CA1050314A - Projection lens system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A projection lens system is disclosed having a total focal distance of at least 400 mm and comprising a triplet. A positive lens is positioned substantially halfway between the triplet and the object to be projected. The triplet has a focal distance of between 1.2 and 2.5 times the total focal dis-tance of the projection lens system. The lens system has a relative opening which is larger than 1:3.5. The end lenses of the triplet and the positive lens consist of glass having a refractive index of less than 1.53. The inter-mediate lens of the triplet consists of glass having a refractive index of less than 1.63.

Description

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The ~resent invention re:Lates to a projection lens system comprising a positive lens or group of lenses, preferably a so-called triplet.
When projecting diffusely radiating pictures having slight luminous intensity and large extension, for example when projecting the picture on the surface of the picture tube of a tele-vision, or a picture or a transparency of overhead type, a lense system having a long focal distance is used, generally over ~00 mmO
In order to obtain sufficient light intensity in the projected picture, the lens must have a large relative opening, preferably more than 1:3.5. Conventionally a lens group is used for ~his purpose which usually comprises two convex and one concave lenses and is called a triplet. A projection lens system consisting of a triplet and having a focal distance of 600 mm and relative opening of 1:3, has a diameter of 200 mm, in which case the thickness at the centre of each convex lens in the triplet is about 50 mmO Such a thick lens presupposes the use of a so-called heavy crown glass having a refractive index of around 1.6 for the lens material since the quality of the picture away from the centre would otherwise be unacceptableO
If instead of this grade of glass a glass were used for the lenses having a refractive index of less than 1.53, for example crown glass with a refractive index of 10525, which is much less expensive, the lenses for this light intensity would have an even greater thickness at the centre and, furthermoreS the so-called zone error (spherical deviation) and other picture dis-tortions outside the optical axis would be extremely disturbingO
One object of the invention is to provide a projection lens which is considerably less expensive to manufacture and which gives better picture quality than a conventional lens wit}

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corresponding performance.
Another object o the invention is to effect a lens which gives the projected picture considerably bet~er light intensity at ~he edges than a conventional lens.
According to the invention there is provided a projection lens system having a total focal distance of at least 400 mm, comprising a triplet characterized in that a positive lens is positioned substantially halfway between said triplet and the object ~o be projected, said triplet having a focal distance of between 1.2 and 2.5 times the total focal distance of the projection lens system, the projection lens system having a relative opening which is larger than 1:3.5, said triplet comprising end lenses and an inter-mediate lens, said end lenses and said positive lens consisting of glass having a refractive index of less than 1.53 and said triplet intermediate lens consisting of glass having a refractive index of less than 1.63.
For the purposes mentioned in the introducti~n it has been found that a lens system having a focal distance of 600 mm is suitable. If the second lens then compriscs a field lens which virtllally enlarges ~he pic~ure twice, the first lens group (the projection lens system), preerably a triplet, should have a ocal distance of 900 mm and the triplet should have a free diameteT of 200 mm in order to give the lens a relative aperture of about 1:3. This composite lens according to the invention, comprising a triplet and a field lens, has a focal distance of about 600 mm and may therefore be compared with a conventional lens comprising only a triple~
having a focal distance of 600 mm, and this comparison shows that the lenses in the triplet used for the lens according to the invention are dioptrically much weaker and correspondingly

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much thinner. This results in advantages not only from the tech~
nical point of vie~ in the manufac~uring process, but also means that the material required, and thus also the material costs for the triplet in the lens according to the invention are considerab-ly lower. Furthermore, ordinary crown glass may be used for the triplet acco~ding to the invention since picture distortion both on the axis and outside will be considerably less since the lenses are dioptrically weaker.
The field lens, which may be simple and in certain con-nections is known as collective lens, is suitably of a size approach-ing the size of the diffusely radiating picture. The "field lens"
may of course be split into two. The field lens has a favourable influence on the light intensity in the outer parts of the pro-jected picture. This is because a considerable part of the diffuse light emanating from the picture object is collected by the field lens and deflected in towards the triple~. In a con~Jentional lens a large part of the light radiating from the picture would be lost due to the diffuse light dispersion. The field lens may be made of inexpensive crown glass, but may even with advanta~e con-sist of plastic, for example acrylic plastic ~polymethylmethacrylate).
In order to reduce the material required the field lens may be in the form of a thin so-called Fresnel lens.
From the point of view of picture quality and manufactur-ing costs for the lens, it has been found favourable to let the focal distance of the first lens or lens group (the triplet) be at the most about 2.5 times the total focal distance of the projection lens, preferably less than about twiceO
~n the following the invention will be further described by way of example with reference to the accompanying drawings in which Fig. 1 sho~s a projection lens according to the invention, Fig. 2 ~SC~331L4 an alternative projection lens according to the invention, Fig~ 3 illustrates a conventional projection lens, Fig.4 illustrates the spherical deviation for a conventional lens in comparison with the lens according to the invention, and Fig. 5 shows schemat-ically an episcope with the ]ens according to the invention~
Figs. 1 to 3 show on the same scale projection systems having a relative aperture of 1:3 and a focal distance of 600 mm for projecting a diffusely radiating picture 1 with a diameter of 300 mm, enlarged five times, using a triplet 3' and 3, respectivelyO
In the conventional system according to Fig. 3 the lenses 4', 5' and 6' of the triplet 3' must be made of heavy crown glass and flint glass and must be of a considerable thickness.
Fig. 1 shows a projection system according to the invention in which a field lens 2 is placed approximately centrally between the diffusely radiating picture 1 and the triplet 3 comprising two convex lenses 4 and 6 and a concave lens 5 between them. The field lens 2 enlarges the picture 1 virtually twice and a simple geometric observation shows that the triplet 3 should have a focal distance of 900 ]nm if the lens composed of the field lens 2 and the triplet 3 is to have a focal distance of ~00 mm. Thus, in the projection lens according to the invention the lenses 4-6 of the triplet 3 may be considerably flatter and thinner, thanks to the greater focal distance of the triplet 3. Furthermore, the lenses 4-6 may be made of a normal crown glass having a refractive index 10525 (in Fig. 1 the lens 5' has a refractive index of 1.626 while the lenses 4' and 6' have a refractive index of 1.613) thanks to the decreased curvature of the lenses 4-6, which is due to the in-creased focal length of ~he triplet 3. Si~ce the lenses 4-6 in the triplet 3 are flatter than the lenses 4'-6' in the triplet 3', a greater number of lenses 4-6 can be manufactured on one and the same tool.

3~L4 A comparison of the cost of material or the triplet 3' and the projection lens according to the invention shown in Fig. 1, an approximate price ratio of 3:1 is revealed with current prices. This also includes the cost of the ield lens.
The projection system according to Fig. 2 is the same as that shown in Fig. 1 except that the field lens is in the form of a so-called Fresnel lens 2A. The use of a Fresnel lens as field lens permits fur~her reductions in cost when mass produced.
Fig. 4 indicates the spherical deviation for the conventional lens according to Fig. 3 (curve A) and the lens of the invention according to ~igs. 19 2 and 5 (curve B), both with a relative aperture of 1:3, the ordinate Y indicating *he zone distance from the optical axis and the abscissa X
indicating the spherical longitudinal deviation as a percentage of the ocal distance. It is clear from Fig. 4 that the lens according to the invention has considerably less spherical distortion than a comparable conventional lens.
The projection lens system 3 and the field lens 2 have been stated above to be a triplet and a simple lens, respectively. However, it should be obvious that they can be replaced as the need, desire and higher or lower requirements regarding distortion of the projected picture arise, by other positive lens systems, for example consisting of a single convex lens or a meniscus lens, or some type of double lens system, preferably colour corrected, for instance including a concave and a convex lens, or some type of objective derived from a triplet.
It should be clear from the above that the lens according to the invention entails considerable technical advantages over known technique in that it provides better light intensity around the edges of the projected picture, permits the use of ordinary lens material, i.e. crown glass, for example, having a refractive 105~3~
index of less than 1.53, which results in considerable savings of the cost of material and yet gives better picture quality than, for example, a corresponding Tessar objective. Furthermore, the lenses in the projection lens system according to the invention have a curvature which is considerably-less than that of the lenses in a conventional projection lens system and the lens making tool can therefore be used for a far greater number of lenses in the system according to the invention than for lenses in a comparable conventional projection lens systemD
By splitting the triplet better performance can also be obtained, which together with the field lens described results in a still greater relative aperture, for instance 1:2050 One or more of the lenses of the triplet may be split and in this case, as well, they can be made of the inexpensive types of glass mentioned earlierO Of course, it is also possible to achieve the favourable relative aperture by choosing a glass having high refractive index for one of the lens components.
It should be mentioned that the whole system is of course counted as a unit, i.e. the field lens cannot be fitted to an already existing projection objective and a result then expected corresponding to that illustrated in the drawingsO
The lens according to the invention is primarily intended for use in projectors of overhead and/or episcope type, in which the beam path is deflected by a mirror 7 as shown schematically in Fig. 5. T~e picture material shown in projectors of overhead/
episcope type is often of a size approaching A4 format. The field lens in this case should have a diameter of at least about 300 mmO
When using the lens in this way it has been found necessary to give the lens a focal distance greater than 300 mm, preferably greater than 400 mm to give the operator free vision into the ~S133~9~
picture material and also to enable manipulation during pro-jectionO
Generally, however, the lens has a focal distance of less than 1000 mm.
~ hen being used in projectors the lens can be easily adjusted to set the focus of the picture in various planes by displacing the field lensO
Usually the lens is set so that a picture placed on a stationary table is projected clearly. However, if the surface of an object having a certain thickness, say 15 mm, which is on the table is to be clearly represented, the field lens may be raised a distance corresponding to said thickness and the triplet need not be adjusted, or not noticeably, thanks to its long focal distance and the projection distance, for example 4 6 m.
This adjustment of the plane of focus also means that the operator can place a picture on the table in advance and only when desired adjust the focus so that a deciferable picture is projected.
The invention is not limited to the specific purposes mentioned above, however, but may also be used for other purposesO

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A projection lens system having a total focal distance of at least 400 mm, comprising a triplet characterized in that a positive lens is posi-tioned substantially halfway between said triplet and the object to be projected, said triplet having a focal distance of between 1.2 and 2.5 times the total focal distance of the projection lens system, the projection lens system having a relative opening which is larger than 1:3.5, said triplet comprising end lenses and an intermediate lens, said end lenses and said positive lens consisting of glass having a refractive index of less than 1.53 and said triplet intermediate lens consisting of glass having a refractive index of less than 1.63.