US1986187A - Television apparatus - Google Patents

Description

Jan. 1, 1935. H. M. DOWSETT E AL 7 TELEVI S ION APPARATUS Filed May 9, 1932 INVENTOR c) H. M. DOWSETT .-WAL a & NLEVIN BY 77 gin/W A '0 NEY Patented Jan. 1, 1935 I UNITED. STATE TE E ISION APrARA'rUs" Harry M elville Dowsett, Winchmore Hill, London, 7 Louis Edward Quintrell Walker, Braintree, and

Nyman Levin, Regents Park, London, England,

' assignors to Radio Corporation of America,a I

corporation of Delaware Application; May 9, 1932,

In Great Britain iMay 2 0, 1931 Sta Nf 610,126 1.1

I 1 Claim. {(131.17 4) This invention relates to television and like apparatus and more particularly to scanning apparatus for use in television and like systems.

The invention has for its object to providean "5 improved scanning system wherein certain pracr' tical limitations met with in most known systems are avoided. r v a l i y.

In the specification accompanying copending application. SerialNo' 592,513, filed February 12, 1932, there isdisclosed a television scanning apparatu's comprising a first driven member having hereon a plurality'of elementary areas adapted tofollov'v one another through the samelpath and. aseconddriven member associated with said 1 first-"member, and having thereon deflecting means adapted to follow one another through a path having an angular relationship to the path of the elementary areas 'on' the said first member,- the relative speeds and arrangement of said members being such that the picture surface to be scanned is explored in a series of parallel lines each due to the movement of an elementary area on the first member through the effective portion of itsv path, said lines being spaced from onejanother' by distances due to the deflection set up by'the movement of the second member.

An. examination of this system will show that, as is the case with many other systems, practical limitations impose themselves when high speeds and fine scanning are desired. Take the case of a picture of a given area scanned a number of timestper' second, by a light spot of the usual square section then the linear dimensions of the scanning lightspot must be inversely proportionalto the number'of scanning'lines perpicture; and'the ratio of the linear dimensions of the spot tothe length of the scanning lines is also inversely proportional to this number of scanning lines. This ratio is thesame as the aperture ratio'il e. the ratio of the aperture dimensions on the light projecting drum to the distance between .the apertures. 1 g V l It will be seen therefore that having given a certain speed of the scanning drum an increase in-the number of scanning lines per picture, i.-e., an increase in the fineness-of scanningwill in volve an increasein the number of-apertures, an 1 increase in the drum diameter and a decrease in theaper-ture ratio, and if thechange in the last factor is obtained by; decreasing the aperture dimensions there is soon found to be a practical limit determined by the'minimum aperturearea necessary toprovide a light-beam of sufiicient intensity to actuate, photo-electric cells satisfactorily. e .I-

This minimum size :dependsupon the light source available and forlight sources such as are now ordinarilyemployed in practice the minimum size is about 0.5 mm; square. If the "aperture dimensions be assumed fixed say. atthis minimum value a further increase in the fineness of scanning -involves a proportional greater increase in the drum diameter and here again practical limits impose themselves and for i a; drum running at 750" R. P; M. the practical mechanical limit is 5 reached with admin diameter-of about 1 metre, the practical mechanicallimit being reached at smaller diameters with higher speeds. v As an extreme practical example of these difiiculties, a picture 24 x24" scannediby 200 lines 12.5 times 1-0 per second'wouldrequire an aperture drum of about :25 diameter having 20 apertureseach 9.5 mma-square, saiddrum running at "a speed of 7,500 R. P. M; 1

."The inter-rel be better understood from the following, quantitative analysis, Suppose the-picture. area to be scanned is of length land breadth b and suppose it is to be scanned by acom bination of apertured drum and mirror wheel at right angles to one 20 another as are the lens drum and mirror wheel of Figure l to be described later. Let the size of the apertures be m square the distance between two-adjacent apertures b1; and the number of scanning lines n. Then the size of the spot on- 25v the screenis v p v 1 if the scanning spot moves parallel to the side of 3 length b." Hence hi on the drum is magnified to l b on the screen. Therefore b-divided by i v r n 35 equals v b1 7 V, l i I 40 1. e.,- i l slink 1 a Let,v 5

where leis a constant generally lying between and 2. Then 50 For n to be large a1 must be large, i e. bimust belarge .or armust be small. I v 60 ationship above referred to will 15 Since bi is directly proportional to the diameter of the drum, n" can onlybe increased'by increasing the diameter or decreasing the aperture.

As above stated, mechanical considerations set a limit beyond which the diameter cannot be increased, this limit being of the order of 1 meter for ordinary slow runningdrums, the said limit becoming, of course, smalleras the rate of rotation of the drum is increased. j

Mechanical and optical considerations also impose a limit below which a1 cannot be decreased, this limit being of the order of .5

The present invention provides an arrangement whereby the apparent size of the apertures can be reduced so that the mechanical'and optical limits above mentioned are not reached.

To put the matter in another way, the present invention avoids the above mentioned practical limits opposin'g an increase in scanningfspeed or fineness of scanning, this result being achieved by substituting forjthe apertures in the drum or other actual scanning member one or more apertures in association with the lenses of a lens wheel, so that in'efiect'the apparent size of an aperture is substituted for the real size thereof as the'dete'rminingfactor in thescanning system.

The invention is illustrated in; the accompany ing drawing in which Figure 1 shows one form of the invention using a fixed aperture, lens drum,

and rotating mirror, and Figure 2 shows a modification in which a movable aperture'and lens drum is substituted for the fixed aperturemovable lens drum 'o'fFigure 1.

Referring to Figure l which shows schematically one way of carrying the invention into effect a television scanning system comprises a'lantern or other convenient screened source 1 of light (the screen is not shown) the light from which is reflected by a mirror 2 and focussed and concentrated-by "any suitable lens system 3 upon a single aperture 4 in a fixed diaphragm or apertured member 5; .,Light passing through this aperture is projected upon a fixed prism or mirror 6 mounted behind the rim of a lens wheel '7, the 'beam'of light incident'upon said prism or mirror beingsubstantially parallel to the axis of the IGIISffWhBEl, and being reflected by said prism or mirror radially outwards with respect to the said wheel, ,so as to pass through a lens 7 carried by said wheel. The light passing through the lens is projected upon the mirrors of an ordinary scanning mirror wheel 8, whence it is reflected as a scanning spot 9 upon a screen 10. The beam of light incident upon the lens wheel 7 covers two of the lenses 7 on the lens wheel. The axis of rotation of the lens wheel 7 is at right angles to the'axis of rotation of the mirror wheel 8, the lens wheel thus providing one component (the high speed component) 91 scanning and the mirror wheel the other cornponent of scanning.

In order to avoid two spots of light appearing on the screen at one time (due to the fact that the beam of light. incident upon the wheel will cover two lenses) a gate or equivalent device (not shown) may be provided, the arrangement being such that as one spot leaves the screen another appears thereon. a

The arrangement in which light from the aperture covers two lenses 7 on the lens wheel 7 has the advantage that throughout the effective traverse of the light the spot upon the screen will be of substantially constant intensity. It will be seen that with the arrangement described, since the spot upon the screen is determined by what may be termed theapparent'size of the aperture, the lens wheel may be of relatively small size, and the lenses thereon may be much closer together than the apertures on an apertured drum could be.

Obviously, if desired, the single aperture 4 in the fixed plate 5 may be replaced by a plurality of apertures which move with the lenses '7' in the lens wheel 7 so that, from the optical point of View, the moving .apertures are equivalent to the single fixed aperture of Figure 1. An ar- 'rangement of this kind is schematically illustrated-'in'Figure 2 which shows part of a com- .bined aperturedand'lens drum. In Figure 2, 1

is a source of light which is arranged within a combined apertured and lens drum' as shown, the said combined drum consisting of two concentric drum elements moving together the inner drum element having apertures 4 and the outer drum element having lenses 9', the apertures and lenses being so arranged that the center line of a beam of light from the source 1 through an aperture 4' will pass radially through the center of a corresponding lens '7'. In Figure 2 an operative beam of light is shown in chain lines, the dotted lines representing radii passing through the series of apertures 4 and lenses 7'.

It is to be understood that the emergent (operative) beam of light shown in chain lines falls upon a mirror wheel such as the wheel 8 of Figure l whence it is reflected upon the television screen.

For a better understanding of the invention a quantitative example with reference to Figure 2 will now be given.

Suppose it is desired to scan a picture surface in 200 lines.

200 apertures 4 equally spaced from one another are punched or otherwise formed in the.

inner drum of the composite drum structure while the outer drumis provided with 200 corresponding lenses 7'. Then if Z1 is the radial distance between the inner and outer drum elements, Z2 the distance between the outer drum element and the picture and ,f the focal length of a lens in the outer drum element, the apparent'size of the scanning spots on the picture will be a function of Z1, Z2, and f while the dimension 1) (breadth) of the picture will be a function only of Z2 and the separation of the apertures 4 in the drum can be made large by increasing Z2 while still keeping Z1 small. The speed of the drum structure may be maintained small, and in the above specific case may be made equal to-the picture frequency of 750 R. P. M.

Having now described our invention, What we claim and desire to secure by Letters Patent is the following: y

In a television system wherein a picture area is scanned by a moving spot of light, a scanning'device comprising a rotary lens wheel supporting a plurality of lens elements, a source of scanning rays, a second "rotary member arranged concentrically with the first rotary member and having

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