Monochromatic imaging apparatus

A monochromatic imaging apparatus comprising a pair of similar diffraction gratings, a pair of focusing lenses, a pair of collimating lenses, an entrance slit, an exit slit and an intermediate slit, and means for concurrently moving the two gratings; wherein a polychromatic source image is placed at the entrance slit, and a collimating lens collimates the light rays from the source image and projects the collimated rays onto the first diffraction grating thereby to disperse the rays. The dispersed rays are then focused by the first focusing lens onto the intermediate slit, and then collimated by the second collimating lens onto the second diffraction grating which thereupon recombines the dispersed light. The recombined rays are then focused by the second focusing lens onto the exit slit. The two gratings may be concurrently moved in opposite or similar directions about an axis in the grating ruling plane and at the centers thereof, and at similar angles, so that the angle of incidence at the first grating is equal to the angle of diffraction at the second grating. In this manner, a monochromatic image of the polychromatic source image will appear at the exit slit and be of a selected band of wavelengths of the visible or other spectrum dependent upon the angular position of the two gratings from zero order of diffraction.

BACKGROUND OF THE INVENTION 
This invention relates to an imaging apparatus which produces a 
monochromatic image of a selected band of wavelengths from a polychromatic 
source image, and more particularly, to such an apparatus wherein a pair 
of monochromators are utilized therein. 
In the prior art, there are known two types of double monochromators, one 
of which has been used for imaging purposes. In that instrument, it was 
only possible to reproduce an image at only one wavelength, to any degree, 
and not over a selected band of wavelengths. For example, for use in human 
spectroscopy, an isolated spectral line was possible of isolation and 
viewing. However, in the art, there is no apparatus which can by simple 
manipulation, view a polychromatic source image, as well as a 
monochromatic image of any selected wavelength or band of wavelengths, 
with faithful reproduction of the source image. 
It has been known to use a double monochromator as a scanning spectrometer 
wherein an entrance slit, collimating mirror, grating, camera mirror, 
intermediate slit, second collimating mirror, second grating, second 
focusing mirror and exit slit were employed. In this prior spectrometer, 
the slits were extremely narrow so as to produce a precise one dimensional 
resolution of the spectral lines. However, there has not yet been produced 
a monochromatic imaging device wherein an accurate reproduction of the 
source image was produced and of a selected single wavelength or band of 
wavelengths. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to overcome the foregoing and 
other deficiencies and disadvantages of the prior art. 
Another object is to provide a monochromatic imaging apparatus wherein by 
simple manipulation of the gratings of a double monochromator, an accurate 
monochromatic image is produced of a polychromatic source image and of a 
selected single wavelength or a band of wavelengths within the spectrum of 
the source image. 
The foregoing and other objects and features of the invention are attained 
in the invention which encompasses a double or multiple monochromator 
comprising two similar diffraction gratings or a suitable number of 
gratings, two or suitable number of similar collimating lenses, two or 
suitable number of similar collimating lenses, two or a suitable number of 
similar focusing lenses, an entrance slit, an exit slit and a suitable 
number of intermediate slits with means for selectively and concurrently 
moving the two gratings, disposed as follows: The entrance slit is 
positioned in front of a first collimating lens which focuses on the 
polychromatic source image at the entrance slit, and collimates the light 
rays from the source object image onto the first grating, which disperses 
the light. The first focusing lens focuses the dispersed light from the 
first grating onto the intermediate slit. The second collimating lens then 
collimates the dispersed light passing through the intermediate slit onto 
the second grating, which thereupon recombines the dispersed light (that 
is, disperse same in the opposite direction). The recombined light is then 
focused onto the exit slit. The two gratings are movable in opposite or 
similar directions rotated about their axis at the ruled surface plane and 
at the centers thereof, and preferably so that the angle of incidence at 
the first grating is equal to the angle of diffraction at the second 
grating. The intermediate slit is of suitable dimensions so as to pass the 
desired bandwidth and image. 
In this manner, by suitable movement of the two gratings, an accurate image 
of the source image will appear at the exit slit, and be of a selected 
single wave length or band of wave lengths within the band of wavelength 
spectrum of the polychromatic source image. The exit slit image will be an 
accurate proportional reproduction of the dimensions of the source image 
since there will be two dimensional spatial resolution. For example, 
employing this invention, it would be possible to obtain, an accurate 
image at the exit slit of the source image in, for example, a green color, 
and then by suitable movement of the two gratings, view the same image in 
a yellow color. The same would be possible in UV and other wavelengths. 
Accordingly, this invention is highly useful and has many different 
applications, such as for example, in identifying and spatially locating 
different components of a vapour or other material composition. 
A feature of the invention is the double monochromator wherein to similar 
diffraction gratings are interconnected to move concurrently about their 
respective axis disposed at the ruling plane and at the centers thereof, 
so that the angle of incidence of the first grating closest to the source 
object is equal to the angle of diffraction of the second grating closest 
to the final image. 
Another feature is the two dimensional resolution of the apparatus, wherein 
high resolution lenses are employed to project images in two dimensions 
accurately, and wherein suitable dimensioned slits are employed to pass 
two dimensions of the source image. 
A further feature is the use of two similar gratings and concurrent 
movement of the two gratings in opposite directions from the zero order 
position and at the same angle to thereby diffract and then recombine 
light waves, with an intermediate slit disposed therebetween for selective 
control of the bandwidth of light waves being transmitted. 
Another feature is use of an entrance slit and exit slit of sufficient 
width to enable passage of desired light from an object image without 
passage of extraneous light. 
Another feature is the use of a lighttight housing for a double 
monochromator to prevent extraneous light from entering the light paths 
being processed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawing, there is depicted an imaging arrangement 
according to the invention, comprising a light and dust tight housing 1, 
wherein are disposed, an entrance slit 2, a collimating lens 3, a 
diffraction grating 4, a focusing lens 5, an intermediate slit 6, a 
collimating lens 7, a second diffraction grating 8, a focusing lens 9 and 
an exit slit 10, and interconnection and moving device 13 which 
interconnects for concurrently movement of the gratings 4 and 8, as 
depicted. A viewer 12 is focused to view the resulting image at the exit 
slit 10. A polychromatic color source 11 may be used to project a 
polychromatic source image onto the entrance slit 2. 
The source image is focused at the entrance slit 2 by the image producer 
11, which may be a TV camera lens, and then the light therefrom is 
collimated by collimating lens 3 onto the first diffraction grating 4. The 
grating 4 diffracts the light and disperses same toward the focusing lens 
5, which then focuses the dispersed light onto the intermediate slit 6. 
The dispersed light passes through the intermediate slit 6 and is then 
collimated by collimating lens 7 onto the second diffraction grating 8. 
All of the wavelengths of the polychromatic source image are spread out on 
the plane of the intermediate slit, the depending upon the angular 
position of the grating 4, selected wavelengths will pass through the 
intermediate slit 6. As will be seen in the later discussions, in this 
manner, selected bands or band of wavelengths will appear at the exit 
slit. The second grating 8, then diffracts the already dispersed light in 
the opposite direction, that is to say, the second grating 8 then 
recombines the dispersed light which reaches it. The recombined light is 
then focused by focusing lens 9 onto the exit slit 10, to produce an image 
thereat which is an accurate reproduction of the object image appearing at 
the entrance slit 2, but of a selected wavelength or band of wavelenghths 
within the spectrum of wavelengths of the source image. The source image 
at the entrance slit 2 may be polychromatic. The spectrum of the source 
image may be of any suitable band, such as within the visible and up to 
IR, or up to X-ray on the other end. The exit image, advantageously, will 
be proportional in dimension and be an accurate reproduction of the image, 
with only the color, that is the wavelength of the light being different 
from the source image, with the exit image wavelength being a single 
wavelength or a band of wavelengths within the spectrum of wavelengths of 
the source image. As will be discussed, by suitable movement of the device 
13, the two gratings 4 and 8 can be moved concurrently in opposite or 
similar directions so that the angle of incidence at the grating 4 will 
equal the angle of diffraction at grating 8, and so that the particular 
wavelength which passes through the intermediate slit 6 will be that which 
is desired to be shown at the exit slit. For example, if the color yellow 
is desired at the exit slit image, the moving device 13 will move the two 
gratings 4 and 8 such that such yellow band will pass through the 
intermediate slit 6 and exclude the others, and then such yellow band will 
appear at the exit slit image. 
As shown in the sole FIGURE, the two gratings 4 and 8 are connectable 
together for concurrent movement by device 13. The gratings have their 
axis on the grating ruled plane and located substantially at the 
horizontal centers thereof. The gratings may be of any suitable size in 
terms of number of grooves and total surface area. The two diffraction 
gratings 4 and 8 should be of the same size and groove density, although 
the invention will still be operative with variations in the two. When 
devices 13 moves the two gratings concurrently, two functions are served: 
first, the wavelength or band of wavelengths which will be passed through 
the intermediate slit 6 is determined. This is done by the angular 
position of the grating 4; with the entire spectrum being spread 
throughout the plane of the intermediate slit 6, positioning the grating 4 
will determine which band of wavelengths will be at the position of the 
slit and thus be passed thereby. Second, the two gratings will be moved 
such that grating 4 will be moved in an opposite or similar rotating 
direction from grating 8, and such that all the time the angle of 
incidence at grating 4 will equal the angle of diffraction at grating 8. 
In this second function, the grating 4 will disperse the image, and the 
grating 8 will disperse the image in the opposite direction, or to put it 
another way, recombine the dispersed light which passes though the 
intermediate slit 6. 
The connecting device 13 (which also causes the movement of the gratings) 
can be of any mechanical means, such as employing connecting arms together 
with a screw adapted interconnection for appropriate moving of both 
gratings 4 and 8 about their respective axis. The device 13 can be 
manually operable or automatically operable using a motor means. When the 
gratings 4 and 8 are turned, they are moved in similar or opposite 
direction about their zero order position. 
The collimating lenses 3 and 7 may be substantially the same, and should be 
of sufficient resolution and quality to provide suitable clear image at 
the exit slit. Similarly, the focusing means lenses 5 and 9 may be 
substantially similar, and should be of sufficient resolution and quality 
to provide a clear image at the exit slit. The housing 1 should be 
substantially light and dust tight. The interior may be coated black to 
prevent extraneous light scatter. Extraneous light would tend to interfere 
with the accurate reproduction of the source image and interfere with the 
accurate passage of the desired selected wavelength or band of 
wavelengths. The entrance slit 2, intermediate slit 6, and exit slit 10 
are of suitable horizontal and vertical dimensions. The entrance slit 6 
must be of sufficient dimensions in both vertical and horizontal 
dimensions to pass the object image without passing extraneous light. 
Similarly, exit slit 6 must be of sufficient dimensions in both the 
horizontal and vertical dimensions to pass the formed final image of the 
size desired and provided by the focusing lens 9. The intermediate slit 
should be of sufficient horizontal and vertical dimensions to allow the 
passage of the image and of the desired band of wavelengths. By suitable 
selection of the dimensions of the intermediate slit 6, a selected single 
wavelength or a selected band of wavelengths may be allowed to pass 
through slit 6 and ultimately appear at the exit slit 10. The minimum 
dimension would be to permit passage of the image and of a single 
wavelength, and the maximum dimension would be to permit passage of the 
image and of a selected band of wavelengths. 
The viewer 12 which may be used to view the exit image appearing at the 
exit slit 10 may be of any suitable device, such as the human eye, TV 
camera, light sensitive detector, etc. The image will be of a selected 
single wavelength, or a selected band of wavelengths within the spectrum 
of wavelengths of the object image. The width of the band of wavelengths 
will depend upon the dimensions of the intermediate slit as discussed 
above. The particular wavelength which is selected at the particular band 
of wavelengths which is selected will depend upon the position of the 
grating 4 vis-a-vis the entrance and intermediate slits 2 and 6. The 
viewer 12 will take into consideration whether a single wavelength is 
viewed or a selected band of wavelengths is viewed. However, any suitable 
viewer can be used. Similarly, any suitable image producing device 11 can 
be used to produce the object image at the entrance slit 2. The source 
image can be monochromatic or polychromatic. Any suitable size reducing 
device may be used consistent with the band of wavelengths desired to be 
within the object image. 
Advantageously, the invention may be used for all ranges of wavelengths 
from X-ray to radio waves utilizing suitable components and limited only 
by the energy of the X-ray on the one side of the spectrum and on the 
other side of the spectrum by the physical size of the apparatus needed. 
In the visible range, the invention may be utilized for many different 
purposes, such as to detect chemical content of a specimen, heat analysis, 
plasma diagnostics, etc, and in any use wherein bands of wavelengths need 
to be isolated, observed and measured. 
Although the above description lenses and a physical arrangement wherein 
the light rays being operated upon cross over, other arrangements and 
devices can be employed, such as for example the lenses may be replaced by 
suitable mirrors, and the gratings may be arranged on a single rotating 
shaft on different planes with the lenses appropriately positioned so that 
a compact apparatus is provided. 
Although the theory of the optics would be understood by the worker skilled 
in the art with only the foregoing description and without further 
elucidation, the following is set forth for illustrative purposes and is 
not intended to be limiting of the invention. Reference is made to the 
light rays diagram in the drawing. 
The difference between the exit image and the source image is that of all 
the wavelengths present in the source image, only part of them is present 
in the exit image. The total wavelength band which is at the exit image is 
given by the dispersion: 
EQU d.lambda./d.beta.=(d.multidot.cos .beta.)/n (1) 
and since 
EQU d.beta.=dl/f (2) 
wherein l is the distance measured in the direction of dispersion on the 
plane of the intermediate slit 6, and f is the focal length of the lens 5: 
EQU d.lambda.=d.beta..multidot.cos .beta.d/n=dl.multidot.d.multidot.cos 
.beta./nf (3) 
In order for the entire image to appear at the exit slit, it is necessary 
that dl&gt;w, wherein w is the source image width. It is clear from equation 
(3) that, in order to restrict d.lambda., dl (limited by w) and d have to 
be made small and n and f to be made large. The d is the grating constant 
namely, the distance between consecutive grooves and n is the order of 
diffraction. 
To show that the angle of incidence at grating 4 must be equal to the angle 
of diffraction at grating 8, the following is set forth: 
The basic equation that relates the angle of incidence .alpha. with the 
angle of diffraction .beta. at the grating withe a grating constant d 
(i.e. the distance between two consecutive grooves) is given by: 
EQU n.lambda.=d(sin .alpha.+sin .beta.) (4) 
wherein .lambda. is the wavelength of light and n is the order of 
diffraction. Angle .alpha. and .beta. are signed and measured from the 
grating normal N, .alpha. being the angle of incidence and .beta. being 
the angle of diffraction. Therefore, angle .alpha. is positive and angle 
.beta. is negative. n may be positive or negative, meaning that there are 
always two diffraction spectra, one spectrum on each side of the zero 
order beam. 
Since the grating 4 has been rotated by an angle .gamma., .alpha..sub.1 
=.alpha..sub.10 +.gamma., and .beta..sub.1 =.beta..sub.10 
-.gamma.=.alpha..sub.10 -.gamma., since .alpha..sub.10 =.beta..sub.10. The 
wavelength passing through the intermediate slit is given by equation (4). 
On grating 8, the angle of incidence is .alpha..sub.2 =.alpha..sub.10 
-.gamma.=.beta..sub.1, and therefore, the angle of diffraction is given by 
equation (4) solved for .beta..sub.2 : sin .beta..sub.2 =nP.lambda./d-sin 
.alpha..sub.2 =n.lambda./d-sin .beta..sub.1 =sin .alpha..sub.1 
Therefore .beta..sub.2 =.alpha..sub.1. 
As previously stated of all the wavelengths emitted by the source image, 
only those will reach the exit slit that passes through the intermediate 
slit. If d.lambda. is too large, the image position at the intermediate 
slit will be off axis and blocked off by the slit jaws. The intermediate 
slit limits the width of the wavelength that is allowed to reach the 
second grating 8. All of the wavelengths in that band which reaches the 
second grating 8 are then recombined into as many directions as there are 
image points, and then lens 9 focuses them into a single image of the 
source image at the exit slit. There are as many images of the source 
image on the intermediate slit as there are wavelengths in the source 
light. If the source is a continuous band of wavelengths, images at the 
intermediate slit will overlap and form an image blurred in the direction 
of the wavelength dispersion. 
A typical arrangement utilized components having the following 
measurements. These measurements and components are given only as 
illustrations and are not intended to be limiting of the invention in any 
manner. 
Focusing lenses: resolution 1.2 .mu.m. 
Collimating lenses: resolution 1.2 .mu.m. 
Gratings: 590 grooves per mm; figure of grating 1/4 wavelength (using Mg 
564 nm). 
Exit Slit: 0.5 mm. 
Entrance slit: 0.5 mm. 
Intermediate Slit: 2 mm. 
Viewer: TV camera. 
Objectimager: TV camera lens. 
The foregoing description is illustrative of the principles of the 
invention. Numerous modifications and extensions thereof would be apparent 
to the worker skilled in the art. All such modifications and extensions 
are to be considered to be within the spirit and scope of the invention.