CRT with specific envelope thickness

Picture display device with an evacuated tube comprising, around a longitudinal axis, a display window, a conical portion and a neck portion, said conical portion being connected to an upstanding wall of the display window. The wall thickness of the conical portion is selectively reduced. In particular, the wall thickness in an area of the conical portion is substantially constant.

BACKGROUND OF THE INVENTION
 The invention relates to a picture display device comprising a display tube
 having an evacuated envelope, which envelope comprises, around a
 longitudinal axis, a display window with a display screen on its inner
 side, a conical portion and a neck portion, the conical portion being
 connected to an upstanding wall of the display window.
 The invention also relates to a conical portion for use in a picture
 display device.
 Picture display devices of the type described in the opening paragraph are
 used, inter alia, in television apparatuses and computer monitors and are
 referred to as cathode ray tubes (CRTs).
 A picture display device of the type described in the opening paragraph is
 known.
 The known picture display device has some drawbacks, notably a large weight
 and high cost price of its conical portion.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a picture display device in
 which said problem is alleviated.
 To this end, the picture display device according to the invention is
 characterized in that the conical portion has a wall thickness, wherein
 the wall thickness d.sub.sa along a short axis of the conical portion as a
 function of a distance z in a first area of the conical portion adjacent
 to the upstanding wall of the display window is defined by the relation:
EQU .delta.d.sub.sa /.delta.z=C.sub.1,
 and, in a second area adjacent to the first area, is defined by the
 relation:
EQU .delta.d.sub.sa /.delta.z=C.sub.2,
 in which .vertline.C.sub.1.vertline.&gt;.vertline.C.sub.2.vertline., and in
 which the first area comprises a part of the conical portion for which
 0.ltoreq.z.ltoreq.a, and the second area comprises a part of the conical
 portion for which a.ltoreq.z.ltoreq.80%, in which z represents a relative
 distance measured with respect to the projection on the longitudinal axis
 between the connection with the upstanding wall of the display window and
 the transition to at least substantially axial symmetry in the conical
 portion, and in which a is in the range of 10%.ltoreq.a.ltoreq.60%.
 The conical portion is provided as a kind of linking element between the
 upstanding wall of the display window of the known picture display device,
 which upstanding wall usually has a relatively large wall thickness, and
 the neck portion which usually has a relatively small wall thickness, the
 wall thickness of the conical portion, measured from the upstanding wall
 towards the neck portion, generally decreasing linearly. In other words,
 in the known display device, the variation (direction coefficient)
 .delta.d.sub.sa /.delta.z of the wall thickness d.sub.sa along the short
 axis of the conical portion as a function of the relative distance z is
 constant across the conical portion. Hitherto, it has been assumed that
 such a linear variation of the wall thickness of the conical portion was
 necessary, both from a design point of view and for reasons of
 moldability. The inventors have recognized that such a linear variation is
 not necessary. Due to the measure according to the invention, the wall
 thickness of the conical portion in the first area undergoes a larger
 variation (decrease) than that in the second area. Consequently, a conical
 portion is obtained which comprises less material (glass) than the conical
 portion of the known picture display device. Due to the reduction of the
 quantity of material, the picture display device as a whole will be less
 heavy. A reduction in weight of picture display devices is notably
 important for those devices having a relatively large picture diameter
 because such apparatuses can otherwise be hardly lifted. The measure
 according to the invention may be used to advantage, notably for picture
 display devices having a large deflection angle (&gt;100%). For picture
 display devices having such a large deflection angle, the tensions in the
 glass of the conical portion are relatively high. To compensate for such
 high tensions, thicker glass is generally used. Due to the measure
 according to the invention, such an increase of the quantity of material
 is not necessary in picture display devices having a large deflection
 angle (&gt;100%). The weight reduction of the conical portion has the
 additional advantage that the picture display device as a whole thus has a
 lower cost price.
 In this application, three main directions (see FIG. 2) are distinguished,
 namely the thickness variation of the conical portion along a
 cross-section of the conical portion parallel to the diagonal (to one of
 the corners of the display window), the thickness variation along a
 cross-section of the conical portion parallel to the short axis (to the
 upper and lower side of the display window), and the thickness variation
 along a cross-section of the conical portion parallel to the long axis (to
 the sides of the display window). The short axis is also referred to as
 "short cross-section" and the long axis is also referred to as the "long
 cross-section".
 It is to be noted that C.sub.1 and C.sub.2 do not need to be constants but
 may be dependent on the relative distance z. Notably in the first area
 adjacent to the upstanding wall of the display window, a strong decrease
 of the wall thickness is preferably realized, so that in the second area,
 adjacent to the first area, the wall thickness in a direction remote from
 the display window does not decrease or hardly decreases. An embodiment of
 the picture display device according to the invention is characterized in
 that .vertline.C.sub.2.vertline..ltoreq.0.02. In the second area, the wall
 thickness d.sub.sa along the short axis as a function of the relative
 distance z is at least substantially constant, as measured with respect to
 the wall thickness in the first area. An alternative embodiment of the
 picture display device according to the invention is characterized in that
 the ratio between the variation of the wall thickness d.sub.sa in the
 second area, divided by the length of the second area, and the variation
 of the wall thickness d.sub.sa in the first area, divided by the length of
 the first area, is larger than 5. In this way, a considerable saving in
 weight of the conical portion is realized.
 According to the invention, the value of the parameter a is in a range of
 10%.ltoreq.a.ltoreq.60%. If a.apprxeq.10%, the first area is
 0%.ltoreq.z.ltoreq.10%, in which the wall thickness d varies (decreases)
 to a relatively large extent, and relatively small with respect to the
 second area of 10%.ltoreq.z.ltoreq.80% in which the wall thickness d
 varies (decreases) to a relatively small extent. If a.apprxeq.60%, the
 first area is 0%.ltoreq.z.ltoreq.60%, which is relatively large with
 respect to the second area of 60%.ltoreq.z.ltoreq.80%. Values for a&lt;10%
 or a&gt;60% are unfavorable for realizing material savings. The value for
 the parameter a is preferably in a range of 25%.ltoreq.a.ltoreq.50%.
 Notably in this range of the parameter a, the conical portion can be
 satisfactorily molded (at a relatively low molding force) and so-called
 folds are prevented when molding the conical portion. A particularly
 suitable value of the parameter is a.apprxeq.40%, at which a minimal
 molding force of the conical portion is realized.
 An embodiment of the picture display device according to the invention is
 characterized in that the wall thickness d.sub.sa in the second area is
 defined by the relation
EQU 0.9xd.sub.pa.ltoreq.d.sub.sa.ltoreq.1.25xd.sub.pa,
 in which d.sub.pa is a wall thickness of the conical portion at the area of
 the transition to at least substantially axial symmetry.
 The upstanding wall of the display window generally has a non-axially
 symmetrical shape with respect to the longitudinal axis, whereas the neck
 portion of the picture display device usually has an axially symmetrical
 shape. Viewed from the display window, the conical portion has a shape in
 the first and the second area which is adapted to the symmetry of the
 display window (for example, an at least substantially fourfold symmetry)
 and has a transition to axial symmetry at an end portion of the second
 area remote from the display window.
 By choosing the ratio of the wall thicknesses d.sub.sa /d.sub.pa to be
 between 0.9 and 1.25 in the second area, a considerable saving of material
 for the conical portion is realized and a satisfactory moldability is
 achieved. Instead of the known linear decrease of the wall thickness, the
 wall thickness d.sub.sa has a size of the same order in the second area as
 the wall thickness d.sub.pa at the area of said transition to at least
 substantially axial symmetry. The wall thickness d.sub.sa for said part of
 the conical portion is preferably defined by the relation:
EQU 0.95xd.sub.pa.ltoreq.d.sub.sa.ltoreq.1.10xd.sub.pa.
 Instead of the known linear decrease of the wall thickness, the wall
 thickness d.sub.sa along the short axis of the conical portion in said
 part of the conical portion is at least substantially equal to the wall
 thickness d.sub.pa at the area of said transition to at least
 substantially axial symmetry. This leads to a further saving of material
 for the conical portion.
 In the known picture display device, the variation of the thickness of the
 conical portion is at least substantially equal for the various directions
 in which the neck portion is reached from the upstanding wall of the
 display window. In other words, the thickness distribution in the
 circumferential direction (measured by determining the section of the
 conical portion with projection planes perpendicular to the longitudinal
 axis) is usually constant along each of the three main directions (the
 short axis, the long axis and the diagonal) of the conical portion. The
 inventors have also recognized that a further saving of material (and
 hence a cost price reduction) of the conical portion is realized by
 allowing a suitable degree of the thickness variation along the various
 main directions. This further recognition may also be considered
 separately from the above-mentioned change of the direction coefficient
 .delta.d.sub.sa /.delta.z at the interface of the first and the second
 area. To this end, an embodiment of the picture display device according
 to the invention is characterized in that, in the second area, the ratio
 of a wall thickness d.sub.diag along a diagonal of the conical portion
 with respect to the wall thickness d.sub.sa along a short axis of the
 conical portion is defined by the relation:
EQU 1.2.ltoreq.d.sub.diag /d.sub.sa.ltoreq.1.6.
 By using such a thickness variation in the circumferential direction of the
 conical portion, a considerable quantity of material is saved.
 Particularly suitable (also with a view to moldability of the conical
 portion) is a ratio value of d.sub.diag /d.sub.sa.apprxeq.1.4. A similar
 ratio may also be given for the wall thickness d.sub.la along a long axis
 of the conical portion. In the second area, a wall thickness d.sub.la
 along a long axis of the conical portion is preferably defined by the
 relation:
EQU d.sub.sa.ltoreq.d.sub.la.ltoreq.1.10xd.sub.sa.
 A particularly suitable thickness variation in the circumferential
 direction of the conical portion, saving a considerable quantity of
 material, is obtained with a cone design for which, in the second area,
EQU d.sub.sa.apprxeq.d.sub.pa,
EQU d.sub.la.apprxeq.1.05xd.sub.sa,
EQU d.sub.diag.apprxeq.1.4xd.sub.sa.
 Notably for these values of the ratio of the wall thicknesses in the
 circumferential direction, measured along the various main directions, a
 conical portion can be manufactured (molded) satisfactorily, while folds
 are prevented during the molding process.

DETAILED DESCRIPTION OF THE INVENTION
 The invention will now be described in greater detail with reference to the
 figure of the drawings.
 FIG. 1 is a diagrammatic cross-section of a picture display device
 comprising a cathode ray tube (CRT) having a longitudinal axis 20 and an
 evacuated envelope 1 comprising a display window 2, a conical portion 4
 and a neck portion 5. A display screen 3 is provided on the inner surface
 of the display window 2. The display screen 3 comprises a large number of
 red, green and blue-luminescing phosphor elements. In this embodiment, the
 neck portion 5 comprises three electron guns 6, 7 and 8 for generating
 three electron beams 9, 10 and 11 which are usually situated in one plane,
 here the plane of the drawing. On their way to the display screen 3, the
 electron beams 9, 10 and 11 are deflected in two mutually perpendicular
 directions (a field and line deflection direction) by the deflection unit
 14 across the display screen 3 and pass a color selection electrode 13
 arranged in front of the display window 2, which electrode usually
 consists of a thin plate having apertures 12 and is referred to as shadow
 mask in this case. The color selection electrode 12 is suspended to the
 inner side of the upstanding wall 15 of the display window 2 with the aid
 of suspension means 16. The transition between the conical portion 4 and
 the upstanding wall 15 of the display window 2 is also referred to as the
 "seal edge" 19 where a (glass) frit is present, which frit serves as a
 sealing material. The three electron beams 9, 10 and 11 pass the apertures
 12 of the color selection electrode 13 at different angles and thus each
 impinge on phosphor elements of one color only. The inner side of the
 conical portion 4 is usually coated with a conducting coating 18.
 FIG. 2 is a perspective elevational view, partly broken away, of a part of
 the cathode ray tube of FIG. 2, with longitudinal axis 20, display window
 2 and upstanding wall 15, conical portion 4 and neck portion 5. For the
 sake of clarity, some of the components shown in FIG. 1 have been omitted
 in this Figure. The thickness variation of the conical portion 4 is
 different for the various directions in which the upstanding wall 15 of
 the display window 2 is reached from the neck portion 5. Generally, three
 main directions can be distinguished, namely the thickness variation of
 the conical portion 4 along a cross-section of the conical portion 4
 parallel to the diagonal 21 (to one of the corners of the display window),
 the thickness variation of the conical portion 4 along a cross-section of
 the conical portion 4 parallel to the short axis 22 (to the upper or lower
 side of the display window), and the thickness variation of the conical
 portion 4 along a cross-section of the conical portion 4 parallel to the
 long axis 23 (to the sides of the display window).
 FIGS. 3, 4A and 4B show examples of the contour of a color picture display
 device. FIG. 3 is a side elevation of a part of a cathode ray tube with a
 longitudinal axis 20, a display window 2 and upstanding wall 15, a conical
 portion 4 and a neck portion 5. The relative distance z is measured with
 respect to the projection on the longitudinal axis 20 from the connection
 with the upstanding wall 15 of the display window 2 to the transition to
 at least substantially axial symmetry in the conical portion 4. In this
 application, the conical portion 4, viewed from the upstanding wall 15, is
 considered to be divided into four contiguous areas which are denoted by
 the reference numerals 41, 42, 43 and 44, respectively. The first area 41
 of the conical portion 4 is bounded by 0.ltoreq.z.ltoreq.a, the second
 area 42 is bounded by a.ltoreq.z.ltoreq.80%, at which the value for the
 parameter a is in the range of 10%.ltoreq.a.ltoreq.60%. In this
 application, the parameter a is introduced so as to be able to choose the
 transition from the first to the second area at different values of the
 relative distance z in the design of the conical portion 4. In the
 preferred case, where a=40%, the first area 41 is bounded by
 0.ltoreq.z.ltoreq.40%, and the second area 42 is bounded by
 40%.ltoreq.z.ltoreq.80%. The third area 43 of the conical portion 4 has
 values of 80%.ltoreq.z.ltoreq.100% for the relative distance, at which
 z=100% corresponds to the transition to at least substantially axial
 symmetry in the conical portion. The fourth area 44 comprises the part of
 the conical portion 4 between said transition to at least substantially
 axial symmetry and the transition to the neck portion 5 of the picture
 display device.
 The upstanding wall 15 of the display window 2 generally has a relatively
 large wall thickness (typical thicknesses are 7-12 mm), whereas the neck
 portion 5 generally has a relatively small wall thickness (typical
 thicknesses are 3-6 mm). The wall thickness of the upstanding wall 15 is
 dependent on the diameter and the (glass) composition of the display
 window 2 and on requirements imposed on the permeability to X-rays of the
 material used. In the first area 41, the second area 42 and the third area
 43, the conical portion 4 has a symmetry with respect to the longitudinal
 axis 20 adapted to the symmetry of the display window 2 (oriented with
 respect to the at least substantially rectangular shape of the display
 window 2; see also FIG. 2). In the fourth area 44, the conical portion 4
 has an at least substantially axially symmetrical shape with respect to
 the longitudinal axis 20. The (at least substantially axially symmetrical)
 deflection unit 14 is generally arranged around this fourth area 44 of the
 conical portion 4. Often, the conical portion 4 at the area of the
 deflection unit 14 (area 44 ) is provided with ducts on its inner side so
 as to increase the effective deflection angle of the picture display
 device. Generally, the conical portion 4 in said fourth area 44 is
 perfectly round (circularly cylindrical), but in some picture display
 devices, the conical portion 4 in the fourth area 44 has a certain extent
 of out-of-roundness, with the deflection unit 14 being adapted thereto as
 regards shape. It is alternatively possible to implement the conical
 portion 4 in the fourth area 44 in a fourfold symmetry.
 The transition in the conical portion 4 between the third area 43 and the
 fourth area 44 is characterized by a transition in the curvature of the
 conical portion to at least substantially axial symmetry, which transition
 (viewed from the neck portion 5) is also referred to as "top of round" by
 those skilled in the art. Said transition usually coincides with the end
 portion of the deflection unit 14 facing the display window 2 and referred
 to as the "outward flaring flange of the deflection unit 14 ". The
 relative distance z measured from the upstanding wall 15 of the display
 window 2 only covers the first, second and third areas (41, 42, 43 ) of
 the conical portion 4. The longitudinal axis 20 is scaled in percents by
 determining the section of the relevant conical portion 4 with projection
 planes perpendicular to the longitudinal axis 20. At the position on the
 longitudinal axis 20 corresponding to z=0%, the projection plane
 intersects the transition between the conical portion 4 and the upstanding
 wall 15, at z=100%, the projection plane intersects said transition to at
 least substantially axial symmetry (to the end portion of the deflection
 unit 14 facing the display window 2). In FIG. 3, the thickness d.sub.se is
 the wall thickness of the conical portion 4 at the area of the transition
 between the upstanding wall 15 of the display window 2 and the conical
 portion 4, indicated by z=0% on the longitudinal axis 20 in FIG. 3.
 Generally, d.sub.se is at least substantially equal to the wall thickness
 of the upstanding wall 15 at the area of the transition between the
 upstanding wall 15 and the conical portion 4. The thickness d.sub.pa is
 the wall thickness of the conical portion 4 at the area of the transition
 in the conical portion 4 to at least substantially axial symmetry
 (interface of the third area 43 and the fourth area 44 ), indicated by
 z=100% on the longitudinal axis in FIG. 4. Wall thicknesses of the conical
 portion 4, such as d.sub.pa and d.sub.se, are measured perpendicularly to
 the curvature at the area of the wall (instead of in alignment with the
 projection plane perpendicular to the longitudinal axis).
 In the example of FIGS. 4A and 4B, the display window 2 has a nominal
 screen diameter of 66 cm, the deflection angle is 106.degree. and the
 aspect ratio is 16:9. Contour (p) in FIG. 4A shows the contour of the
 cathode ray tube, measured along the short axis (22), contour (q) shows
 the contour measured along the long axis (23) and contour (r) shows the
 contour measured along the diagonal (21) (see FIG. 4B). Viewed from the
 upstanding wall 15 (seal edge 19) up to the transition to at least
 substantially axial symmetry in the conical portion, this conical portion
 is divided as a function of the relative distance z into a number of
 sections 51, 52, 53, . . . , 66, 67, (in this example: 17 sections), all
 of which sections have the same thickness with respect to the longitudinal
 axis 20. FIG. 4B shows the angle dependence of the contours of a number of
 sections 51, 52, 53, etc., as shown in FIG. 4A: along the longitudinal
 axis (23), the angle is 0.degree.[contour(q)], along the short axis (22),
 the angle is 90.degree.[contour(p)]; in this example, the angle along the
 diagonal direction is 29.36.degree.[contour(r)]. Table I shows the maximum
 distances of the relevant sections as a function of the angle for each of
 the sections 51, 52, 53, etc., as shown in FIG. 4A. The side of section 51
 facing the display window 2 is connected via the seal edge 19 to the
 upstanding wall 15 of the display window 2. It is apparent from Table I
 that for section 67, corresponding to the transition to axial symmetry
 (front end of the deflection unit), the distances to the longitudinal axis
 20 are at least substantially equal for all angles. On the side of section
 67 remote from the display window 2, the conical portion 4 is (perfectly)
 round: the distances of the section to the longitudinal axis 20 deviate by
 less than 5% as a function of the angle.
 TABLE I
 Contour of a conical portion
 Nominal distance maximum distance from the
 from section 51 the longitudinal axis (mm)
 z (p) (r) (q)
 section (mm) 0.degree. 29.36.degree. 90.degree.
 51 0 312.7 350.4 191.8
 52 10 311.8 348.8 191.2
 53 20 309.8 344.5 189.9
 54 30 306.3 335.8 187.5
 55 40 301.2 322.8 184.0
 56 50 294.8 308.2 179.2
 57 60 287.1 292.8 173.3
 58 70 277.9 276.6 166.5
 59 80 266.9 259.5 159.1
 60 90 253.5 241.0 151.2
 61 100 237.2 221.5 142.8
 62 110 217.5 201.5 134.0
 63 120 193.8 180.5 124.6
 64 130 166.4 157.4 114.4
 65 140 136.9 132.1 103.1
 66 150 106.8 105.5 90.8
 67 160 77.3 77.3 77.5
 FIG. 5 shows a graph of the wall thickness d.sub.sa of the conical portion
 4 as a function of the relative distance z measured along the short axis
 22 in a direction from the upstanding wall 15 of the display window 2 to
 the transition to at least substantially axial symmetry in the conical
 portion 4. As shown above, d.sub.se corresponds to z=0% and d.sub.pa
 corresponds to z=100% (in the example of FIG. 5, d.sub.se =12 mm and
 d.sub.pa =4 mm). Curve (a.sub.1) represents the (linear) variation of
 thickness in the conical portion 4 of the known picture display device.
 Curves (a.sub.2) and (a.sub.3) show examples of the decrease of thickness
 of the wall of the conical portion 4 according to the invention. In curve
 (a.sub.2), there is a kink at z=40%, which kink is denoted by reference
 numeral 71 in FIG. 2 and at which the gradient of the wall thickness
 .delta.d.sub.sa /.delta.z changes. The position of the kink 71 is
 determined by the value of the parameter a in curve (a.sub.2) and may be
 chosen in the range of 10.ltoreq.z.ltoreq.60%. With a view to the
 possibility of manufacturing (molding) the conical portion 4 and
 preventing folds during the molding process, it is desirable to situate
 the kink in curve (a.sub.2) in the range of 10%.ltoreq.a.ltoreq.60%. A
 minimal molding force is realized at a value of the parameter
 a.apprxeq.40%. An example of such a variation of the wall thickness of the
 conical portion 4 is shown in curve (a.sub.3). In curve (a.sub.3), the
 gradient of the wall thickness of the conical portion 4 is at least
 substantially constant for values of z.apprxeq.20%, and the wall thickness
 d.sub.sa along the short axis is at least substantially equal to the wall
 thickness d.sub.pa at the area of the transition to at least substantially
 axial symmetry (z=100%). In curve (a.sub.3), the direction coefficient of
 the wall thickness changes as a function of the relative distance z at
 z.apprxeq.20% (denoted by reference numeral 72 in FIG. 5). In curve
 (a.sub.3), there is a gradual change of the gradient of the wall thickness
 for values of z.ltoreq.20%, so that a satisfactory transition between the
 conical portion 4 and the upstanding wall 15 of the display window 2 is
 realized.
 FIG. 6 shows a graph of the wall thickness d.sub.sa, d.sub.la and
 d.sub.diag of the conical portion 4 as a function of the relative distance
 z measured along the short axis 22, the long axis 23 and the diagonal 21,
 respectively, in a direction from the upstanding wall 15 of the display
 window 2 to the transition to at least substantially axial symmetry in the
 conical portion 4. At z=0% and z=100%, the wall thickness
 d.sub.sa.apprxeq.d.sub.la.apprxeq.d.sub.diag.apprxeq.d.sub.pa. Curve
 (a.sub.1) represents the (linear) thickness variation in the conical
 portion 4 of the known picture display device (see also FIG. 5). Curves
 (b.sub.1), (b.sub.2) and (b.sub.3) show examples of the decrease of
 thickness of the wall of the conical portion 4 in the three main
 directions according to the invention. Curve (b.sub.1) shows the decrease
 of thickness of the wall along the short axis 22 of the conical portion 4,
 curve (b.sub.2) shows the decrease of thickness along the long axis 23 and
 curve (b.sub.3) shows the decrease of thickness along the diagonal 21. In
 curves (b.sub.1), (b.sub.2) and (b.sub.3), there is a kink at
 z.apprxeq.25%, at which kink the gradient of the wall thickness
 .delta.d.sub.sa /.delta.z, .delta.d.sub.la /.delta.z and .delta.d.sub.diag
 /.delta.z changes. In the curves (b.sub.1), (b.sub.2) and (b.sub.3), the
 gradient of the wall thickness of the conical portion 4 is at least
 substantially constant for values of z.gtoreq.25%, and the wall thickness
 d.sub.sa along the short axis is at least substantially equal to the wall
 thickness d.sub.pa at the area of the transition to at least substantially
 axial symmetry (z=100%). In the curves (b.sub.1), (b.sub.2) and (b.sub.3),
 there is a gradual change of the wall thickness gradient for values of
 z.ltoreq.25%, so that a satisfactory transition between the conical
 portion 4 and the upstanding wall 15 of the display window 2 is realized.
 In FIGS. 5 and 6, the curves (a.sub.2), (a.sub.3) and the curves (b.sub.1),
 (b.sub.2) and (b.sub.3) are situated below curve (a.sub.1): the area
 enclosed by the afore-mentioned curves illustrates the saving of material
 for the conical portion. Such a reduction of required material leads to a
 considerably lower cost price of the picture display device. A measure of
 the material saving for the conical portion 4 is the weight of this
 portion. For example, a conical portion 4 of a picture display device
 having a screen diameter of 55 cm, with the wall thickness of the conical
 portion 4 decreasing similarly as in the known picture display device, and
 as described by curve (a.sub.1) in FIG. 5, weighs 4.83 kg. A conical
 portion 4 according to the invention, whose wall thickness decreases as
 described by curve (a.sub.2) in FIG. 5, weighs 3.88 kg. A conical portion
 4 according to the invention, whose wall thickness decreases as described
 by curve (a.sub.3) in FIG. 5, weighs 3.72 kg. In the latter case, the
 total weight of glass of a conical portion with respect to the conical
 portion of the known picture display device has decreased by approximately
 25%.
 It will be evident that many variations within the scope of the invention
 can be conceived by those skilled in the art. Generally, measures
 according to the invention, relating to the z direction (the "height" of
 the conical portion) lead to a reduction of the weight of the conical
 portion. Measures according to the invention, relating to the
 circumferential direction (sections of the conical portion with projection
 planes transverse to the z direction) lead to a satisfactory moldability
 or to an improved moldability of the conical portion.