength (for example in the case of protective glasses against laser radiation). In this case, the photodetector will have a sensitivity range centered on this wavelength. In other cases, this sensitivity range may cover a wider spectral band going for example from near ultraviolet (about 0.3 ssm) to near infrared (about 1 Bm). In still other cases, a particular range (for example the ultraviolet) will be preferred if, for medical or other reasons, one wishes to protect oneself only from the radiation located in this range.

Claims (9)

 1. Protective glasses comprising at least one liquid crystal cell (10) with adjustable transmission and a control means for this cell, characterized in that this means comprises at least one photodetector (12) capable of receiving the light flux transmitted by the cell and an adjustable voltage generator (18) controlled by the photodetector and connected to the cell, the photodetector-generator-cell assembly constituting a control loop capable of automatically maintaining the light flux transmitted at a regulated intensity.  2. Protective glasses according to claim 1, characterized in that the generator delivers an alternating voltage with adjustable duty cycle, the photodetector acting on this generator to vary this duty cycle.  3. Protective glasses according to any one of claims 1 and 2, characterized in that they comprise an electric means with manual control allowing the regulation of the regulated intensity of the transmitted light flux  4. Protective glasses according to claim 1, characterized in that they comprise an additional photodetector (80) disposed in front of at least one of the cells and receiving the incident light flux, this photodetector being connected to the generator.  5. Protective glasses according to any one of claims 1 to 4, characterized in that at least one of the photodetectors has a particular range of sensitivity located between the near ultraviolet and the near infrared.  6. Protective glasses according to any one of claims 1 to 5, characterized in that they comprise means for supplying electrical energy constituted by solar cells integral with the glasses.  7. Protective glasses according to any one of claims 1 to 6, characterized in that the cell has at least two zones defining different fields of vision, zones for which the intensities of the transmitted light fluxes are different.  8. Protective glasses according to claim 7, characterized in that the zones are in the form of strips.  9. Protective glasses according to claim 7, characterized in that the zones are in circular form.