This invention relates to a leading mark, more precisely to an instrument for marking a definite direction for an obsr erver. Instruments utilizing light interference technique for indic¬ ating a plane are previously known a.o. through the Swedish patent specifications 7611513-8 and 354 354, which relate to an instrument utilizing moire technique for bringing about for an observer the indication of a certain horizontal or vertical plane.

The instrument acoording to said patents comprises three screens , each of which consists of opaque lines separated by transparent interstices. When viewing the instrument, indic¬ ation is obtained on that the observer is outside a certain plane defined by the instrument, in that an interference pattern, a so-called moire pattern arises which includes- lines with a discontinuous angular deviation. In the Swedish patent specification 354 354 an instrument is disclosed, -at which two screen plates are provided angularly relative one another, and the division of the screens, i.e. the number of opaque lines per length unit perpendicular to the opaque lines, re selected so that an interference patt¬ ern arises which is to be read in respect of the symmetry of the pattern. An accurate determination of the indicated plane by means of such a pattern is extremely difficult.

The instruments above referred to per ^u se .render ' very high accuracy, but this type of indication does not meet certain requirements at leading marking. One of these requirements is that upon viewing the instrum¬ ent it must be clearly apparent therefrom, in a simple way, preferably by marking in the form of arrows, in which direct¬ ion a deviation from the leading line" prevails . A further requirement is very uniform quality in respect of the active elements of the instruments and of their locat¬ ion, in order thereby to obtain identical interference patt-

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ems produced by different instruments for the same deviat ^¬ ion from the leading line.

The present invention solves the aforesaid problems by a comb¬ ination of the design of the screens and their attachment means. The screens according to the invention are formed so that the precision of the position in a first direction is less essential, and so that the necessary precision in respec of their position in a direction perpendicular to the first direction is obtained automatically by the attachment means. The present invention relates to an instrument for marking leading lines, comprising a front screen and a behind screen, which include opaque lines separated by transparent interstic¬ es, in such a manner, that upon viewing the instrument an interference pattern, a so-called moire pattern, arises,said screens having different division, i.e. different distances between the opaque lines. The invention is characterized in that each of said screens comprises a plate, which centrally is bent to form an obtuse angle, that two such screen plates, are positioned by attachment means so that the obtuse angles either face toward each other, in which case the front screen plate is provided with a screen division of greater density than the behind screen plate, or alternatively so that the obtuse angles face away from each other, in which case the front screen plate is provided with a screen division more sparse than the behind screen plate.

The invention is described in greater detail in the following, with reference to the accompanying drawings, in which

Figs.'l and 6 schematically show screen settings according to the invention, Fig. 2 shows the instrument seen along its stated leading line

Fig. 3 is a view where the right-hand side piece in Fig. 2 has been removed,

Fig. 4 shows the instrument observed to the left of its stated leading line, Fig. 5 shows the instrument observed to the right of its state leading line.

In Fig. 1 a screen setting according to the invention is show schematically, where each screen 1,2 includes opaque lines 3 separated by transparent interstices 4. The screens 1,2 are arranged overlapping so that an observer viewing the screens from a direction indicated schematically by the arrow 5 looks _# through front and behind screens. Hereby an interference patt ern, a so-called moire pattern, is formed when viewing the instrument.

The screens 1,2 have different division, i.e. different numb- ers of opaque lines per length unit perpendicular to the opaque lines.

Each screen l,-2 comprises a plate, which at its centre is bent to form an obtuse angle v, see Fig.. 3.

The angle v may be be"tween 120° and 170°, or below or above these values, depending on the desired sensitivity, as will be explained below.

According to one embodiment of the invention, two such screen plates 1,2 are positioned so that the obtuse angles face tow- ward each other, as shown in Fig. 1, and according to a sec- ond embodiment said plates are positioned so that the said angles face away from each other, as shown in Fig. 6. According to the first entioned embodiment, the front screen plate 1 is provided with a division, which is more dense than that of the behind screen plate 2. For applying the instrument within a preferred distance inter¬ val from about 25 m to 250 m, the opaque lines 3 of the front screen, for example, may have a width of 3 mm while the dist¬ ance between two adjacent opaque lines, i.e. the transparent interstice 4 has a width of 1,5 m. T e opaque lines 3 of the behind screen 2 also have a width of 3 mm while its transpar¬ ent interstices have a width of 1,75 mm. These measurement values are stated only as example.

The screen plates 1,2 are supported by attachment means, which comprise ^■• two lateral pieces 6,7, for example metal sheets or corresponding ones, which are _* arranged in parallel with each other and perpendicularly to the plane surfaces of the screen plates 1,2.

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The screen plates 1,2 are clamped between the lateral pieces 6,7 by means of set bolts 8 or corresponding ones extening between the lateral pieces 6,7.

The lateral pieces 6,7 are provided with indications in the form of upright pins 9 or the like arranged in pairs , or in the form of grooves. Between each pair of indications the edge 10 of a screen plate 1,2 extends. •The screen plates 1,2 are rectangular or square plane plates, which after having been provided with a screen extending in parallel with two sides are bent to the angle v perpendicul¬ ar to the screen lines. This implies that each screen plate 1,2 has parallel sides where every screen line is in par¬ allel with two opposed sides. Each of the screen plates 1,2 includes in respect of the plate a symmetrically located opaque line 3 or a transparent interstice 4.

When, thus, the screen plates 1,2 are positioned as shown schematically in Fig. 1 and are surrounded by the lateral pieces 6,7 and fitted-in between the pairs of indications 9, the screens are located so that their screen lines 3 are in parallel with each other, and so that said symmetrically loc¬ ated opaque line 3 or said transparent interstice 4 are loc¬ ated directly in front of each other, seen in a direction of observation which is perpendicular to the plane surfaces of the screen plates 1,2.

It is extremely essential that the screen lines 3,4 are in parallel relative to each other, and that the position of said symmetric line 3 or interstice 4 is such as stated above, in order to give rise to the formation of an inter- ference pattern consisting of dark strips 11 in parallel rel¬ ative to each other, as shown in Fig. 2, when the instrument is viewed perpendicularly to the plane surfaces of the screen plates 1,2 in a direction of observation indicated by the arrow 12 in Fig. 3, i.e. along the leading line shown by the instrument.

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The interference pattern or the accuracy expressed as the deviation from the leading line where the interference patt¬ ern shown in Fig. 2 is. disturbed, is affected relatively little by the fact that the screen plates 1,2 are slightly offset in vertical direction from the position shown in Fig. 3.

The first direction mentioned in the introductory portion above is to be understood as a vertical displacement in Fig. 2 of one screen plate relative to the second one. The second direction mentioned above in the introductory portion is to be understood as a displacement perpendicularly to the plane of the paper in Fig. 2, so that the position of the symmetric lines is not the one intended.

Due to the fact that the screen plates 1,2 are plane rectang- ular plates, which are provided with screen lines 3,4 extend.- ing in parallel with two sides and thereafter are bent to a definite angle, thus, a high accuracy of the instrument, also in respect of the angle v, is obtained when the screens are clamped between the plane lateral pieces 6,7. The afore-described screen plates 1,2, in a setting according to Figs. 1 and 3, yield an interference picture as shown schematically in Fig. 2 when the instrument is viewed along its leading line, i.e. in a direction perpendicular to the plane surfaces of the screen plates 1,2. When the instrument according to Figs. 1 and 3 is viewed from a direction located to the left of the leading line, the interference pattern shown in Fig. 4 is formed. It also con¬ sists of dark strips 13, but these strips 13 form arrows,which indicate the correction direction for coming to the leading line. When the instrument is viewed from a direction located to the right of the leading line, in a corresponding manner the interference pattern shown in Fig. 5 is formed which con¬ sists of arrows formed of dark strips 14 -which also indicate the correction direction. By designing the screen plates 1,2 and by positioning them as stated above, a simple and accurate instrument for marking leading lines and for indicating the correction direction is obtained.

When the screen plates are set as shown in Fig. 6, where the screen plates are designated 15,15, the front screen plate 15 is provided with a screen having a division sparser than that of the behind screen 16. The screen divisions here may be such as stated above. Hereby the interference patterns in Figs. 4 and 5 form in connection to the deviations from the leading line described in these Figures.

The angle v affects the sensitivity of the instrument. At a small angle v the interference pattern consisting of arrows forms for a very small deviation from the leading line. When the angle v is greater, this sensitivity is lower. Practical tests have shown that for an intended distance interval of 25 to 250 m an angle v of 145 is to be preferred. In the foregoing the terms leading line and leading mark have been used. These terms were used in order to express that the instrument alone, i.e. without using a further instrument or mark, shows a specific line when viewing the instrument, and also to express that the instrument is intended to be preferably used for indicating leading lines, especially at navigation.

According to Figs. 2 and 3, the instrument comprises, in addition to what was mentioned above, a front piece 17 of a transparent material, for example a plastic material, and a rear piece 18, preferably of sheet metal. It further co - prises preferably an inner wall 19 of transparent material, fo example plastic material, which inner wall is intended to serve as a diffusor for the light emitted by one or several light sources, preferably fluorescent tubes 20, which are located between the inner wall 19 and the rear piece 18. In order to increase the contrast of the interference pattern, the light sources are capable to emit yellow light. Alternat¬ ively the light sources 20 are capable to emit white light while the inner wall 19 is coloured so as to emit yellow light to the screen plates 1,2. An electric heating element 21 of known type further may be provided for heating the instrument and thereby to prevent

frost formation when the instrument is placed outdoors. When using a heating element 21, preferably a thermostat 22 is located in the upper part of the instrument for controlling the temperature within the instrument. The present invention can be used for a great number of purp¬ oses, for example leading marking for navigation. A special application is to arrange two instruments spaced from one another for marking each its leading line, which lines inter¬ sect each other in a point. Such an arrangement can be used, for example, for docking-in or parking aircraft, in which case the pilot follows the leading line of one of the instrum¬ ents to the point where he also is on the leading line of the second instrument, whereby the intended position for the aircraft is obtained. The present invention, thus, renders possible a simple instrum ent, which has a uniform quality in respect of the interferenc patterns produced by various instruments, and which shows a leading line and, in addition, the correction direction in the case when the observer is not on the leading line. The attachment means, of course, can be varied, and" the design of the screens can be altered without abandoning the idea of the invention.

The present invention, thus, must not be regarded restricted to the embodiments set forth above, but can be varied within the scoDe of the attached claims.

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