U.S. patent number 3,802,763 [Application Number 05/278,216] was granted by the patent office on 1974-04-09 for beam splitting prisms.
This patent grant is currently assigned to The Rank Organisation Limited. Invention is credited to Gordon Henry Cook, John Anthony Fawcett, Gordon Whitehead.
United States Patent |
3,802,763 |
Cook , et al. |
April 9, 1974 |
BEAM SPLITTING PRISMS
Abstract
An optical colour separating arrangement comprising a first
plane colour-selective dichroic semi-reflector inclined at an angle
of incidence .alpha. to the axis of the beam incident on said
semi-reflector, .alpha. being less than 30.degree., a second plane
colour-selective dichroic semi-reflector inclined at an angle of
incidence .beta. to the axis of the beam transmitted through the
first semi-reflector, .beta. lying between 20.degree. and
50.degree. and being greater than .alpha., a single plane
total-reflector receiving the reflected beam from the second
semi-reflector 0and directing said beam along an axis at least
approximately parallel to the axis of the transmitted beam, and at
least one total reflector for the beam reflected from the first
semi-reflector, the arrangement being such that all the colour
constituents undergo either zero or an even number of
reflections.
Inventors: |
Cook; Gordon Henry (Leicester,
EN), Fawcett; John Anthony (Leicester, EN),
Whitehead; Gordon (Leicester, EN) |
Assignee: |
The Rank Organisation Limited
(London, EN)
|
Family
ID: |
10416468 |
Appl.
No.: |
05/278,216 |
Filed: |
August 7, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Sep 1, 1971 [GB] |
|
|
40756/71 |
|
Current U.S.
Class: |
359/633; 359/583;
348/338; 359/634 |
Current CPC
Class: |
G02B
27/1013 (20130101); G02B 27/145 (20130101) |
Current International
Class: |
G02B
27/14 (20060101); G02b 027/14 () |
Field of
Search: |
;350/173 ;178/5.4E |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rubin; David H.
Attorney, Agent or Firm: Brisebois & Kruger
Claims
We claim:
1. An optical color separating prism system comprising a first
prism element having a first polished surface on which the beam of
light to be split into color components is incident at an angle of
incidence .lambda., .lambda. being less than 40.degree., and having
a second polished surface inclined at an angle .alpha. to the
incident axis, .alpha. being less than 30.degree., a second prism
element having a first polished surface cemented to the second
polished surface of the first element and having a second polished
surface inclined at an angle .beta. to the incident axis, .beta.
lying between 20.degree. and 50.degree., a first dichroic layer
contained between the cemented surfaces of the first and second
prism elements, the color component reflected from said first
dichroic layer being totally reflected at the first polished
surface of the first prism element to emerge from said element
through a third polished surface thereon along an axis inclined at
an acute angle to the incident axis, a third prism element having a
first polished surface cemented to the second polished surface of
the second prism element and having said second polished surface
normal to the incident axis, a second dichroic layer contained
between the cemented surfaces of the second and third prism
elements, the color component transmitted by both the first and
second dichroic layers emerging along the incident axis through the
second polished surface of the third prism element, and the second
prism element having a third polished surface at which the color
component reflected from the second dichroic layer is totally
reflected along an axis substantially parallel to the incident axis
to emerge normally through a fourth polished surface on the second
prism element.
2. An optical system according to claim 1, wherein the second prism
element comprises a basic prism unit carrying the first and second
polished surfaces and an auxiliary prism unit carrying the third
and fourth polished surfaces of said second prism element, the two
units being cemented at a plane interface normal to the axis of the
color component reflected from the second dichroic layer.
3. An arrangement according to claim 1, wherein .beta. is not less
than 30.degree. and not greater than 45.degree..
4. An arrangement according to claim 1, in which .lambda. is equal
to zero.
5. An arrangement according to claim 1, including an entrance prism
element in front of the first prism element having a first polished
surface normal to the axis of the incoming beam of light and a
second polished surface spaced by an air gap in front of the first
polished surface of the first element at the angle .lambda. to the
incident beam axis.
6. A colour television camera optical system comprising an optical
objective and a colour separating prism arrangement, said colour
separating arrangement comprising a first prism element having a
front polished surface normal to the incident optical axis and a
second polished surface inclined to the incident axis by an angle
.lambda.; a second prism element separated from the first by a thin
parallel air gap so that its first polished surface is also
inclined to the incident axis by the angle .lambda., a second
polished surface of said second element closely adjacent to a first
colour selective partially reflecting surface which is inclined to
the incident axis by an angle .alpha.; a third prism element
closely adjacent to the second prism element so its first polished
surface is inclined to the incident axis by the angle .alpha., a
second polished surface of said third element closely adjacent to a
second colour selective partially reflecting surface which is
inclined to the incident axis by an angle .beta.; a fourth prism
element closely adjacent to the third prism element so that its
first polished surface is inclined to the incident axis by the
angle .beta., said element having a second polished surface normal
to the incident optical axis; and wherein the second prism element
has a third polished surface normal to the optical axis of the beam
reflected by its second and first surfaces, whilst the third prism
element cooperates with a total reflector for the beam reflected
from the second partial reflector; the beam emerging through a
polished surface normal to the optical axis of the beam reflected
by the second partial reflector and said total reflector, the
arrangement being such that:
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf)] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) -
2.alpha.]
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf)] > .alpha. >
[Sin.sup..sup.-1 (1/2nf)]
Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) > .beta. >
.alpha.;
where f is the f number or relative aperture of the camera
objective, and n is the refractive index of the prism glass
material.
7. A colour television camera optical system as claimed in claim 6,
in which
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf)] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 2.alpha.]
1)
Sin.sup..sup.-1 (1/n)/2 > .alpha. > Sin.sup..sup.-1 (1/2nf),
(2)
Sin.sup..sup.-1 (1/n) > .beta. > .alpha.. 3)
Description
This invention relates generally to a method of splitting a beam of
light into three components, especially colour constituents, and to
a beam splitting and colour separating arrangement for carrying out
said method, and more particularly to a beam splitting prism more
especially for use in combination with an optical objective in a
colour television camera optical system.
In a colour television camera, it is necessary to split the beam of
light received through the objective into three components
respectively having different spectral characteristics. The three
images associated with these components are received by three
image-receiving tubes, which are of appreciable axial length, and
therefore predominantly determine the overall dimensions of the
camera. Compactness of layout is assisted if two of the image
receptors are parallel to one another.
When, as is usual, the three image planes have different angular
relationships with respect to one another, the necessarily long
receiving tubes are disposed in different directions, impairing the
compactness of the overall arrangement. Since in preferred
arrangements there is insufficient axial space to introduce further
reflecting surfaces solely for the purpose of re-orientating one or
more image planes to lie in the same or similar angular
relationship to another, one or more primary images must be relayed
to a secondary image by an optical system within which the required
angular change can be incorporated. Sometimes there is a preference
to use the blue component for this purpose, particularly when it is
desirable to incorporate an optical reduction in format size to
adjust the relative image brightness in a manner which reduces the
lag characteristics when viewing movements in the object space.
This preference may arise in presence of studio incandescent
illumination, deficient in blue light.
One beam splitting arrangement for a colour television camera is
known from British Specification No. 983933. In this known
arrangement, the incident beam is successively incident on two
dichroic layers inclined in opposite senses to the axis of the
incident beam, in each case at angles of 251/2.degree. and
13.degree. respectively. The first dichroic layer is carried on the
rear surface of a wedge prism having a front polished surface
normal to the incident beam. Spaced behind the first dichroic layer
by a small air gap is the front polished surface of a triangular
prism having a rear surface carrying the second dichroic layer.
Behind the second dichroic layer is the front surface of a second
wedge prism having its rear polished surface normal to the incident
axis. The portion of the beam transmitted through the two dichroic
layers, the green constituent, emerges through this rear surface of
the second wedge prism. The beam reflected at the first dichroic
layer, the blue constituent, is internally reflected at the front
polished surface of the first wedge prism and emerges normally
through a lateral polished surface of said prism at an acute angle,
about 51 degrees, to the axis of the transmitted beam. The beam
reflected at the second dichroic layer, the red constituent, is
reflected at the air gap behind the first dichroic layer to emerge
normally through the third surface of the triangular prism at a
larger acute angle, about 78 degrees, to the axis of the
transmitted beam.
The arrangement has the advantage that all three emergent beams
have suffered zero or an even number of reflections, which means
that the three images are optically handed vertically and
left-to-right in the same sense. This facilitates the problem of
achieving registration of the colour signals to be generated,
because provided the geometry of electron beam scanning in the
three image receptors is identical there is no need for such
geometry of scanning to be perfect in itself. This remains true
even when a relay lens is introduced into the blue channel in order
to give a reduced image format. The blue beam is inverted by such
relay lens both vertically and horizontally, and this can be
compensated for by rotating the blue image receptor and its
deflection coils through 180.degree. about its optical axis.
However, the known arrangement is very disadvantageous with regard
to compactness of layout. In order to bring two of the emergent
beams into parallel relationship, it is known to bring the blue
beam out of the lateral surface of the first prism into a fourth
prism which has a plane polished surface at which the blue beam is
internally reflected along an axis parallel to the axis of the
transmitted beam, said fourth prism having another polished surface
through which the blue beam emerges normally. This creates the
difficulty, however, that the third reflection introduces an
inversion into the blue beam in one direction only. As a result, to
solve the resultant registration problem, the geometry of scanning
in the image reflectors must be made geometrically perfect, which
is difficult and expensive to achieve.
The last-mentioned difficulty can be overcome by replacing the
additional plane reflector for the blue beam by a compound
reflector of the roof-prism type. Unfortunately, a difficulty of
similar severity then arises. Not only is a roof prism difficult
and expensive to manufacture, but also it occupies a longer path
length than the corresponding plane reflector. The distance between
the primary image and the relay lens has to be increased to
accommodate the roof prism so that for a given reduction factor,
the focal length of the relay lens has to be increased, further
increasing the distance between the primary and secondary images,
and adding significantly to the problem of aberration correction in
design of the lens. This detracts appreciably from the compactness
of layout which is being sought.
The present invention has for its general object to provide an
improved method and arrangement for splitting a beam of light into
three colour constituents, and for a more specific object, to
provide a colour separating arrangement for a colour television
camera which enables a compact layout to be achieved without
incurring difficulties as to image registration.
In one aspect, the present invention provides a method of splitting
a beam of light into three components, especially colour
constituents, according to which the beam is successively incident
on two plane semi-reflectors, especially colour-selective dichroic
semi-reflectors, with an angle of incidence at the first
semi-reflector of less than 30.degree. and an angle of incidence at
the second semi-reflector lying between 20.degree. and 50.degree.
and greater than the angle of incidence at the first
semi-reflector, where the angle of incidence is defined as that
angle between the optical axis and the normal to the
semi-reflector, and the reflected beam from the second
semi-reflector is then incident on a single plane surface affording
total reflection, preferably to cause said second reflected beam to
be directed along an axis at least approximately parallel to the
axis of the transmitted beam, the method being such that all the
colour constituents undergo either zero or an even number of
reflections. The most important respect in which this method
differs from the known arrangement is that the angle of incidence
at the second dichoroic semi-reflector is increased. In practice,
as will be explained later, it is this feature that enables the
beam reflected from this second semi-reflector then to be totally
reflected by a single plane reflector along an axis parallel to
that of the transmitted beam. Further, for application to a colour
television camera of high quality, especially with regard to
colorimetry, the method is only practical when the first dichroic
semi-reflector is adapted to give peak reflection at a wavelength
spectrally positioned between the wavelengths of peak reflection
and peak transmission at the second dichroic semi-reflector. This
will mean that the first reflected beam is a green constituent. The
transmitted beam is preferably a red constituent and the second
reflected beam a blue constituent.
Preferably, the reflected beam from the first semi-reflector is
subsequently incident on a single plane surface affording total
reflection to cause said first reflected beam to be directed along
an axis at an acute angle to the axis of the transmitted beam.
Thus, one of the emergent beams undergoes zero reflections and the
other two emergent beams each undergo two reflections. Difficulties
over image registration are thus avoided, and the introduction of a
relay lens into the blue channel to reduce the blue image format
can be catered for, as with the known arrangement, by rotation
through 180 degrees of the blue channel image receptor.
The three emergent beams may be contained in a single plane or,
alternatively, the first reflected beam, the green channel, may be
brought out by means of a single plane surface affording total
reflection so that the axes of the first reflected beam and the
transmitted beam lie in a plane normal to the plane containing the
axes of the second reflected beam and the transmitted beam.
In another aspect, the present invention provides an optical colour
separating arrangment comprising a first plane colour-selective
dichroic semi-reflector inclined at an angle of incidence .alpha.
to the axis of the beam incident on said semi-reflector, .alpha.
being less than 30.degree., a second plane colour-selective
dichroic semi-reflector inclined at an angle of incidence .beta. to
the axis of the beam transmitted through the first semi-reflector,
.beta. lying between 20.degree. and 50.degree. and being greater
than .alpha., a single plane surface affording total reflection
receiving the reflected beam from the second semi-reflector, and
preferably directing said beam along an axis at least approximately
parallel to the axis of the transmitted beam, and at least one
surface affording total reflection for the beam reflected from the
first semi-reflector, the arrangement being such that all the
colour constituents undergo either zero or an even number of
reflections. The arrangement preferably includes a single plane
surface affording total reflection for the reflected beam from the
first semi-reflector and directing said beam along an axis inclined
at an acute angle to the axis of the transmitted beam. .beta. will
usually be not less than 30.degree. and not more than
45.degree..
A practical construction for the arrangement comprises a first
prism element having a first polished surface on which the beam of
light is first incident at an angle of incidence .lambda., .lambda.
being less than 40.degree., and having a second polished surface
associated with the first semi-reflector at the angle .alpha. to
the incident axis, and a second prism element behind the first
element having a first polished surface associated with the first
semi-reflector at the angle .alpha. to the incident axis and having
a second polished surface associated with the second semi-reflector
at the angle .beta. to the incident axis. Conveniently, in this
construction, the beam reflected at the first semi-reflector is
subsequently totally reflected internally at the first polished
surface of said element, said first prism element having a third
polished surface normal to the axis of said totally reflected beam
and through which said beam is emergent. A third prism element is
preferably provided behind the second element, this third prism
element having a first polished surface associated with the second
semi-reflector at the angle .beta. to the incident axis, and a
second polished surface normal to the incident axis through which
the transmitted beam is emergent.
In one embodiment, the second prism element has a third polished
surface at which the beam reflected at the second semi-reflector is
reflected along an emergent axis substantially parallel to the axis
of the incident beam through a fourth polished surface on said
element which lies normal to said emergent axis. In this
embodiment, .lambda. is conveniently equal to zero.
In a more useful embodiment particularly suitable for application
in a colour television camera, the second prism element carries an
auxiliary prism element into which the beam reflected from the
second semi-reflector enters normally through a plane contact
surface, said auxiliary element having a first polished surface at
which said beam is totally reflected along an emergent axis through
a second polished surface on said auxiliary element which lies
normal to said emergent axis. This embodiment conveniently includes
an entrance prism element in front of the first prism element
having a first polished surface normal to the axis of the incoming
beam of light and a second polished surface spaced by an air gap in
front of the first polished surface of the first element at the
angle .lambda. to the incident beam axis. The small air gap behind
the entrance prism enables the angle of incidence on the first
dichroic semi-reflector to be reduced whilst still achieving total
internal reflection at this air gap of the beam reflected from the
first semi-reflector.
In a colour television camera, the colour separating arrangment
receives the beam of light to be split from the camera objective.
In this instance, or in other applications where the colour
separating arrangement is operating in conjunction with an optical
objective, the parameters of the colour separating arrangement need
to be related to the relative aperture of f-number of the
objective. Preferably therefore, in such a combination:
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf) ] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 2.alpha. ],
1)
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf)/2 ] > .alpha.
> [Sin.sup..sup.-1 (1/2nf) ] 2)
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) ] > .beta. >
.alpha.; 3)
where f is the f-number of the objective and n is the mean
refractive index of the material of which the prism elements are
made, all said elements being made of the same material. Within
these broad limits, more specific relationships will usually be
appropriate, according to which:
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf) ] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 2.alpha. ],
1)
Sin.sup..sup.-1 (1/n)/2 > .alpha. > Sin.sup..sup.-1 (1/2nf) ,
2)
Sin.sup..sup.-1 (1/n) > .beta. > .alpha.; 3)
where f is the f-number of the objective and n is the mean
refractive index of the material of which the prism elements are
made, all said elements being made of the same material.
In a third aspect, the present invention provides a colour
television camera optical system comprising an optical objective
and a colour separating prism arrangement, said colour separating
arrangement comprising a first prism element having a front
polished surface normal to the incident optical axis and a second
polished surface inclined to the incident axis by an angle
.lambda.; a second prism element separated from the first by a thin
parallel air gap so that its first polished surface is also
inclined to the incident axis by the angle .lambda., a second
polished surface of said second element carrying or being closely
adjacent to a first colour selective partially reflecting surface
which is inclined to the incident axis by an angle .alpha.; a third
prism element cemented to or closely adjacent to the second prism
element so its first polished surface is inclined to the incident
axis by the angle .alpha., a second polished surface of said third
element carrying or being closely adjacent to a second colour
selective partially reflecting surface which is inclined to the
incident axis by an angle .beta.; a fourth prism element cemented
to or closely adjacent to the third prism element so that its first
polished surface is inclined to the incident axis by the angle
.beta., said element having a second polished surface normal to the
incident optical axis; and wherein the second prism element has a
third polished surface normal to the optical axis of the beam
reflected by its second and first surfaces, whilst the third prism
element either carries a total reflector for the beam reflected
from the second partial reflector or cooperates with a further
prism element carrying such total reflector; the beam emerging
through a polished surface normal to the optical axis of the beam
reflected by the second partial reflector and said total reflector,
the arrangement being such that:
]Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf) ] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 2.alpha.
]
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf)/2 ] > .alpha.
> [Sin.sup..sup.-1 (1/2nf) ]
Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) > .beta. >
.alpha.;
where f is the f number or relative aperture of the camera
objective, and n is the refractive index of the prism glass
material.
When a high standard of colorimetry is essential, the more specific
relationships previously stated will usually be adhered to. These
more specific relationships will invariably apply when, as
mentioned earlier, the first dichroic semi-reflector is adapted to
give peak reflection at a wavelength spectrally positioned between
the wavelengths of peak reflection and peak transmission at the
second dichroic semi-reflector
Practical embodiments of colour separating arrangement in
accordance with the invention will now be described by way of
example, more particularly with reference to their application in a
colour television camera, and referring to the accompanying
drawings, in which:
FIG. 1 shows a preferred embodiment when viewed in a direction
normal to the plane containing the optical axes of the incident
beam and of the split constituent beams, which are all
coplanar;
FIG. 2 shows an alternative and simpler arrangement suitable for
less exacting applications, from a viewpoint corresponding to that
of FIG. 1;
FIG. 3a shows a modification of the embodiment of FIG. 1 from a
similar viewpoint, in which part of the arrangement is turned
through 90 degrees about the axis of the incident beam, so that a
particular one of the split constituent beams emerges towards the
viewpoint; and
FIG. 3b shows the modification of FIG. 3a viewed in a direction
normal to the plane containing the axes of the incident beam and
the said particular one split constituent beam.
In a first embodiment shown in FIG. 1, the beam of light to be
split is incident normally on the front polished surface 1a of an
entrance prism 1 in the form of a wedge, which has a rear polished
surface 1b inclined at an angle .lambda. to the incident beam,
.lambda. being equal to 24.degree.. Spaced by a small air gap
behind the rear surface of the entrance prism is the front polished
surface 2a, also inclined to the optical axis at the angle
.lambda., of a first main prism 2 also of wedge form which on its
rear surface 2b carries a first plane dichroic semi-reflector
inclined at an angle .alpha. to the optical axis, in the opposite
sense to the inclination .lambda., .alpha. being equal to
15.degree.. Adjoining the first semi-reflector is the front
polished surface 3a of a second main prism 3, of quadrilateral
form, which has a rear surface 3b carrying a second plane dichroic
semi-reflector inclined at an angle .beta. to the optical axis, in
the opposite sense to the inclination .alpha., .beta. being equal
to 33.5.degree.. Behind the second main prism is a third main prism
4 having a front polished surface 4a adjoining the second and a
rear polished surface 4a normal to the incident axis.
The first dichroic semi-reflector 2b is adapted to reflect the
green constituent G of the beam, and transmit the blue and red
constituents to the second dichroic semi-reflector, where the blue
constituent B is reflected. The red constituent R is transmitted
along the optical axis to emerge normally through the rear surface
of the third main prism to be received directly by the image
receptor R.sub.1 for the red channel.
The green constituent G reflected at the first semi-reflector 2b is
totally reflected internally at the air gap 1b, 2a behind the
entrance prism 1, to emerge from the first main prism 2 along an
axis inclined at about 70.degree. to the optical axis, passing
normally through a polished lateral surface 2c on the first main
prism to the image receptor G.sub.1 for the green channel.
The blue constituent B reflected at the second semi-reflector 3b
emerges directly from the second main prism 3, passing normally
through a lateral surface 3c thereof cemented to a first polished
surface 5a of a supplementary prism 5 adjoining and carried by said
second main prism. The supplementary prism 5 has a second polished
surface 5b at which the blue constituent is totally reflected along
an axis parallel to the incident axis, passing normally through a
third polished surface 5c on said supplementary prism, through a
relay lens B.sub.2 for reducing the image format of the blue image,
to the image receptor B.sub.1 for the blue channel. Thus, with this
arrangement, the image receptors R.sub.1 and B.sub.1 for the red
and blue channels lie in parallel relationship.
The arrangement is especially intended for use in a colour
television camera in combination with an optical objective, )]it
receives the light beam to be split, having a relative aperture
f/1.6. The prisms are all made of a glass having a refractive index
2.alpha.= 1.518. The advantages of the arrangement can best be
understood by making a comparison with the known arrangement
previously described. The fundamental difference is the increase in
the angle of incidence at the second semi-reflector. This increase
in angle enables the blue constituent to be brought directly out of
the second main prism and reflected at a single plane reflector
back along an axis parallel to the incident and transmitted beam.
In other words, reflection of the beam at an air gap behind the
first dichroic semi-reflector is avoided. However, for utilisation
in an efficient colour television optical system, which has
relatively stringent colorimetry requirements, two further
differences from the known arrangement are of prime importance.
Firstly, the entrance prism is introduced to provide a plane air
gap at the angle .lambda. to the optical axis. The angle of
incidence at the first semi-reflector can thus be reduced whilst
still achieving total internal reflection, at said air gap, to
cause the green constituent to emerge directly from the first main
prism. In this connection, it will be understood that the spectral
characteristics of dichroic layers vary with the angle of incidence
and, generally speaking, disadvantageous effects with regard to
colorimetry become more pronounced when the angle of incidence is
increased. A reduction in this angle not only improves the quality
of the beam in the reflected green channel, but also those of the
beams transmitted to the blue and red channels.
Secondly, the order of split of the incident beam is changed from
blue-red-green, as in the known arrangement, to green-blue-red.
Again, it is well known that the spectral response of a dichroic
semi-reflector varies with angle of incidence and polarisation of
the incident beam. adverse effects are concentrated around the
cut-off edge, and become more pronounced with an increasing angle
of incidence
In the known arrangement, the dichroic semi-reflectors are
inevitably working near the cut-off edges which separate one
channel from another, and it is therefore difficult to achieve high
colorimetric standards when the angle of incidence at the
semi-reflector exceeds 20 degrees.
However, in the present arrangement, having taken off the green
part of the spectrum first, it is possible to locate the cut-off
edge of the second dichroic semi-reflector separating the red and
blue constituents in the green part of the spectrum. Thus,
increased adverse effects around the cut-off edge do not affect the
red and blue channels. This fact taken in conjunction with the
greater freedom afforded by the reduced angle of incidence at the
first semi-reflector enables the angle of incidence at the second
semi-reflector to be considerably increased, as previously
described. Even with the increased angle of incidence at the second
semi-reflector, the overall colorimetry characteristics of the
present arrangement are an improvement on the known arrangement.
The required spectral shape for the green channel can be reflected
directly, with only a simple filter for secondary trimming and to
absorb some unwanted out of band responses. Having extracted the
green component, the red and blue channels are not shaped by the
second dichroic semi-reflector, but by trimming filters acting in
combination with the responses of the camera objective, relay lens
and image receptors. The amplitude in the green channel is reduced,
as is known to be desirable, possibly without the use of a neutral
density filter. A further advantage is that both dichroic
semi-reflectors are located between glass, and can be cemented
between glass, giving greater spectral stability and less risk of
contamination than an air-backed dichroic layer.
In fact, the advantages of the above-described arrangement,
relative to the known system, are most readily apparent when, as in
practical application to a colour television camera, the effects of
polarised light and/or light from off-axis object points are
considered.
In an optimised form of the present arrangment in which the various
parameters are selected for best overall performance an average
error for twenty six test colours, which can conveniently be used
to assess colorimetric fidelity, of less than 1.25 j.n.d. units is
achievable where a "j.m.d. unit" is a unit representing the
magnitude of the average colour step which produces a "just
noticeable difference" to the human eye), for unpolarised light
from an axial object point. Optimisation of the known system can
only achieve a substantially comparable result in unpolarised light
and on axis. However, considering an off-axis point, for example at
the image corner of a typical image format, the optimised form of
the present arrangement, for the same test colours, gives rise to
an average error of about 2.5 j.n.d. units, compared with about 5.5
j.n.d. units for the known system. For polarised light from an
axial object point, the comparative values are 2.5 j.n.d. units
with the arrangement according to the present invention and 3.5
j.n.d. units for the known system. For off axis object points and
polarised light these differences are appreciably larger.
In general, the variation in colour rendering over the image
(colour shading) can be to some extent negated by electronic
shading correction circuitry. Similarly, the effects of polarised
light may be reduced by an optical retardation plate. However, in
addition to their cost and added complexity, the provision of these
features can impair the overall camera performance in other
respects, such as signal to noise ratios and optical quality. The
significance of the above comparative figures is that with the
present arrangement the need for such measures is greatly reduced
or even eliminated.
In association with the above, both signal strength and spectral
shape tend to change appreciably with variation in field angle in
the known arrangement, the former effect is substantially avoided
and the latter effect greatly reduced in the above-described
arrangement in accordance with the invention.
It can readily be determined that the above-described arrangement
complies with the relationships previously mentioned:
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf)] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 2.alpha.],
1)
[Sin.sup..sup.-1 (1/n)]/2 > .alpha. > Sin.sup..sup.-1
(1/2nf),
Sin.sup..sup.-1 (1/n) > .beta. > .alpha.. (3)
FIG. 2 shows a second embodiment wherein corresponding references
are used to denote elements and surfaces corresponding to similar
elements and surfaces in the embodiment of FIG. 1.
In this second embodiment applicable where less high standards of
colorimetry are necessary, the entrace prism, and the
supplemenetary prism carried by the second main prism are dispensed
with. The angle .lambda. thus becomes equal to zero. The angle
.alpha. is equal to 25.5.degree. and the angle .beta. is equal to
45.degree.. The refractive index of the prism glass is 1.518 and
the arrangement is adapted for use with an optical objective of
relative aperture f/2.2. It can thus be ascertained that the
arrangement complies with the relationships previously
mentioned:
[Sin.sup..sup.-1 (1/n) - Sin.sup..sup.-1 (1/2nf) ] > .lambda.
> [Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) - 20.alpha.
], (1)
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) ]/2 > .alpha.
> [Sin.sup..sup.-1 (1/2nf) ], 2)
[Sin.sup..sup.-1 (1/n) + Sin.sup..sup.-1 (1/2nf) ] > .beta. >
.alpha.. (3)
In this second embodiment, the incident beam enters the first main
prism 2 directly, through a front polished surface 2a normal to the
optical axis. The green constituent G reflected at the first
semi-reflector 2b is totally reflected internally at said front
polished surface 2a to emerge directly from the first prism 2. The
blue constituent B is reflected at the second semi-reflector 3b at
right angles to the incident axis, to be internally reflected at a
lateral polished surface 3c formed directly on the second main
prism, thereby to emerge normally through a rear polished surface
3d on said second prism 3 to pass through a relay lens B.sub.2 to
the blue channel image receptor B.sub.1. As before, this receptor
B.sub.1 lies parallel to the red channel image receptor R.sub.1,
which is aligned with the incident and transmission axis. Further
relaxation of colorimetric requirements may make this second
embodiment useful without changing the order of colour split from
that used in the known arrangement.
In the above-described arrangements, the colour constituents are
brought out along axes lying in a common plane. However, in a
useful modification shown in FIGS. 3a and 3b, the green constituent
is brought out on an axis lying normal to the plane containing the
red and blue channel axes. In this modification, as applied to the
first arrangement above-described, the entrance prism and the first
main prism, together with the first dichroic semi-reflector and the
front surface of the second main prism, are turned through
90.degree. about the incident axis relative to the remainder of the
system. The arrangement of FIGS. 3a and 3b will be clear without
further description from a study of the references employed, which
correspond with those used in FIG. 1.
* * * * *