Untitled record

Holder: Idem ©) Agent:

Stray light trap.

Invention of: Jacques Vulmiere and Huguette Vulmiere.

(33) (32) (31) Conventional priority:

Sale of booklets at IMPRIMERIE NATIONALE. 27. rue de la Convention - PARIS (15^e)

19263 2088198

The present invention relates to a stray light trap.

In many optical instruments, an illumination device, the principle of which is well known, is used for lighting the field of observation, and which comprises a semitransparent plate sending part of the light beam coming from a source towards the object to be illuminated and allowing part of the return beam to pass to the observer.

The region opposite to the lighting system is directly illuminated by the light beam and becomes visible in the field after reflection on the slide. The details of the image of the object are thus drowned in stray light.

To reduce this stray light, various systems have been proposed, the most basic of which, still in use, consists in placing a black absorbent and matt surface perpendicular to the axis of the beam. Such a surface retains an appreciable diffusion factor, the indicator of which has a maximum directed in the direction corresponding to the law of reflection. To reduce the amount of flux diffused towards the blade, it is therefore advantageous to tilt said surface by 45 ° for example, and to collect the maximum of flux in an absorbent and diffusing enclosure.

A light trap of this type always leaves reflections and has other disadvantages such as:

1) difficulties of realization:

an enclosure with an inclined bottom and welded to the edges of a tube is more difficult to produce than a part of revolution.

2) cleaning difficulties:

such an enclosure is also difficult to clean; however the efficiency of the trap decreases with the dust which is deposited on the inclined surface,

5) clutter is sometimes a pitfall.

The object of the present invention is to produce a light trap which, compared with the device described above, has a higher efficiency, which is easier to produce and clean and has a small footprint.

To this end, the subject of the invention is a lens flare characterized in that it comprises a chamber of revolution delimited by a side wall with an absorbent internal surface, a bottom provided with a cone coaxial with said chamber, facing inside of it and with an absorbent and shiny surface and an inlet diaphragm constituted by a part secured to the pa2

19263 2088198 lateral king, and coaxial with said enclosure.

According to a particularly interesting embodiment, the diaphragm is determined by the intersection of two conical surfaces coaxial with said enclosure and oriented in the same direction as said cone, that of the two conical surfaces facing outward being advantageously made absorbent and shiny. .

The internal face of the lateral wall of the enclosure can be cylindrical or frustoconical.

In the case where the conicity of said internal face is in the opposite direction to that of the cone, this internal face is made absorbent and shiny and the face of the part delimiting the inlet diaphragm turned towards the interior of the enclosure is made absorbent and matte.

In the case where the conicity of the internal face is in the same direction as that of the cone, this internal face is made equally absorbent and shiny and the part of the bottom surrounding the cone is made absorbent and matt.

Furthermore, in order to make the assembly easily cleanable, the three elements: side wall, conical bottom and inlet diaphragm, are preferably removable and assembled by screwing.

Other characteristics and advantages will emerge from the following description of embodiments of the device according to the invention, description given by way of example only and with reference to the appended drawings in which:

Figure 1 shows an axial sectional view of a first embodiment of the device according to the invention.

FIG. 2 represents a view in axial section of another embodiment, and

Figure 3 shows an axial half-section of another embodiment.

The device of FIG. 1 comprises an enclosure of revolution E delimited by a cylindrical side wall 1, a bottom 2 screwed to one of the ends of the cylinder 1 and an inlet diaphragm 3 coaxial with the cylinder and constituted by a part of revolution 4 secured to the other end of the cylinder 1 and which further comprises an external threaded connection 5 for mounting the trap on optical devices.

The bottom comprises a cone 6 turned towards the inside of the enclosure and coaxial with it.

The diaphragm 3 © is delimited by the intersection of two sur3

19263 2088198 conical faces oriented in the same direction as the last supper 6. The diameter of the base of the cone 6 is substantially greater than the diameter of the diaphragm 3 and the angle at the top of the cone 6 is less than or equal to 90 °.

The wall of the cone 6 and the external frustoconical surface 7 of the part 4 are made absorbent and shiny while the internal surface 1a of the wall 1 and the internal cylindrical wall 5a of the connector 5 are made absorbent and mat.

In this device, an incident beam such as the beam 8 1 ° penetrates into the enclosure E and falls on the surface of the aosorbent and reflective cone 6. The beam 8 is therefore partly absorbed and reflected towards the wall 1, the surface 1a of which is absorbent and matt.

The beam 8 is reflected with absorption a first time on the cone 6, is diffused with absorption on the surface 1a then is reflected again with absorption on the cone 6 and finally is almost completely absorbed.

Another beam 8 ′ which is stopped by the diaphragm 3, strikes the frustoconical absorbent and shiny conical surface 7 and is referred 2θ chi towards the surface 5 j which is absorbent and mat. The beam 8 'is therefore also almost completely absorbed.

FIG. 2 represents a variant of the trap of FIG. 1 according to which the internal surface T'a of the side wall 1 'is frustoconical and flares upwards from the base of the cone

6 'to the level of the part 4' defining the diaphragm 3, this part 4 'being removable and hoisted on the wall 1.

The internal surface 1'a of the wall is also made absorbent and 'shiny and the internal face 4'a_ of the piece 4' is made absorbent and matt.

5θ Thus a beam 9 coming to strike the cone 6 'is reflected on this one then on the face 1'a to come to strike the surface 4'a. The beam 9 is thus almost completely absorbed.

FIG. 3 represents another variant according to which the internal surface 1a of the wall 1 is frustoconical and in the same direction as the cone 6. This surface 1a is made absorbent and shiny and the bottom wall 2si surrounding the cone 6 is made absorbent and mat.

The cone 6 consists of a block of polished black glass crimped into the bottom 2. In addition, the internal face 5a of the connector 5 is inclined inward and is made absorbent and preferably 4/0 19263 2088198 gloss rence in which case the part of the part 4 between the connector 5 and the frustoconical surface 7 is made absorbent and mat. The beam 10 striking the cone 6 is reflected with absorption a first time on the cone 6 and on the surface Va, then diffused with absorption on the surface 2a and reflected again with absorption on the surface 1a and on the cone 6 and is finally considerably weakened.

The beam 10 'striking the surface 7 is reflected with absorption successively on the surfaces 7 and 5a then diffused with absorption on the face of the part 4 then reflected again with absorption on the surfaces 5 a and 7 and is considerably weakened.

The surface 2a is flat or inclined, as is the external face of the part 4 lying between the surface 7 and the connector 5.

In the embodiments described above the angle at the top of the cone (5,6 ', 6) is equal to or less than 90 °.

The more this angle decreases the more the surface of the cone increases and the more the illumination of this surface decreases which results in a lower parasitic luminance of said surface.

The cone (6) is for example cut in black glass - but in this case the realization of a perfect point is difficult or indeed in metal (6, 6 '). If it is for example durai, it will be black aluminite and covered with a black and shiny paint. In the case of a metal other than durai the surface of the cone will be optically polished and covered with a deposit of thin black layers, shiny, absorbent and non-diffusing.

The shape of the internal surface of the side wall of the trap can be arbitrary, cylindrical, frustoconical, etc., however the shape allowing the best yield to be obtained is that of the traps of FIGS. 2 and 3.

The realization of the inlet diaphragm, by the intersection of two conical surfaces, allows to have a non-shiny sharp edge and allows the absorption of marginal beams such as 8 'or 10'.

The three main elements of the trap, namely the side wall, the bottom and the inlet diaphragm are parts of revolution and therefore easy to make. In addition, they are advantageously removable and can therefore be cleaned easily and correctly.

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The size of such a trap is smaller than that of conventional traps. Thus the trap of FIGS. 2 and 3 in which the cone has an apex angle of 90 ° is approximately twice as short as the conventional traps with inclined ellipse.

The traps according to the invention can be used as a stray light trap in the self-adhesive systems such as those used for example in self-adhesive glasses, in self-adhesive microscopes (intended either for examining objects illuminated by reflection, or for reading of rules ...).

They can be used as a black standard in various photometric assemblies such as those for example, intended for:

a) the measurement of the stray light of an optical instrument ,. To make the black area, the trap should preferably be placed at the bottom of a tube which will limit the opening of the light beam to a low value angle and which will be provided, if necessary, with an intermediate diaphragm.

b) zeroing of instruments for measuring very low fluxes, reflection factors or diffusion factors by means of integrating spheres. In this case, an absorbent product in the wavelength range for which the measurements are made will be chosen as the black paint of the various elements of the trap.

Of course, the present invention is not limited to the embodiments described and shown above but on the contrary covers all variants.

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