Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
1999-03-08
2001-10-16
Chang, Audrey (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S584000, C359S835000, C359S834000
Reexamination Certificate
active
06304395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a viewing optical instrument having a roof prism, which is adapted to invert an object image in the vertical and horizontal directions. The present invention also relates to a roof prism.
2. Description of the Related Art
In viewing optical instruments, such as a monocular or binocular telescope, those using a roof prism having a pair of reflection surfaces intersecting at a nominal face angle of 90 degrees can be made small since an object image formed by an objective optical system can be inverted in the vertical and horizontal directions by a single roof prism. However, since the pupil of the objective optical system is split by an edge line of the reflection surfaces, the face angle must be highly precise. If the face angle is not accurate, a viewable double image occurs.
As is well known, in the case where light is reflected by a reflection surface, a phase difference is produced between two orthogonal P-polarized and S-polarized light components of a light wave before and after the reflection takes place. The phase difference has no adverse influence on the image to be viewed in an optical system having a Porro prism or the like in which the pupil is not split. To the contrary, the image to be viewed can be deteriorated in an optical system having a roof prism or the like in which the pupil is split by the edge line, due to wavefront aberrations caused by a difference in the polarization state between the lights emitted from the pair of reflection surfaces. This is because a change in the polarization state due to a phase difference caused by the reflection surfaces of the roof prism differs between the light incident on one of the reflection surfaces (first reflection surface) and emitted from the other reflection surface (second reflection surface), and the light incident on the second reflection surface and emitted from the first reflection surface. The larger the phase difference produced by the reflection surfaces, the greater the difference in the polarization state caused thereby. If the difference in the polarization state becomes large, the wavefront aberrations can increase, and consequently, a double image occurs and is viewed, and the contrast is reduced to the same as a pair of reflection surfaces having an inaccurate face angle.
To reduce the phase difference caused by the reflection surfaces of the roof prism, it is known to coat the reflection surface with a metal layer such as an aluminum or silver layer. It has been confirmed that the metal layer can reduce the phase difference in question. However, it has been found that the metal coating layer cannot sufficiently reduce the phase difference, particularly in accordance with an increase in the machining precision of the roof prism. Namely, if the machining precision is not high enough, it cannot be determined whether the deterioration of the image to be viewed is caused by the face angle error or by the phase difference. Consequently, the quality of the image is evaluated based on a combination of the face angle error and the phase difference. However, if the machining precision is increased, significance is chiefly placed on the phase difference.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a viewing optical instrument using a roof prism which has a highly accurate face angle, wherein the change in the phase difference produced before and after reflection by light incident on each of the pair of the reflection surfaces (located on opposite sides of an edge line) of the roof prism is restricted to reduce wavefront aberrations to thereby improve the quality of an image to be viewed.
Another object of the present invention is to provide a roof prism to be used in the viewing optical instrument.
According to the present invention, there is provided a viewing optical instrument having a roof prism which is provided with a pair of reflection surfaces intersecting at a nominal face angle of 90°, so that a pupil is split by an edge line of the reflection surfaces of the roof prism, wherein the reflection surfaces are provided with a multiple-layer coating.
According to another aspect of the present invention, there is provided, a roof prism including a pair of reflection surfaces intersecting at a nominal face angle of 90°, and the reflection surfaces being provided with a multiple-layer coating.
With the multiple-layer coating, the change in the phase difference between the P-polarized light component and the S-polarized light component, produced before and after the light is incident on the reflection surfaces of the roof prism, can be restricted within an allowable limit to thereby prevent an image to be viewed from being deteriorated.
Preferably, in the viewing optical system (or the roof prism) according to the present invention, the following condition (1) is satisfied: (1) DXE×&egr;≦60 (mm·seconds); wherein, D designates the diameter (mm) of light flux in section, perpendicular to the direction of incidence thereof, incident on the reflection surfaces, and &egr; designates manufacturing error (seconds) of the face angle of the pair of the reflection surfaces which deviate from the nominal angle of 90°.
Condition (1) was obtained through experimentation, and if condition (1) is not satisfied, even if a multiple-layer coating is provided, image quality is decreased and the effect of a reduction in the amount of change in the phase difference before and after reflection cannot be confirmed.
Preferably, the precision of the nominal face angle of the roof prism is in the range of 90°±10″.
In an optical system (or the roof prism) in which the machining precision of the nominal face angle of the roof prism is not within 90°±10″, it cannot be confirmed whether the amount of change in the phase difference before and after reflection is reduced by the presence of the dielectric layers since the image is considerably deteriorated when the dimensional precision of the nominal face angle is not good.
Preferably, in the viewing optical instrument, there is provided at least one lens positioned in front of the roof prism. In this optical instrument, the multiple-layer coating preferably has optical properties wherein the change in the phase difference of the P-polarized component and the S-polarized component produced before and after reflection by light incident on each of the pair of the reflection surfaces of the roof prism, at an angle of incidence of ±1° with respect to the optical axis of the at least one lens, is within 90°. According to the inventor's analysis, if the difference (change) in the phase differences before and after the reflection is within 90°, no unacceptable image deterioration occurs. More preferably, the multiple-layer coating has optical properties to restrict the change in the phase differences before and after the reflection to within 20°.
In the aspect of the present invention which relates to a roof prism, the change in the phase difference of the P-polarized component and the S-polarized component produced before and after reflection by light incident on each of the pair of the reflection surfaces of the roof prism, at an incident angle of 47.74 to 49.74°, is within 90°.
The multiple-layer coating can include dielectric layers which do not absorb visible light (approximately 400 nm to 700 nm). Numerous multiple-layer coatings (dielectric layers) which do not absorb visible light are known.
More specifically, when the refractive index n of the Pechan prism (roof-prism) is 1.46<n<1.60, the multiple-layer coating can be composed of first to ninth dielectric layers superimposed on the reflection surfaces of the Pechan prism (roof-prism) in that order, the first to ninth dielectric layers having the following optical thicknesses d
1
to d
9
(nm) and refractive indexes M
1
, M
2
, M
3
, respectively, wherein 2.00<M
1
<2.10, 1.35<M
2
<1.40, 1.45<M
3
<1.50:
1st layer: 34.0<d
1
<42.5 (M
1
)
2nd layer:
Ito Taku
Noguchi Masato
Asahi Kogaku Kogyo Kabushiki Kaisha
Chang Audrey
Greenblum & Bernstein P.L.C.
Winstedt Jennifer
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