CA1310721C - Light filter with automatic regulation of optical transmission - Google Patents

Abstract

Abstract The invention relates to improvement in a light filter with automatic regulation of the optical transmission. The filter comprises a filter element (1) with a liquid crystal layer located between two transparent plates (2) and assigned polarizers. The optical transmission of the light filter is varied by applying a variable electric voltage.
A first optical sensor (4) is arranged behind the filter element (1) in the radiation emitting direction, and another sensor (8) is arranged beside or in front of the filter element (1) in the radiation emitting direction.
The two sensors are connected to a regulating circuit which comprises a subtraction circuit (15) which subtracts the signal generated by the other sensor (8) from the signal of the optical sensor (4), or the signal of the optical sensor (4) from the signal of the other sensor (8), so that the output signal of the regulating circuit is at least approximately proportional to the amount of visible light. As the other sensor (8) a sensor sensitive only to infrared radiation, may be used or, if this is also sensitive to other radiation, an infrared band-pass filter may be placed in front of it.

Description

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t,IGI IT F cr,T~1~ WI'l'~ UTOMl~T:[C
Rl;~GULl~'l'ION O OP'ïICAL TR NSMISSION

l'he present invetltioll relates to a light Eilter having autolllatic regul.ation of the opticaL transmission.
~ rlle Ei:Lter has a Eilter element adjustable in relation to vary its optical translllis.sion o.E visible light. ~n optica:L sensc~r cletectY lncidellt and/or baekground light transmitted througll the Eilter and is connected to a regulatiny circuit. Tl-le circu.it produces an electric signal necessary for the regulation oE the optical transmission oE
the Ei:lter element ciependent 011 the radiation Ealling on the sensorO Ti~e sensor is arranged behind the filter element in the radiation emittilly direction.
'I'he invenkioll also contemplates a light Eilter with autolllatic regulation of the optical transmission which has two ~ilter elements adjustable in relation to th,eit optieal trasmlssiorl c~E v.isible light, and an optical sensor which detects the ineidellt and/or background light anc3 is connected to a regul,ati.llcJ ci~cuit whicl; produces the ~ ' .

-~- 1310721 electric siyna:L nee~e~l ~or the reyulation oE the optical trans,lllis~ioll oE the Eilter elemellt dependent on the radlatiorl ~al]iny on the sensor.
Sucll ligl-t Eilters are used, Eor example, as protective Eilters in tlle pcotective shields or helmets, etc., oE
wek~ers, ln order to protect the person operatiny the weldlny apparatus Erom the extremely briyht radiation occurriny during we.ldiny. On tlle other hand, other uses are also possible, Eor example, in sunglasses, window panes Eor bulLdings, auto or aircraEt windows etc.
~ slgl-ltllly wirl(low with automatic reyulation of light translllissioll is knowrl Erom German Laicl-Open Patent ~pplicatiotl No. 2,~42,998 among others. There, a so-called 13(1ukl crystal cell, that i9, a liquid crystal element is used Eor light screeniny. The amount of light behind the sight winclow is measured by a photosensor and a signal is Eecl as a regulating variable to a control circuit which adjusts the control voltage Eor the crystal ].ayer so that the amoullt oE :Light allowed to pass through corresponds to a preselected desired value.
Theoretical].y, sucll an arranyement may Eunction, but it has beerl Eound that the occurrence oE inErared radiation can sensitively disturb this regulating mechanislll. But sincej in weldiny applicatlons, increasinyly and almost a].ways, a stroncJ inErared radiation occurs, the device proposed in the above-lnentiond German disclosure has only _3_ 131~72~

].im;.tecl ll~Se Eor wel~ler's protective shields or l)elmets, since strollg sullsl)llle, but especia:l.ly tlle work oE a nearby EeLlo~ weldeL, can badly impair the reyulation, because of a strong ;.nfrared racl:iation wh:ich can unexpectedly blank the protective sl~ield or helmet.
Invis.ible ul.traviolet radiation, also present, afEects littl.e or not all an arrangelllent with a sensor placed behillcl a liquic3 crystal cell since liquid-crystal cells, as we know, let throuyll inErared radiation almost unhindered, but largely screen out the ultraviolet radiation, especia:l1y wherl yrovided, as usual, with polarizers in Eront oE and behirld thelll. Moreover, usual yhotosensors, beslcles their sensitivity in the visible range oE the spectrum, have little or 110 sensitivity to ultraviolet radlatlon, but are very sensitive to inEr:ared radiation.
The object of the present invention is to improve a liyht Eilter of the kind mentiolled so that in its regulating behavic)r, it responds mainly to visible light but hardly at all to inErared radiation, which latter is mucll less harmEul to persons.
Accordillg to the Eirst embodimellt oE the invention, this object is achievec] in a liyht Eilter oE the kincd Eirst mentiollecl by provicling another sensor in an arrangement with a sincJle llght Ei.lter element which is adjustable in its optical transllllssion. The anotller sensor i8 arranged beslde or in Eront oE the Eilter element, in the direction ,,- 131072~

c~E racli~tiorl emi~ssi.oll ~ subtraction circuit that Eorms part c~E ll-~e reyulatilly circuit is provlded. The subtraction clrcuit subtlacts the signal yenerated by the other sensor from the siyllal oE the optica]. sensor, or the siynal oE the optical sensor Erom the signal of the other sensor, ~so that the output signal oE the regulating circuit is at :least approxilllately proportional to the amount oE
visible light in ~ront of the Eilter element.
In an arrangemellt witll two light Eilter elernents, switclled 011 in series and adjustable in their optical traslnission, the object oE ~he invention is achieved accordinc3 to c].aim 2, by arranging an opti~cal sensor functiona:Lly behilld the Eirst Eilter element in the directioll of radiation ernis.sion and by providing another sensor whicll is arranged beside or in Eront oE the Eirst Eilter elerment in the radiation emission direction, and by providi:ng a subtractlon circuit in the regu:lating cicuit which subtracts the signal generated by the other sensor ~
Erorn the sigral of the optical sensor, or the signal oL the opti.cal sensor Erom the signal oE the other sensor, so that the output signal oE the regulatiny circuit is at least approximately proportional to the amount of visible lig-ht in Erollt oE the filter elernent.
In each case, there~ore, it is assured that, throuyl the suhtraction, the i~nEluence oE inErared radiation is elimirlated and that the regulatiny circuit Eunctioniny .

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-~5~ ~310721 deperldq only on the ~moullt oE visible :llght ~s it has alrea(~ly been melltiolle(l, ultraviolet radiation has on]y a sliyht inEluerlce, ancl can be Eurther reduced, i~ desired, by a~rang;llg arl ultravlolet blocking filter in Eront of one or botll oE the sensors.
Witll a view to sirnpLicity oE constructiorl, it may be aclvalltayeous iE the optical sensor and the other sensor are identical. This provides ~ Eurther advantacJe oE their sensitivity beiny the same. This recluces expenses ~or the reyulatiny circuit and improves regulation. Witll this arrangelllellt a neyative or positive control signal is obtained according to the kind oE circuit. The signal correspollds to the cli~Eerence between the light actually present and the Eiltered visible light, that isl to the amoullt oE visible ligllt behind the light filter element without the in~rared component.
~ nother solution may consist in that the sensor arranged behilld the liyt-t Eilter element is sensitive to inErared racliatioll and visible l;ght, but the other sensor is sensitive only to inErared radiation. ~ter subtraction, accordincJ to the circuit, a positive or negative signal is obtalned, wilich is directly proportional to the visible liyht component without inErared radiation. The disadvalltage here is that two diEEerent sensors, possibly with diEEerent sensitivity, or sensors with difEerent characteristics, must be usecl. This can be compensated, however, by arrang;.llg an adj-lstable arnpliEier after one or boLI~ sensors. Irhe aclvantage o~ the Eirst mentioned reyul.ating circuit ls that it is simp]er.
Other embodilnel-ts ancl further Eeatures of the subject invelltioll are deE:ined in the dependent claims.
Tlle invelltioll wi:Ll be explained in detail below, with reEerence to the attached drawings, which ~show schematically several embocl:ilnents:
F;.y. 1 shows a Eirst embodirnent with a single filter elelnellt;
l~ly. 2 shows a modiEied Eorm oE the Eirst embodiment;
Fig. 3 shows a second embodiment witll two Eilter elemellts conllected one aEter the other? and E'iy. 4 shows a modiEied ~orm oE the second embodiment.
~ ccordlng to liig. 1, the l.lght Eilter according to the lnvelltion includes, Eor example, a liquid crystal cell l;
that ls, a llquid crysta:L element consisting, in principle, oE two parallel glass.palles 2 with a liquid crystal layer 3 between theln. 'rhe ylass panes may themselves act as polarizers, or separate polarizers may be provided Eor the llqui.d crystal elements. Such elements are known and are characterized by the Eact that such a filter element, by applyiny a variable alternating voltage to the liquid crysta.l layer 3, can be changed Erom a substantially transparellt cond.itioll to a substantially opaque condition.
Behind the liquid crystal cell 1 is arranged a photosel-sor 4 which pick5 Up the radlation let through by t:he ce:l:L l, and gerlera~e3 an vutput siyndL proportional to the receive~ intells;ty of the radlation. ~ difEerential alllpl.i~i.er 5, WhiCIl will be discussed later, transmits the output signa] o~ the sensor ~1 to a reyulatiny circuit 6, wh.icl) processes the signa.l. and, in turn, yenerates an output control signal directed thro~lgll a line 7 to the ].iquid crystal cell 1. '.I'he arranyement is such that the cel.L :l is blanked when the sensor 4 receives much light, ancl vice versa; such a device is known and need not be explained here.
'rhe arrangement showl) in ~ig. 1 includes also anotller sensor ~, which in the pres.ent case is arranged beside the cell 1. It might equally well be arranged in Eront of the cell 1 since it is on.l.y essential that it always receives the total amount oE existiny radiation without being inE.luenced by the cell :l. The sensor a generates an output signa:l dependellt on the received radiation and which is transmitted to an impedance transEormer 9 the outlet of ~which is connected to the second inlet of the differential ampliEler 5. Ultraviolet b].oclcing Eiltees miyht be instalJ.ed in front oE the sensor 4 and/or the sensoe ~ to filter out any disturbilly ultraviolet radiation.
'rhe circuit Eunctions as Eollows:
lladiation, Erom a we].dinc] spot, for example, and thus contain.ing a (relative].y strony) infrared componellt, a vi.s.i.ble liyht componellt and, possibly, an ultraviolet .

component, also is emitted in the direction Pl as shown by arrows. 'rhe cell l ;s in an open condition, and the sensor 4 rece;ves almost a:ll the radiation. In any ease, it should be noted tllat a l;quid crystal cell, ev~n in an open concli~ion, already acts as an ultraviolet blocking filter, so that, practically, at Eirs~, the total amount of inrared racliatlon and visible light is received by the ~sensor 4.
The reyulating circuit 6 is deslyned in the known way, 50 that it de:livers at once an output signal to the cel] 1, whictl acts witllill mil.Lisecollds, so that the eell 1 is ;~
blanked; that is, its radiation perrnèability is redueed. A
control circuit, which may be provided, permits to ~set the regulatillg circuit 6 to a certain preseleeted blanked value.
'[he distinction oE sueh arl arrangernent, however, is that, even with the occurrence oE inErared radiation alone, OL with a strong compollellt oE radiation oE such wavelengtlls, ~;
a response oE the regulatiny circuit and, thus, blanking oE
visible light takes ylace. I~owever, in many cases, this is hiyhly undesirable; Eor example, in ease oE a strong sullligllt or witll an inErared radiation caused by a neiyhborillg workplace.
Provi~ lg a remedy here is the objeet oE the present illVellt iOIl. 'l'his takes place, in the Eirst discu~ssed embodimer-t and shown schematically in Fig. 1, in the followillg way:
Le~ us assume that the two sensoes nalllely sensors 4 and 8, are identieal; that is, equally senslti.ve to in~rared -~- 1310721 radiati.~ and to v.i.sib.le light. The sensor 4 receives all, or pl:act.ically al,,l. i.nCrared radiation and a component of vi~sible ;ligllt when the :L.;quid crystal cell is blanked. The sensor ~, on the other halld, receives al]. the in~rared racl;.ation as we:L1 as a:ll the radiation in the visible range oE tl1e spectrum, since, accordiny to the invention, it is p].aced in Eront o[ or beslde the liquid crystal cell 1. The ultraviolet radiation compolleni: i9 practically negligibleO

L,et:
IR = IR radiation component Sv = component oE visible radiation behilld the liquid crystal cell Sk = component oE visible radiation in Eront oE the liquid crystal cell Ao1 = ~rop:ortiollality Eactor oE the output siynal oE the subtraction circuit.
Therl, accordiny to whetller the sensor 4 is connected to the nollillvertilly inlet, and the sensor 1' to the inverting inlet oE the diEEerential ampl.LEier 5, or conversely:

(Sv ~ IR) - ~Sk -~ IR) = Sv ~ Sk = -A 1~ or ~(Sv ~ IR) -~ (Sk -~IR) = =Sv ~ Sk = ~A 1~

In both ca~ses, thereEore, a Eactor is obtained, indepell(1ellt oE the inErared component, whicll is used to control. the i.iqu.i.d crystal cell 1 and whicll is a].so inEluellced by the visible light component behind the cell 1, so that it can be used as a regulating variable.

..., ~ ~.

-lO- 1310721 rlle arranye1nent discus~sed above has the advantage that two ide11tical sensors (4 and B) can be used, oE w11ic1l the characteristics may also be equal or very simLlar; however, tl-e costs o~ evaLuatiny the output signal, Aol or -~ol~
i9 somew11at greater.
In order to avoid these costs, lt i5 posslble to use two dirEere11t sensors ~ and 8. ~I'he sensor ~, aEter the cel] l, i9 sensitive to ;.nErared radiation and to visible light; t11e sen.sor ~, beside or in front oE the cell l, on the other hand, is sensitive ~y to inErared radiation.
Ther1, based 01l the above deEinitions, dependent, again, on tlOW the two sensors are connected to the difEerential ampliEiers:
(Sv ~~ IR = Sv ~o2' or ~(Sv ~ IR) ~ l) = Sv ~02' 11ere again, the output signals ~02 or -~02~ are Eree oE any inErared componerlt and also directly proprotional to the amour1t oE visible light received behind the cell l. Thus, they may be processed simply and directly, in the reyulatiny circuit.
With this arranyement, two difEerent sensors might be used as sensors 4 and 8, or, as in Fig. 2, identical sensors 14 and l8 miy11t be used, with an inErared band-pass Eilter beiny p:Laced in Eront oE the sensor 18. The rest oE the ~., ~, .

3~0721 arrangelllellt, witll a liquicl crystal cel]. 11, consi.stiny oE
two pa~allel gla~ss plate~s 12, a l:iquid crystal layer 13 Lying betweell them, possible separate polari%ers (not sllowll)~ a ~ilolosellsor 1~ arranyed behirld a diEEerentia:l amp:LiE;er 15, a regulatlllcJ circuit lG, connected througll a l.ine 1-1 with the cell l:l, and an impedance transEormer/
amp:LiEier :I9, i.s, ilt pTinciple~ exactly the same. Ilowevee, it may be recommellde(l here that the .switchillg circuit :L9 be a~justable in its ampliEication to be able to compensate any losses oE sensltivity caused by tlle filter 20.
In Fiy. 3 is shown a second embodiment oE the lnventioll. Unlike the desigM in Fiys. 1 and 2, two liquid crystal cells 2]a and 21b, connected one after the other, are usecl here, in order to obtain a stronyer dampiny oE
light whicll ls sometimes desirable. For the rest, these structures are the same, witll a liquid crystal layer 23a or 23b, betweell two parallel glass plates 22a or 22br in each case, WlliCIl themselves act as polarizers, or with separate ~o.l.arizers (not shown) located in Eront oE and behind the cells. The yllotosellsor 24 is arranged aEter tlle Eirst cell 21a b-lt in Eront oE the cell 21b, in the radiation emitting ~lirectioll Pl, so tllat a suEEicient residual amount of light Ealls on the sensor, Wtl ich is necessary for actuation oE
the regulatiny circuit 26.
Corresporldillgly, anotller sensor 28 is arranged beside or in Eront oE tlle Eirst cell 21a, which, as it was already i -.l2-melltlolle(3, receives eitl-ler the who:le radiation spectrum or onl.y the inrrared colllpollerlt, alld conduct6 its output signal throllcJIl a posslbly adjus~ab]e impedance transformer-amp]iEier 29 to a diEEerential ampliEier 25. The output oE
the reglllatiny circuit 26 i.5 conducted through a line 27 to the two cell.s 21a and 2:Lb, conllected one aEter ~he other, wlli:le rneans (not shown) may possibly be provided Eor the opposite-pllase ac~uation oE the ce.l.ls.
~ 5 to the spectruln sensitivity oE the cells 24 and 2B, or the Eilters (not shown), arrancJed in Eront oE them in all cases, what has beer- said above applies in principle.
t~, Eor physical reasons or Erom commercial consicleratiorls, it is impossible to pIace the sensor 24 between the cells 2Ja and 21b, a solution according to Fig.
4 may be considered. Eleee, two liquid crystal cells 31a and 31b, arranyed more c].osely to each other and arranged one behilld tl-e other in ~he licJht-emittillg direction Pl, are providecl. ~ach ce].:l CollSiSts oE two parallel glass plates 32a arld 32b ancl a liquid crystal layer 33a or 33b between them. As to polarizers, what has been saicd beEore app].Les.
The pllotosensor 34 can be mounted behind the Eirst cell 31a, brlt can be arranyed, pllysically, aEter the second cell, the second cell 31b is provided with an opening 40, whicll always remains openso that no changes in optical transmission behavior occur even whell tlle cell 31b is ' blanked. This is advantayeous whenever the two liquid crysl.al cells 31a ar-l 3]1 unlilce the schematic reLrese~ atlon in tlle drawillg are very narrow or must be placecZ very close to each other. The small opening 4~ at tl~e etlcJe oE the ce:Ll 31b hardly di.sturbs the behavior and eEEect oE the tota] arranc~ement or not at all. For the re~st the str~lcture oE the arrangement with another sensor 3~ the impelallce trallsEormer-ampliEier 39 the diEferential ampliEier 35 and the reyulating circuit 36 connected to tlle cel.:l.s 3:La and 31b througll the line 37 are the same as described beEore.
Natural].yr in the arrangelllellt accordinc3 to Fig. 3 and Fi.g. 4 respectively the sensors 24 and 34 on the one hand and 2~ and 3~ on tlle ottler hand are oE the same type possibly wlth band-pass Eilter (not shown) beEore the sellsor 2~ or 38. ~lternat:ively dieEerent types may be uscl; that ls sensor 24 or 34 may be sensitive to inrared an(l to vlsible liglt and sensor 28 or 3~3 on the other hanc3 may be sensltive only to inErared component. What has been sai3 beEore as to the adjus-table ampllEication Oe the impedallce transEormer-arrlpliEier 25 or 35 also applies ill pr.illcip] e.
J.ina.lly it should be melltioned that polari~ation f.ilters must be located in Eront oE and behind the llquid crystal cell.s Eor eYample as Icnown in the state oE the art Çor example Eroln U.S. Patent 4 039 354.

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~310721 ", lt :is importarlt accorcling to tl)e invention that the inEluence oL inErared radiatioll on the regulating circuit be elimll-latecl, and tllat an .inErared blockiny Eilter be e:lectrica:LLy si~ lated. ln thls way, an eEfective, rel.iabl.e regulation oE the li.ght permeability of tlle light Eilter, whetller it consists oE one cell or oE two cells located one behil-ld the other, is always assured, even when strony inErared radiatio1l prevails in the backyround, since tile reyulatiny circuit now responds only to visible light.
rhe u]traviolet racliati.on, also present, harclly interferes, since most photosellsors, as already mentioned, hardly respollcl to this, and since a liquid crystal cell is in itselE already a good ultraviolet blocking filter.

Claims (15)

1. Light filter means with automatic regulation of optical transmission comprising:
a liquid crystal filter element which is adjustable to vary the optical transmission of visible light therethrough;
a first optical sensor for detecting incident and back-ground light, said first optical sensor producing an electric signal for regulating the optical transmission of said liquid crystal filter element in accordance with the radiation received by said first sensor, said first optical sensor being located behind said liquid crystal filter element in the direction of radiation emission;
a second optical sensor being arranged beside or in front of said liquid crystal filter element in the radiation emitting direction;
a regulating circuit for controlling the optical trans-mission of visible light through said liquid crystal filter element including a subtraction circuit which subtracts the signal generated by one of said first and second optical sensors from the signal of the other of said first and second optical sensors, the output signal of said regulating circuit being at least approximately proportional to the amount of visible light in front of said filter element.

2. Light filter means according to claim 1 wherein said first optical sensor and said second optical sensor have substantially the same sensitivity characteristics.
- Page one of claims -

3. Light filter means according to claim 2 wherein said first optical sensor and said second optical sensor are sensitive to infrared radiation and to visible light.

4. Light filter means according to claim 2 wherein at least one of said first and second optical sensors is connected through an amplifier to said subtraction circuit of said regulating circuit.

5. Light filter means according to claim 2 wherein at least one of said first and second optical sensors is connected through an impedance transformer to said subtraction circuit of said regulating circuit.

6. Light filter means according to claim 2 further including a neutral density filter for visible light arranged in front of at least one of said first and second optical sensors.

7. Light filter means according to claim 2 wherein said subtraction circuit includes a differential amplifier, said first optical sensor being connected to the noninverting input thereof and said second optical sensor being connected to the inverting input thereof so that a negative signal is fed to said regulating circuit, the negative signal being proportional to the visible light component and being independent of the infrared radiation.

8. Light filter means according to claim 2 wherein said subtraction circuit includes a differential amplifier, said first optical sensor being connected to the inverting input thereof and said second optical sensor being connected to the noninverting input thereof so that a positive signal is fed to said regulating circuit, - Page two of claims -the positive signal being proportional to the visible light component and being independent of the infrared radiation.

9. Light filter means according to claim 1 wherein said first optical sensor is sensitive to infrared radiation and visible light and said second optical sensor is sensitive only to infrared radiation.

10. Light filter means according to claim 9 wherein said subtraction circuit includes a differential amplifier, said first optical sensor being connected to the noninverting input thereof and said second optical sensor being connected to the inverting input thereof so that a negative signal is fed to the regulating circuit, the negative signal being proportional to the visible radiation component passing through said filter element and being independent of the infrared radiation.

11. Light filter means according to claim 9 wherein said subtraction circuit includes a differential amplifier, said first optical sensor being connected to the inverting input thereof and said second optical sensor being connected to the noninverting input thereof so that a positive signal is fed t the regulating circuit, the positive signal being proportional to the visible radiation component passing through said filter element and being independent of the infrared radiation.

12. Light filter means according to claim 1 wherein said first optical sensor and said second optical sensor have the same sensitivity characteristics, and further including an optical band-pass filter arranged in front of said second optical sensor.
- Page three of claims -

13. Light filter means according to claim 1 further including an ultraviolet blocking filter arranged in front of at least one of said first and second optical sensors.

14. Light filter means with automatic regulation of optical transmission comprising:
first and second liquid crystal filter elements which are adjustable to vary the optical transmission of visible light therethrough;
a first optical sensor for detecting incident and back-ground light, said first optical sensor producing an electrical signal for regulating the optical transmis-sion of said liquid crystal filter elements in accordance with the radiation received by said first optical sensor, said first optical sensor being arranged behind said first filter element in the radiation emitting direction;
a second optical sensor being arranged beside or in front of said first filter element in the radiation emitting direction;
a regulating circuit for controlling the optical trans-mission of visible light through said liquid crystal filter elements including a subtraction circuit which subtracts the signal generated by one of said first and second optical sensors from the signal of the other of said first and second optical sensors, the output signal of said regulating circuit being at least approximately proportional to the amount of visible light in front of said filter element.
- Page four of claims -

15. Light filter means according to claim 14 wherein said second filter element has a partial area in which the optical transmission does not vary, said first optical sensor being arranged in the radiation emitting direction behind said second filter element and the partial area of said second filter element.