This invention relates to a leading mark indicator, more precisely to an instrument for indicating to an observer a definite direction. A device of the kind here concerned is disclosed previously in SE-PS 8006622-8.

Said device comprises a screen- in front and a screen behind, which include opaque lines separated by transparent inter¬ stices, whereby an interference .pattern, so-called moire pattern, arises when an observer views the device. Each of the screens comprises a plate, which is bent centrally to form .an obtuse angle, and by attachment means the screen plates are positioned so that the obtuse angles face toward each other. The front screen plate is provided with a screen division which is denser than the screen division of the screen plate behind. Alternatively, the obtuse angles face away from each other, and the front screen plate is provided with a screen division sparser than that of the screen plate behind. According to said known device, a number of dark interference strips are produced, which are in parallel with each other when the device is viewed in a direction perpendicular to the plane of the screen plates, i.e. the direction or leading mark, which the device is intended to indicate. When the device is viewed from a direction lying outside the -leading mark, which" he device is intended to indicate, an interference pattern is formed which consists of a great numb of parallel strips ^' , which form a number of arrows, or more properly a fishbone pattern, where. interference strips produc by the upper screen surfaces are angular to the interference strips produced by the lower screen surfaces.

Devices of this kind here referred to are known also from SE-PS Nos. 7611513-8 and 35^ 35^, which relate to an : ^• instrum at which the moire pattern is utilized for bringing about in- dications of a certain horizontal or vertical plane for an observer.

The firstmentioned SE-PS 7611513-8 relates to a device, at which three screens are utilized,_ and a predetermined plane is indicated in that a great -number of interference strips are in parallel, and the observer being outside said ^" plane is indicated in that an interference pattern including lines with.? discontinuous angular deviation arises.

The second one of the aforesaid patents, SE-PS 35 ¹ * 35 ¹ !, relates to a device, at which two screen plates are arranged angularly relative to each other, and the division of the screens, i.e. the number of opaque lines per length unit perpendicular to the opaque lines, is- chosen so that an interference pattern arises, which is intended to be read in respect of the pattern symmetry. It is extremely difficult to accurately determine the indicated plane by means of such a pattern.

All of said known devices , thus , are designed to form a great number of interference strips , the angles of which in one direction or the other form discontinuous ^' angle variations or asymmetric patterns when an observer is outside the plane or direction, . which the device is intended to indicate .

The total width of opaque lines normally is such that it ex¬ ceeds or is substantially equal to the tot l.yϊixii-h of trans¬ parent interstices .

This implies that such an instrument is difficult to read , especially on a large distance . The reasons substantially are two. A first reason is that an observer is met only by the light ^* which is transmitted through the screens , i. e . light of a surface of about half the device surface perpend¬ icular to the viewing direction. _ i is , for example for navigation purposes , essential that as great a proportion as possible of luminous surface is obtained in order to increase the visibility of the device , especially at a long distance or in bad weather .

A second reason Is due to the fact that a large number of dark i nterferenc stri ps and l ight i nterspaces by ^' necess i ty gives that each ^' interference stri and each i πtersoace i s relatively narrow in comparison with its length, wh reading of the interference pattern i s made more di ffi cult, es pec i a IJ- _^ ^g T

greater d stances or n a weat er. -

The present invention relates to a device of the aforesaid kind, by means of which a substantially more distinct indicat is obtained, and which is substantially easier to read at 5 a longviewing distance.

The present invention, thus, relates to a device for indicati a definite direction, comprising a acreen in front and a screen behind, which include opaque lines separated by trans¬ parent interstices, whereby an interference pattern, so-calle

10 moire pattern, arises when the device is viewed, which screen have different divisions, i.e. different distances between the opaque lines, which lines have the same width in both screens, and which transparent interstices have the same width within a screen, of which screens at least one is bent

Ϊ centrally to form a ^" n obtuse angle. The invention is character ized in that one of the screens is provided with only one opaque line less than the other one of the screens, that in said one screen the width of a transparent interstice plus the width of an opaque line is greater than or about equal

²⁰ to 1,5 times the width of the opaque line, but smaller than 1,9 times the width of the opaque line, preferably 1,5 times to 1,8 times the width of the opaque line, and that-the two screens are located symmetrically relative to each other, so that a central opaque line of one of the screens sym etric

² 5 ally overlaps a central transparent interstice of the second screen.

The invention is described in greater detail in the following, with reference to the accompanying _. drawing, in which Fig. 1 shows- a screen set according to a first embodiment, 30 Fig. 2 shows a screen set according to a second embodiment, Fig. 3 shows a screen set according to a third embodiment, Fig. 4-4 is a schematic view of the location of the screen lines relative to each other in the front and, resp¬ ectively, behind screen, 35 Fig. 5 is a view ^' of the device seen along is indicated leading mark, Fig. 6 is a view of the device seen to the right of its indicated leading mark,

Fig. 7 is a view of the device seen to the left of its__

•MO indicated leadin mark. f

: In Fig. 1 a screen set according to a first embodiment of the invention is schematically shown, at which each screen 1,2 includes .opaque lines 3 separated by transparent inter¬ stices . The screens 1,2 are positioned so overlapping that

5 an -observer viewing the screens from a direction indicated schematically by the arrow 5- looks through screens located in front and behind. Hereby an interference pattern, so-calle moire pattern, arises when the observer views the device.

The screens 1,2 have different divisions, i.e. different ¹⁰ numbers of opaque lines per length unit perpendicular to the .opaque lines. According to this set, the screen plate 1 in front is provided with a division which is denser than that o the screen plate 2 behind. Each screen 1,2 comprises a plate, , which is bent centrally to form an obtuse angle. The angle - 5 can be between 120 and 170 or smaller or greater, dependin on the desired sensitivity to changes in the interference pattern in response to the deviation of an observer from the leading mark. When the angle is small, the interference pattern is changed for a very small deviation from the leadin 0 mark. At a greater angle the interference pattern changes fir after a greater deviation from the leading mark.

. In Fig. 2 a second screen set is shown, at which the screens 6,7 are arranged so that the obtuse angles face away from eac other. 5 In this case the screen plate 6 in front is provided with a sparser division than the screen plate 7 behind.

In Fig. 3 a third screen set is shown, at which the screen 8 located behind is formed as a screen plate _which, as the scre plates 1,2,6,7 according to Figs. 1 and 2, consists of rect- 0 angular or square plane plates, which after having been prov¬ ided with a screen running in parallel with two sides is bent said angle perpendicularly to the screen lines. This implies, that each screen p-late has parallel sides where each screen line'is in parallel with two opposed parallel sides. The scree 59 in front according to Fig. 3 consists of a plane screen pla

which also is provided with screen lines in parallel with its sides. According to this embodiment, the screen plate 9 front has a denser screen division than the screen plate 8 . behind. The angle between screens in the screen plate 8 . _. located behind is chosen so that the tangent (tan) for half the angle is half the tangent (tan) for half the angle for the screens in Figs. 1 and 2, the sensitivity thereby remain the same.

According to the present invention, -the screens are designed so that only one interference strip is obtained across the entire surface of the device. See Fig. 5-

When the device is view ^τ ed from a direction located to the ri of the leading mark, the interference pattern. according to Fig. 6 arises, and when the device is viewed from a direction located to the left of the leading mark, the interference pattern shown in Fig. 7 arises.,The arrow-shape of the inter¬ ference patterns,thus, indicates the correction direction for coming to the leading mark.

This is effected according to the invention, in that one.of the screens 1,6,8 and, respectively, 2,7,9 is provided with only one opaque line more than the other one of the screens, whereby only one interference strip 10- is obtained. This is illustrated in Fig. -I where the screen 9 in front includes opaque lines with a width of b_. and transparent lines with a width of d ₁ . The screen 8 located behind also includes opaque ^• lines with a width of b. , but transparent interstices with a slightly greater width d .

In order to obtain the interference strip 10 be located symmetrically over the screen surface, the opaque lines of the screen in front are offset at the outer edges 10 of the screen relative to the opaque lines of the screen located behind.

This symmetry, for the interference strip 6 to arise in symmetric position on the screen when the observer is on the leading mark, implies that the screens shall be positioned so in relation to each other, that the central opaque line -

„on one of the screens shall overlap symmetrically a trans-, parent interstice on the second screen, which interstice constitutes the centre of the second screen. According to a preferred embodiment, at the screen 8 located behind an opaque line is located immediately close to each of its edges 11, while at the screen 9 in front the opaque line located closest to the respective edge 10 of the screen 9 runs at a distance d ₁ from the edge 10.

When Ϊ designates the number of lines in the screen 9 in front, and N _? is the number of lines in the screen 8 located behind, and when B is the width of the screens, according to the invention the screens, with the designatio in Fig. 4-44-., are designed so that the following relations are me . - -

B - d_. w = i (1) ^b l ^{+ d} l

₂ = N ₁ - 1 (2)

B - b

'1 +' *"2 = (3)

N ₂ - 1

The aforesaid applies depending on which of the embodiments according to Figs. 1,2 or 3 it refers to. Furthermore, what is said concerning the screen in front applies to the screen located behind, and vice versa, when it is desired that the deviation direction, instead of the correction direction, shall be indicated by the interference strip 10.

According to the invention, furthermore, the-width of the interference strip 10 is at maximum one third of the width B of the device, i.e. of the screen plates 1,2,6,7,8,9-

This is brought about in that the transparent interstice i the ^" screen in front is at least about 0,5 times the width of the opaque line , i.e. that one period (b_ _] +d ₁ ) shall be at least 1,5 x b_, . By increasing the transparent interstice so that one period is 1,6 x b.. , the width of the interference s decreases to about one fourth of the width B of the scr

The period under all circumstances shall be smaller than 2,0 x b.., because a period of 2,0 b implies transparent in stices being as wide as opaque lines. According to the presen invention, the period shall be smaller than 1,8 x b.., and 5 preferably between about 1,5 x b_. and 1,6 x b . - When a period exceeding 1,8 x b ₁ is chosen, the interference strip for most of the applications will be too narrow in respect of the visibility, which then decreases.

An increase in the width of the transparent interstice implies 10 that the difference in division between the screen in front ^• and the screen behind increases, which in its turn implies a lower sensitivity - measured as deviation from the leading mark necessary for the interference strip to deflect. However, a greater contrast between the interference strip and surround 15 ing light surfaces is obtained.

According to a preferred embodiment, the screens are designed so that the screen in front has ^" a division period of 1,5 x -r> _* to 1,6 x b..

This design, thus, yields a relatively narrow interference ²⁰ strip, with very good contrast to surrounding light surfaces, and a very high sensitivity at the aforesaid angle of the scre ^• •according to Figs. 1 and 2 of about 150 and a corresponding angle of the screens according to Fig. 3 of about 120.

Practical experiments have shown, that the eye sees much ■25 easier a deviation when there is only one interference strip, . especially at long distances, compared with when the inter¬ ference pattern consists of a great number of interference strips. The visibility, further, increases .substantially in that the light surfaces at a device according to the invention 30 are proportionally greater than at known devices of the kind here referred to.

A device according to the invention preferably is provided behind the screen located behind with light sources in the form of sodium lamps or high-pressure lamps, so that yellow 35 light is transmitted through the screens in the direction to the observer.

The present invention must not be regarded restricted to the embodiments set forth above. The screen located behin _d , for example, can be designed wider -than the screen in front, whereby opaque lines are located in those portions of the screen located behind which project out at the side of line-free portions on the screen in front, in order thereby additionally to increase the area of the light surfaces.

The opaque lines, further, instead of being designed on a plate, can consist of clamped strips or corresponding means. The invention, thus, can be varied within the scope ^' of the attached claims.