GB2043944A

GB2043944A - Optical Magnifying Instruments Incorporating Optical Attenuators

Info

Publication number: GB2043944A
Application number: GB8003976A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1979-02-08
Filing date: 1980-02-06
Publication date: 1980-10-08
Also published as: GB2043944B

Abstract

A fast-acting switchable diffusing screen 12 is used to provide protective optical attenuation in an optical magnifying instrument, e.g. a telescopic gun sight or astronomical telescope (shown), in response to high intensity flashes. The screen 12 is positioned across the field of view of part of the optical system of the telescope which has an angle of acceptance smaller than the image field angle of the instrument. Typically the screen 12 will be positioned across the objective lens of the instrument for maximum efficiency, and moderately efficient switched diffuse scattering screens can be used to provide a considerable change in the overall instrument attenuation. In a preferred embodiment, the switched diffusing screen 12 comprises a plate PLZT ceramics material operated in its longitudinal scattering mode by application of switching potentials in a direction perpendicular to its major surfaces. Using this material, switching speeds of less than 50 microseconds can be achieved. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Optical Magnifying Instruments Incorporating Optical Attenuators This invention relates to optical magnifying instruments such as telescopes and sighting instruments, and in particular is concerned with protecting the users of such instruments against the harmful effects of high intensity incident infra red, visible or ultraviolet radiation.

Harmful eye damage effects such as retinal burn as well as temporary "flash-blindness" or afterimaging dazzle effects can result from direct viewing of a high intensity source of thermal or optical radiation. These effects are increased when the same source is viewed through an optical magnifying instrument such as a telescope, due largely to the retinal image magnification provided by the instrument.

It is an object of the present invention to provide means whereby the user of an optical magnifying instrument can be protected against the harmful effects produced by the sudden occurrence of high intensity incident radiation in the instrument's field of view.

According to the present invention, an optical magnifying instrument incorporates a fast-acting shutter device comprising an attenuating screen switchable in response to incident radiation levels above a predetermined intensity from a substantially specularly transmitting state to an optically diffuse scattering state, and positioned across the field of view of a part of the optical system of the instrument the angle of acceptance or object field angle of which, as determined by the limiting aperture of the instrument, is smaller than the image field angle of the instrument.

Because light incident on a diffusive transmitting screen is scattered into a wide scattering angle, for a given incident intensity the smaller the object field angle, or angle of acceptance, of an optical system (or part of an optical- system) through which scattered light transmitted by the screen is viewed, the lower will be the intensity of light passing through the optical system. Assuming incident light is uniformly scattered, the overall attenuation of the instrument achieved by switching of the screen from its specularly transmitting state of its optically diffuse scattering state will be increased by a factor of approximately R2, where R is the ratio of the scattering angle of the screen to the object field angle of the part of the optical system through which the screen is viewed.

Advantageously therefore, the attenuating screen may be placed across the field of view of that part of the optical system having the smallest angle of acceptance or object field angle, e.g. across the field of view of the objective lens in the case of a telescopic viewing instrument.

The degree of attenuation achieved may be further increased by positioning the screen as far as practicable away from the part of the instrument's optical system through which it is viewed, thereby reducing the width of the scattering angle from which light transmitted through the screen is accepted by the instrument.

In order to be effective, the shuttering device must be capable of responding within 1 millisecond, and preferably within 50 microseconds, to an increase in the intensity of incident radiation above the predetermined value.

In a preferred embodiment, the shuttering device comprises an opto-electronic attenuating screen comprising a plate of PLZT (lead-lanthanum-zirconate-titanate) ceramics material, and a switching circuit responsive to an increase in the intensity of incident radiation above said predetermined level, to switch the plate from a specularly transmitting state to an optically diffuse scattering state.

If a suitable electric field is applied to a plate of, for example 9/65/35 (La/Zr/Ti) coarse-grained PLZT ferroelectric ceramics material is applied in a direction normal to its major surfaces, incident light is multiply scattered on transmission, and the intensity of scattered light is dependent upon the ferroelectric remanent polarisation state. In its zero-remanence state, the material is specularly transmitting, but the degree of induced polarisation and hence the degree of scattering, increases (positive or negative) with increasing electric field strength field up to saturation remanence. The degree of scattering for a given remanent polarisation increases also with increasing grain size, and a useful optical scattering effect can only be induced in PLZTferroelectrics ceramics materials having a grain size greater than 2-3 ym.Switching from maximum to minimum transmission, i.e. from zero to saturation remanence can be accomplished extremely rapidly, within tens of microseconds, on application of electric fields up to 7 KV/mm, the switching speed depending on the applied electric field strength.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic diagram of a telescopic viewing instrument incorporating an optoelectronic shuttering device in accordance with the invention; Figure 2 is block circuit diagram of a switching circuit for the opto-electronic switching device; and Figure 3 is a graphical representation of a voltage waveform appearing at point A in the circuit of Figure 2.

Referring to Figure 1 of the drawings, the viewing instrument is in the form of an astronomical telescope, comprising a converging objective 1, having a focal length fO, and a converging eye piece 2 having a focal length fe The aperture stop of the instrument is constituted by the objective 1, which thus also provides the entrance pupil. The second focal plane of the objective 1 and the first focal plane of the eyepiece 2 coincide in the plane of a field stop diaphragm 6, producing an exit pupil 8 on the observer's side of the eyepiece.

Figure 1 shows rays coming from an object at infinity entering the telescope at an angle d and emerging from the telescope and entering an observer's eye, the entrance pupil of which coincides with the exit pupil of the telescope, at an angle fl. The magnification M of the telescope is given by 0/.

This magnifying power of the telescope, in addition to increasing the energy intensity concentration at the exit pupil by a factor of approximately M2T (where T is the percentage optical transmissivity of the instrument) relative to that at the entrance pupil, also projects an M-times larger image of the object on the observer's retina than would be produced by the naked eye without the telescope. Although the intensity per unit area of this magnified retinal image is smaller by a factor T, the transmissivity of the instrument, the ability of the retina to dissipate the incident energy decreases as the area of the retinal image increases.The increased size of the magnified retinal image thus increases the risk of retinal burn and flash blindness, while the increased energy concentration at the exit pupil (where the cornea of the observer's eye will usually be positioned) increases the risk of corneal damage due to high intensity incident radiation.

In accordance with the present invention, the viewing instrument includes a fast-acting shuttering device comprising an opto-electronic screen 12 exhibiting an electrically controlled scattering effect placed across the entrance pupil of the telescope on the object side thereof, and a switching circuit 1 3 responsive to incident optical intensities above a threshold level to rapidly switch the screen from a clear specularly transmitting state to a diffuse optically scattering state.

In the present example, the screen 12 comprises a plate 14 of coarse-grained (grain size greater than 2 from, optimum 7-10 ,um) rhombohedral-phase PLZT (lead-lanthanum-zirconate-titanate) ceramics material carrying on its major surfaces transparent electrodes 1 5, 1 6 of tin oxide-doped indium oxide (In203) formed by sputtering. The PLZT plate may be formed by the CP-OX-HP (chemically prepared oxygen-atmosphere hot-pressed) ceramics process, although improved results can be achieved using the MO-OX-HP (mixed-oxide oxygen-atmosphere hot-pressed) ceramics process.

In its thermally depoled zero-remanence condition, i.e. zero electric field strength, the PLZT plate 14 is specularly transmitting. However, in the presence of an electric field normal to its major surfaces (longitudinal), a remanent polarisation state is induced in the material causing it to become optically diffuse, incident light being multiply scattered on transmission, producing in this "anomalous" scattering mode, a substantial attenuation of the incident beam. The degree of scattering, and the rate at which the plate 14 can be switched from its unpolarised specularly transmitting state to its polarised optically diffuse state, depends on the magnitude of the applied electric field.

Figure 2 shows a suitable form of switching circuit 13 capable of switching the opto-electronic screen 12 to its optically diffuse scattering state within 50 ztS of receiving a high intensity light pulse. In the present example, the thickness and diameter of the PLZT plate 14 are 0.8 mm and 50 mm respectively, requiring a voltage of approximately 750 volts across the electrodes 15, 1 6 to produce a longitudinal electric field strength of 1000 volts/mm. This provides an open state transmission of approximately 70% (at 560 mm) in its unpolarised state, and an effective optical density change of approximately 1.0 to 1.5 upon switching to the saturated remanent polarisation state, using 9/65/35(La/Zr/Ti) PLZT having a grain size of 7 to 10 ,um and prepared by the MO-OX-HP process.

Factors which affect the switching speed are the diameter and thickness of the PLZT plate, and the resistance of the electrodes, the higher these values the slower the switching speed.

The switching circuit 1 3 comprises a photo-diode 20 connected in series with a resistor R1 between a 10 volt DC supply line and earth, and arranged to produce a triggering pulse along input line 21 to a monostable circuit 22 when illuminated with incident light above a predetermined intensity. On receipt of the triggering pulse, the monostable circuit 22 changes state, producing a 5-second output pulse. The output of the monostable circuit controls the switching of two transistor switches T1, T2; the first T1, via a drive circuit 23, and the second T2, via an opto-isolator circuit 26 and a drive circuit 25.

The transitor switch T1 is connected in series with the opto-electronic screen 12 between the output of an EHT (750 Voit) power supply 24 and earth, and a capacitor C is also connected between the output of the power supply and earth, in parallel with the screen 1 2 and transistor switch T1.

The opto-isolator circuit 26 comprises a resistor R2 and a light-emitting diode (LED) 27 connected in series between the output of the monostable circuit 22 and earth, and optically coupled to a photo-transistor 28 connected in series with a resistor R3 across the output terminals 29, 30 of an isolated power supply 31 which supplies the drive circuit 25 for switch T2. The isolated power supply 31 and the opto-isolator circuit 26 are provided to protect the relatively low voltage drive circuit 25 associated with switch T2 against interference from the EHT sections of the circuit.

The switch T2 is connected in parallel with a diode D across the opto-electronic screen 12, the circuit also including a coil represented by inductor L and resistor R4 connected between points A and B, which in combination with the capacitance of the opto-electronic screen 1 2 form a series LCR "ringing" circuit.

The operation of the switching circuit 1 3 will now be described with reference to Figure 3 which shows the voltage waveform appearing at point A in the circuit. In its quiescent state, the monostable circuit 22 is off, holding switch T1 off and switch T2 on, and maintaining point A at EHT potential (0 volts across the opto electronic screen 12). On receipt at time t1, of a triggering pulse from the photodetector diode 20 along line 21, the monostable circuit changes state for 5 seconds, switching T1 on via its drive circuit 23. At the same time LED 27 is illuminated, switching photo-transistor 28 on and short-circuiting the input to drive circuit 25, thus causing T2 to switch off.

During the initial 30,us after T1 has switched on, t1 to t2 in Figure 3, T1 provides a zg amp charging current to the capacitance (40 nf) of the screen 12 building up the potential across the electrodes 1 5, 16 to the full EHT potential of 750 volts and causing the screen to switch to its saturation remanence scattering state.

At the end of the 5 second period (t3), the monostable circuit 22 switches off, switching T1 off and T2 back on. The diode D and switch T2, when closed, effectively provide a short circuit allowing the LCR ringing circuit, consisting of the coil L, resistor R4 and the capacitance of the screen 12, to produce a damped voltage oscillation (t3 to t6) across the electrodes 1 5, 1 6. The diode D and switch T2 alternatively conduct as the oscillating voltage swings between positive and negative values with respect to the EHT potential. Thus during the period t3 to t5 switch T2 is conducting, and during period t5 to t6 the diode D is conducting.

The purpose of the "ringing" circuit is to hasten the depolarisation of the opto-electronic screen 12 to its (zero remanence) specularly transmitting state by providing a reverse-polarity de-poling field across its terminals as the voltage at point A overswings to voltage V2 at time t4. In the absence of such a reverse-polarity depoling field, the screen would take a considerably longer time to reach its zero-remanence condition being dependent on the loop characteristic of the ferroelectric hystersis behaviour. Ideally the value of the overswing voltage V2 should be just below the minimum polarising voltage of the screen 12, in this case about 350 volts, so that substantially all the remanent polarisation originally induced by the excursion of point A to zero voltage is cancelled, and no opposite polarisation remanence is induced.

The component values, or manufacturing specification of the components of a typical circuit are as follows.

Resistors Value (ohms) .

R1 1 K Phototransister 28 y Optoisolator R2 470 LED 27 ) 6N135 R3 1 K Diode D T36E4T20 or R4 0-1 OK IN4007 Photodiode 20, ZPA27 (Ferranti) Capacitors Value

T1) BU209A C O.4F T2 Although the shuttering device is capable of switching the screen from its specularly transmitting state to its optically diffuse scattering state within 50 microseconds, sufficiently fast to prevent any harmful eye damage effects in most instances, it is capable on its own of providing only a limited attenuation.

However, when the screen is placed on the object side of the objective lens of a magnifying optical instrument, as in the present example, it will when switched to its optically diffuse scattering state, increase the overall instrument attenuation by a factor of approximately R2, where R is the ratio of the scattering angle of the screen to the angle of acceptance # of the instrument. Thus for a Lambertian scattering screen having a 180 scattering angle (corresponding to a solid scattering angle of 2 radians) placed in front of a telescope having an angle of acceptance 0 of 6 , the overall attenuation of the instrument will be increased by a factor of 900.In practice however, the actual scattering angle of a PLZT shutter will be substantially smaller than 1 SOC (typically 20 30 ), but nevertheless attenuation factors of the order of 30 can readily be achieved, corresponding to an attenuation of 15 dB or an optical density change of 1.5.

By comparison, placing the same screen on the observer's side of the eyepiece will increase the overall instrument attenuation by a factor approximately equal to the square of the ratio of the scattering angle of the screen to the angle of acceptance of the eye, which angle is approximately equal to the image angle of the instrument. Thus, the attenuation factor achieved by positioning the screen on the object side of the telescope rather than on the observer's side is greater by a factor of approximately M2, due to the relatively small angle of acceptance 0 of the instrument compared with its image field angle 0.

In some applications an infra-red absorbing/reflecting filter may be placed on the object side of the screen to protect it against thermal overloading.

The screen 12 may alternatively be placed between the objective 1 and the eyepiece 2, preferably spaced away from the first principal focus of the eyepiece so that the screen is not in focus when observed through the eyepiece. Although the change in the overall instrument attenuation obtained on switching the diffusing screen 12 when so positioned is smaller, being related to the square of the magnification of the eyepiece alone, screens of substantially smaller area be used. In optical systems where the aperture stop lies at an intermediate point in the optical system, the screen may advantageously be positioned across the aperture stop if smaller physical screen size is an advantage.

The embodiment of the invention described above employs a PLZT diffusing screen 12 exhibiting an electrically controlled scattering mode in which the scattering condition is induced by application of a polarising potential across the electrodes 1 5, 1 6. A converse scattering mode has been observed in PLZT ceramics material (see W. D. Smith Er C. E. Land, Applied Physics Letters, Vol. 20 No. 4, 15 February 1972) in which the material exhibits maximum transmittance in its two extremes of positive and negative remanence, passing through a minimum value, i.e. maximum scattering, in its electrically depoled or zero remanence state. This effect was observed using 7/65/35 (La/Zr/Ti) material, while 9/65/35 (La/Zr/Ti) material was used in the present example.The invention may be implemented using screens exhibiting either of these two scattering modes, although screens exhibiting the electrically induced scattering effect are preferred because they do not rely on the presence of a polarising voltage to maintain the screen in its specularly transmitting state. The instrument may still be used even if the switching circuit fails.

PLZT compositions in which between 7 and 7.9 atom percent of the lead in substituted by lanthanum are particularly suitable for the purposes of the present invention. Preferably, also the aggregate mole ratio of lead-lanthanum zirconate to lead-lanthanum titanate is in the range 65:35 to 70:30, and one suitable example is Pb0,9La0,07 (Zr0,70Ti0,30)03, or PLZT 7/70/30.

Although the use of PLZT shuttering screens is preferred, owing to their inherently fast switching speeds and good thermal hardness, it will be appreciated that other forms of fast-acting switched diffusing screen, such as liquid crystal devices may be used within the scope of the present invention.

Claims

1. An optical magnifying instrument incorporating a fast acting shutter device comprising an attenuating screen switchable in response to incident radiation levels above a predetermined intensity from a substantially speculariy transmitting state to an optically diffuse scattering state, and positioned across the field of view of a part of the optical system of the instrument the angle of acceptance or object field angle of which, as determined by the limiting aperture of the instrument, is smaller than the image field angle of the instrument.

2. An optical instrument as claimed in Claim 1, wherein the attenuating screen is placed across the field of view of that part of the optical system having the smallest angle of acceptance or object field angle.

3. An optical instrument as claimed in Claim 2, wherein the optical instrument is a teiescopic instrument and the attenuating screen is placed across the field of view of the objective lens.

4. An optical instrument as claimed in Claim 1, 2 or 3, wherein the shuttering device comprises an opto-electronic attenuating screen and a switching circuit responsive, in use, to an increase in the intensity of incident radiation directed into said optical instrument above a predetermined level, to switch the attenuating screen from a specularly transmitting state to an optically diffuse scattering state.

5. An optical instrument as claimed in Claim 4, wherein the opto-electronic attenuating screen comprises a plate of PLZT (lead-lanthanum-zirconate-titanate) ceramics material formed with transparent electrodes on its major surfaces, and the switching circuit is arranged to apply a suitable switching voltage across said electrodes to switch the screen to its optically diffuse scattering state in response to incident radiation intensities above said predetermined level.

6. An optical instrument as claimed in Claim 5, wherein the transparent electrodes are of tin oxide doped indium oxide material.

7. An optical instrument as claimed in Claim 5 or Claim 6, wherein the grain size of the PLZT material is greater than 2 microns.

8. An optical instrument as claimed in Claim 7, wherein the grain size of the PLZT material is between 7 and 10 microns.

9. An optical instrument as claimed in Claim 5, 6, 7 or 8, wherein the PLZT material is prepared by a mixed-oxide-oxygen-atmosphere-hot-pressed (MO-OX-HP) ceramics process.

10. An optical instrument as claimed in any one of Claims 5 to 9, wherein the substituent lanthanum for lead content of the PLZT material lies between 7.0 and 7.9 atom per cent.

11. An optical instrument as claimed in any of Claims 5 to 10 wherein the aggregate mole ratio of lead-lanthanum-zirconate to lead-lanthanum-titanate is in the range 65:35 to 70:30.

1 2. An optical instrument as claimed in any one of Claims 5 to 11 wherein the composition of the PLZT material is Pb0,93La0,07 (Zr0,70Ti0,30)03, i.e. PLZT7/70/30.

1 3. An optical instrument as claimed in any one of Claims 4 to 12, wherein the switching circuit is arranged to maintain the attenuating screen in its diffuse scattering state for a predetermined period of time in response to incident radiation intensities above said predetermined level, and to automatically switch the screen back to its specularly transmitting state at the end of said period.

14. An optical instrument as claimed in Claim 13, wherein the attenuating screen comprises a plate of PLZT ceramics material and the switching circuit is arranged in use switch the PLZT plate from a specularly transmitting state to an optically diffuse scattering state by changing the potential applied across transparent electrodes formed on opposite major surfaces of the plate from a first potential to a second potential, and to automatically switch the screen back to its specularly transmitting state after said predetermined time by discharging the capacitive energy stored in the plate through an LCR resonant circuit of which the capacitive component comprises the capacitance of the screen, whereby the potential across the two electrodes of the screen returns to said first potential after a period of damped oscillation during which the potential across the electrodes swings positive and negative with respect to said first potential.

1 5. An optical instrument as claimed in Claim 14, wherein the maximum amplitude of the damped oscillations with respect to said second potential is just below the minimum potential required to switch the screen to a diffuse scattering state.

1 6. An optical instrument incorporating a fast acting optical attenuating device substantially shown in, and as hereinbefore described with reference to the accompanying drawings.