Objective with several fields

An objective providing at least two fields of view and/or of surveillance from a single sighting axis dividing into two channels over one of which two magnifications and different fields can be obtained. The objective comprises a set (31) of at least one lens centered on a common optical axis (41) and, behind this set, with respect to the direction of propagation of the light, a reflector element (5). A movable component (61) constituted by at least one lens is movable between, on the one hand, a position between the set (31) of lenses and the reflector element (5) so as to be centered on the common optical axis, and, on the other hand, a position located after the reflector element (5) so as to be centered on the optical axis (43) symmetrical with the common optical axis (41) with respect to the reflecting surface of the reflector element (5). An instrument for viewing and/or surveillance, such as a periscope, can be equipped with this objective.

FIELD OF THE INVENTION 
The present invention relates to an objective providing at least two fields 
of view and/or of surveillance from a single sighting axis dividing into 
two channels, over one of which two magnifications and different fields 
can be obtained. This objective can equip an instrument for viewing and/or 
surveillance such as a periscope. 
BACKGROUND 
A periscope comprises a head prism which is pivotable, for sighting, about 
a horizontal axis and which deviates light coming from viewed objects 
towards an objective disposed in the upper part. After the objective, the 
beam of light passes through a set of image-conveying media. The beam of 
light is then reflected, at the foot of the periscope, at one or several 
reflectors, towards one or several eyepieces. 
Equipping periscopes with two objectives, one adapted to day viewing and 
the other to night viewing, has been tried. These two objectives are then 
centered on two parallel and separate optical axes. The images given by 
these two objectives are then collected by the set of image-conveying 
media which is common to the two channels. The space these objectives 
occupy is a disadvantage in making a submarine periscope head. 
SUMMARY OF THE INVENTION 
The subject of the present invention is an objective providing at least two 
fields of view or surveillance with a single sighting axis, one of these 
fields being relatively small and corresponding to a high magnification, 
the other field being relatively large and corresponding to a low 
magnification. The advantage of the common sighting axis is that it 
requires only a single aiming prism for sighting. In particular, the 
objective allows three separate fields to be obtained with a single 
sighting axis via two channels, over one of which two different fields can 
be obtained. Because it needs only a single head prism, this objective 
consequently takes up very little space. It can be incorporated in a 
periscope with one sighting channel allowing night viewing and the other 
channel providing two different magnifications and allowing day viewing. 
The objective according to the invention comprises a set of at least one 
lens centered on a common optical axis and, behind this set, with respect 
to the direction of propagation of the light, a reflector element and it 
is essentially characterized by the fact that it includes a medium 
constituted by at least one lens and capable of being moved between, on 
the one hand, a position between the set of lenses and the said reflector 
element so as to be centered on the common optical axis and, on the other 
hand, a position located after the said reflector element so as to be 
centered on the optical axis symmetrical with the common optical axis with 
respect to the reflecting surface of the reflector element. 
In order that the invention may be more clearly understood, reference will 
now be made to the attached drawings, wherein an embodiment of the 
invention is shown for purposes of illustration.

DETAILED DESCRIPTION 
The objective represented by FIGS. 1 and 2 is mounted in a periscope 
comprising, at the top, a prism 1 which can pivot, for sighting, about a 
horizontal axis. The objective is disposed behind this head prism 1 with 
respect to the direction of propagation of the light. It has an optical 
system for day viewing comprising a front set 31 of at least one lens and 
a rear set 32 of at least one lens. One set of lenses at least, such as 
34, is disposed behind the set 32. The objective also includes an optical 
system for night viewing comprising a set 21 of at least one lens and a 
rear set 22 of at least one lens. The set 22 can also be followed by a set 
of lenses, if necessary. The two front sets 21 and 31 are centered on the 
common optical axis 41. The front set 21 of the objective for night 
viewing is constituted by annular lenses which form a location in which 
the front set 31 of the objective for day viewing is disposed. In the 
embodiment represented by the drawings, each of the two sets 21 and 31 is 
constituted by a doublet formed of a converging lens coupled with a 
diverging lens. 
The objective includes a reflector element 5 which reflects the light beam 
from one of the viewing channels. This reflector element is located after 
the front set 31 of the objective for day viewing. It is equipped with a 
reflecting surface 51. This reflecting surface reflects the light beam 
coming from the front set 31. This reflecting surface therefore has a 
substantially elliptical shape centered on the common optical axis 41. 
This reflecting surface is preferably inclined at 45.degree. with respect 
to the common optical axis 41. This reflector element 5 is preferably 
constituted by a thin plate 52 with parallel faces on which a localized 
reflecting deposit 51 has been laid, centered on the common optical axis 
41. 
The front set 31 of the optical system for day viewing cooperates with a 
rear set 32 composed of at least one lens. This rear set 32 is centered on 
the axis 43 which is symmetrical with the axis 41 with respect to the 
normal to the reflecting surface of the reflector element 5. The front set 
21 of the optical system for night viewing cooperates with a rear set 22 
which is constituted by at least one lens. This set 22 is centered on the 
optical axis 42 which is parallel and substantially aligned with the 
common optical axis 41. In the embodiment represented by the attached 
figures, each rear set 22 or 32 is constituted by a doublet formed by a 
converging lens coupled with a diverging lens. Each set 22 or 32 is 
located behind the reflector element. 
The objective includes a medium 61 of negative power, comprising at least 
one lens. This medium is more precisely constituted by two diverging 
doublets each formed of a diverging lens and a converging lens. This 
medium is capable of being moved between, on the one hand, a position 
between the front set 31 of lenses and the reflector element 5 so as to be 
centered on the optical axis 41 and on the other hand a position located 
after the said reflector element 5 so as to be centered on the optical 
axis 43. More precisely, when the movable medium 61 is centered on the 
axis 43, it occupies a position between the reflector element 5 and the 
rear set 32 of the objective for day viewing. It will be noted that the 
medium is the only lens between the set 31 and the reflector 5 or between 
the latter and the set 32. Movement of the movable medium 61 is effected 
by rotation about an axis 62 of rotation passing through the point of 
coincidence of the optical axes 41 and 43 and perpendicular to these 
optical axes 41 and 43. This axis 62 of rotation is parallel to the 
reflecting surface of the reflector element and more precisely is 
contained in the plane of this reflecting surface. The distances between 
the movable medium 61 and the axis 62 of rotation corresponding 
respectively to the position centered on the axis 41 and the position 
centered on the axis 43 are equal. 
FIG. 3 represents an embodiment of the rotation mechanism for the movable 
medium. The medium 61 is housed in a mounting solid with a bent arm 63. 
This arm is guided by an articulation 64 whose axis 62 is perpendicular to 
41 and 43. This articulation 64 is disposed so as to be set off with 
respect to the optical axis 41 and with respect to the reflector element 5 
so as not to reduce the field of the objective. The rotary movement of the 
arm 63 is limited by adjustable stops 65 and 66 which determine the two 
positions of the medium 61 centred on the axes 41 and 43. 
A light-intensifying tube 71, centered on the optical axis 42, is 
positioned behind the rear set 22 of the objective for night viewing. The 
light beam is then reflected by the reflectors 73 and 74. In the 
embodiment represented by the drawings, these reflectors are constituted, 
for example, by right-angled isosceles triangle prisms termed "total 
reflection prisms". The reflector 74 is retractable so as to clear the day 
viewing channel. If necessary, the reflector 73 can be retracted at the 
same time as the reflector 74. 
Behind the rear set 32 of the objective for day viewing, the light beam is 
reflected by a reflector 33 towards the reflector 74. The two reflectors 
33 and 74 are aligned along the optical axis 45 parallel to the optical 
axis 41. The image given by the day viewing channel passes through the 
reflector 74 constituted by a total reflection prism. Behind the prism 74, 
the night viewing channel and the day viewing channel follow through a 
common optical system. This common optical system is constituted by the 
set of the media and of the eyepiece unit and it has a set 8 of lenses in 
front which is centered on the optical axis 45. A micrometer 9 is disposed 
on the day viewing channel. The optical distances between the set 8 and 
the exit of the tube and between the set 8 and the reticle 9, 
respectively, are equal. 
The principle of operation and the optical characteristics of the objective 
whose practical embodiment has just been described will now be explained 
in detail. 
The light beam which passes through the front wall set 31 of central lenses 
is reflected by the reflecting surface of the reflector 5. The reflected 
beam is directed towards the rear set 32. The light beam has to pass 
through the medium 61 which is positioned either on the optical axis 41 
between the reflector 5 and the front set 31 or on the optical axis 43 
between the reflector element 5 and the rear set 32. Behind the rear set 
32, the beam is reflected by the reflector 33 towards the set 8. 
The light beam which passes through the front set 21 of annular lenses 
passes through the thin plate 52 with parallel faces without being 
reflected by the reflecting surface 51. Behind the reflector element 5, 
the beam passes through the rear set 22 from which it is focussed on the 
inlet of the light-intensifying tube 71. The night viewing optical axis is 
merged with the day viewing optical axis 45, in front of the set 8, by 
means of the reflectors 73 and 74. 
The movable medium 61 which is involved in day viewing moves from the 
position between the reflector element 5 and the front set 31 (FIG. 2 and 
the representation in full line of FIG. 3) and a position between the 
reflector element 5 and the rear set 32 (FIG. 1 and the representation in 
dotted lines of FIG. 3). When the medium 61 is in front of the reflector 
element 5 and centered on the common optical axis 41, magnification is 
relatively small and the field relatively large. Conversely, when the 
medium 61 is behind the reflector element 5 so as to be centered on the 
reflected optical axis 43, magnification is relatively high and the field 
relatively small. The magnifications of the medium 61 which correspond 
respectively to the position in front of the reflector element 5 and the 
position behind the reflector element 5 have reciprocal values. When the 
medium 61 occupies either of the two positions, the object plane and the 
image plane (conjugate planes) relating to this medium are fixed. 
In the case of day viewing, the reflector 74 is retracted, change in 
magnification being effected by rotating the medium 61. When a change to 
night viewing is required, the reflector 74 has to be positioned on the 
axis 45 and the medium 61 has to be retracted to the axis 43 where it does 
not block night viewing.