Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-12-07
2001-10-09
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S631000, C359S633000, C359S834000
Reexamination Certificate
active
06301052
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical combiners for superimposing two images.
BACKGROUND OF THE INVENTION
It is a frequent requirement to superimpose one optical image on another. For example, in a pilot head-up display, aircraft instruments may be imaged and projected on to the helmet visor as a virtual image emanating from infinity, so as to be superimposed on the landscape image seen through the visor. As is well known, such a technique obviates the need for the pilot to remove his gaze from the landscape in order to read the instrument panel.
FIG. 1
is an optical ray diagram showing schematically a typical prior art optical combiner depicted generally as
10
for combining an image of an instrument
11
on to an external image seen by an observer
12
. The instrument panels is imaged by an imaging objective
13
so as to project an incident beam of light
14
on to a beam splitter
15
having a partially reflective coating. A first portion
16
of the incident beam is reflected by the beam splitter
15
toward a concave reflector
17
which reflects the first portion
16
back through the beam splitter
15
as a second portion
18
towards the observer
12
.
It is thus apparent that the beam splitter constitutes a major limitation in the overall efficiency of the optical system
10
. This limitation derives from the fact that the beam splitter both reflects and transmits the incident beam and that any improvement to the reflectivity is at the expense of transmissivity and vice versa. For this reason optimal results are achieved when the reflectivity and transmissivity of the beam splitter are equal,
Thus, in an optimally configured system the beam splitter
15
may be assumed to reflect and transmit 50% of the light incident thereon. On this basis, the first portion
16
of the beam reflected by the beam splitter
15
has only 50% of the intensity of the original beam emanating from the instrument
11
. The second portion
18
of the incident beam which finally reaches the observer
12
after yet again passing through the beam splitter
15
has a net intensity equal to only 25% of the intensity of the original light beam. This figure assumes 100% transmission through the imaging objective
13
and the concave reflector
17
, which can never be met in practice. Thus, allowing for only 90% reflection by these two optics, the net intensity of the beam reaching the observer is close to 20% of the original beam. The net intensity can be increased somewhat to between 30 to 50% by employing polarizers or &lgr;/4 plates for the concave reflector
17
. However, any improvements to the transmissivity of the imaging objective
13
and to the reflectivity of the concave reflector
17
are marginal compared to the 25% net propagation of the beam through the beam splitter
15
.
EP 0 303 742 discloses a head-up display for aircraft cockpits comprising an optical system for directing an image produced by a CRT on to a partly-reflective screen in the pilot's field of view. A prism having at least one curved face that acts as a lens replaces the fold mirror in previous systems allowing the CRT image to be directed towards the observer from a different direction than the external scene. The prism allows the optical system to be kept compact. In such a system, at least one combiner screen, or beam splitter, must still be used to present both images to the observer simultaneously.
SUMMARY OF THE INVENTION
It is thus an object of the invention to provide an improved optical system for combining two images whilst avoiding the need for a beam splitter.
According to the invention there is provided an optical system for directing an auxiliary image towards an observer so as to be superimposed on to a main image, the optical system comprising:
a pentagonal prism having a first surface for intercepting the auxiliary image and directing an auxiliary image beam toward an opposing second surface so as to be internally reflected by the second surface towards a third surface so as to be internally reflected by the third surface and propagated out of the pentagonal prism through a fourth surface opposite the third surface towards the observer, and
optical means proximate and external to the third surface for directing a main image beam derived from the main image through the third surface into the pentagonal prism and out of the fourth surface thereof towards the observer.
In essence, the invention substitutes a pentagonal prism for the beam splitter used in conventional optical combiners. Within the pentagonal prism, light is internally reflected by both the second and third surfaces using internal reflection, such that the effective reflectivity of each surface can be close to 90% (subject to other possible limitations). Likewise, the effective transmissivity of the main image through the third and fourth surfaces, can be kept relatively high (especially when using a dichroic reflective coating on the third surface) as can be the transmissivity of the auxiliary image through the first surface. Thus, the auxiliary image can be very efficiently projected (with more than 75% efficiency) while keeping the main image with very high (depends on the dichroic coating on the third surface) transmission coefficient. This represents a significant improvement over hitherto proposed systems based on conventional beam splitters.
REFERENCES:
patent: 4697882 (1987-10-01), Sato et al.
patent: 5052800 (1991-10-01), Mimmack et al.
patent: 5708522 (1998-01-01), Levy
patent: 6094287 (2000-04-01), Li
patent: 3809656-A1 (1989-10-01), None
patent: 38 09 656 (1989-10-01), None
patent: 3917200 (1990-11-01), None
patent: 0 303 742 (1989-02-01), None
patent: 2 222 892 (1990-03-01), None
patent: 2222892-A (1990-03-01), None
patent: 3917200-A1 (1990-11-01), None
Browdy and Neimark
Mack Ricky
Vectop Ltd.
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