DE4219187A1 - Four element objective lens for CCD imaging measurement chamber - comprises two meniscus collimating elements, each with bi-concave flint glass component, on opposite sides of iris - Google Patents

Abstract

The meniscus lens element (1) on the object side has only a low refractive power (negative or positive) and is formed of three components, including a biconcave lens of short flint glass between two collecting lenses of heavy crown glass. An iris (2) separates it from another three component meniscus lens element (3) which divides the image plane and is matched to the focal surface. The structure affords sufficient refractive power for fully apochromatic performance. USE/ADVANTAGE - For remote sensing of the Earth or other planets, an extremely compact lens of high resolution in the 400 to 1000 nm wavelength range covers a field of view of at least plus or minus 20 deg.

Description

The present invention relates to a compact, vignettie smooth apochroma consisting of only 4 lens elements table lens of high resolution, which is mainly for Application in CCD measuring chambers is provided.

In aerial photography technology, especially spectral Remote sensing of the earth or other planets from airplanes or spacecraft, lenses are as low as possible of large mass and with high resolving power. The fig Recently, this has been done on CCD matrices or CCD lines arrangements, the object continuously according to the so-called ten push-broom scanning principle line by line or in complexes is covered by several lines. The required resolution The lenses' assets depend on the pixel size of the CCD rows or arrays and this is of the order of magnitude 7 to 14 µm.

In general, to be a landscape scenery with high Detect resolution using the push-broom principle, narrow Angle lenses with about 10 ° field angle are sufficient, because the CCD arrays in their dimensions or the CCD lines in their Length are limited.

However, if you want to fly over a wider area strips or a stereogrammetric evaluation possible, then you have to add further CCD lines or - Arrays on the side or for stereogrammetry in front of and behind the attach the central line, which then acts one after the other sam and the object under different viewing angles make appear.

In such cases, however, are normal or wide-angle lenses required to achieve the required image quality large lengths of the order of magnitude much more than that Half of the focal length and thus large masses have what is undesirable. Such lens designs are e.g. B. in  the patent US 4,206,972 (= DD 1 29 696) and DE-OS 40 03 421 specified.

On the other hand, it is practically un with large lens dimensions possible a completely apochromatic image correction for to obtain a range of approx. 400 to 1000 nm, since at large lens diameters for apochromatic color errors correction required partial powers with the ent speaking pronounced curvatures of the surface radii not at all rea can be lized. The above lens according to the Pa US 4,206,972 is therefore only for a range of 460 to 650 nm usable and has the disadvantage of a large length of more than 1.5 times the focal length.

The other way of realizing an apochromatic correction consists in the use of calcium fluoride (CaF ₂ ) as an op tical medium, such as z. B. in the lens according to DE-OS 40 03 421, which also has a large length, is applied.

However, glasses similar to CaF ₂ or this medium have the after part of a high thermo-optical constant (ie a strong dependence on the refractive index depending on the temperature) and a high thermal expansion coefficient. Both thermal effects together lead to a significant change in focal length and focus shift when the temperature changes and thus to a blurred image.

A highly apochromatic lens according to the DE-OS 24 61 111 for a wavelength range from 466 to 1014 nm, in which normal optical glasses are used also has a length longer than the focal length and is only for a narrow limited field of view of + -3.5 ° suitable.

The aim of the invention is to create an extremely compact high-resolution lenses for a spectral range of approx. 400 to 1000 nm, which has a field angle of at least + -20 ° recorded and for registration on CCD line arrangements or arrays is suitable.  

Research has shown that such a registration necessarily an imaging system in the form of a photographic refractive lens is required, taking out weight establish its overall length a maximum of half the focal length should wear. Furthermore, such a lens is essential have an apochromatic correction.

Research has further shown that there are basically none refractive lenses with a short overall length exist, which at one required for high contrast resolution relative opening from 1: 5 to 1: 6 a good field correction for allow a field of + -20 °.

The triplet or the Tessar, which are in general characterized by good contrast rendition already measured a too strong branch for small field angles matism.

Another short type of lens that does not have the astigmatically unfavorable properties of the triplet variants is the orthometar. This type of lens is described in the handbook of scientific and applied photography (ed. M. v. Rohr) Springer Verlag Vienna 1932, on pages 316-322 and Fig. 292 with regard to its essential imaging properties. Its basic structure consists of a periscope, which is enclosed on the object and image side by a cemented, weakly dispersing and almost concentric meniscus in terms of its external appearance, both of which face their cavity towards one another.

With regard to the field mapping, the orthometar shows only a slight astigmatism (ie a small difference between the meridional and sagittal scales), but the field curvature is considerable and is at least 0.7% of the focal length for a field angle w = 20 °. Even if you subject this lens to an image error correction up to the required angle (the curve of the image field curvature then shows the characteristic reversal with an inflection point corresponding to the shape shown in FIG. 2), an image field curvature of approximately 0.15% of the focal length remains with this type of lens cannot be eliminated. With the focal lengths of 150 to 200 mm in question, this value is still too large, since it exceeds the depth of field range, which is of the order of magnitude + -0.06 to + -0.07 mm, by twice.

A high resolution over the required large field is achieved with a lens with a focal curve according to FIG. 2 and CCD arrays or line arrays as receivers by arranging these flat receivers in larger areas in the image plane so that they correspond to the shape the focal curve always falls in the area of the sharp image. In the focal curve according to FIG. 2, the respective flat receivers in larger areas are to be arranged in accordance with the positions marked with 1, 2 and 3.

Strictly speaking, this constellation is basically three nested narrow-angle lenses, each of which, based on its own receiver level, captures the enlarged image field with high imaging quality: the central imaging system (center of the focal curve in Fig. 2 with receiver arrangement 1 ) adjusts normal lens with intrafocal image field curvature, the outer imaging systems with the receiver arrangements 2 and 3 can be viewed as separate imaging systems with extrafocal curvature. Since the receiver levels 1 , 2 and 3 do not necessarily have to coincide, there is actually no uniformly closed imaging system, but the whole problem of capturing a large object field is realized with a single, extremely compact lens!

In order to obtain a high spectral bandwidth, in particular for multispectral analysis, an apochromatic correction for a range from approximately 400 to 1000 nm is also required. This is achieved with optical glasses according to the invention in that the first and initially weakly diverging meniscus 1 of the objective is constructed as a triple compound group, the outer lenses being ordinary heavy crown glasses (SK) and the middle lens being biconcave and made from a short flint (KzF ) or a special short flint glass (KzFS) is that the meniscus 3 standing behind the aperture 2 of the original periscope is also a triple compound, the outer converging lenses also being ordinary heavy crown glasses (SK) and the central lens being biconcave in shape and consists of a special short flint glass (KzFS), and the diverging lens contained in the last diverging meniscus advantageously consists of a short flint glass.

With this lens construction, in particular the design of the triple composite links, it is possible to introduce the high partial powers required for short flint glasses to achieve full apochromasia. Because of the inevitably larger thicknesses of the lens members, the original refractive power distribution necessarily changes. The first cemented meniscus 1 , which originally has a weakly dispersing effect, can also assume a weak positive refractive power, which is compensated for by a stronger negative refractive power of the last cemented meniscus or a weaker positive refractive power of the collecting meniscus of the original periscope standing in front of the aperture 2 . This meniscus can contain an additional cemented area to increase the equivalent Abbe's v-value of this lens.

An advantageous embodiment of the lens according to the invention is shown in the sectional drawing in FIG. 1 with the data according to Table 1. It has a curved focal surface with the section shown in FIG. 2. The first and the third meniscus of the objective are designed as a triple compound element, the middle biconcave lens being made of Schott glass KzF2 in the first and the middle biconcave lens being made of Schott glass KzFS2 in the second triple compound element. Furthermore, KzF2 is also contained in the last divergent meniscus as a dispersive medium. The apochromasia is achieved in that only low-refractive short glasses are used in the dispersive elements, their refractive power sum being more than 8 times the total refractive power of the objective. The chromatic longitudinal aberration is shown in Fig. 3 and shows the complete correction of the secondary spectrum.

Another possible embodiment of a lens according to the invention is shown in FIG. 4 and the data in Table 2. Here only a short flint glass, namely KzFS2, is used in the two triple connecting elements and the chromatic correction is otherwise effected by normal flint glasses and a cement surface in the collecting meniscus standing in front of the screen. The chromatic aberration achieved is almost the same as that in FIG. 3.

The focal area for both exemplary embodiments is shown in FIG. 2. According to the designations 1 , 2 and 3, the CCD receivers are at different levels within the depth of field. In principle, it is also possible to arrange the receivers on the same level and to compensate for the difference using different glass paths.

Both lens variants are characterized by good thermal stability, in that the expansion coefficients of the adjacent optical media differ from one another by no more than the order of magnitude 10 ^-6 .

As glass types were used, which are equally in the Catalog of the Jenaer Glaswerk GmbH as well as in the company's catalog Schott & Gen. Mainz are offered.  

Table 1

Table 2

Claims (7)

1. Four-section lens according to the orthometer type, in which a periscope, consisting of two collecting menisci, which enclose a diaphragm with their cavity, is enclosed both on the object and on the image side by a cemented and externally almost concentric meniscus, characterized in that
that the object-side meniscus ( 1 ) has only a little negative or positive refractive power and is constructed as a triple compound group in such a way that a biconcave lens made from a short flint or short flint special glass is enclosed by two converging lenses made from heavy-duty glass,
that the collecting Me niskus ( 3 ) standing directly behind the panel ( 2 ) is also constructed as a triple composite group in the form
that a biconcave lens from a short flint special glass is enclosed by two converging lenses made from heavy-duty glass and that the image plane is further divided and adapted to the focal surface of the lens. 2. Lens according to claim 1, characterized in that the image plane is embodied by flat receiver assemblies becomes. 3. Lens according to claim 2, characterized in that the receiver assemblies by lateral displacement within the depth of field to the focal surface of the Lens are adjusted. 4. Lens according to claim 2, characterized in that the adaptation of the receiver assemblies to the focal area by correspondingly equivalent glass paths in front of the recipient assemblies is effected. 5. Lens according to claim 1, characterized by the following data: 6. Lens according to claim 1 to 4, characterized in that the right in front of the aperture and with the cavity to this standing collecting meniscus a chromatic putty area contains. 7. Lens according to claim 6, characterized by the following design data: