Combination optical low pass filter capable of phase and amplitude modulation

A combination optical low pass filter adapted for use in a single or double tube color television camera is provided with a phase retarding pattern on a transparent substrate. The pattern includes optical elements having a transmissivity which varies with the wavelength of light passing there through so that the optical low pass filter functions as both a phase diffraction and an amplitude diffraction filter. Medium indexed filler material can further supplement the pattern to eliminate any light scattering.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical low pass filter to be used in 
an optical system for a single tube or double tube color television camera 
and more particularly to an optical filter selectively capable of both 
phase and amplitude modulation. 
2. Description of the Prior Art 
There have been proposed and known various types of optical low pass 
filters in the prior art which are used in single or double tube color 
television cameras for eliminating cross-talk between luminance and 
chrominance signals. One example is a phase grating filter in which 
transparent optical elements such as laminae, dots, strips or the like of 
given size are regularly or random disposed on a transparent substrate to 
cause phase retardation, see for example, U.S. Pat. No. 2,733,291, U.S. 
Pat. No. 3,681,519 and U.S. Pat. No. 3,756,695. These type of optical 
filters can provide a signal low pass effect without a substantial loss of 
light during the transmission, and further desired OTF (optical transfer 
function) characteristics can be obtained in these optical filters by 
selecting the size of the optical elements. The OTF characteristic 
obtained through these phase grating low pass filters, however, varies 
depending on the wavelength of light passing there through because the OTF 
characteristic is a function of optical thickness of the optical elements 
which is related to the wavelength. 
The prior art has also used amplitude modulated optical filters such as 
shown in U.S. Pat. No. 3,566,013 which discloses an amplitude type low 
pass filter having alternative and parallel strips of different 
transmissivity. According to this type of optical filter, the OTF 
characteristics will not vary in accordance with the wavelength. In 
addition, the OTF value can be designed to become zero at a predetermined 
cutoff spatial frequency. This latter property may be of advantage for the 
primary purpose preventing interference and false signals between the 
chrominance and luminous signals. However, it sometimes is desirable or 
necessary that the OTF does not become zero for all wavelengths, for 
instance, as pointed out in U.S. Pat. No. 3,911,479. Further, an amplitude 
type of filter has the disadvantage of causing light loss when the light 
passes there through. 
Since the television industry is extremely cost competitive, there is a 
continual desire to improve the video encoding performance while reducing 
cost. Thus, any optical element that can perform more than one task would 
be highly desirable. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide an improved optical 
low pass filter having the joint advantages of both the phase type and the 
amplitude type of optical filters. Another object of the present invention 
is to provide an optical low pass filter having different OTF 
characteristics for different spectral bands or wavelengths of light. 
Still another object of the present invention is to provide an optical low 
pass filter in which the cutoff frequency varies depending on wavelength 
or spectral band of light passing there through but which does not require 
a complex pattern of gratings. Still another object of the present 
invention is to provide an optical low pass filter as mentioned above and 
for which various patterns of optical elements are available. A still 
further object of the present invention is to provide an optical low pass 
filter which can reduce the amount of light of a designated spectral band 
passing there through. 
In order to accomplish these objects a combination phase and amplitude 
optical filter for modulating light in a color television video system is 
provided and includes a transparent substrate supporting a phase retarding 
layer. The layer is capable of providing an optical transfer function 
value characteristic of cutting off the transmittance of high spatial 
frequency signal components of at least one or more wavelengths while 
passing at least another wavelength above the cutoff frequencies. The 
phase retarding layer further includes a plurality of optical elements 
having a plurality of sublayers of respective different indices of 
refraction. The relative optical thicknesses and index of refraction of 
each layer is selected to provide a wavelength variance in the 
transmissivity of light energy passing through each optical element for at 
least two different bandwidths in the visual spectrum. 
The objects and features of the present invention which are believed to be 
novel are set forth with particularity in the appended claims. The present 
invention, both as to its organization and manner of operation, together 
with further objects and advantages thereof, may be be understood by 
reference to the following description, taken in connection with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is provided to enable any person skilled in the 
optical design and video transmission art to make and use the invention 
and it sets forth the best modes contemplated by the inventors in carrying 
out their invention. Various modifications, however, will remain readily 
apparent to those skilled in the above arts, since the generic principles 
of the present invention have been defined herein specifically to provide 
a relatively economical and easily manufactured optical filter. 
Referring to FIG. 1, a cross-sectional view of a conventional phase 
retardation optical low pass filter is disclosed. The low pass filter has 
a rectangular cross-sectional wave shape pattern 2 supported on a 
transparent substrate 1. The phase elements 2 can cause phase retardation 
in the light energy, Lp, that passes there through and thereby causes a 
phase difference between the light Lp, and the light Lo which passes 
through only the substrate 1. The height, d, and the width, a, of each 
phase element 2, along with the period, x, of the gratings can be 
determined in accordance with the desired cutoff frequency and the 
intended position of the low pass filter in the color television optical 
system. 
The individual optical phase elements 2, that form the grating pattern on 
the conventional low pass filter, are usually formed of a single 
transparent material for example magnesium floride, MgF or silicon 
dioxide, SiO.sub.2, which preferably has a refractive index lower than 
that of the substrate so that the individual optical phase elements will 
function as anti-reflection layers as well as preventing or avoiding light 
loss due to reflection at the surface and by absorption there through. By 
selecting material of a relative low index of refraction for the optical 
phase elements, the intensity of the light, Lp passing through the optical 
phase element in the substrate will be substantially equal to that of the 
light, Lo, passing through only the substrate. 
In accordance with the present invention, the optical phase elements, 2' 
that are positioned on a transparent substrate 1, as shown in FIG. 2 and 
FIG. 3 are formed from a plurality of sublayers to provide a multi-layer 
construction of respectively different refractive indices. By an 
appropriate selection of the optical thickness of these layers, the 
transmissivity of the optical elements can vary depending upon the 
incident wavelength of light traversing the element. Accordingly, an 
optical low pass filter of the present invention can function as a complex 
amplitude grating or black and white grating for certain wavelengths of 
light and as a phase grating for other wavelengths. 
Referring specifically to FIG. 2, a symbol A, can represent the ratio of 
the amplitude Ap of the light Lp passing through both the optical phase 
element 2', and the substrate 1 to the amplitude Ao of the light Lo 
passing through only the substrate as follows: 
EQU A = (Ap/Ao) (1) 
If the optical phase elements 2' are appropriately designed, as known in 
the prior art, to consist of multiple layers of appropriately chosen 
indices of refraction with appropriate thickness, then the ratio A will 
change with the wavelength or the spectral band of the light passing 
through the optical elements, see U.S. Pat. No. 3,922,068. Generally, a 
prior art optical phase element will have the ratio A equal to 1 or 
approximately 1 for phase gratings that have transparent optical phase 
elements. 
In accordance with the present invention, the ratio of our optical phase 
elements 2' will have the value of A less than 1 for certain preselected 
spectral bands so that our grating is capable of functioning as a complex 
amplitude grating. When the complex amplitude grating of the present 
invention is appropriately positioned in an image forming optical system, 
the intensity distribution, Cm, in an image of a line formed by the 
optical system will be as follows; 
##EQU1## 
wherein m = order of spectrum 
x = distance between adjacent phase elements 
a = width of each phase element 
.delta. = phase difference between the lights Lp and Lo 
As can be seen from the above equation (2), the intensity distribution Cm, 
transmitted through the complex-amplitude grating pattern is a function of 
.delta., a/X and A and further the complex-amplitude grating can function 
as an optical low pass filter providing the following conditions are 
satisfied; 
EQU 1 - 2(a/X) + 2A a/X cos.delta. .ltoreq. 0 (3) 
##EQU2## 
When the conditions of equations (3) and (4) are satisfied, the optical 
transfer function response will be as set forth in FIG. 4. Accordingly, 
the value of the OTF P will be zero or negative at the point Sc. 
Since A and .delta. are a function of the wavelength of light, the complex 
amplitude grating will change its cutoff characteristic as an optical low 
pass filter as a function of the wavelength of the light passing there 
through. It is necessary, however, that the conditions set forth in 
equations (3) and (4) be satisfied for at least some proper spectral 
bandwidth in the visual light region. Hence, in case .delta. = .+-. m.pi. 
(wherein m may be any positive integer) then the following condition can 
be derived from equations (3) and (4); 
EQU 1/4 .ltoreq. (a/X) .ltoreq. 3/4 
in addition, for a spectral band where A can be considered zero, equation 
(3) above will be reduced to; 
EQU (a/X) .gtoreq. (1/2) 
therefore, if the following condition is met; 
EQU 1/2 .ltoreq. a/X .ltoreq. (3/4) (5) 
in a phase grating as disclosed in FIGS. 2 and 3, then that grating can 
operate as a black and white grating type low pass filter, i.e., amplitude 
type low pass filter. This particular limit for a/X, set forth in equation 
(5), is also effective to provide a filter that can function as a phase 
grating low pass filter for a spectral band of light wherein A can be 
considered one. For an additional understanding of the background theory 
references is made to U.S. Pat. No. 3,756,695 which is incorporated by 
reference herein. 
Thus, the phase grating optical elements of the present invention can 
operate as a phase grating low pass filter for the spectral band wherein A 
can be considered one, that is for a band where the phase elements are 
transparent and as a black and white, i.e., amplitude type low pass filter 
for a spectral band wherein, A can be considered zero, that is for a band 
wherein the phase elements are opaque. It is necessary that the optical 
phase elements be composed of a plurality of sublayers of different 
indices to selectively transmit light with respect to wavelength and 
further that the ratio of width a, of each phase element to the distance 
or grating period X, between each pair of adjacent phase elements satisfy 
the above equation (5). In those cases wherein, A, is neither one nor 
zero, the phase grating elements can function as optical low pass filter 
provided that values of A, .delta. and a/X satisfy the conditions of 
equations (3) and (4) for at least some spectral band of visual light. 
A specific embodiment of the present invention can utilize a rectangular 
wave-shape grating pattern as disclosed in FIG. 2 with a multi-layer 
structure for the optical elements as disclosed in FIG. 3. The width, a, 
of the phase element can be set forth as follows: 
EQU a = 2/3 X 
wherein X is the distance between each pair of adjacent optical phase 
elements 2'. 
In addition, each optical phase element 2' comprises at least two kinds of 
sublayers 2a, 2b respectively overlayed for a total of 14 sublayers having 
individual optical thicknesses of 175m.mu.. Alternate layers are composed 
of a high index of refraction material for example, N.sub.H equals 2.3 and 
a low index of refraction material, for example, N.sub.L equals 1.38. 
Thus, the total geometric thickness d, of each optical phase element 2' is 
1420m.mu.. Since the optical thickness of the individual layers 2a and 2b 
are respectively N.sub.H .times. dH and N.sub.L .times. dL. The total 
geometric thickness d, of each phase element 2' is (dH + dL) .times. 7, 
with a total optical thickness of each phase element 2' being; 
EQU (nHdH + nLdL) .times. 7 = 2 .times. 175 (m.mu.) .times. 7 = 2450 (m.mu.) 
Accordingly, the optical path difference between the light passing through 
the optical element phase portion and the light passing through the 
nonphase portion will be; 
EQU 2450m.mu. - 1420m.mu. = 1030m.mu.. 
The resultant transmissivity will change with wavelength as disclosed in 
FIG. 5 with the boundary or transformation point occurring at about 
580m.mu. to 600m.mu.. Thus, the transmissivity is 100% for light of a 
wavelength smaller than the border value, i.e., blue and green regions, 
while the transmissivity is substantially zero for the light energy of a 
wavelength larger than the border value, i.e., in the red region. 
Accordingly, this optical phase grating can function as a phase grating 
low pass filter for light in the blue and green spectral regions where A 
equals 1 and is a black and white grating low pass filter for the light in 
the red region. 
If we accept light of 450 as a wavelength represented of the blue light, 
and 540m.mu. as a wavelength represented of green light then the phase 
difference .lambda. for these lights are respectively 4.6.pi. and 3.8.pi. 
and the response function or OTF (optical transfer function) 
characteristic for these wavelengths will be respectively as shown in FIG. 
6 as curves (b) and (a). If we accept 620m.mu. as a wavelength 
representative of light in the red region, then the phase grating 
functions as a black and white grating low pass filter and the OTF 
characteristic will be as shown in curve (c) in FIG. 6. 
As can be seen from FIG. 6, the grating of this specific embodiment of the 
present invention will change its high-frequency cutoff characteristics 
depending on the wavelength of light passing there through. That is, the 
grating can cutoff the high frequency component in the image formed by the 
image forming optical system including the above grating, in the blue and 
red region of the spectral band but does not cutoff the green region. As 
an additional feature of the present invention, it should be noted that 
the grating of this embodiment allows only about one-third of the red and 
infrared light region to pass through the grating. The optical phase 
portions of the grating which occupy two-thirds of the total area of the 
grating (since a/X equals 2/3), reflect the light of the wavelength larger 
than 600m.mu. and does not allow it to pass there through. This provides 
an additional advantage because it is known that the image pickup tubes 
for color television cameras have a higher sensitivity for light in the 
red or infrared region than in the other regions. The optical grating 
filter of the present invention is therefore capable of compensating the 
sensitivity of the image pickup tube by reducing red and infrared 
components of the light which would normally reach the photosensitive 
surface of the tube. Because of this feature of the present invention, it 
is possible to utilize an optical image forming system without the 
necessity of including a red compensating cutback filter in the color 
television optical system to balance the spectral sensitivity of the color 
television camera. 
Referring to FIG. 7, a schematic side view of an image pickup tube 11 is 
disclosed to show an example of one arrangement of the optical grating 
filter as mentioned above. The optical phase elements 2', as explained 
above, are formed on a transparent and plane parallel substrate 13 to form 
the optical low pass filter. The substrate, as shown, can be attached 
directly to the faceplate 12 of the tube 11. In this arrangement, if the 
width of each optical phase elements are 40.mu. and the distance between 
each adjacent pair of phase elements 60.mu. (i.e., a/X = 2/3), and 
further, the thickness tF, of the faceplate is 1.0mm with the image pickup 
tube being 2.54Cm, i.e., of a 1-inch size, then the OTF characteristic of 
the optical low pass filter will be as disclosed in FIG. 6. Additionally, 
this optical low pass filter can cutoff sufficiently the components of the 
spatial frequency higher than about 15 lines per millimeter for light in 
the red and blue region while allowing the light in the green region to be 
formed on the faceplate 12 up to high frequency components so that 
luminous signals can be detected up to the high frequency component of the 
image in the green light region and a sharp image of high resolution can 
be obtained. 
Although the above description is directed for the embodiment shown in FIG. 
7, wherein the transparent substrate 13 is directly attached to the 
faceplate 12 it should be realized that substantially the same result can 
be obtained when the low pass filter is positioned in the image forming 
optical system apart from the faceplate. In that case, the specific values 
of a/X can be changed depending upon the distance from the image plane on 
the faceplate to the plane of the low pass filter in the optical system. 
A second embodiment of the present invention can have the same construction 
and parameters as that of the first embodiment with the exception that the 
ratio a/X = 1/2. The OTF characteristic of the second embodiment can be 
seen in FIG. 8 wherein curves (a), (b) and (c) represent respectively the 
OTF characteristics for blue (.lambda. = 450m), green (.lambda. = 540m) 
and red (.lambda. = 620m). As can be seen from FIG. 8, the respective 
values reach a minimum value for a specific wavelength and then increase 
as the spatial frequency increases. An optical low pass filter is possible 
if the carrier frequency for the color signals is selected or set at or 
near the spatial frequency capable of providing a cutoff such as where 
curve (c) assumes a minimum value at a spatial frequency, fr. 
For example, if the low pass optical filter is positioned as shown in FIG. 
7 with tF = 2.4mm, tL = 2.4mm and the refractive index of the transparent 
substrate is 1.5 with a = 30.mu. and x = 60.mu., then the spatial 
frequency fr, where the OTF for red light is a minimum, will be 15 lines 
per millimeter. This would set the carrier frequency for the color 
signals. As can be seen in FIG. 8, the low pass filter in the second 
embodiment shows less of a decrease of the OTF of the green light 
(540m.mu. wavelength) and therefore permits a greater transmittance of 
this light. At a/X = 1/2, the low pass filter will allow 50% of the red 
and infrared wavelengths to pass there through. Again this property of 
reducing the amount of the red and infrared light is capable of balancing 
the higher sensitivity of the color television camera 2 for the red and 
infrared light. 
FIG. 9(A) shows a schematic cross-section of a pickup tube with the 
combined optical filter element of the present invention mounted thereon. 
In the above embodiments, the optical phase elements were surrounded by an 
air medium. In FIG. 9(A), these optical phase elements are sandwiched 
between a plane parallel transparent plate 15 and a substrate 13 with the 
space therebetween being filled with a transparent cementing material 14 
with a relatively medium index of refraction of 1.56. 
In the modified embodiment of FIG. 9(B), the substrate 13 is on the object 
side and the optical phase elements 2' are mounted immediately adjacent 
the faceplate of the image pickup tube 12. Again a transparent cement of 
the same refractive index as that of FIG. 9(A) can be utilized. The medium 
refractive index filler material can minimize light scattering. 
In both of these embodiments, the optical path difference between the light 
passing through the phase portion and the light passing through the 
nonphased portion will be 235m.mu. and accordingly, the phase difference 
is 1.04.pi. for a blue light of 45 m.pi. wavelength and 0.87.mu. for a 
green light of 540m.mu. wavelength. 
If the optical low pass filter of the first embodiment is employed in these 
arrangements, the OTF characteristic will be represented as shown in FIG. 
10 wherein all of the curves (a), (b) and (c) respectively for blue 
(.lambda. = 450m.mu.), green (.lambda. = 540m.mu.) and red (.lambda. = 
620m.mu.) will experience a cutoff effect. It should be noted that in 
these embodiments the spectral transmissivity characteristic of the 
optical phase portions are assumed not to be affected by the cement 
material that surrounds the phase elements. 
If the optical low pass filter of the second embodiment is adopted in the 
arrangement of either FIG. 9(A) and/or FIG. 9(B), the OTF characteristics 
will be as represented in FIG. 11. Again, in this case, the filter will 
work as a low pass filter for eliminating cross-talk between luminous and 
chrominance signals in the visual spectral region, if the carrier 
frequency for the color signals is set to or near the spatial frequency 
fo, at which the OTF for the green light of 540m.mu. wavelength becomes 
zero. It should be noted that in these arrangements, the optical low pass 
filter does not permit the transmission of a high frequency component of 
light to provide an improved resolution, however, the low pass filter does 
have a sharply defined low pass effect and still has the effect of 
reducing the amount of red and infrared light that is transmitted to the 
image plane of the image pickup tube. 
Although the specific examples disclose optical phase portions capable of 
blocking the light of a wavelength longer than 600m.mu., the actual 
spectral transmissivity characteristics of the present invention can be 
determined in accordance with a subjective spectral sensitivity 
characteristic of the image pickup tube of a particular color television 
camera in which the low pass optical filter is to be employed. As can be 
fully appreciated by a person skilled in this art, the phase elements may 
block, instead of red and infrared wavelengths, the light of blue or the 
green spectral region as desired. 
The actual multi-layer structure of the optical phase elements can be 
manufactured through various methods. For example, those methods employed 
for forming dichroic or color encoding filters can be used. Generally, the 
material for the sublayers can be selected from TiO, CeO, ZrO, etc., as 
high refractive index material and from MgF, SiO.sub.2, etc., as low 
refractive index materials. The depositing of the materials on the 
substrate can be accomplished by ordinary evaporating techniques and 
photoresist techniques. As can be appreciated, the present invention is 
applicable not only to a one dimensional grating, but also to two 
dimensional low pass filters such as shown in U.S. Pat. No. 3,756,695. The 
present invention is also applicable to a filter wherein the phase 
retarding elements are arranged at random with respect to their size and 
spaces. 
It is also possible within the parameters of the present invention to form 
the individual optical elements from a light absorbing type of color 
filter material as opposed to that of multiple layers. 
While the above embodiments have been disclosed as the best mode presently 
contemplated by the inventors, it should be realized that these examples 
should not be interpreted as limiting, because artisans skilled in this 
field, once giving the present teachings, can vary from these specific 
embodiments. Accordingly, the scope of the present invention should be 
determined solely from the following claims in which we claim.