High-gain , photoelectronic image intensification is applied under conditions of low incident light levels whenever the integration time required by a sensor or recording instrument exceeds the limits of practicability .
Examples of such situations are ( aerial ) night reconnaissance , the recording of radioactive tracers in live body tissues , special radiography in medical or industrial applications , track recording of high energy particles , etc. .
High-gain photoelectronic image intensification may be achieved by several methods ; ;
some of these are listed below : ( A )
Cascading single stages by coupling lens systems , ( B )
Channel-type , secondary emission image intensifier , ( C )
Image intensifier based upon the `` multipactor '' principle , ( D )
Transmission secondary electron multiplication image intensifiers ( TSEM tubes ) , ( E )
Cascading of single stages , enclosed in one common envelope .
Cascading single stages by coupling lens systems is rather inefficient as the lens systems limit the obtainable gain quite severely .
Channel-type image intensifiers are capable of achieving high-gain values ; ;
they suffer , however , from an inherently low resolution .
Image intensifiers based upon the multipactor principle appear to hold promise as far as obtainable resolution is concerned .
However , the unavoidable low-duty cycle restricts the effective gain .
TSEM tubes have been constructed showing high gain and resolution .
However , electrostatic focus , important for many applications , has not been realized for these devices .
Resolution limitations with electrostatic focus might be anticipated due to chromatic aberrations .
Furthermore , the thin film dynodes appear to have a natural diameter limitation wherever a mesh support cannot be tolerated .
Cascaded single stages enclosed by a common envelope have been constructed with high gain and high resolution .
These tubes may differ both in the choice of the electron optical system and in the design of the coupling members .
The electron optical system may be either a magnetic or electrostatic one .
The magnification may be smaller , equal , or larger than unity .
An electrostatic system suffers generally from image plane curvature leading to defocusing in the peripheral image region if a flat viewing screen ( or interstage coupler ) is utilized , while a magnetic system requires accurate adjustment of the solenoid , which is heavy and bulky .
As it will be discussed later , peripheral defocusing can be improved on by utilizing curved fiber couplers .
It should be noted , however , that the paraxial resolution is quite similar for both electron optical systems .
It is felt that fiber-coupled double- ( and multi- ) stage image intensifiers will gain considerable importance in the future .
Therefore , we shall consider in this paper the theoretical gain and resolution capabilities of such tubes .
The luminous efficiency and resolution of single stages , fiber couplers , and finally of the composite tube will be computed .
It will be shown theoretically that the high image intensification obtainable with such a tube and contact photography permits the utilization of extremely low incident light levels .
The effect of device and quantum noise , associated with such low input levels , will be described .
After these theoretical considerations , constructional details of a fiber-coupled , double-stage X-ray image intensifier will be discussed .
Measured performance characteristics for this experimental tube will be listed .
The conclusion shall be reached that fiber-coupled , double-stage tubes represent a sensible and practical approach to high-gain image intensification .
Basic design of a fiber-coupled , double-stage image intensifier
The tube design which forms the basis of the theoretical discussion shall be described now .
The electron optical system ( see fig. 14-1 ) is based in principle on the focusing action of concentric spherical cathode and anode surfaces .
The inner ( anode ) sphere is pierced , elongated into a cup , and terminated by the phosphor screen .
The photoelectrons emitted from a circular segment of the cathode sphere are focused by the positive lens action of the two concentric spheres , pass through the ( negative ) lens formed by the anode aperture , and impinge upon the cathodoluminescent viewing screen .
The cylindrical focusing electrode permits adjustment of the positive lens part by varying the focusing potential .
The anode potential codetermines the gain , G , and magnification , M , of the stage .
Both the photocathode and the image plane of such an electrode configuration are curved concave as seen from the anode aperture .
The field-flattening property of the biconcave fiber coupler can be utilized to alleviate the peripheral resolution losses resulting with a flat phosphor-screen or coupling member .
For the same reason , the output fiber plate is planoconcave , its exposed flat side permitting contact photography if a permanent record is desired .
As it will be shown later , the field-flattening properties of the interstage and output fiber coupler comprise indeed the main advantage of such a design .
The second photocathode and both phosphor surfaces are deposited on the fiber plate substrates .
The photocathode sensitivities S , phosphor efficiencies P , and anode potentials V of the individual stages shall be distinguished by means of subscripts 1 , and 2 , in the text , where required .
Both stages are assumed to have unity magnification .
Theoretical discussion of flux gain
flux gain of a single stage
The luminous gain of a single stage with Af ( flux gain ) is , to a first approximation , given by the product of the photocathode sensitivity S ( amp / lumen ) , the anode potential V ( volts ) , and the phosphor conversion efficiency P ( lumen/watt ) .
In general , P is a function of V and the current density , but it shall here be assumed as a constant .
The luminous efficiency Af of a photocathode depends on the maximum radiant sensitivity Af and on the spectral distribution of the incident light Af by the relation : Af where Af is normalized radiant photocathode sensitivity .
Af is standard visibility function .
The luminous flux gain of a single stage is given by : Af .
If the input light distribution falls beyond the visible range , Af as expected , since Af .
Such cases are not considered here .
Efficiency of fiber couplers
The efficiency of fiber optics plates depends on four factors : ( A )
numerical aperture ( N.A. ) ; ;
( B )
end ( Fresnel reflection ) losses ( R ) ; ;
( C )
internal losses ( I.L. ) ; ;
( D )
packing efficiency ( F.R. ) .
The numerical aperture of the fibers is given by : Af where Af .
The angle Af is measured in the medium of index Af .
Settled phosphors , as generally used in image intensifiers , have low optical contact with the substrate surface , hence Af shall be assumed .
The numerical aperture should be in general close to unity .
This condition can be satisfied , e.g. , with Af and Af or equivalent glass combinations .
A sufficiently good approximation for determining the end reflection losses R can be obtained from the angle independent Fresnel formula : Af .
For phosphor to fiber and fiber to air surfaces , and assuming Af , we obtain Af percent .
This value may be reduced to 4.6 percent by means of a ( very thin ) glass layer of index 1.5 .
Hence , the Af factor for the output fiber coupler is Af .
As the index of refraction of photosensitive surfaces of the SbCs-type lies around 2 , the Fresnel losses at the fiber-photocathode interface are about 0.5 percent and the Af factor for the interstage coupler is 0.95 .
It might be anticipated that multiple coatings will reduce end reflection losses even further .
The internal losses are due to absorption and the small but finite losses suffered in the numerous internal reflections due to deviations from the prescribed , cylindrical fiber cross-section and minute imperfections of the core-jacket interface .
These losses depend on fiber diameter and length , absorption coefficient , the mean value of the loss per internal reflection and last , but not least , on the angular distribution of the incident light .
Explicit expressions ( integral averages ) are given in the literature .
Lacking reliable data for some of the variables , we are relying on experimental data of about 20 percent internal losses for 1/4-inch long , small ( 5 - 10 M ) diameter fibers .
This relatively high value is probably due to the small fiber diameters increasing the number of internal reflections .
Since we are considering here relatively small diameter ( 1 - 1.5 inches ) fiber plates , their average thickness can be kept below 1/4 inch and their internal losses may be assumed as 15 percent ( per plate ) .
The packing efficiency , F.R. , of fiber plates did not receive much attention in the literature , probably as it is high for the larger fibers generally used , until rather recently .
For circular fibers in a closely packed hexagonal array , the packing efficiency is given by : Af where Af , and 0.906 is the ratio of the area of a circle to that of the circumscribed hexagon .
For the small diameter fibers now technically feasible and required for about 100 Af resolution , Af .
The cladding thickness is about 0.5 M , hence , Af and Af .
Thus , the efficiency **yt couplers is given by -- Af or approximately 50 percent each .
It must be remembered that the fiber plates replace a glass window and a ( mica ) membrane , in addition to an optical output lens system .
The efficiency Af of an Af lens at the magnification Af is : Af .
Neglecting absorption , the end losses of the coupling membrane and the output window Af would be 6 percent and 8 percent .
Thus , the combined efficiency of the elements replaced by the two fiber plates ( with a combined efficiency of 0.25 ) is 0.043 or about six times less than that of the two fiber plates .
Gain of fiber coupled image intensifiers
Including the brightness gain Af due to the Af area demagnification , the overall gain of a fiber coupled double stage image intensifier is : Af .
It is obvious that the careful choice of photocathode which maximizes Af for a given input E ( in the case of the second stage , for the first phosphor screen emission ) is very important .
The same consideration should govern the choice of the second-stage phosphor screen for matching with the spectral sensitivity of the ultimate sensor ( e.g. , photographic emulsion ) .
We have evaluated the `` matching integrals '' for two types of photocathodes ( S-11 and S-20 ) and three types of light input .
The input light distributions considered are P-11 and P-20 phosphor emission and the so-called `` night light '' ( N.L. ) as given by H.W. Babcock and J. J. Johnson .
The integrals ( in units ) are listed in table 14-1 , below .
Theoretical discussion of paraxial device resolution
resolution limitations in a single stage
The resolution limitations for a single stage are given by the inherent resolution of the electron optical system as well as the resolution capabilities of the cathodoluminescent viewing screen .
The resolution capabilities of an electrostatic system depend on both the choice of magnification and chromatic aberrations .
It has been stated previously that a minifying electrostatic system yields a lower resolution than a magnifying system or a system with unity magnification .
Furthermore , the chromatic aberrations depend on the chosen high voltage .
In general , a high anode voltage reduces chromatic aberrations and thus increases the obtainable resolution .
The luminous gain of the discussed tube was calculated from Eq. ( 6 ) for the 16 possible combinations of S-11 and S-20 photocathodes and P-11 and P-20 phosphor screens , for night light and P-20 light input .
( The P-20 input is of interest because it corresponds roughly to the light emission of conventional X-ray fluorescent screens ) .
The following efficiencies obtained from JEDEC and RCA specifications were used : Af
The following table ( 14-2 ) lists the ( luminous ) gain values computed according to Eq. ( 6 ) with Af .
The possibility of a space charge blowup of the screen crossover of the elementary electron bundles has been pointed out .
It is obvious that such an influence can only be expected in the final stage of an image intensifier at rather high output levels .
Space charge influences will also decrease at increased voltages .
Electrostatic systems of the pseudo-symmetric type have been tested for resolution capabilities by applying electronography .
A resolution of 70 - 80 line-pairs per millimeter appears to be feasible .
The inherent resolution of a cathodoluminescent phosphor screen decreases with increasingly aggregate thickness ( with increasing anode voltage ) , decreases with decreasing porosity ( thus the advantage of cathodophoretic phosphor deposition ) and might be impaired by the normally used aluminum mirror .
Thus , in general , elementary light optical effects , light scatter , and electron scatter determine the obtainable resolution limit .
It should be noted that photoluminescence , due to `` Bremsstrahlung '' generated within the viewing screen by electron impact , appears to be important only if anode voltages in excess of 30 KV are utilized .
It has been stated that settled cathodoluminescent phosphor screens may have a limiting resolution of 60 Af at high voltage values of approximately 20 Aj .
For the further discussion , we shall thus assume an electron optical resolution of 80 Af and phosphor screen resolution of 60 Af .