Patent Publication Number: US-7896056-B2

Title: Manually operated venetian blind

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of Venetian blinds. More particularly, the invention relates to a manually operated Venetian blind that can be lowered/raised and tilted effortlessly and speedily by means of a novel, reliable, and cost-effective operating mechanism. 
     BACKGROUND OF THE INVENTION 
     Shading devices have been traditionally used to reduce the area of glazing which is exposed to an influx of solar radiation. 
     Operable shading devices mounted exterior to windows are designed to control the incoming solar radiation. They may have a complex mechanism which requires maintenance or replacement, or may have an expensive automatic control system. 
     Other types of shading devices, such as roller shades, curtains or Venetian blinds, are installed within the interior of a building and are adapted to reflect incoming solar radiation back through the window before it can be absorbed and converted to heat. Recently, internal blinds, i.e. Venetian blinds that are arranged in a spaced parallel relationship and are placed between glass sheets in a double glazing unit or between the frames of a double window, have been employed. 
     Prior art Venetian blinds, must remain in an upright position, such that a longitudinal axis coinciding with each slat is essentially perpendicular to horizontally disposed flooring, during transportation, installation and while in use so that the slats and cords will not be in a state of disarray and be rendered unusable. If the longitudinal axis of the blind arrangement were not in an essentially vertical disposition, some of the blinds would touch the glazing, causing the cords which support the blinds to become entangled. 
     Venetian blinds are generally provided with at least two ladder braids, the number of ladder braids depending on the length of the slats. Each ladder braid is composed of two outer cords for tilting the slats, an inner cord for raising and lowering the slats, and a plurality of cross ladders. Each of the cross ladders is connected between the two outer cords and supports a corresponding horizontally disposed slat. The outer cords of prior art Venetian blinds, which provide a tilting motion, are directed to the tilting mechanism by means of a spacer placed on the outside of the uppermost slat. As a result, the weight of the slats is concentrated disproportionally more on the spacer and the uppermost slat than on the other slats, causing the slats to change their relative position, or even to fail, over the course of time. The tilting mechanism is usually a gear train that adds to the cost of the blind unit. Also, due to the angle of the cords the blinds cannot completely close and invariably only 75% of the incoming light is blocked. 
     Concerning the raising and lowering of prior art internal Venetian blinds, the actuation is generally by means of a magnet external to the glazing which linearly translates another magnet imbedded within the raising mechanism. The degree of raising or lowering of the blind is completely dependent on the displacement of the external magnet, and may take up to 2 minutes to raise the blind from a lowered position to a raised position. Due to the configuration of the cord and spacers, the area of the blind is limited to 2 sq. meters. A considerable force must be applied in order to raise the blind. 
     U.S. Pat. No. 3,702,040 discloses a Venetian blind structure that is adapted to be mounted within the air space of a double glazed sealed window unit. A plurality of elongated slats are pivotally mounted at their ends in a support frame, such that pins which protrude from a wheel secured to the pivot shaft of each slat abut transverse edges of a reciprocable actuating member driven by a gear train and therefore limit the angular position of the slats. The gear train is adapted to tilt the slats, but the Venetian blind structure is not provided with a means to raise or to lower the slats. Also the addition of the gear train adds to the cost of producing and assembling the blind structure. 
     U.S. Pat. No. 6,059,006 is another actuation device for adjusting the inclination of Venetian blinds arranged inside a double glazing unit. A first magnet is slidingly movable inside the sealed space of the double glazing unit, in response to the movement of a second magnet, which is located inside a box-like body connected outside the double glazing unit and driven by a cord stretching in a loop between two pulleys and connected to an actuation rod. The first magnet is fixed to a bush which is internally shaped complementarily to the helical profile of a shaft, so that translatory motion of the first magnet results in rotation of the shaft and of vertical adjustment chords to thereby synchronously adjust the inclination of the blinds about a longitudinal axis thereof. The complexity and cost of the apparatus is increased, due to the transmission system, as well as the actuator that is external to the double glazing unit. 
     U.S. Pat. No. 6,095,223 discloses an actuation unit that includes a magnetic kinematic coupling device which is connected to an electric motor and to a kinematic system for moving an internal Venetian blind. The apparatus of the actuation unit adds to the cost of the Venetian blind. 
     Other manually operated Venetian blinds mounted in a double glazing unit are disclosed in EP 0 245 811, EP 1 087 095, EP 0 902 155, U.S. Pat. Nos. 4,685,502, 4,768,576, US 2003/0089462, and US 2004/0211528. 
     It is an object of the present invention to provide a Venetian blind assembly which is incorporated in a permanently sealed double glazed window unit. 
     It is an object of the present invention to provide an internal Venetian blind assembly that provides for the tilting, raising and lowering of the slats. 
     It is an additional object of the present invention to provide an internal Venetian blind assembly in which means for tilting, raising and lowering of the slats are disposed internally to the double glazed window unit. 
     It is an additional object of the present invention to provide an internal Venetian blind assembly for which means for tilting, raising and lowering of the slats are manually actuated. 
     It is an additional object of the present invention to provide an internal Venetian blind assembly for which the tilting, raising and/or lowering of the slats can be reliably completed within approximately 2 seconds. 
     It is an additional object of the present invention to provide an internal Venetian blind assembly for which means for tilting, raising and lowering of the slats are relatively frictionless and can be easily assembled. 
     It is an additional object of the present invention to provide an internal Venetian blind assembly in which the means for tilting, raising and lowering of the slats do not incorporate a gear train, a set of pulleys or couplings. 
     It is yet an additional object of the present invention to provide an internal Venetian blind assembly that can be inverted without resulting in cord entanglement and can be operable immediately thereafter. 
     It is yet an additional object of the present invention to provide an internal Venetian blind assembly by which the slats may be tilted, raised and lowered while the blind assembly is disposed at an inclination of approximately 45 degrees with respect to the floor. 
     It is yet an additional object of the present invention to provide an internal Venetian blind assembly that blocks approximately 90% of the incoming light. 
     It is yet an additional object of the present invention to provide an internal Venetian blind assembly that overcomes the disadvantages of the prior art. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a manually operated internal Venetian blind which comprises an arrangement for raising/lowering a plurality of slats. The arrangement comprises:
     a) A frame comprising a transversally extending headrail and lower support member, each of which having at least one planar support surface, and two longitudinally extending side members positioned at each transversal end of said blind, said frame being interposed between two glass sheets;   b) A plurality of transversally extending, equally sized slats suspended from said headrail by means of at least two ladder braids, each ladder braid comprising two longitudinally extending outer cords, each of which being disposed laterally outward from a different lateral edge of the slats, and a plurality of laterally extending cross ladders, each cross ladder being connected to the two outer cords and supporting a corresponding slat, wherein two outwardly positioned ladder braids further comprise a longitudinally extending central cord passing through each slat;   c) A transversally oriented slat support disposed underneath said plurality of slats to which each of said ladder braids is affixed;   d) A bearing housing for housing a ball bearing, wherein said bearing housing is secured to the upper longitudinal end of a first side member;   e) A bifurcated bearing block longitudinally displaceable within said first side member which houses a ball bearing;   f) A counterweight longitudinally displaceable within said first side member and integrally formed by a central portion and two outward portions, each of said outward portions being disposed laterally outwardly from said central portion, at least one internal magnet being housed in an outward portion, said bearing block and said counterweight being interconnected;   g) a stabilizer affixed proximate to the upper longitudinal end of said first side member; and   h) a first linearly displaceable actuator disposed outwardly to one of said glass sheets and provided with at least one external magnet in opposed relation to, and at a fixed distance from, said internal magnet which is housed in said counterweight, said external and internal magnets being magnetically coupled,   wherein each central cord is transversally oriented within said headrail, is wound around the ball bearing housed in said bearing housing, is further wound around the ball bearing housed in said bearing block, and the ends of each central cord are affixed to said stabilizer, said stabilizer thereby applying a reactive force to each of the central cords during displacement of said central cords,   said counterweight and said bearing block being longitudinally displaceable upon linear displacement of said first actuator, the displacement of said first actuator being significantly less than the distance to which the longitudinally lowest slat is raised or lowered due to the winding of said central cords around the ball bearing of said bearing block.   

     The following are definitions for various terms that are referred to herein:
     “internal Venetian blind”—a unit comprising a plurality of slats, frame, and operating mechanism, all interposed between two sheets of glass;   “external Venetian blind”—a unit comprising a plurality of slats, frame, and operating mechanism, which is not interposed between two sheets of glass;   “longitudinal”—in the direction between the headrail and lower rail or lower support member, generally denoting the direction to which the plurality of slats are raised and lowered;   “transversal”—in the direction to which the slats extend;   “lateral”—in the direction perpendicular to the longitudinal and lateral directions, in the direction between the two glass sheets of an internal Venetian blind;   “extending”—concerning an element having one side longer than the other side, in the direction of the longer side;   “oriented”—the disposition of a wall or face of an element which coincides with a longitudinally, transversally, or laterally positioned plane; for example, a wall may be longitudinally oriented while being transversally extending;   “inner” and “inwards”—in a direction towards the cavity between the two glass sheets;   “outer” and “outwards”—in a direction away from the cavity between the two glass sheets;   “upper,” “upwards,” and “above”—in a longitudinal direction closer to the headrail;   “lower,” “downwards,” and “below”—in a longitudinal direction away from the headrail;   “front”—in a lateral direction towards a user that manipulates the actuator; and   “rear”—in a lateral direction away from a user that manipulates the actuator.   

     Preferably, the bearing housing has a laterally oriented base and two parallel, flexible rings which longitudinally protrude from said base, a ball bearing and a pair of plain bearings for supporting and positioning said ball bearing being housed in said bearing housing. 
     Preferably, the bifurcated bearing block comprises two symmetrical rectilinear sections for housing a ball bearing therebetween, each of said sections being formed of a laterally thicker portion and a laterally narrower portion, said thicker portion being above said narrower portion, each of said thicker portions being formed with a circular recess to receive therein a corresponding bearing flange and an annular portion outwardly protruding from the back of said recess in which a corresponding bearing axle is seated, wherein the narrower portions are mated such that each upper portion is spaced one from the other with only a bearing rim located between said bearing flanges being visible, a hole being bored through said mated narrower portions. 
     Preferably, a hole is bored through a portion of the central portion of the counterweight longitudinally protruding from said outward portions, said bearing block and counterweight are interconnected by means of a rope tied through the hole bored through said lower portions of said bearing block and through the hole bored through said central portion of the counterweight; 
     By winding the central cords about the ball bearings housed in the bearing housing and bearing block, respectively, and then affixing the ends of each central cord to the stabilizer, the central cords are subjected to a frictional force significantly less than that of the prior art. While prior art blinds require a full 100% stroke length to achieve a complete raising/lowering of the slats, the displacement of the actuator of the present invention is advantageously only 40% of the distance to which the longitudinally lowest slat is raised or lowered. Consequently, the slats of a blind having a surface area of up to 5 square meters can be lowered or raised effortlessly and speedily. 
     In one aspect, the outer cords and central cord of each ladder braid is disposed at essentially 90 degrees with respect to the support surface between the headrail and lower rail. 
     One embodiment of the present invention provides a manually operated internal Venetian blind which comprises an arrangement for tilting a plurality of slats. The tilting arrangement comprises:
     a) Frame components comprising a transversally extending headrail and lower support member, each of which having at least one planar transversally oriented support surface, and two longitudinally extending side members positioned at each transversal end of said blind, a frame assembled from said frame components being interposed between two glass sheets;   b) A plurality of transversally extending, equally sized slats suspended from said headrail by means of at least two ladder braids, each ladder braid comprising two longitudinally extending outer cords, each of which being disposed laterally outward from a different lateral edge of the slats, and a plurality of laterally extending cross ladders, each cross ladder being connected to the two outer cords and supporting a corresponding slat, wherein two outwardly positioned ladder braids optionally further comprise a longitudinally extending central cord passing through each slat;   c) Device for tilting said plurality of slats to which two outer cords of a corresponding ladder braid are affixed, each of said tilting devices comprising two laterally oriented sides, a rectangular face perpendicular to said laterally oriented sides, and an annular protrusion transversally extending outwardly from each laterally oriented side, wherein said tilting device is rotatable about a transversal axis coincident with the axis of said annular protrusions, said annular protrusions being rotatably supported by a suitable assembly secured to a support surface of said headrail or said lower support member which is internal to said glass sheets;   d) Means for rotationally driving each of said tilting devices which is internal to said glass sheets; and   e) A linearly displaceable actuator in communication with said driving means,   wherein an outer cord is partially wound around, and tensioned by, a corresponding tilting device during rotation thereof following displacement of said actuator, said outer cord being subsequently longitudinally and laterally displaced, causing uniform tilting of each of said slats.   

     Several prior art internal Venetian blinds comprise a tilting mechanism disposed in the headrail by which outer cords of a ladder braid are wound around a cylindrical element, which is generally driven by gears, or any other type of transmission in communication with an actuator external to the Venetian blind. Since this cylindrical element is housed within the headrail, the circumference of the cylindrical element is laterally spaced from the lateral periphery of the slats across which the outer cords extend. Spacers placed on the transversal periphery of the uppermost slat are therefore needed to direct the outer cords to the cylindrical element. 
     In contrast, the tilting device of the present invention is configured such that two outer cords are affixed to a corresponding tilting device by two affixation means, respectively, that retain the cords in an essentially vertical disposition from the headrail to the lower support member. The weight of the slats is therefore more evenly distributed than in the prior art, allowing the slats of a blind having a higher surface area than has been known heretofore to be tilted with greater reliability and without use of a gear train. 
     When the slats are in a completely open position, the outer cords have equal tension. Upon displacement of the actuator, a first outer cord is partially wound around, and tensioned by, a tensing portion of the tilting device during rotation thereof to a first angular distance, while a second outer cord is slackened. The first outer cord is then drawn upwards and inwards, causing the slats to change their inclination with respect to a horizontal plane. Following additional displacement of the actuator, the first outer cord is further drawn upwards and inwards, causing an additional change in the inclination of the slats, and the second outer cord is drawn downwards. When the slats are in a closed position, the spacing between the two outer cords is at a minimum. During this stage, the tension of the first outer cord is maximum, and this maximum tension further assists in closing the slats by pressing on the top of each slat, thereby increasing the inclination thereof. 
     The blind may also be provided with an arrangement for raising/lowering the plurality of slats. 
     In one embodiment, the two outer cords are affixed to a corresponding tilting device by two affixation means, respectively, positioned along the transversal centerline of the rectangular face thereof in such a way that an outer cord contacts one of said two opposed faces when said rectangular face is parallel to said at least one support surface. Each affixation means comprises an aperture bored through the rectangular face and a corresponding nail having radial protrusions, said nail being received in said aperture. The radial protrusions are adapted to press an outer cord onto the wall of the aperture and to thereby retain the pressed portions of the outer cord in a fixed position relative to the rectangular face of the tilting device. 
     In one embodiment, the tilting device comprises:
     a) a core member provided with—   i. two laterally oriented walls;   ii. a convex periphery disposed between, and integral with, said laterally oriented walls in such a way that the axis of said periphery is transversally extending;   iii. a toothed key protruding from, and having a similar curvature as, said periphery;   iv. at least one coaxial protrusion extending transversally from each of said laterally oriented walls, wherein one of said coaxial protrusions extending from a corresponding laterally oriented wall is adapted to receive a D-shaped shaft for driving the tilting device and one of said coaxial protrusions is rotatably supported by a suitably sized opening formed in a cord guide assembly secured to a headrail or lower rail of the blind;   v. two separated coplanar elements defining a rectangular face which truncate said laterally oriented walls and said periphery such that the diameter of said at least one coaxial protrusion is greater than the maximum radial dimension between said periphery and said coplanar elements;   vi. two transversally extending grooves of bilateral symmetry having a rectangular cross section and obliquely oriented with respect to said coplanar elements, each of said grooves being formed at a different lateral side of said at least one coaxial protrusion;   vii. a portion for tensing an outer cord formed between one of said grooves and the corresponding planar element; and   viii. a circumferential, rectangularly shaped recess formed between said toothed key and a corresponding laterally oriented wall;   b) two oppositely oriented wing members which are engageable with said core member, each of said wing members being provided with—   i. a convex shell having a curvature substantially equal to that of the periphery of said core member and being positioned such that the axis thereof is transversally extending;   ii. a first transversal end of said shell formed with two openings so as to define a central shell portion and two peripheral portions and with a lip circumferentially extending along the inner wall of said shell, each of said peripheral portions being considerably thicker than said central portion and being configured to be received within a corresponding groove of said core member;   iii. a second transversal end proximate to said toothed key having a laterally oriented circumferential wall extending between said two peripheral portions along the inner wall of said shell; and   iv. a plurality of radially extending teeth formed in said laterally oriented circumferential wall, each of said teeth being configured with a sufficient length and spacing therebetween so as to abut complementary recessed sharp edge junctions of said toothed key;   wherein said teeth of each wing member retain a corresponding outer cord of a ladder braid of the blind which is placed on the periphery of said core member in pressed relation with said toothed key,   wherein the lip of each of said wing member is adapted to engage a corresponding recess of said core member with a snapping action to prevent detachment of said corresponding outer cord from the tilting device,   wherein an outer cord is partially wound around, and tensioned by, a corresponding tilting device during rotation thereof following displacement of a linearly displaceable actuator, said outer cord being subsequently longitudinally and laterally displaced, causing uniform tilting of a plurality of transversally extending slats each of which is supported by at least two ladder braids.   

     The central and outer cords are received in a cord guide assembly in immobilized engagement with the headrail or lower support member. A partitioned cord guide for separating each cord is secured to, or integrally formed with, a transversally oriented face of the cord guide assembly. The cord guide abuts the wall of an equally sized aperture formed in the support surface of the corresponding headrail or lower support member, thereby preventing movement in the transversal and lateral directions. The cord guide is substantially coplanar with the corresponding support surface. 
     The present invention is also directed to a linearly displaceable actuator for use in an internal Venetian blind, comprising:
     a) An actuator guide mounted by a press fit between a glass sheet of the blind and a frame element of the wall opening in which the blind is placed, a linear track having an arcuate cross section being formed within said actuator guide;   b) An external magnet housing having a body with an inwardly facing cavity for the insertion therein of an external magnet and an L-shaped appendage protruding from said body, said L-shaped appendage being adapted to be slidingly displaceable within said track; and   c) An internal magnet housing positioned within a frame component of said internal Venetian blind, said internal magnet housing being formed with an outwardly facing cavity for the insertion therein of an internal magnet in opposed relation to, and at a fixed distance from, said external magnet, an elongated element parallel to the back of said outwardly facing cavity, a bridge connecting between said elongated element and the back of said outwardly facing cavity, and coplanar abutment plates extending from each of the elongated element and cavity back to such a length that a gap is formed between said abutment plates,   wherein a drive means for tilting or raising/lowering a plurality of slats is in communication with one of said abutment plates, said drive means operable upon displacement of said body.   

     In one aspect, the external magnet housing is suitable for adjusting the applied magnetic force by changing the number of magnets housed within the magnet retaining compartment. 
     The present invention is also directed to a driving assembly for a manually operated Venetian blind, comprising:
     a) a rotatable cylindrical casing positioned within a headrail or side member of a blind frame, a hole being bored through the periphery of said casing;   b) means for receiving a transversally oriented shaft by which a tilting device of said blind is rotated;   c) an actuator cord fed through said hole bored through the periphery of said driving assembly in such a way that two portions of said actuator cord dangle in disengageable fashion from a diametrically opposite periphery of said casing; and   d) an actuator in communication with said two portions of said actuator cord, wherein said casing and receiving means are an integral non-detachable unit, wherein the tensile force of at least one of said two portions of said actuator cord upon displacement of said actuator is sufficiently high to cause rotation of said driving assembly, whereby to drive said tilting device.   

     In one aspect, the casing is rotatably supported within the walls of a circular aperture formed within a first longitudinal end of a laterally oriented wall of a side member. The actuator cord is partially wound about a bearing element rotatingly supported by a bearing housing secured to a second longitudinal end of said side member and is fed through a linearly displaceable actuator. The ends of the actuator cord are tied to each other, the driving assembly and the shaft received therein being rotatable in unison upon displacement of said linearly displaceable actuator. 
     In one aspect, the receiving means is a D-shaped core to which a plurality of ribs radially extend from the inner face of the casing, said core coinciding substantially with the axis of the casing. 
     The present invention is also directed to an apparatus for the lateral centering of central cords of a manually operated Venetian blind, comprising:
     a) A frame comprising a transversally extending headrail and lower support member, each of which having at least one transversally oriented planar support surface, and two longitudinally oriented side members positioned at each transversal end of said blind, said frame being interposed between two glass sheets;   b) A plurality of transversally extending, equally sized slats suspended from said headrail by means of at least two ladder braids, each ladder braid comprising two longitudinally extending outer cords, each of which being disposed laterally outward from a different lateral edge of the slats, and a plurality of laterally extending cross ladders, each cross ladder being connected to the two outer cords and supporting a corresponding slat, wherein two outwardly positioned ladder braids further comprise a longitudinally extending central cord passing through each slat; and   c) A cord guide assembly secured to a headrail or to a lower support member, said cord guide assembly being provided with transversally and laterally oriented walls, a rectangular opening being formed in each lateral wall of said cord guide assembly, at essentially the lateral centerline thereof,   said cord guide assembly being further formed with internal laterally oriented walls extending from a corresponding transversally oriented wall which are suitably configured so as to allow each axle of a bearing element to be rotatingly seated between a pair of said internal laterally oriented walls wherein the axis of said axles is laterally oriented,   said cord guide assembly being further formed with a pair of parallel, transversally extending partitions, the spacing between said partitions being substantially equal to the lateral dimension of said rectangular opening, such that a central cord is transversally fed through said cord guide assembly via said rectangular openings and said partitions,   wherein said partitions are adapted to limit the lateral movement of a central cord fed through said cord guide assembly and in contact with said bearing element when said blind is tilted,   wherein a longitudinal wall of said rectangular opening is adapted to contact a central cord fed therethrough when the blind is completely inverted and to further urge said central cord to the lateral centerline of said cord guide assembly.   

     The present invention is also directed to a manually operated external Venetian blind, comprising:
     a) A frame comprising a transversally extending headrail having a planar transversally oriented support surface;   b) A plurality of transversally extending, equally sized slats suspended from said headrail by means of at least two ladder braids, each ladder braid comprising two longitudinally extending outer cords being disposed laterally outwardly from a different lateral edge of the slats and a plurality of laterally extending cross ladders, each cross ladder being connected to the two outer cords and supporting a corresponding slat, wherein two outwardly positioned ladder braids further comprise a longitudinally extending central cord passing through each slat;   c) A cord guide assembly in immobilized engagement with said headrail comprising a transversally oriented and inwardly positioned face, two longitudinally oriented sides, and two laterally oriented sides, each of said laterally oriented sides being formed with an arcuate opening outwardly separated from said transversally oriented face;   d) A device for tilting said plurality of slats to which two outer cords of a corresponding ladder braid are affixed, each of said tilting devices comprising two laterally oriented sides, a rectangular face perpendicular to said laterally oriented sides, and an annular protrusion transversally extending outwardly from each laterally oriented side, wherein said tilting device is rotatable about a transversal axis coincident with the axis of said annular protrusions, said annular protrusions being rotatably supported by a corresponding annular opening of said cord guide assembly;   e) A transversally extending D-shaped shaft received in an annular protrusion of each of said tilting devices, two adjacent tilting devices being interconnected by means of one of said shafts;   f) a driving assembly having a cylindrical casing in which a hole is bored through the periphery thereof, and means for receiving one of said shafts;   g) an actuator cord fed through said hole bored through the periphery of said driving assembly in such a way that two ends of said actuator cord dangle in disengageable fashion from a diametrically opposite periphery of said driving assembly; and   h) a cord lock for securing each central cord and thereby retaining the plurality of slats in a raised or lowered position,   wherein one end of said actuator cord is displaced in one longitudinal direction following displacement of the other actuator cord end in the opposite longitudinal direction, causing said driving assembly and each of said tilting devices to rotate in the same rotational direction,   wherein an outer cord is partially wound around, and tensioned by, a corresponding tilting device during rotation thereof, said outer cord being subsequently longitudinally and laterally displaced, causing uniform tilting of each of said slats.   

     The present invention is also directed to a method for darkening a room, comprising:
     a) Providing a manually operated Venetian blind, comprising:   i. providing at least one frame component comprising at least a headrail, and optionally a cover for each of said frame components;   ii. a plurality of transversally oriented, equally sized slats suspended from said headrail by means of at least two ladder braids, wherein each ladder braid comprises two longitudinally extending outer cords, each of which being disposed laterally outward from a different lateral edge of the slats, and a plurality of laterally extending cross ladders, each cross ladder being connected to the two outer cords and supporting a corresponding slat, wherein two outwardly positioned ladder braids optionally further comprise a longitudinally extending central cord passing through each slat;   iii. one or more devices for tilting said plurality of slats, two outer cords of each ladder braid being affixed to a corresponding tilting device, each of said tilting device positioned within one of said frame components and comprising a core member comprising a convex periphery, a toothed key protruding from said periphery, two separated coplanar elements defining a rectangular face which truncate said periphery, two transversally extending grooves obliquely oriented with respect to said coplanar elements, a portion for tensing an outer cord formed between one of said grooves and the corresponding planar element, and a circumferential, rectangularly shaped recess formed between said toothed key and a corresponding laterally oriented wall of said core member; and two oppositely oriented wing members which are engageable with said core member, each of said wing members being provided with a convex shell having a curvature substantially equal to that of the periphery of said core member and being positioned such that the axis thereof is transversally extending, a first transversal end of said shell formed with two openings so as to define a central shell portion and two peripheral portions and with a lip circumferentially extending along the inner wall of said shell, each of said peripheral portions being considerably thicker than said central portion and being configured to be received within a corresponding groove of said core member, a second transversal end proximate to said toothed key having a laterally oriented circumferential wall extending between said two peripheral portions along the inner wall of said shell, and a plurality of radially extending teeth formed in said laterally oriented circumferential wall, each of said teeth being configured with a sufficient length and spacing therebetween so as to abut complementary recessed sharp edge junctions of said toothed key, wherein said teeth of each wing member retain a corresponding outer cord of a ladder braid of the blind which is placed on the periphery of said core member in pressed relation with said toothed key and the lip of each of said wing member is adapted to engage a corresponding recess of said core member with a snapping action to prevent detachment of said corresponding outer cord from the tilting device; and   iv. an actuator for causing rotation of each of said tilting devices;   a) Displacing said actuator a first distance, thereby causing each tilting device to rotate to a first angular distance;   b) Allowing a first outer cord to be partially wound around, and tensioned by, the tensing portion of a corresponding tilting device during rotation thereof to said first angular distance, while a second outer cord is slackened;   c) Allowing said first outer cord to be longitudinally and laterally displaced by the pressing of the tensing portion of a corresponding side of said corresponding tilting device onto said first outer cord during rotation of said corresponding tilting device, causing uniform tilting of each of said slats;   d) Displacing said actuator a second distance, causing each tilting device to rotate to a second angular distance, the inclination of the slats to further increase to a maximum value and the spacing between said first and second outer cords to be a minimum value, significantly less the spacing between said first and second outer cords when the slats are at a completely open position; and   e) Releasing said actuator when said maximum value of the slat inclination results in the darkening of a room to a desired degree, wherein said actuator is displaced said first and second distances by a continuous hand motion.   

     The first outer cord presses the plurality of slats and thereby assists in further increasing the inclination of the slats. Due to the increased inclination of the slats, the spacing between the first and second outer cords changes from 17 mm when the slats are in a completely open position to 3 mm when the slats are in a completely closed position. 
     By the method of the invention, the slats may block up to 93% of the incoming light, and up to 95% of the incoming light when a lower support member of the frame has two longitudinally oriented mounting surfaces. 
     The present invention is also directed to a method for assembling a Venetian blind, comprising:
     a) providing at least one frame component comprising at least a headrail and optionally a cover for each of said frame components;   b) providing a plurality of tilting devices comprising two laterally oriented sides, a rectangular face perpendicular to said laterally oriented sides, and an annular protrusion transversally extending outwardly from each laterally oriented side, wherein each of said tilting devices is rotatable about a transversal axis coincident with the axis of said annular protrusions, the number of employed tilting devices depending on the transversal dimension of said headrail;   c) providing a plurality of ladder braids, wherein each ladder braid comprises two longitudinally extending outer cords and a plurality of laterally extending cross ladders, each of said outer cords being disposed laterally outwardly from a different lateral edge of a plurality of transversally extending slats and each cross ladder being connected to the two outer cords and adapted to support a corresponding slat;   d) forming apertures in an inner transversally extending planar surface of said at least one frame component;   e) securing a plurality of cord guide assemblies to a frame component while placing each corresponding cord guide of said plurality of cord guide assemblies in each of said apertures such that a cord guide is substantially coplanar with the corresponding planar surface;   f) mounting the two annular protrusions of each of said tilting devices on corresponding arcuate walls of each of said cord guide assemblies such that each of said protrusions is rotatable within a corresponding arcuate wall;   g) interconnecting adjacent tilting devices by means of a D-shaped shaft insertable within a corresponding annular protrusion;   h) introducing the two outer cords of a ladder braid through a corresponding outer section of the cord guide such that each outer cord is essentially longitudinally disposed;   i) affixing the two outer cords of a ladder braid to a corresponding tilting device;   j) inserting a drive means adapted to tilt the plurality of slats in one of said frame components and connecting one of said tilting devices by means of a D-shaped shaft to said drive means;   k) connecting an actuator to said drive means; and   l) optionally, interlocking a cover with each corresponding frame component by a snapping motion.   

     In one aspect, the step of securing a cord guide assembly to a frame component comprises the steps of:
     a) providing a substantially rectilinear cord guide assembly having a transversally oriented and inwardly positioned face from which the cord guide longitudinally protrudes, two longitudinally oriented sides, two laterally oriented sides, and four legs, each of said legs abutting a corresponding corner of said cord guide assembly and the longitudinally outward end of each of said legs terminating with a wedge-shaped portion, wherein each of said laterally oriented sides is formed with an arcuate opening on which an annular protrusion of a tilting device is to be mounted and with a substantially rectangular opening which is formed laterally outwardly from the corresponding arcuate opening, thereby affording the corresponding laterally oriented side with increased flexibility;   b) providing a frame component having at least one planar transversally oriented support surface, two longitudinally oriented and transversally extending walls perpendicular to said support surface, and mutually parallel, laterally oriented and transversally extending legs which perpendicularly protrude from the inner side of each of said transversally extending walls such that a first leg has a longer lateral dimension than a second leg, said first leg terminating with a longitudinally oriented abutment surface;   c) flexing a laterally oriented side of said cord guide assembly by bringing two opposed legs corresponding to said laterally oriented side towards each other;   d) inserting each of said wedge-shaped portions of said two opposed cord assembly legs between a corresponding longitudinally oriented abutment surface and wall of said frame component, so as to be compressed and snapped in secured, undetachable relationship with respect to a corresponding set of first leg, wall and abutment surface of said frame component; and   e) repeating steps c) and d) for the other laterally oriented side of said cord guide assembly.   

     The method for assembling an internal Venetian blind further comprises the steps of:
         a) providing two glass sheets of specified dimensions;   b) providing frame components of specified dimensions, said frame components consisting of a headrail, lower support member, and two side members, and a cover for each of said frame components;   c) positioning a first internal magnet housing formed within an outwardly facing cavity in which an internal magnet is placed and two spaced coplanar abutment plates such that one of said abutment plates is in communication with the drive means;   d) interlocking a cover with each corresponding frame component by a snapping motion;   e) connecting each of side members to said headrail and said lower support member to produce a blind frame;   f) bonding said two glass sheets to the front and rear of said blind frame;   g) inserting said blind frame in a suitably sized frame of a wall opening;   h) mounting a first actuator guide by a press fit between a glass sheet and a frame element of said wall opening in such a way so as to correspond to an intended displacement path of said internal magnet; and   i) mating a first external magnet housing containing at least one external magnet with said first actuator guide such that said at least one external magnet is in opposed relation to, and at a fixed distance from, said internal magnet of said first internal magnet housing and that said first external magnet housing is slidingly displaceable within said first actuator guide, whereby said plurality of slats are uniformly tiltable upon linear displacement of said first external magnet housing.       

     In one aspect, the method further comprises the steps of:
     a) additionally providing two outwardly positioned ladder braids with a longitudinally extending central cord passing through each slat;   b) introducing each central cord of a ladder braid through a corresponding inner section of the cord guide such that each central cord is essentially longitudinally disposed;   c) affixing a stabilizer proximate to the upper longitudinal end of a first side member;   d) securing a bearing housing which houses a ball bearing to the upper longitudinal end of said first side member;   e) interconnecting a bifurcated bearing block which houses a ball bearing with a counterweight which houses at least one inner magnet in an outward portion thereof;   f) introducing said bearing block and said counterweight within said first side member such that said bearing block and said counterweight are longitudinally displaceable within said first side member;   g) transversally orienting the two central cords through the headrail;   h) winding the two central cords around the ball bearing housed in said bearing housing;   i) winding the two central cords around the ball bearing housed in said bearing block and then affixing the ends of each central cord to said stabilizer;   j) mounting a second actuator guide by a press fit between said glass sheet and said frame element of said wall opening in such a way so as to correspond to an intended displacement path of said at least one internal magnet housed within said counterweight; and   k) mating a second external magnet housing containing at least one external magnet with said second actuator guide such that said at least one external magnet is in opposed relation to, and at a fixed distance from, said at least one internal magnet of said counterweight and that said second external magnet housing is slidingly displaceable within said second actuator guide, whereby said bearing block and said counterweight are longitudinally displaceable upon linear displacement of said second external magnet housing.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIGS. 1A and 1B  are front views of manually operated blinds in accordance with the present invention, having one and two linearly displaceable actuators, respectively; 
         FIG. 2  is a perspective view of the lower rail of an internal Venetian blind, showing the drive means according to one embodiment of the invention; 
         FIG. 3  is a perspective view of a cord guide assembly according to one embodiment of the invention; 
         FIG. 4  is a perspective view of one embodiment of a tilting device; 
         FIG. 5  is a transversal cross-sectional view of the blind, corresponding to the embodiment of  FIG. 2 ; 
         FIG. 6  is an exploded, perspective view of the drive means, corresponding to the embodiment of  FIG. 2 ; 
         FIG. 7A  is a longitudinal cross-sectional view of the tilting device of  FIG. 4 , showing the means by which an outer cord is affixed thereto, and  FIG. 7B  is an enlarged view of  FIG. 7A ; 
         FIGS. 8A-C  are perspective views of the tilting device of  FIG. 4  in three angular positions, respectively; 
         FIG. 9  is an exploded, perspective view of a linearly displaceable actuator, with an enlarged view of an actuator guide shown in Detail A; 
         FIG. 10  is a perspective view of means for tensioning a central cord, according to one embodiment; 
         FIGS. 11A and 11C  are perspective views of an assembling method for one embodiment of a blind frame, and  FIG. 11B  is a side view of a frame corner; 
         FIG. 12  is a perspective view of a lower rail and rail cover; 
         FIGS. 13A and 13B  are perspective views of two longitudinal ends, respectively, of a side member; 
         FIGS. 14A-C  are side views of the tilting device of  FIG. 4  and a corresponding ladder braid, showing a change in inclination of the slats during three angular positions of the tilting device, respectively; 
         FIG. 15  is a perspective view of a side member and headrail, showing a driving assembly; 
         FIG. 16  is a perspective view of a bearing housing; 
         FIGS. 17A-D  are perspective views of the steps by which one embodiment of a driving assembly is produced; 
         FIG. 18  is a perspective view of a side member, showing the arrangement for lowering and raising a plurality of slats within a manually operated internal Venetian blind; 
         FIGS. 19A and 19B  are perspective views before assembly and after assembly, respectively, of a longitudinally displaceable bearing block; 
         FIG. 20A  is a perspective view of a stabilizer affixed to a side member when the central cords are removed,  FIG. 20B  is a perspective, cross sectional view of an internal blind frame, showing a corner being secured thereto and a portion of the raising/lowering arrangement,  FIG. 20C  is a plan view of a blind in which central cords extend substantially through the lateral centerline of the headrail thereof, and  FIG. 20D  is an enlarged view of  FIG. 20C ; 
         FIG. 21A  is a side view of one embodiment of a counterweight and  FIG. 21B  is a perspective view of another embodiment of a counterweight, both counterweights being suitable for the arrangement of  FIG. 18 ; 
         FIGS. 22A-C  are perspective views of means for centering the central cords to the lateral centerline of a headrail; 
         FIGS. 23A-C  are a perspective view, side view, and a partially removed perspective view, respectively, showing the affixation of the outer cords thereto of another embodiment of a driving assembly; 
         FIG. 24  is a perspective view of another embodiment of a blind frame; 
         FIG. 25  is a side view of the headrail of the frame of  FIG. 24 ; 
         FIG. 26  is a side view of the headrail of  FIG. 25  and a corresponding cover in interlocking relation; 
         FIGS. 27A and 27B  illustrate the engagement of the lower supporting member of the frame of  FIG. 24  with a corresponding cover, wherein the lower supporting member and cover are in a transversally offset relation in  FIG. 27A  and are transversally aligned in  FIG. 27B ; 
         FIG. 28  is a top view of a side member of the frame of  FIG. 24 ; 
         FIG. 29A  is a perspective, exploded view which illustrates one method for assembling the frame of  FIG. 24 ; 
         FIG. 29B  is a perspective, exploded view of a blind which employs the frame of  FIG. 24 , illustrating another assembly method therefor; 
         FIG. 29C  illustrates the increased darkening capability of the blind of  FIG. 29B  as a slat support is lowered onto the lower supporting member of  FIG. 27A ; 
         FIGS. 30A-D  illustrate in perspective view the method for assembling another embodiment of the external magnet housing; 
         FIG. 31  illustrates a perspective, exploded view of another embodiment of a tilting device; 
         FIG. 32  illustrates a perspective view of a core member of the tilting device of  FIG. 31 , showing a shaft receiving channel formed therewithin; 
         FIG. 33  illustrates another perspective, exploded view of the tilting device of  FIG. 31 ; 
         FIGS. 34 and 35  illustrate two stages in perspective view of assembling the tilting device of  FIG. 31 , illustrating the method of securing an outer cord of a corresponding ladder braid thereby; 
         FIG. 36  is a perspective view of an external blind in accordance with the present invention; 
         FIG. 37  is a perspective, cross-sectional view of an actuator guide being secured between a wall opening frame and an internal blind; and 
         FIG. 38  is a perspective, schematic view of the blind of  FIG. 1B  being mounted in a skylight. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is a novel manually operated internal Venetian blind with a surface area of up to 5 square meters that can be lowered/raised and tilted effortlessly and speedily, whose operating mechanism can be adjusted to the dimensions of the blind, and which can be inverted without disarray of the slats and of the cords. 
     The first embodiment of the present invention relates to an internal Venetian blind for tilting a plurality of slats. Several prior art internal Venetian blinds comprise a tilting mechanism in the headrail in which outer cords of a ladder braid are wound around a cylindrical element, which is generally driven by gears, or any other type of transmission in communication with an actuator external to the Venetian blind. Since this cylindrical element is housed within the headrail, the circumference of the cylindrical element is laterally spaced from the lateral periphery of the slats across which the outer cords extend. Spacers placed on the transversal periphery of the uppermost slat are therefore needed to direct the outer cords to the cylindrical element. Such an arrangement for the tilting of Venetian blinds suffers from several drawbacks. Firstly, the weight of the slats is concentrated disproportionally more on the spacers and on the uppermost slat than on the other slats, causing the slats to change their relative position, or even to fail, over the course of time. Secondly, each outer cord is affixed to the cylindrical element by means of a clasp whose two legs are pressed onto the outer cord. Over the course of time, an outer cord may be released from its clasp, due to the weight concentration on a spacer or a change in the relative positions of the slats. Also, due to the angle of the outer cords resulting from the addition of the spacers, the slats cannot completely close and invariably only 75% of the incoming light is blocked. 
     In contrast, the tilting device of the present invention is rectangular, and the outer cords retain an essentially vertical disposition from the headrail to the lower rail. The outer cords are not affixed to the outer periphery of the tilting device, but rather to apertures formed within a planar surface of the tilting device, as will be described hereinafter. The weight of the slats is therefore more evenly distributed than in the prior art, allowing the slats of a blind unit with a higher surface area than has been known heretofore to be tilted, with greater reliability and without use of a gear train or any other transmission. 
     Referring now to the drawings,  FIG. 1A  illustrates an internal Venetian blind generally indicated by numeral  10  which is adapted for tilting a plurality of slats inserted between a double glazing unit, such that each slat is tiltable about a horizontal axis. Internal blind  10  comprises frame  49 , e.g. made from aluminum, a plurality of slats  14  suspended from headrail  2  of frame  49 , ladder braids  13  and  13 A to support and tilt slats  14 , and horizontally displaceable actuator  40  which actuates the tilting device, as will be described hereinafter. 
     As further shown in  FIG. 2 , frame  49  comprises headrail  2 , lower rail  52 , side members  3 , and covers  5  that interlock with each of the aforementioned frame components. Side members  3  are formed with a plurality of apertures  12 , so that hydrophilic pellets  9  placed within each side member  3  communicate with the interior of the blind via apertures  12  and absorbs any moisture that infiltrates to the interior of the blind. 
     The structure of lower rail  52 , which is arranged in mirror symmetry with respect to headrail  2 , is illustrated in  FIG. 12 . Lower rail  52  is a rectilinear aluminum component provided with transversally extending mounting plate  53  and wall  123 , which are bonded to glass sheets  59  and  60  ( FIG. 2 ), respectively. Inner transversally oriented planar surface  65  is perpendicular to, and located below, upper longitudinal edge  54  of the mounting plate, and laterally extends to wall  123 . Wall  122 , i.e. the portion of mounting plate  53  which is below surface  65 , is symmetrical to wall  123 . Mutually parallel, laterally oriented legs  90  and  91  perpendicularly protrude from the inner side of each of walls  122  and  123 , with leg  90  being longer than leg  91 . Longitudinally oriented abutment surface  87  is perpendicular to the lateral end of each leg  90 . Legs  90  and  91  interlock with corresponding leg  125  of rail cover  5 , such that a longitudinal abutment surface  127 , which is perpendicular to leg  125 , is coplanar with corresponding abutment surface  87  of the lower rail. Inner planar surface  66  of rail cover  5 , which serves as the bottom of the lower rail, is interposed between, and perpendicular to, the two abutment surfaces  127 , with the lateral dimension of surface  66  being less than that of surface  65 . 
     With reference to  FIGS. 2 and 3 , ladder braids  13  and  13 A are composed of three essentially vertically disposed cords, e.g. made from nylon: central cord  45  passing through an equally sized and similarly located aperture  55  formed within each slat  14 , outer cords  46  and  47 , each of which is interspersed between the slats and glass sheets  59  and  60 , respectively, and a plurality of essentially horizontally disposed cross ladders  48  for supporting each corresponding slat  14 . As well known to those skilled in the art, the outer cords  46  and  47  are adapted for tilting slats  14  in a desired angular direction by being either raised or lowered, while each of the cross ladders connects the two outer cords  46  and  47 . The three cords  45 - 47  extend from headrail  2  to lower rail  52 , passing through upper and lower cord guide assemblies  36 . 
     When slats  14  are of an increased transversal length, e.g. 1.5 m due to a corresponding increased surface area of the blind, more than two ladder braids are employed. The two outer ladder braids are composed of three cords as described hereinabove wherein central cord  45  thereof is passed through similarly located aperture  55  formed within the slat. The remaining ladder braids are composed of outer cords  46  and  47  and a plurality of cross ladders  48  for supporting each corresponding slat  14 , without need of a central cord. Accordingly, the inner portions of each slat  14  are formed without an aperture  55 . 
     Each cord guide assembly  36  is substantially rectilinear, wherein face  75  is perpendicular to transversal sides  68  and to lateral sides  64 . Trapezoidal opening  63 , or an opening of any other convenient shape, is formed in each transversal side  68 , such that a portion is removed from the transversal edge of face  75 . Rectangular cord guide  61  is centrally located within, and protrudes from, face  75 . Cord guide  61  is divided into three sections by dividers  62 A and  62 B so that each cord passes through a different section and is essentially vertically disposed. The cord guide  61  of each cord guide assembly  36  is internally positioned, i.e. it faces the cavity of internal blind  10 , and is substantially coplanar with the inner planar surface  65  of either headrail  2  or of lower rail  52 , protruding from a similarly shaped aperture formed in said outer planar surface so as to be accessible to the cords  45 - 47 . 
     Cord guide assembly  36  is adapted for rotatably supporting tilting device  31  ( FIG. 4 ). Protrusion  85  extending from each laterally oriented side  79  of tilting device  31  is received and rotatable within arcuate opening  67  formed on the outer end of sides  68  of cord guide assembly  36 , thereby allowing tilting device  31  to rotate. Tilting device  31  is prevented from being dislodged from the two arcuate openings  67  of cord guide assembly  36  by means of inner planar surface  66  of rail cover  5  ( FIGS. 8C and 12 ), the latter being adapted to contact a lateral side  80  of a fully rotated tilting device  31 , the rotation of which will be described hereinafter. Arcuate opening  67  is formed by two straight walls  70  and  71 , each of which has an equal length and is separated by a distance substantially equal to the diameter of protrusion  85 , and by arcuate wall  73  tangential with walls  70  and  71 , whose curvature is substantially equal to that of protrusion  85 . Each transversal side  68  is also formed with a rectangular opening  58 , laterally inwards from arcuate opening  67 , thereby defining leg  57  of a small lateral length. Cord guide assembly  36  is provided with four legs  57 , one on each corner thereof, with the outward end of each leg terminating with a wedge-shaped portion  76 , which facilitates the securing of cord guide assembly  36  to the headrail or to the lower rail, as shown in  FIG. 2 . Wall  70  is flexible, due to the existence of rectangular opening  58 , and therefore wall  70  may be slightly flexed as the cord guide assembly is secured, or during insertion of protrusion  85  of the tilting device within arcuate opening  67 . 
     Cord guide assemblies  36  are immobilized within lower rail  52  ( FIG. 12 ) and within headrail  2  ( FIG. 1A ) by means of each corresponding cord guide  61 , which abuts the wall of an equally sized aperture (not shown) formed in inner planar surface  65  of the lower rail, and therefore is prevented from moving in the transversal and lateral directions. Longitudinal movement of a cord guide assembly  36  is prevented by means of wedge-shaped portions  76 , each of which is inserted between longitudinally oriented abutment surface  87  and wall  122  of a headrail or lower rail, so as to be compressed and snapped in secured, undetachable relationship with respect to a corresponding set of leg  90 , wall  122  and abutment surface  87 . 
     As shown in  FIG. 4 , tilting device  31  has a rectilinear body comprising planar face  78  which is perpendicular to its transversal sides  79  and to its lateral sides  80 . Face  78  is formed with circular apertures  81  and  82 , such that the center of each aperture coincides with centerline A-A of face  78 . Sides  79  are formed with coaxial annular protrusions  84  and  85 , wherein protrusion  85  has a greater outer diameter than protrusion  84  and axis C-C passing through protrusions  84  of each side is parallel to, and located directly underneath, the second centerline B-B of face  78 . Apertures  81  and  82  are bored through face  78  and are adapted for receiving a corresponding nail  26  having radial protrusions  43 . 
     Referring now to  FIGS. 7A and 7B , wall  83  of apertures  81  and  82  extends throughout body  88  of the tilting device, so that end  77  of each outer cord may be easily introduced within a corresponding aperture. Radial protrusions  43  of nail  26  have curved ends, with the spacing between the curved ends of opposing radial protrusions  43  substantially equal to, but slightly less than, the diameter of apertures  81  and  82 . These curved ends are adapted to press an outer cord (for example the illustrated outer cord  47 ) onto wall  83  and to retain the pressed portions of the outer cord in a fixed position relative to face  78  of the tilting device, even during rotation of tilting device  31  about axis C-C ( FIG. 4 ). Radial protrusions  43  may encircle the entire periphery of nail  26 , or a portion thereof. Nail  26  is inserted into a corresponding aperture after the introduction of the corresponding outer cord in such a way that radial protrusions  43  press the outer cord and head  42  of the nail contacts, or is close proximity to, face  78 . If nail  26  is incorrectly inserted into an aperture, e.g. head  42  is not parallel to face  78 , head  42  will contact abutment surface  87  ( FIG. 12 ) of headrail  2  or of lower rail  52  and tilting device  31  will therefore not be able to rotate to a full extent. 
     As shown in  FIGS. 8A-C , face  78  of each tilting device  31  is outwardly positioned with respect to face  75  of cord guide assembly  36  ( FIG. 3 ). Outer cords  46  and  47  are retained in a fixed relative position within corresponding upper and lower tilting device by nails  26  after having been introduced through each corresponding cord guide  61  and through each corresponding circular aperture of the tilting device while brushing against a corresponding lateral side  80  of each tilting device. Since the lateral dimension of tilting device  31  is substantially equal to that of slats  14 , outer cords  46  and  47  are in an essentially vertical disposition from headrail  2  to lower rail  52 . 
     Therefore the weight of slats  14  is substantially evenly distributed throughout the length of the outer cords, resulting in added reliability without danger of slat failure. Also, the circular apertures  81  and  82  ( FIG. 4 ) are formed along the transversal centerline A-A of face  78  so that tilting device  31  is well balanced during rotation. 
     When the slats are in a completely open position as illustrated in  FIGS. 8A and 14A  such that a maximum amount of solar radiation is admitted through glass sheets  59  and  60  ( FIG. 2 ), outer cords  46  and  47  have equal tension and are located proximate to the lateral ends, respectively, of the cord guides  61 . Upon displacement of actuator  40  ( FIG. 1A ) slightly to the left, for example, cord  46  is tensed by the tilting device, as will be described hereinafter, and cord  47  is slackened. Cord  46  is then drawn upwards and inwards, causing the slats that are supported on cord  46  by means of each cross ladder  48  to change their inclination with respect to a horizontal plane, as shown in  FIG. 8B . Following additional displacement of actuator  40 , cord  46  is further drawn upwards and inwards, causing an additional change in the inclination of the slats, achieving an angle of E, and cord  47  is drawn downwards, as shown in  FIGS. 8C and 14B . 
     As previously mentioned, the tilting of the slats is accomplished by rotating tilting device  31  about axis C-C ( FIG. 4 ). While outer cords  46  and  47  have a nominal tension, due to their fixation by nails  26 , when face  78  is substantially parallel to the inner planar surface  65  ( FIG. 12 ), as in  FIG. 8A , the tension of the outer cords may be increased by rotating the tilting device. The rotation of tilting device  31  causes a lateral side  80  to press against a corresponding outer cord, depending on the direction of rotation, thereby increasing the tension of that outer cord. With respect to the illustrated rotational direction of tilting device  31  shown in  FIGS. 8B and 8C , the pressing of outer cord  46  by the corresponding lateral side  80  of the upper tilting device also draws that outer cord upwards and inwards, being wound around lateral side  80  and partially around face  78 . It will be appreciated that outer cord  47  is slackened at this stage, since the corresponding lateral side  80  is no longer in contact with cord  47 . At the rotational position of the tilting device depicted in  FIG. 8C , the tension of outer cord  46  is substantially at a maximum, while the tension of outer cord  47  is increasing, since a corresponding lateral side  80  of the lower tilting device begins to press against outer cord  47 . At the minimum spacing between the two outer cords, cord  46  is tensed due to the action of the upper tilting device and cord  47  is tensed due to the action of the lower tilting device. 
     The drive means for this embodiment is illustrated in  FIGS. 5 and 6 , and is positioned within lower rail  52  ( FIG. 2 ). In contrast with prior art internal blinds wherein the tilting mechanism is usually a gear train that adds to the cost of the device, the drive means of the present invention does not employ a gear train or clutch, but rather is manufactured from inexpensive components. Adapter  30 , tilting device  31 , and cord guide housing  36 , for example, may be molded from acetal polyoxymethylene copolymer (POM) or polyacetal for its high rigidity and low coefficient of friction. Linear transversal displacement of actuator  40  is converted into rotational displacement by means of helical member  33 , e.g. made of stainless steel. Helical member  33  is a metallic rectangular sheet that is twisted in such a way that its curvature can generate a volume of solid in the shape of a cylinder. Helical member  33  is provided with transversal ends  34 , e.g. of plate-like shape, which are insertable within adapter  30  having an annular cross-section. Adapter  30  in turn receives steel shaft  29 , which is insertable within annular protrusion  84  of the tilting device ( FIG. 4 ). Shaft  29  is D-shaped or of any other suitable configuration that allows the periphery of shaft  29  to engage the inner wall of annular protrusion  84  and to thereby cause tilting device  31  to rotate about axis C-C. As shaft  29  rotates following the displacement of actuator  40  and the resulting rotation of helical member  33 , as will be described hereinafter, tilting device  31  is rotated as well. If so desired, ends  34  may be inserted directly into a corresponding annular protrusion  84 , for the direct driving of the tilting device. 
     Helical member  33  is placed between two coplanar, laterally oriented abutment plates  92  of internal magnet housing  23 . Following transversal displacement of actuator  40 , and consequently of abutment plates  92  as well, one of the abutment plates contacts a twist  94  of helical member  33 . The twist in contact with an abutment plate is in turn transversally displaced in a similar direction, and then that twist follows a helical path, due to the shape of member  33 . Helical member  33  therefore is therefore rotationally displaceable since all portions thereof follow a helical path. It will be appreciated that the rotational displacement of helical member  33  per stroke of actuator  40  is dependent on the pitch, or numbers of twists for a given length, of the helical member. Therefore a shorter helical member  33  can be rotationally displaced as much as a longer helical member if the shorter member is provided with a greater pitch. 
     Two sets of tilting devices are provided, wherein one set is positioned in headrail  2  and the other in lower rail  52 . By utilizing transversally extending D-shaped shaft  29  to connect between adjacent tilting devices, the spacing between a pair of tilting devices  31  is adjustable. A tilting device  31  or adapter  30  may be effortlessly pushed along shaft  29 , whereby to define a transversal position of the tilting device within the corresponding rail. Following adjustment of the spacing between adjacent tilting devices, an end  96  of D-shaped shaft  29  may inwardly protrude within the cavity of tilting device  31  or adapter  30 . If the transversal length of an internal blind is relatively long, more than two tilting devices, with suitable connections thereto such as an additional helical member or shaft, may be employed. If so desired, the drive means may be positioned in headrail  2 , for example if the lower rail is in an inaccessible location. Therefore the drive means may be effortlessly customized to any desired transversal length, e.g. from 2 cm to 8 m, without any compromise in reliability, rate of actuation, comfort or safety. 
     The slats of the present invention can be tilted effortlessly and speedily from one angular position to another within less than 2 seconds regardless of the surface area of the Venetian blind. Such quick tilting is realized due to the configuration of actuator  40 , which is shown in  FIGS. 2 ,  5 , and  9 . Since the Venetian blind is located internally to the double glazing, an actuator external to the glass sheets is needed to transmit a force to the tilting mechanism disposed within the double glazing. One prior art method comprises the engagement of an actuating rod with a gear train, which causes the rotation of the tilting mechanism. To reduce the costs associated with a gear train, another prior art method involves the use of an externally disposed and hand-held magnet, which is placed on a glass sheet, to displace another magnet housed within the double glazing causing a force to be transmitted to the tilting mechanism. The movement of the externally disposed magnet is often slow since the location of the internal magnet may not be known, and even if it were known, inevitable delays in guidance of the magnets occur due to uncertain displacement paths, muscle spasms while holding the external magnet, etc., and therefore actuation of the slats from one position to another may last as much as two minutes. In contrast, actuator  40  is coupled to the frame of the Venetian blind, and therefore external magnet  9  is guided within a fixed path, such that the distance from external magnet  9  to internally disposed magnet  89  is a constant value. 
     As shown in  FIG. 9 , external magnet  9  is placed within external magnet housing  19  and internal magnet  89  is placed within internal magnet housing  23 . External magnet housing  19  has a rectangular body  98  formed with an inwardly facing cavity for the insertion therein of external magnet  9 , wherein body  98  is suitably sized so that external magnet  9  is in contact with glass sheet  60 . External magnet housing  19  is provided with L-shaped appendage  99 , which extends from the inner lateral edge of external magnet housing  19  and is longitudinally spaced from, extends the entire transversal length of, and has a lateral dimension considerably less than that of, body  98 . L-shaped appendage  99  is adapted to be slidingly displaceable within arcuate track  102  of actuator guide  6 , which extends the entire transversal length of internal blind  10 , and is sized so that outward face  97  of body  98 , which is held by a user during actuation of the blind, is substantially coplanar with outward face  105  of actuator guide  6 , when L-shaped appendage  99  is inserted within track  102  and the inner planar face of mounting plate  108  of the actuator guide is mounted by a press fit between glass sheet  60  and the frame of the wall opening in which blind  10  is placed. As shown in Detail A of  FIG. 9 , the outer side of mounting plate  108  is formed with transversally extending arcuate recess  109 , planar transversally extending contact element  111  which is parallel to the inner planar face of mounting plate  108 , and oblique side  115  being inclined with respect to contact element  111 , the purpose for all of which will be described hereinafter. 
     Internal magnet housing  23  is integrally formed with longitudinally oriented stopper  110  and cavity back  116 , transversally oriented bridge  112 , and laterally oriented abutment plates  92  and legs  114 . The longitudinal dimension of stopper  110 , which is sidable along wall  122  of lower rail  52  ( FIG. 12 ), of legs  114  and of abutment plates  92  is substantially equal to that of the spacing between planar surface  65  and abutment surface  87  of the lower rail, to prevent slippage of the stopper when being transversally displaced. Bridge  112  is contiguous with, and has a similar lateral dimension as, planar surface  65 . Legs  114  laterally extend from cavity back  116  to bridge edge  113 , with helical member  33  positioned below the remaining portion of bridge  112 . Abutment plates  92  laterally extend from stopper  110  and cavity back  116 , respectively, to such a length such that a gap is formed between the two abutment plates. This gap allows helical member  33  to be positioned approximately at the lateral centerline of the lower rail, yet allows for contact between the helical member and the abutment plates during transversal displacement of the actuator, as described hereinabove. 
     The outwardly facing cavity in which internal magnet  89  is placed is defined by legs  114 , cavity back  116  and bridge edge  113 , and is suitably sized so that the internal magnet contacts wall  123  of lower rail  52  ( FIG. 12 ). External magnet housing  19  is initially transversally displaced along track  102  to such a position that external magnet  9  and internal magnet  89  are in opposed relation, at a fixed distance ranging from 6-10 mm. Since external magnet  9  and internal magnet  89  produce a relatively high magnetic force, suitable for tilting the slats or raising them (in another embodiment) and are of opposite polarity, they quickly move in unison upon transversal displacement of external magnet housing  19  along track  102 , which is initiated by a user. Since lower rail  52  is made from aluminum, the magnetic force exerted by external magnet  9  and internal magnet  89  is not significantly reduced by wall  123 , which is interspersed between the two magnets. Likewise glass sheet  60  interposed between the two magnets does not adversely affect the magnetic force produced by the two magnets. 
     The transversal displacement of actuator  40 , and therefore the rotation of tilting device  31  as well due to the kinematic relationship described hereinabove between external magnet housing  19  and tilting device  31 , is limited by stopper  110 , as the stopper abuts leg  57  and straight wall  70  of cord guide assembly  36  ( FIG. 3 ). The length of stopper  110  is selected to be compatible with the transversal length of internal blind  10 , so that the rotation of tilting device  31  will not be greater than a predetermined angle. 
     In contrast with prior art internal blinds with which great care has to be taken so that the slats remain horizontally disposed during shipping, handling and installment in order to avoid transversal cord movement and subsequent entanglement in other cords and/or slats, the present invention employs a central cord tensioning means. By applying a tensile force, e.g. of 0.5 kg, to the central cord, the central cord, as well as the outer cords connected thereto by means of the cross ladders of a ladder braid, will remain essentially perpendicular to planar surface  65  of lower rail  52  ( FIG. 12 ), even if the blind is inverted. Due to the tensile force applied to the central cord, the slats may be tilted even if the blind is disposed in an inclined disposition, e.g. 45 degrees with respect to a horizontal plane, or even in a horizontal disposition, depending on the applied tensile force. 
       FIG. 10  illustrates an exemplary tensioning means which comprises base  119  having an elliptical cross section, or any other suitable long and thin shape, and an annular projection  120  perpendicularly protruding from base  119 . Base  119  is placed on the outward face of laterally oriented transversal side  68  ( FIG. 3 ), and as central cord  45  is pulled and tied onto annular projection  120 , base  119  is pressed against side  68  of cord guide housing  36 , which is in immobilized, pressed engagement within a corresponding rail  2  or  52 . Central cord  45  can be tied in any convenient way that applies a constant, long-lasting and sufficient tensile force to base  119 . 
       FIG. 1B  illustrates an internal Venetian blind generally indicated by numeral  130 , which is adapted for both tilting and raising/lowering a plurality of slats inserted between a double glazing unit with the use of lubrication-free bearing elements. Slats  14  are suspended from headrail  4 , while ladder braids  13  and  13 A are secured from below to slat support  7 , e.g. made from aluminum, by means of an element (not shown) which is inserted into the slat support and presses the various cords of a corresponding ladder braid. Tilting is effected by longitudinally displacing actuator  140 A and raising/lowering is effected by longitudinally displacing actuator  140 B. 
     After suspending the slats from a suitably sized headrail  4 , the aluminum frame of blind  130  is assembled in the manner illustrated in  FIGS. 11A-C ,  12  and  13 A-B. Headrail  4 , which is longitudinally longer than headrail  2  ( FIG. 1A ), is structured in a similar fashion as the latter, such that headrail  4  can be interlocked with rail cover  5  whereby longitudinal abutment surface  127  of the rail cover is coplanar with a corresponding abutment surface of headrail  4 . Similarly rail cover  5  can be interlocked with transversally extending lower rail  52 , as shown in  FIG. 12 , and with longitudinally extending side member  3 , the latter being illustrated in  FIGS. 13A-B . 
     It will be appreciated that cover  5  need not be interlocked with any of the aforementioned components by being inserted at an end of the component, e.g. a longitudinal end of side member  3 . Cover  5  is advantageously adapted to be interlocked with a corresponding component by a single snap, e.g. at the lateral centerline of the cover when interlocked with a side member after one leg  125  has been inserted between legs  90  and  91  of a corresponding side member. 
     Longitudinally extending side member  3 , as illustrated in  FIGS. 13A-B , is a rectilinear component which comprises two longitudinally oriented walls  132 , one laterally oriented wall  133  interposed between, and perpendicular to, walls  132 , two parallel fins  134  transversally extending from the exterior of lateral wall  133  and laterally spaced from a corresponding longitudinally oriented wall  132 , and two sets of mutually parallel, laterally oriented and longitudinally extending legs  90 ,  91  and  136  which perpendicularly protrude from the inner side of a corresponding wall  132 , wherein the spacing between, and the dimensions of, legs  90  and  91  being equal to those of rails  2  and  52 . The lateral dimension of leg  136  is preferably equal to that of leg  90 . At one longitudinal end of side member  3 , transversal wall  132  is formed with cut-out  135 , as shown in  FIG. 13B , which accommodates the fixation of bearing housing  8  ( FIG. 16 ), as will be described hereinafter. At the other longitudinal end of side member  3 , lateral wall  133  is formed with a circular aperture  137 , as shown in  FIG. 13A , which allows for the rotation of driving assembly  139  ( FIG. 15 ) therethrough, as will be described hereinafter. Accordingly, each side member  3  is interlocked with a corresponding longitudinally extending cover  5 , such that each cover  5  is at a transversal end of blind  130  ( FIG. 1B ) and fins  134  of one side member  3  face the fins of the other side member. It will be appreciated that side members  3  of blind  10  shown in  FIGS. 1A and 2  may also have fins  134 . 
     A transversally extending cover  5  is then connected to headrail  4  and lower rail  52 . As illustrated in  FIG. 11A , lower rail  52  has an equal transversal dimension as headrail  4 , while transversally extending cover  5  is longer than the two rails. A corner  25  made of a thermoplastic molded material, e.g. polypropylene, is then inserted into the upper and lower ends of each longitudinally extending cover as depicted in  FIG. 11A , in such a way that laterally oriented surface  121  of the corner shown in  FIG. 11B  is adjacent to outer planar surface  120  of cover  5  ( FIG. 12 ). While corner  25  is inserted into a corresponding longitudinally extending cover  5 , wedge-shaped teeth  128  formed on the inner side of laterally oriented surface  121  of corner  25  snap in place by a press fit with inner planar surface  66  of longitudinally extending cover  5  ( FIG. 20B ). The lateral spacing between adjacent fins  134  of a side member  3  is substantially equal to, and slightly greater than, the lateral spacing between walls  122  and  123  of headrail  4  and lower rail  52  ( FIG. 12 ). 
     After being secured to the longitudinally extending covers, corners  25  are then secured to the transversally extending covers. As shown in  FIG. 11C , the two side members  3  together with the corresponding longitudinally extending cover  5  and corner  25  are inwardly displaced until wedge-shaped teeth  128  formed on the inner side of transversally oriented surface  124  of each corner  25  snap in place by a press fit with inner planar surface  66  of a corresponding transversally extending cover  5 , thereby preventing disengagement of a corner from a cover and resulting in a sturdy frame which is then bonded to glass sheets, e.g. by means of adhesive strips. Furthermore, walls  122  and  123  of headrail  4  and lower rail  52  ( FIG. 12 ) are received between a corresponding pair of fins  134  of side member  3 , thereby restricting movement of the plurality of slats (not shown) suspended by headrail  4  and increasing the capability of the blind to darken a room. Alternatively, side members  3  may be provided without fins, while laterally oriented wall  133  of side member  3  ( FIGS. 13A-B ) is bonded to adjacent walls  122  and  123  of headrail  4  and of lower rail  52 . 
     By assembling the blind frame in the manner described hereinabove, each transversally extending cover  5  abuts two side members  3  and rail  4  or  52 , as illustrated in  FIG. 1B . As a result the frame may be painted in a uniform color, in contrast to the prior art wherein the various components are connected by screws and silicon, thereby necessitating the frame to have a non-uniform color. Also, the frame, as well as other components of the blind, may be easily assembled with several simple motions, and therefore may also be assembled by means of a robot. 
     In this embodiment, one set of tilting device is employed, and is positioned within headrail  4 , as illustrated in  FIG. 15 . Each tilting device is directly driven by driving assembly  139 , and a greater degree of slat tilting may therefore be realized, as will be described hereinafter. D-shaped shaft  29  connects each tilting device  31  of the set with driving assembly  139 , so that when actuator  140 A is longitudinally displaced and driving assembly  139  is consequently rotated, all tilting device  31  connected to shaft  29  rotate an equal angular displacement. Driving assembly  139  in turn is driven by actuator cord  142 , which is wound about a plain bearing (not shown) housed within bearing housing  8 . Actuator cord  142  is affixed to driving assembly  139  and is tied to a hole bored through an abutment plate  92  of the actuator, while the ends of the actuator cord are tied to each other. Consequently, longitudinal displacement of actuator  140 A results in corresponding longitudinal displacement of actuator cord  142  and in rotation of the plain bearing housed in bearing housing  8  and of driving assembly  139 . The provision of a plain bearing and a rotating driving assembly therefore reduces the frictional resistance to the actuator cord and consequently, the required force needed to tilt slats  14 . 
       FIG. 16  illustrates bearing housing  8 , e.g. made of polyacetal. Bearing housing  8  is formed of laterally extending base  144  and two parallel rings  145 , which longitudinally protrude from base  144  and are spaced from each corresponding lateral end  148  thereof to define shoulder  146 . The plain bearing is mounted within bearing housing  8  by flexing rings  145 . Bearing housing  8  is secured to the bottom of side member  3 , as shown in  FIG. 15 , by forcing each ring  145  to be in pressed engagement with corresponding leg  136  and lateral wall  133  of the side member while shoulder  146  is supported by the transversal wall of cut-out  135  ( FIG. 13B ), as further shown in  FIG. 18 . 
     Driving assembly  139  is both a means to drive the tilting device and to receive D-shaped shaft  29 , and is adapted to convert longitudinal linear motion of the actuator into rotational motion. Driving assembly  139  is produced by the steps illustrated in  FIGS. 17A-D . After the slats have been suspended from headrail  4  ( FIG. 15 ) by affixing the outer cords to corresponding tilting device  31  and the central cords to the centering means, as will be described hereinafter, and after the tilting device has been transversally positioned on shaft  29 , shaft  29  is then inserted into receiving means  31 A, which is the same component as tilting device  31 , as shown in  FIG. 17A . Cylindrical casing  18 , e.g. made from aluminum, is then placed around receiving means  31 A and pushed transversally inwards over shaft  29 , as shown in  FIG. 17B . Actuator cord  142  is then fed through aperture  138  bored in a central location within the periphery of casing  18 , drawn through the interior of the casing, inwardly fed through aperture  81  of receiving means  31 A, outwardly fed through aperture  82 , drawn once again through the interior of casing  18 , and outwardly fed through aperture  138 , with ends  150  of the actuator cord dangling over casing  18  and facing longitudinally inwards. Receiving means  31 A is then repositioned to be completely within the interior of casing  18 , as shown in  FIG. 17C , after which actuator cord ends  150  are pulled so that actuator cord  142  is in contact with the inner face of receiving means  31 A. Hot, fast-drying glue  151  is then injected into casing  18 , whereupon driving assembly  139  is produced such that casing  18 , receiving means  31 A and actuator cord  142  become one integral non-detachable unit. The actuator cord ends are then tied to each other, after the actuator cord has been tied to the actuator and wound around the plain bearing, as described hereinabove. Therefore driving assembly  139  and shaft  29  are rotatable in unison upon longitudinal displacement of actuator cord  142 . 
     Since the tilting devices are directly driven by the driving assembly, a greater rotational displacement by the tilting device may be realized.  FIGS. 14A-C  illustrate the rotation of tilting device  31  from a completely open position of the slats in  FIG. 14A  to a completely closed position in  FIG. 14C . During the stage of  14 B, tilting device  31  is displaced to an intermediate rotational position and the slats achieve an inclination of E. When the slats are in a closed position, i.e. at a maximum inclination of F, as shown in  FIG. 14C , the spacing between cords  46  and  47  is at a minimum, as shown by the arrows. During this stage the tension of outer cord  46  is maximum, and this maximum tension further assists in closing the slats by pressing on the top of each slat, thereby increasing the inclination thereof. Consequently, approximately 90% of the incoming solar radiation can be blocked, due to the increased inclination of the slats relative to prior art internal blinds. 
     The maximum rotational displacement of the tilting device shown in  FIG. 14C  is approximately 120 degrees clockwise from the rotational position shown in  FIG. 14A . Although the tilting device can be further rotationally displaced, additional tensioning of the outer cords is liable to result in failure thereof. To limit the rotational displacement of tilting device  31  and the longitudinal displacement of actuator  140 A, silicon may be injected into track  102  of actuator guide  6  ( FIGS. 1B and 9 ) at predetermined locations, thereby producing stoppers (not shown). 
     An alternative embodiment of a driving assembly, which is adapted to rotate a tilting device to such an extent so as to block up to 90% of the incoming solar radiation, is illustrated in  FIGS. 23A-C . Driving assembly  210  comprises cylindrical casing  215 , radial ribs  218  and central core  220 , all of which are made from a lightweight and strong material such as polyacetal or aluminum. The three ribs  218 , or any other suitable number of ribs, extend radially from inner face of casing  215  to core  220 . The inner wall of core  220  is D-shaped, and is adapted to receive similarly shaped shaft  29 . Ribs  218  extend transversally from the inward end  205  of casing  215  until approximately the transversal centerline thereof. By employing such a driving assembly configuration, a casing having a relatively small diameter ranging from approximately 13 to 19 mm may be employed, since the driving assembly is formed with an integral receiving means. The internal Venetian blind may therefore have an even thinner configuration, with an air gap between the glass sheets of only e.g. 17 mm when a casing diameter of 13 mm is employed. 
     In order to affix actuator cord to casing  215 , knot  224  is tied at the middle of actuator cord  142 . The ends of actuator cord  142  are then introduced into the outward end of casing  215 , through the interior thereof, and fed through aperture  222  bored in the periphery of casing  215 . After the ends of actuator cord  142  are pulled, actuator cord  142  divides into two portions which dangle over outer wall of casing  215  and extend longitudinally inwardly, and knot  224  engages the inner wall of casing  215 . Consequently, actuator cord  142  will not be released from casing  215  as the actuator is longitudinally displaced and driving assembly  210  is thereby rotated. 
       FIG. 18  illustrates the raising/lowering arrangement of blind  130 . By employing bearing elements, over which two central cords are wound, the frictional force that the central cords encounter is significantly reduced relative to the prior art, and therefore a lower actuating force is required to raise or to lower the slats. As a result, a plurality of slats with a surface area of up to 5 square meters can be lowered/raised and tilted effortlessly and speedily. Furthermore, the use of the bearing elements allows for a shorted path of the central cords, and therefore a stroke length of only approximately 40% of the longitudinal dimension of the blind is required to achieve complete raising/lowering of the slats, in contrast to the prior art which require a full 100% stroke length of the longitudinal dimension of the blind to achieve complete raising/lowering of the slats. 
     The raising/lowering arrangement includes bearing housing  8 , bifurcated rectangular bearing block  20  and lead counterweight  1 , all positioned within the interior of side member  3 . Ball bearing  35  having opposed axles  38  ( FIG. 19A ) is housed in each of bearing housing  8  and bearing block  20  for reducing the frictional resistance to the central cords and for reducing the stroke length of the actuator, respectively. Counterweight  1  is attached to bearing block  20 , e.g. by a rope, so that when actuator  140 B is longitudinally displaced, counterweight  1  and bearing block  20  are displaced in a similar direction as the actuator, while slats  14  are longitudinally displaced in an opposite direction as the actuator. 
     In addition to ball bearing  35 , a pair of plain bearings (not shown) is housed in housing  8 . Each plain bearing encircles, supports and centers a corresponding axle  38  of the ball bearing. Each plain bearing presses against a corresponding transversal wall  132  ( FIG. 13B ) of side member  3 . 
       FIGS. 19A and 19B  illustrate the assembly of bearing block  20 . Bearing block  20  comprises two symmetrical rectilinear sections  153 A and  153 B. Each section is formed of an upper, laterally thicker portion  155 , whose transversal dimension is substantially equal to that of transversal wall  132  of side member  3  ( FIG. 13B ) and therefore can be guided by wall  133  and leg  136  during longitudinal displacement, and a lower, laterally narrower portion  156 . Upper portion  155  is formed with a circular recess  158  to receive therein a corresponding bearing flange  161  and annular portion  159  outwardly protruding from recess  158  in which is seated bearing axle  38 . Lower portion  156  of section  153 B is provided with a plurality of pins  163  which are snapped in place within corresponding seats  164  formed within the lower portion of section  153 A. 
     When sections  153 A and  153 B are mated, as shown in  FIG. 19B , the two lower portions  156  in opposed relation are in contact with each other. Block  20  is configured so that each upper portion  155  is spaced one from the other, with only bearing rim  166  being visible, so that the central cords wound around the bearing rim are free to be longitudinally displaced but are limited in terms of their lateral movement due to the existence of the upper portions  155  which may contact a central cord if it is moves in a lateral direction. For clarity, one central cord  45  is shown, but it is understood that two central cords are wound around ball bearing  35 , one from ladder braid  13  and the other from ladder braid  13 A ( FIG. 1B ). 
     Referring now to  FIGS. 20A-D , central cords  45  are wound around ball bearing  35  mounted in housing  8 , and after being wound around ball bearing  35  mounted in block  20  as shown in  FIG. 19A , the ends of the central cords are affixed to side member  3 . That is to say, central cords  45  are tied to stabilizer  16 , e.g. made of three annular elements wherein the central element has a smaller diameter than the outer elements, and which is adapted to be slid under pressure within longitudinal groove  167  cut in wall  133  of the side member. Central cords  45  are inserted into an aperture formed within stabilizer  16  and are tied at the inner side of the stabilizer. Therefore stabilizer  16  applies a reactive force to the central cords as the latter are longitudinally displaced. Since the cords are wound around ball bearing  35  of block  20 , the distance that the central cords traverse is consequently shortened, thereby resulting in a shorter required stroke length for the actuator in order to completely raise or lower the slats and in reduced frictional resistance to the central cords. 
       FIG. 21A  illustrates a side view of lead counterweight  1 , e.g. having an ellipsoidal shape as shown in  FIG. 18  Counterweight  1  is formed by three integral portions: central portion  168 , whose transversal dimension is substantially equal to that of transversal wall  132  of side member  3  ( FIG. 13B ) and therefore can be guided by wall  133  and leg  136  during longitudinal displacement, and two portions  169 , each of which is laterally outward from said central portion. The lateral dimension of counterweight  1  is substantially equal to that of side member wall  133 , and each outward portion  169  faces a corresponding transversal wall  132 . The counterweight is preferably suitably sized such that there will be a small clearance, e.g. of 2 mm, between the counterweight and the walls of the corresponding side member, so as to allow for thermal expansion during the summer months. 
     Internal magnets  89  are housed in the outward portion  169  closest to the external magnet housed in external magnet housing  19  ( FIG. 9 ) of actuator  140 B ( FIG. 1B ). It will be appreciated that counterweight  1  is essentially the internal magnet housing for actuator  140 B, and as external magnet housing  19  is longitudinally displaced along track  102  of actuator guide  6 , counterweight  1  is similarly displaced and the slats are raised or lowered. 
     Bearing block  20  ( FIG. 19A ) and counterweight  1  are interconnected by a rope (not shown) that passes through hole  157  bored through the lower portions of bearing block  20  and through hole  170  bored through central portion  1678  of the counterweight. Alternatively, as shown in  FIG. 18 , bearing block  20  and counterweight  1  are interconnected by means of a longitudinally extending plastic rod  580  having two pairs of laterally extending contractible elements  581  at each longitudinal end of the rod. A pair of contractible elements  581  are press fit into each of hole  157  of bearing block  20  ( FIG. 19B ) and hole  170  of counterweight  1 . 
     Alternatively, the counterweight may be embodied by a plurality of rectangular metallic plates, e.g. steel, as shown in  FIG. 21B . The illustrated longitudinally extending and oriented counterweight  180  comprises three elongated plates  182  and four shortened plates  184 , all of which having an essentially equal width. The three elongated plates  182  are stacked one in front of the other in longitudinal and transversal alignment, while two shortened plates  184  are stacked in longitudinally offset arrangement in front of the elongated plates  182 . Elongated plates  182  are bored with four holes (not shown). Shortened plates  184  are bored with three holes  185 ,  186  and  187  such that the distance between holes  186  and  187  is considerably less than the distance between holes  185  and  186 . Each pair of two shortened plates  184  are arranged in opposite orientation such that hole  185  of a first shortened plate is aligned with hole  186  of a second shortened plate. Shortened plates  184  are positioned relative to elongated plates  182  such that aligned holes  185  and  186  of oppositely oriented shortened plates  184  are also aligned with a corresponding set of holes bored in elongated plates  182 . Therefore counterweight  180  is provided with four sets of aligned holes wherein each set consists of three holes bored in elongated plates  182  and two holes bored in shortened plates  184 . A rivet, or any other suitable fastener, passes through the holes of each set of aligned holes, to mutually secure each elongated and shortened plate so that counterweight  180  will be provided with sufficient rigidity and structural strength for achieving reliable blind raising and lowering. It will be appreciated that any other suitable number of elongated and shortened plates may be employed as well. 
     While each longitudinal edge  181  of elongated plates  182  is longitudinally aligned with the corresponding longitudinal edge  183  of the frontmost shortened plate  184 , longitudinal edge  188  of the other shortened plate protrudes from longitudinal edge  181  of elongated plates  182  to such a degree that hole  187  remains uncovered by the frontmost shortened plate. Consequently, the upper hole  187  of the second shortened plate  184  is available for affixing thereto the means for interconnecting counterweight  180  and bearing block  20  ( FIG. 19A ), whether a rope, plastic rod, or any other suitable interconnecting means. The volume between the longitudinal edges  188  of the two frontmost shortened plates, respectively, constitutes magnetic retaining compartment  189 . Internal magnets inserted within magnet retaining compartment  189  are magnetically coupled to the frontmost elongated plate  182 . The number of magnets inserted within magnet retaining compartment  189  may be changed, in order to adjust the magnetic force applied by the linear actuator. 
       FIG. 37  illustrates actuator guide  6  in perspective, cross sectional view as it is installed between an internal blind and the frame of a wall opening. Wall opening frame  550 , which delimits the opening in which an assembled blind  290 , or any other blind, is inserted, comprises side elements  552 , lower element  554 , a top element (not shown), and removable securing elements  558 . Wall opening frame  550  is made of wood, as shown, or of any other suitable load bearing material such as aluminum, and is fixedly attached to a wall formed with a suitable opening, e.g. a window opening, a door opening and a skylight opening. 
     Blind  290  is sized to substantially correspond to the dimensions of the wall opening. Blind  290  is inserted in the wall opening when securing elements  558  are removed from frame  550 . Silicone sealant  69  ( FIG. 5 ), or any other sealing material, may be applied to the blind frame. Actuator guide  6  is then positioned such that the planar inward face of mounting element  108  abuts front glass sheet  60  and arcuate track  102  extends in the intended direction of displacement for body  98  of external magnet housing  19 . To retain actuator guide  6  in the desired orientation, securing element  558  is placed in abutting relationship with side element  552  of the wall opening frame and with contact element  111  of actuator guide  6  ( FIG. 9 ) by a press fit and is then fixedly attached to side element  552  by means of a fastening element (not shown). Actuator guide  6  is provided with oblique side  115  to allow for access to contact element  111 . The corresponding corner of securing element  558  is received in, and bears on, the wall of arcuate recess  109  of the actuator guide. External magnet housing  19  is coupled to arcuate track  102  of actuator guide  6  after the latter is immobilized by means of securing element  558 . Actuator guide  6  is installed in situ, since a securing element  558  is generally not produced with standard dimensions. 
       FIGS. 30A-D  illustrate another embodiment of an external magnet housing. External magnet unit  310  is shown in  FIG. 30D  after being assembled by the three stages shown in  FIGS. 30A-C , respectively. External magnet unit  310  comprises body  320 , magnet cover  330 , and external magnets  315  housed within body  320 . Body  320  and magnet cover  330  are preferably made from polyacetal. 
     Body  320  shown in  FIG. 30A  is a rectilinear component having a magnet retaining compartment defined by inwardly facing back  322  adapted to contact the front glass sheet of the blind and two laterally oriented walls  325  outwardly extending from back  322 , L-shaped appendage  326  having a transversal length less than that of back  322  and adapted to be slidingly displaceable within arcuate track  102  of actuator guide  6  ( FIG. 9 ), and two dirt removing channels  327  which are external to the magnet retaining compartment. By employing back  322 , external magnets  315  are advantageously separated from the front glass sheet of the blind, thereby achieving a lower coefficient of friction for the actuator. The use of a back  322  also prevents magnets  315  from rusting, since grease is unnecessary. 
     Each dirt removing channel  327  is defined by a transversally oriented plate  328  extending from approximately the middle of a corresponding compartment wall  325 , a laterally oriented channel wall  329  extending inwardly from plate  328 , and wall  325 , wherein the inward edge of a wall  325  and the corresponding leg  329  are coplanar. Two opposed wiper elements  324 A and  324 B, which are essentially coplanar with back  322 , extend transversally from channel wall  329  and compartment wall  325 , respectively, towards the interior of the corresponding dirt removing channel  327 . As external magnet unit  310  is displaced, wiper elements  324 A and  324 B contacting the front glass sheet of the blind scrape any dirt that has accumulated on the glass sheet. A brush (not shown), e.g. having bristles made from Teflon, is adapted to remove the collected dirt from channel  327 . The brush handle may be structured to be insertable in the groove between wiper elements  324 A and  324 B and plate  328 , for storage when not in use. 
     Magnet cover  330  shown in  FIG. 30C  has an outward, longitudinally oriented surface  332 , two transversally oriented walls  334 , and two laterally oriented walls  336  such that each laterally oriented wall  336  is connected with outward surface  332  by curved portion  337 , which facilitates manipulation of the magnet cover by a user during actuation of the blind. The transversally oriented wall  334  facing L-shaped appendage  326  is formed with a notch  339 . 
     After body  320  is coupled to track  102  of actuator guide  6 , magnets  315  are inserted into the magnet retaining compartment as shown in  FIG. 30B . Each magnet  315  has a transversal dimension approximately half of that of the magnet retaining compartment, so that two magnets in side-by-side relation contact the two walls  325 , respectively. External magnet unit  310  is suitable for adjusting the magnetic force applied by the actuator by advantageously allowing a different number of magnets to be inserted within the magnet retaining compartment. Two, three, or four magnets  315  may be inserted within the magnetic retaining compartment, and when more than two magnets are employed, the magnets are arranged in two layers. Cover  330  is secured to body  320  by means of pins  331 , each of which protrudes from the middle of a corresponding plate  328 . By pressing on outward surface  332  as indicated by arrow  343  in  FIG. 30C , pins  331  are received in corresponding recesses (not shown) formed in cover  330  and cover  330  is secured to body  320  by a snap fit. If a user feels that he has difficulty in actuating the blind, he simply removes cover  330  without having to disassemble the blind, inserts at least one additional magnet  315  into the magnet retaining compartment, and secures the cover to body  320 . 
     As previously mentioned, the internal blind of the present invention may be inverted without loss of operability when reoriented to a working position. In this embodiment, reliable operation of the blind is ensured by retaining the central cords at substantially the lateral centerline of headrail  4 , as shown in  FIGS. 20C-D . 
     A means for centering central cord  45  illustrated in  FIGS. 22A-C  may be used. Cord guide assembly  37  is configured in such a way that central cords  45  are continuously urged to the lateral centerline of the headrail. The external structure of cord guide assembly  37  is identical to that of cord guide assembly  36  shown in  FIG. 3 , with the exception of longitudinally oriented walls  176  transversally extending from one laterally oriented side  68  to the other, trapezoidal opening  172  and narrow rectangular opening  173 , the last two being formed in each laterally oriented side  68 . Internal laterally oriented walls  174 A-D extend from a corresponding wall  176  and are suitably configured so as to allow each axle  38  of ball bearing  35  to be rotatingly seated between a pair of internal laterally oriented walls wherein the axis of the axles is laterally oriented. Internal transversally extending walls  175  extend from each of laterally oriented walls  174 B and  174 D, defining the housing of ball bearing  35 . Transversally extending partitions  178  extend from one laterally oriented wall  68  of cord guide assembly  37  and terminate with a corresponding internal lateral wall  177 , which is perpendicular to the end of the corresponding internal transversal wall  175 . The spacing between partitions  178  is substantially equal to the lateral dimension of rectangular opening  173 . 
     As shown in perspective view in  FIG. 22C  and in plan view in  FIGS. 20C-D , two cord guide assemblies  37 A and  37 B are employed for ladder braids  13  and  13 A, respectively. Central cord  45  of ladder braid  13 A longitudinally extends through the central section of cord guide  61  defined by dividers  62 A and  62 B ( FIG. 3 ), as further shown in  FIG. 10 , and is wound around ball bearing  35 , between the bearing and walls  177 . The cord then exits cord guide assembly  37 B via rectangular opening  173 , and then is in contact with the rim of the ball bearing  35  associated with cord guide assembly  37 A, before being wound around ball bearing  35  of bearing housing  8 . Similarly, central cord  45  associated with ladder braid  13  longitudinally extends through cord guide assembly  37 A and is wound around its corresponding ball bearing before exiting the cord guide assembly via rectangular opening  173 . Thus two central cords exit cord guide assembly  37 A, as shown in  FIGS. 20C ,  20 D, and  22 C. 
     Partitions  178  of cord guide assembly  37 A advantageously urge the central cord associated with ladder braid  13 A to remain in the center of rectangular opening  173 . When the internal blind is tilted to an angle of 45 degrees, for example, relative to the ground, central cord  45  contacts one of the partitions  178 , and therefore additional lateral movement is prevented. When the blind is completely inverted, each central cord contacts the wall at the longitudinal end of the corresponding rectangular opening  173 . After the blind is reoriented to its original position the central cords contact again the ball bearing of each cord guide assembly  37 , as described hereinabove, and are thereby centered within the headrail. The lateral centering of the central cords is additionally facilitated by the ball bearing  35  of bearing housing  8 , which is centrally positioned within side member  3  by means of plain bearings, as described hereinabove, and by ball bearing  35  of bearing block  20 , which is centered by its two sections  153 A and  153 B ( FIG. 19B ). 
     After annular protrusions  85  of tilting device  31  ( FIG. 4 ) are mounted in annular openings  67  ( FIG. 22A ), respectively, of cord guide assembly  37 , the outer cords may be affixed to tilting device  31 . The two outer cords are inserted within a corresponding outer section of cord guide  61  ( FIG. 3 ) and then pulled through open region  179  between longitudinally oriented wall  176  and corresponding internal transversal wall  175  ( FIG. 22B ), whereupon the outer cords are affixed to tilting device  31 , as described hereinabove. 
     A blind  130  may be advantageously raised and lowered even when the blind is at an inclination I of up to 45 degrees relative to the ground, such as when mounted within a skylight  570 , as shown in  FIG. 38 . As shown in  FIG. 15 , slat support  7 , which has an arcuate profile, is insertable between fins  134  of side member  3  after headrail  4  and lower rail  52  are inserted within fins  134 . Slat support  7  is accordingly guided by the two fins during longitudinal displacement with minimal clearance therefrom, e.g. 4.5 mm. Fins  134  are therefore adapted to prevent contact between the slats and the glass sheets when the blind is inclined, thereby preventing scratching of the glass and disarray of the slats, or even a malfunctioning of the blind. Due to the clearance between slat support  7  and fins  134 , and furthermore between slats  14  and the glass sheets, a thermal break is created, whereby heat is not directly conducted from the exterior of the blind to its interior. Plastic inserts (not shown) are preferably inserted within the transversal ends of slat support  7 , so that the slat support may slide when longitudinally displaced, such as when the blind is at an inclination I. The blind is operable to raise and lower the slats at an increased incline, e.g. 45 degrees, or an incline of 60 degrees if the slat support  7  is weighted, such as with lead. 
       FIGS. 31-35  illustrate another embodiment of a tilting device which is generally designated by numeral  400 . As shown in  FIG. 31 , tilting device  400  comprises core member  420  and two oppositely oriented wing members  460 A and  460 B which are engageable with core member  420 , so as to retain outer cords  46  and  47  in a pressed relation with respect to core member  420 . 
     With reference to  FIGS. 31 and 32 , core member  420  comprises two laterally oriented walls  415 , convex periphery  430  between walls  415  and subtending an angle of approximately 180 degrees such that the axis thereof is transversally extending, toothed key  435  protruding outwardly from, and having a similar curvature as, periphery  430 , and two separated coplanar elements  442  defining a rectangular face and a chord of walls  415  and of periphery  430 . By providing core member  420  with a convex periphery, the thickness and therefore the structural integrity of the core member may be advantageously increased. Also, outer cords  46  and  47  may be affixed more easily when a core member  420  with a convex periphery  430  is employed. 
     The transversal dimension of periphery  430  is longer than the combined length of the two wing members  460 A and  460 B. Coaxial protrusions  416  and  417  extend transversally from each laterally oriented wall  415 , wherein protrusion  416  has a greater outer diameter than protrusion  417  and laterally oriented face  418  of protrusion  417  is formed with a D-shaped aperture  419 . The two D-shaped apertures  419  of core member  420  are aligned and are adapted to receive a D-shaped shaft, which may extend internally within channel  448  formed between elements  442 . Elements  442  face outwardly when the slats of the blind are in a completely open position. 
     The entire length of convex periphery  430  including walls  415  is formed with two transversally extending grooves  404 , each of which being formed at a different lateral side of protrusions  416 . Each groove  404  has a rectangular cross section which is obliquely oriented with respect to coplanar elements  442 . The two grooves  404  are separated by an angular distance of approximately 120 degrees and have bilateral symmetry. Portion  425  of convex periphery  430  between a groove  404  and the corresponding rectangular element  442  serves to tension an outer cord, as will be described hereinafter. 
     Convex periphery  430  is also formed with two circumferential, rectangularly shaped recesses  438  proximate to the transversal ends, respectively, of periphery  430 . Recesses  438  and toothed key  435  have a substantially equal circumferential length and are equally spaced between grooves  404 . Toothed key  435  is formed at the transversal centerline of periphery  430  and is integral therewith. Toothed key  435  has a series of laterally separated toothed serrations  458  of equal length and of similar configuration with alternating protruding pointed ends and recessed sharp edge junctions, on each transversal end thereof. 
     Wing members  460 A and  460 B are illustrated in  FIGS. 31 ,  33 , and  34 . Each wing member has a convex shell  465 , which has a curvature substantially equal to that of convex periphery  430  of core member  420 , subtending an angle of approximately 120 degrees and being positioned such that the axis thereof is transversally extending. Transversal end  478  of shell  465 , which is proximate to toothed key  435 , terminates with a rim  467  adjacent thereto having a radius slightly larger than that of periphery  465 . The other transversal end  479  of the wing member shell is formed with two U-shaped openings  469 , the curved wall of which faces rim  467 , so as to define a central portion  471  and two peripheral portions  473 . Each peripheral portion  473  is considerably thicker than central portion  471 , and is configured to be received within a corresponding groove  404  of core member  420 . 
     Transversal end  478  also has a laterally oriented circumferential wall  481 , which extends between the two peripheral portions  473  along the inner wall of periphery  465 . A plurality of radially extending teeth  485  are formed in a central region of circumferential wall  481 . Teeth  485  are configured with a sufficient length and spacing therebetween so as to abut the recessed sharp edge junctions of toothed serrations  458  of toothed key  435 . Transversal end  479  of wing member periphery  465  has a lip  492  which circumferentially extends along the inner wall of shell  465 . Lip  492  has a circumferential length substantially equal to recess  438  of core member  420 , and is adapted to engage the same with a snapping action. 
     In order to affix outer cords  46  and  47  to tilting device  400 , outer cords  46  and  47  are first placed on periphery  430  of core member  420  in contact with the two transversal ends of serrations  458 , respectively, while in a lateral orientation and dangling from a corresponding tensing portion  425  of core member  420 , as shown in  FIG. 32 . Wing members  460 A and  460 B are then transversally displaced towards toothed key  435  such that peripheral portions  473  thereof are received in a corresponding groove  404  of core member  420 . The wing members are displaced until teeth  485  thereof abut complementary serrations  458  of key  435  and lip  492  thereof snaps into corresponding recess  438  of core member  420 . Outer cords  46  and  47  are thereby affixed to core member  420  by means of teeth  485  of a corresponding wing member as they abut complementary serrations  458  of key  435  by a pressing relation, and the outer cords are prevented from being dislodged from the tilting device since lip  492  is secured to a corresponding recess  438  by a snapping fit. 
       FIG. 34  illustrates core member  420  as wing member  460 B is engaged therewith while outer cord  46  is not yet affixed.  FIG. 35  illustrates core member  420  as wing members  460 A and  460 B are engaged t herewith and outer cord  46  is affixed to core member  420 . If tilting device  400  were not assembled properly and outer cord  46  is therefore not sufficiently affixed to core member  420 , rim  467  of wing member  460 A retains outer cord  46  in contact with the periphery of core member  420 , until tilting device  400  is reassembled. Wing member  460 A is detachable from core member  420  upon manipulation of a suitable tool with one of the U-shaped openings  469 . 
     It will be appreciated that tilting device  400  is suitable for tilting the slats associated with a blind of any of the aforementioned embodiments, for example the blind illustrated in  FIGS. 2 ,  15  and  29 B. Protrusions  416  ( FIG. 31 ) of core member  420  are rotatably supported by the cord guide assembly illustrated in  FIG. 3  or  FIG. 22A , the latter being immobilized to headrail  4  as shown in  FIG. 15 , to headrail  254  as shown in  FIG. 29B , or to lower rail  52  shown in  FIG. 2 . 
     With reference to  FIG. 31 , outer cord  46  is pressed by portion  425  of core member  420  when tilting device  400  is rotated clockwise about a transversally extending axis by a corresponding D-shaped shaft, thereby increasing the tension thereof. As outer cord  46  is pressed by portion  425 , outer cord  46  is displaced upwards and inwards, being wound around portion  425  and then around the two planar elements  442  ( FIG. 32 ). As clearly shown in  FIG. 32 , tilting device is advantageously configured such that the diameter of protrusion  416  is greater than the maximum radial dimension of laterally oriented wall  415  from periphery  430  to coplanar elements  442 , particularly due to the provision of channel  448  formed between elements  442  to accommodate the D-shaped shaft, thereby precluding the need of a greater radially dimensioned wall  415  which would be needed to enclose the D-shaped shaft. Tensing portion  425  of core member  420  is consequently also of a relatively small thickness, e.g. no greater than 3 mm. As a result, tilting device  400  may achieve an increased angular displacement of up to 140 degrees while outer cord  46  contacts coplanar elements  442 . Such an angular displacement of tilting device  400  assists in increasing the tension of outer cord  46 , and the increased tension of outer cord  46  is instrumental in the improved closing of the slats by pressing on the top of each slat. By employing tilting device  400 , the maximum inclination of the slats is such that the spacing between outer cords  46  and  47  changes from 17 mm when the slats are in a completely open position to only 3 mm when the slats are in a completely closed position. Consequently, approximately 93% of the incoming solar radiation can be blocked, due to the increased inclination of the slats relative to prior art internal blinds. The solar radiation blockage may be up to 95% when the arrangement shown in  FIG. 29C  is employed, as will be described hereinafter. 
       FIGS. 24-29  illustrate another embodiment of a blind frame, which is designated by numeral  250 . While prior art frames for internal Venetian blinds are made from aluminum, the frame of this embodiment is made entirely from polyvinyl chloride (PVC), providing the blind with the following advantages:
     1) A blind frame from PVC significantly reduces the cost of the blind. Since painted aluminum costs approximately $4/kg and PVC costs approximately $1.40/kg and the density of PVC is approximately one-half of aluminum, the cost of a blind frame made from PVC is approximately one-sixth of one made from aluminum.   2) The thermal conductivity of PVC is significantly less than that of aluminum (237 W/mK for aluminum and only 0.16 W/mK for PVC at 20° C.), and therefore a blind frame from PVC serves as thermal insulation, being well suited to very hot or very cold climates.   3) PVC is produced by extrusion. During the extrusion process, a PVC component may be produced with two layers wherein the base layer has a thickness ranging from 1-2 mm and the outer layer has a thickness ranging from 0.2-0.3 mm. The outer layer may be dyed to any desired color, and therefore the blind frame may be custom made. In contrast, aluminum needs to be painted separately after being produced, further increasing its price.   

     As shown in  FIG. 24 , frame  250  comprises headrail  254 , side members  253 , lower support member  259  to support the slats when lowered, transversal covers  255 A, and longitudinal covers  255 B. A transversal cover  255 A is the same component as longitudinal cover  255 B, albeit with a different orientation. Headrail  254  has a similar configuration as side members  253 , although the front and rear wall length of the two components is different. An additional flexible corner element, which may compromise the dimensional stability (i.e. the ability to avoid substantial unpredictable dimensional alteration after being subjected to extreme conditions such as heat, cold and moisture upon return to ambient conditions), is unnecessary since transversal covers  255 A and longitudinal transversal covers  255 B are connected together by screws  251 . Another advantage of the configuration of frame  250  is that a lower rail is unnecessary since tilting is effected by longitudinally displacing actuator  140 A and raising/lowering is effected by longitudinally displacing actuator  140 B ( FIG. 1B ). The field of vision visible through the glass sheets is therefore increased. Furthermore, frame  250  is assembled from only three components, thereby reducing manufacturing cost and time. 
     A side view of headrail  254  is illustrated in  FIG. 25 . Headrail  254  is a rectilinear component provided with transversally oriented rear wall  261  and front wall  263 , which are bonded to the glass sheets  262  and  264  ( FIG. 29B ) of the blind. Planar surface  265  is perpendicular to, and located above, lower longitudinal edge  267  of rear wall  261 , and laterally extends to front wall  263 . Mutually parallel, laterally oriented legs  270  and  271  perpendicularly protrude from the inner side of each of walls  261  and  263 , with leg  271  being longer than leg  270 . The upper surface of leg  270  defines the upper longitudinal edge of wall  261 . Abutting element  266  longitudinally extends from the inward lateral end of leg  271  towards planar surface  265  for a sufficient distance to allow each wedge-shaped portion of a cord guide assembly, e.g. cord guide assembly,  37  as shown in  FIG. 29B , to be inserted between longitudinally oriented abutting element  266  and the corresponding rear wall  261  or front wall  263 , so as to be compressed and snapped in secured, undetachable relationship with respect to a corresponding set of leg  271 , abutting element  266 , and a wall of headrail  254 . 
     A perspective view of transversal cover  255 A is illustrated in  FIG. 27A . Transversal cover  255 A comprises laterally oriented side walls  272 , transversally oriented base portion  273  which is chamfered to coincide with each side wall  272 , two laterally oriented inner walls  275 , transversally oriented abutment surface  279 , and transversally oriented leg  276  perpendicularly protruding from each inner wall  275 , the lateral dimension of which being less than that of abutment surface  279 . The spacing between abutment surface  279  and leg  276  is substantially equal to the thickness of headrail leg  270  ( FIG. 25 ). Cover  255 A also comprises transversally oriented dislodging preventing element  274 , which is interposed between the two inner walls  275  and is substantially coplanar with abutment surfaces  279 . Laterally oriented connecting element  278  extends from substantially the centerline of base portion  273  to dislodging preventing element  274  and is connected to the latter by a T-shaped connection. 
     A side view of a transversal cover  255 A being interlocked with headrail  254  is illustrated in  FIG. 26 . Transversal cover  255 A is adapted to be interlocked with a headrail  254  by a single snap after inserting each leg  276  of transversal cover  255 A between corresponding legs  270  and  271  of headrail  254 . After transversal cover  255 A is interlocked with headrail  254  whereby legs  270  and  271  support abutment surfaces  279  and  277  ( FIG. 27A ), respectively, of transversal cover  255 A, rear and front walls  261  and  263 , respectively, of headrail  254  are coplanar with the corresponding side wall  272  of transversal cover  255 A. Dislodging preventing element  274  contacts a tilting device, e.g. tilting device  31  of  FIG. 4  or tilting device  400  of  FIG. 31 , as the tilting device is rotated, to retain the tilting device in the corresponding cord guide assembly in which it is rotatably supported. 
     Support member  259 , as illustrated in  FIGS. 27A-B , is an H-shaped rectilinear component comprising two longitudinally oriented mounting surfaces  281  and transversally oriented plate  283  extending between the two mounting surfaces  281 . The total longitudinal dimension of support member  259  is significantly less than lower rail  52  of  FIG. 12 , and therefore provides an increased field of vision visible through the glass sheets. The transversal length of support member  259  is substantially equal to that of the headrail. The distance between the lower longitudinal edge  286  of each mounting surface  281  and plate  283  is slightly greater than the distance between abutment surfaces  277  and  279 . The lateral dimensional of support member  259  is substantially equal to that of transversal cover  255 A. The lower longitudinal edge  286  of each mounting surface  281  terminates with an inwardly and upwardly oriented protuberance  288 , which is so configured that when mounting surfaces  281  of support member  259  are placed on corresponding abutment surfaces  279  of transversal cover  255 A, protuberance  288  contacts the underside of leg  276  as shown in  FIG. 27B . Accordingly, support member  259  is afforded the ability of sliding over transversal cover  255 A, so as to be displaced to a desired position. Alternatively, support member  259  may be snapped in place to transversal cover  255 A by inserting the protuberance  288  of one mounting surface  281  between the corresponding leg  276  and abutment surface  279 , and then flexing the other mounting surface such that its protuberance  288  is inserted by a press fit between the corresponding leg  276  and abutment surface  279 . 
     During the extrusion of support member  259 , plate  283  thereof is formed with a plurality of apertures  291 . Consequently, the holding chamber defined by the volume between plate  283  of support member  259  and transversally oriented base portion  273  of transversal cover  255 A, when support member  259  and transversal cover  255 A are coupled, communicates with the interior of the blind via apertures  291 . Hydrophilic pellets  293  placed within the holding chamber absorb any moisture that infiltrates to the interior of the blind. 
     Side element  253  is illustrated in  FIG. 28 . Although side element  253  and headrail  254  have a similar configuration, front wall  263  of side element  253  has a length of I, which is less than the length J of the front wall of headrail  254  shown in  FIG. 25 . Portion  269  of rear wall  261  between planar surface  265  and transversal edge  267 , which serves as a fin, has the same length for both side element  253  and headrail  254 . For example, the side element front wall has a length I of 26 cm, the headrail front wall has a length J of 40 cm, and portion  269  has a length of 5 cm. A side element  253  having only one fin is employed for a blind having a surface area greater than 2 m 2 . 
     As shown in  FIG. 24 , transversal cover  255 A is longer than headrail  254  or support member  259 , with the protruding portion on both sides of headrail  254  or support member  259  being equal to the combined transversal length of a side element  253  and the corresponding longitudinal cover  255 B interlocked therewith. After headrail  254 , support member  259 , and each side element  253  is interlocked with the corresponding cover, the two transversal ends of each transversal cover  255 A are connected to a longitudinal end of a corresponding longitudinal cover  255 B. 
       FIGS. 29A and 29B  illustrate two alternative methods of connecting a transversal cover  255 A to a longitudinal cover  255 B. 
     In  FIG. 29A , longitudinal cover  255 B is positioned such that side walls  272  are longitudinally extending and transversally oriented, base portion  273  is longitudinally extending and laterally oriented, and leg  276  is laterally oriented. The longitudinal length of longitudinal cover  255 B and side element  253  are essentially equal. Longitudinal cover  255 B has a concave screw insertion portion  295  formed in the inner wall of base portion  273  and the corresponding side wall  272 . Screw insertion portion  295  subtends an angle of approximately 300 degrees and is truncated by the corresponding inner wall  277  perpendicular to leg  276 . 
     Transversal cover  255 A is positioned such that side walls  272  are transversally extending and longitudinally oriented, and base portion  273  is transversally extending and oriented. Apertures  298  bored within base portion  273  and surface  279  ( FIG. 27A ) of transversal cover  255 A are aligned with a corresponding screw insertion portion  295  of a longitudinal cover  255 B. When screws  251  are inserted within apertures  298  and are threadedly engaged to a fullest extent with the walls of the corresponding screw insertion portion  295 , each leg  276  of a transversal cover  255 A contacts the longitudinal edge of base portion  273  of the corresponding longitudinal cover  255 B, thereby securing the fully assembled frame. 
     Alternatively, as shown in  FIG. 29B , apertures are bored within base portion  273  and surface  279  ( FIG. 27A ) of longitudinal cover  255 B. Screws  251  are inserted within the apertures bored within longitudinal cover  255 B and are threadedly engaged to a fullest extent with the walls of the corresponding screw insertion portion of transversal cover  255 A. 
       FIG. 29B  illustrates a blind  290  in perspective, exploded form which employs frame  250 A and is suitable for tilting and raising/lowering slats  14 . It will be appreciated that frame  250 A may also be employed for a blind that is suitable only for tilting the plurality of slats. Frame  250 A is identical to frame  250  of  FIG. 24  with the exception of side elements  253 A. As shown, an alternative configuration of a side element has two parallel longitudinally oriented fins  285  of a length substantially equal to, and laterally spaced from, a corresponding side element wall. A side element having two fins is suitable for a blind having a surface area of less than 2 m 2 . Fins  285  extend transversally from laterally oriented, longitudinally extending planar surface  265  ( FIG. 28 ). As frame  250 A is assembled, slat support  7  having an arcuate profile is received between fins  285 . Slat support  7  is accordingly guided by fins  285  during longitudinal displacement of slats  14 . Fins  285  are therefore adapted to prevent contact between slats  14  and glass sheets  262  and  264  when the blind is inclined, thereby increasing the darkening capability of the blind and preventing scratching of the glass and disarray of the slats, or even a malfunctioning of the blind. 
     When slats  14  are completely lowered, as shown in  FIG. 29C , slat support  7  contacts plate  283  of lower support member  259 , or is in close proximity thereto. The two longitudinally oriented mounting surfaces  281  of lower support member  259  advantageously block the incoming solar radiation between slat support  7  and plate  283 , thereby increasing the darkening capability of the blind. 
       FIG. 36  illustrates another embodiment of the invention, which is an external blind, i.e. slats  14  are not retained between two glass sheets, and is designated by numeral  500 . Blind  500  is suitable for both tilting and raising/lowering the plurality of slats  14 , and comprises the blind frame embodied by headrail  254  and a corresponding transversal cover (not shown), cord guide assemblies  37  secured to headrail  254  for each ladder braid  13 , tilting devices  31  and/or  400 , driving assembly  210 , slat support  7  to which one end of each ladder braid  13  is attached, and cord lock  510  positioned within headrail  254  for securing each central cord  45  and thereby retaining the plurality of slats in a raised position. A D-shaped shaft  29  connects two adjacent tilting devices and connects driving assembly  210  with its neighboring tilting device. Driving assembly  210  is conveniently and compactly housed within headrail  254 . Actuator cord  515  is affixed to driving assembly  210 . The two ends of actuator cord  515  may be attached to a suitable cord end (not shown), which serves as an actuator for driving assembly  210 . As one end of actuator cord  515  is displaced, the other end thereof is displaced in an opposite direction, causing driving assembly  210  and tilting devices  31  and/or  400  to rotate in the same rotational direction and thereby causing slats  14  to tilt. Headrail  254  is provided with two cornered openings (not shown) to accommodate actuator cord  515  and central cord ends  45 A, respectively. As central cord ends  45 A are manipulated as well known to those skilled in the art to release the same from cord lock  510 , the central cord  45  of each ladder braid  13  may be lowered or raised according to the discretion of the operator and then cord ends  45 A are secured again to cord lock  510 . 
     Blind  500  advantageously provides tilting and raising/lowering arrangements that are directly driven, without need of a gear train or any other transmission, as has been employed heretofore in prior art external blinds, thereby lowering the cost of the blind. Additionally, tilting devices  31  and/or  400 , about which the outer cords are wound upon rotation of driving assembly  210 , are configured to simplify the mounting thereof on cord guide assembly  37  and the affixation of the outer cords thereto, and to furthermore allow the outer cords to be essentially longitudinally disposed from headrail  254  to slat support  7 , thereby affording the blind with increased reliability with respect to the prior art. Also, tilting devices  31  and/or  400  can rotate an increased angular distance with respect to those of the prior art, and therefore blind  500  can block at least 90% of the incoming light. Moreover, cord guide assembly  37  provides a lateral centering capability of the central cords, so as to prevent entanglement of the cords during transportation of the blind. 
     It will be appreciated that any other aforementioned configuration of the cord guide assembly, driving assembly, headrail, and transversal cover may similarly be employed in conjunction with blind  500 . 
     Reliability Tests 
     A manually operable internal Venetian blind adapted for tilting and lowering/raising a plurality of slats was tested for reliability by Hollis Metal Industries Ltd., Industrial Zone Alon-Tavor, Israel between May 29, 2005 and Jun. 21, 2005. The Quality Assurance Group supervised the measurement of the tolerance tests, and the Electronic Group supervised the counting of the testing cycles and verified the operation of the testing apparatus. 
     The blind had a height of 200 cm and a width of 100 cm, and the slats had a width of 16 mm. The lowering/raising arrangement had a stroke length of 80 cm, i.e. 40% of the distance to which the longitudinally lowest slat is raised or lowered. The tilting arrangement had a stroke length of 4 cm. The arrangement for tilting the slats included a driving assembly having a D-shaped core and a plurality of radially extending ribs, and tilting devices to which the corresponding outer cords were affixed by nails having radial protrusions. A laterally centering cord guide assembly was used. 
     The blind was mounted without its glass sheets onto a custom made testing apparatus. The testing apparatus had a motor for actuating the tilting arrangement and the lowering/raising arrangement simultaneously, an electronic control system for the motor, sensors for detecting the end of the actuator stroke, and an electronic counting system. A first pulley and a second pulley having a considerably smaller diameter than that of the first pulley were mounted on the drive shaft of the motor, and a third pulley was driven by a cord wound around the second pulley. A cord wound around the first pulley was connected to the counterweight of the lowering/raising arrangement and simulated the action of a linear actuator. The cord wound around the third pulley was connected to the internal magnet housing of the tilting arrangement, to which was connected another counterweight, and simulated the action of a linear actuator. The transmission of the first, second and third pulleys was such that the stroke length ratio of the tilting arrangement counterweight to the lowering/raising counterweight was 1:20, and the tilting and lowering/raising arrangements underwent the same number of testing cycles. The bearing housing was removed from the tilting arrangement since glass sheets were not used. The testing conditions did not simulate solar radiation radiating on the blind. 
     Prior to the first testing cycle, selected components of the blind were visually inspected and measured. Each measurement had a tolerance of 0.001 mm. The following components were selected for inspection and measurement:
     a) bearing housing;   b) driving assembly;   c) cord guide assemblies;   d) 3 tilting devices;   e) nails with radial protrusions;   f) 2 bearing blocks;   g) interconnecting plastic rod;   h) 2 plain bearings: housed in a bearing block;   i) 4 bearings made from polyacetal: housed in a bearing housing of the tilting arrangement and in a cord guide assembly; and   j) 2 ball bearings: housed in a bearing block and bearing housing.   

     After inspection, the components were assembled and a first group of 26,680 testing cycles was performed. Each testing cycle included the following steps applied to the lowering/raising arrangement: a) the counterweight was raised for a duration of 13 seconds; b) a delay of 3 seconds; c) the counterweight was lowered for a duration of 10 second; and d) a delay of 2 seconds. The aforementioned components were removed from the blind, and a second visual inspection and measurement of the components was performed. The measured level of wear of the aforementioned components did not exceed the permissible value of 1 mm, with greatest measured level of wear being no more than 0.4 mm. 
     The aforementioned components were assembled and a second group of 20,180 testing cycles was performed. The aforementioned components were removed from the blind, and a third visual inspection and measurement of the components was performed. The measured level of wear of the aforementioned components ranged from 0-0.7 mm. Some small signs of wear were noticeable on the metallic components which did not negatively influence the operation of the blind. With the exception of the interconnecting plastic rod, no signs of wear were noticeable on the plastic components. Some small signs of wear were noticeable on the interconnecting plastic rod which did not negatively influence the operation of the blind. 
     The following are three examples of the measured wear:
     A) Diameter of the arcuate opening of the cord guide assembly—   1. Measurement 1-6.85 mm   2. Measurement 2-6.85 mm   3. Measurement 3-6.85 mm   B) Diameter of the inner portion of the ball bearing housed in the bearing block, over which the central cords were wound—   1. Measurement 1-15.73 mm   2. Measurement 2-15.68 mm   3. Measurement 3-15.68 mm   C) Diameter of the connecting element which transversally extends between the longitudinally rod and the contractible elements of the interconnecting plastic rod—   1. Measurement 1-4.96 mm   2. Measurement 2-4.95 mm   3. Measurement 3-4.88 mm   

     The diameter of the connecting element remained greater than the accepted tolerance of 2 mm. 
     Since the blind is actuated on the average 3 times a day, it can be concluded, after a total number of 46,860 testing cycles without noticeable wear, that the blind of the present invention can operate reliably for at least 42 years without failing. 
     While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without, departing from the spirit of the invention or exceeding the scope of the claims.