SPIE Proceedings [SPIE SPIE Defense, Security, and Sensing - Baltimore, Maryland, USA (Monday 29...

Characterization of domestic and foreign image intensifier tubes

Edward J. Bender, Michael V. Wood, Daniel J. Hosek, Steve D. Hart

US Army RDECOM CERDEC Night Vision & Electronic Sensors Directorate, Fort Belvoir, VA

22060

ABSTRACT

The market for military-use Generation 2 and Generation 3 image intensifier (I2) tubes has become truly global, with

major manufacturers and customers spanning five continents. This worldwide market is becoming increasingly

important to U.S. manufacturers, with the majority of U.S. Army intensifier fielding having been completed in 2012.

Given this keen global competition, it is not surprising that the advertised tube performance of a given source is often

discounted by competitors, and the customers have no objective "honest broker" to determine the relative accuracy of

these claims. To help fill this void, the U.S. Army RDECOM CERDEC NVESD recently measured a number of

domestic and foreign image intensifier tubes, using consistent test equipment/procedures with which the U.S. industry

must correlate for Army tube deliveries. Data and analysis will be presented for the major tube parameters of luminance

gain, equivalent background input (EBI), signal-to-noise ratio (SNR), limiting resolution, halo, and modulation transfer

function (MTF). The bright-light resolution provided by various auto-gated and non-gated tubes will also be addressed,

since this area has been an important factor in the international market. RDECOM CERDEC NVESD measurement data

will be compared to the corresponding manufacturer specifications whenever possible.

Distribution A: Approved for public release; distribution is unlimited.

Keywords: Generation 2, Generation 3, image intensifier tubes, I2, luminance gain, EBI, SNR, resolution, halo, MTF,

auto-gating, non-gated

1. INTRODUCTION

The proximity-focused (i.e., “wafer”) image intensifier tube has served as the premier visible/near-infrared (VNIR)

mobility sensor of the U.S. military for the past forty years. The major reasons for its popularity are environmental

robustness, relative design simplicity, frugal power consumption, and low cost. The Generation 2 (or “Gen 2”) wafer

intensifier tube was introduced in the mid-1970s and widely deployed until it was supplanted in the U.S. by Generation 3

(or “Gen 3”) devices in the mid-1980s. Other countries have for the most part continued to fabricate only Gen 2 devices

in large-scale production. The fundamental difference between Gen 2 tubes and Gen 3 tubes lies in the photocathode:

Gen 2 photocathodes utilize an evaporated, amorphous tri-alkali film, whereas Gen 3 photocathodes employ single-

crystal gallium arsenide to achieve negative electron affinity (NEA) and much higher quantum efficiency. A second

important difference is that Gen 3 tubes typically require an ion-barrier film to be placed over the input side of the

microchannel plate (MCP), in order to prevent positive ion feedback during operation that would damage the NEA layer

of the photocathode and greatly shorten tube life. Gen 2 tubes generally do not incorporate this feature. The general

configuration of the Generation 2/3 image intensifier tube is illustrated in Figure 1 below. This device has three active

components placed in close proximity within a high-vacuum envelope: the photocathode, the MCP, and the phosphor

screen. In response to a dim nighttime image focused upon the photocathode, photoelectrons are emitted at a peak

quantum efficiency of 0.2 – 0.5, and are then accelerated across a small gap by an applied electric field, such that a high

proximity focus (>100 lp/mm resolution) is achieved. These photoelectrons then enter one of over 10,000,000

microchannel openings of the MCP and strike a microchannel wall, thereby initiating an avalanche of secondary

electrons that propagates down the microchannel. Several hundred to over one-thousand electrons can exit a

microchannel for every photoelectron entering it. These multiplied electrons are drawn across another small proximity

gap by an electric field to the phosphor screen. This component converts every impinging electron into ten or more

photons that exit its fiber optic substrate. As a result, the Gen 2/3 intensifier tube achieves a “photon” gain that

approaches 2,000x - 5,000x at peak spectral sensitivity. Its key strength, however, lies in its conversion of invisible NIR

radiation into visible light, typically green, to which the eye has its greatest sensitivity.

Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXIV, edited by Gerald C. Holst, Keith A. Krapels, Proc. of SPIE Vol. 8706, 870607 · © 2013 SPIE

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1,

Figure 1. General Layout of the Generation 2/3 Wafer Image Intensifier Tube Module.

The intensifier tube module, or “bare” tube, consists of the glass/ceramic vacuum package containing the three active

elements of the intensifier tube, plus a glass-window input faceplate and an output fiber-optic faceplate; it does NOT

include an integral, high-voltage power supply for powering the intensifier tube from simple batteries. The tube module

plus integral miniature power supply is conventionally termed the intensifier tube “assembly.” The intensifier testing

reported herein exclusively utilized such tube assemblies.

At high inputs (typically full-moon scene illuminations and higher), the imagery provided by these devices is limited

primarily by the electro-optical losses associated with the device’s components and proximity focusing. As the light

level decreases, however, imagery also degrades due to the increasing temporal noise associated with the “shot” (or

Poisson) noise of the photocathode, and the MCP’s noise figure. This has led to the emergence of approaches for

characterizing overall tube performance by suitably combining high-light metrics (e.g., limiting resolution) and low-light

metrics (e.g., SNR) in a single Figure of Merit (FOM).

2. MAJOR I2 TUBE PERFORMANCE PARAMETERS - DESCRIPTION

The major performance parameters of I2 tubes can be considered to fall within two groups. The first group (luminance

gain, EBI, limiting resolution, and SNR) comprises parameters that are considered fundamental, and are typically

measured on every individual tube; the second-group parameters (bright-light resolution, halo, and MTF), though

considered important, are often measured on a sample basis. This section will next describe and explain the major I2

tube parameters, with the most attention given to those parameters that are considered fundamental. More detail on

these parameters can be found in several sources1,2

in open literature.

2.1 LUMINANCE GAIN

The tube conventionally operates upon an input image formed by an objective lens to provide an output image that is

viewed via a magnifying eyepiece. In the U.S., the tube’s luminance gain is simply the quotient of the output image’s

luminance (in footlamberts) divided by the input image’s illumination (in footcandles), as produced by a 2856K

blackbody irradiance distribution. This luminance gain value can be converted to SI units of nit/lux by dividing it by pi.

If one were to combine this with the focal ratio and transmission of the objective lens and the eyepiece's transmission,

one could arrive at the overall gain of a direct-view system. This system gain is a measure of how much brighter the

intensified imagery appears to the user than the corresponding unaided eye perception (without dark adaptation). An

implicit assumption is that the scene radiance conforms to a 2856K blackbody distribution, which has been found to

fairly well correlate with the general VNIR irradiance of the no-moon night sky3.

Glare Absorber (either

matrix or surface layer)

Clear Glass Faceplate

Gallium Arsenide (Gen 3) or Tri-

Alkali (Gen 2) Photocathode

MCP Ion-Barrier Film

(Gen 3)

Phosphor screen

Microchannel Plate (MCP)

Fiber Optic Inverter



The conventional tube power supply runs the tube at a fixed luminance gain at low input illuminations. As input

illuminations increase, however, two control mechanisms are activated to reduce tube gain. The first mechanism, called

Automatic Brightness Control (ABC), typically engages at input light levels roughly corresponding to quarter-moon

scene illuminations; a second group of controls (Bright Source Protection, Auto-gating) engage at yet higher scene

illuminations, and are described in paragraph 2.4 below. The ABC circuit senses the total electron current striking the

phosphor screen, and a feedback loop then adjusts the MCP voltage (and thus the MCP’s electron gain) such that the

total screen current does not exceed a certain predetermined threshold. The ABC circuit thereby acts to limit average

output image brightness for input illuminations ranging up to those encountered in daytime scenes. There is typically

great flexibility in choosing and changing the luminance gain of a given tube. In fact, the luminance gain and the ABC

threshold can generally be adjusted via two respective controls on the back plate of the tube assembly. The ABC

threshold as measured in footlamberts is conventionally known as the tube's "saturation" value.

From a system standpoint, increasing the gain within reasonable limits generally results in greater user comfort and

improved eye performance; the tradeoff lies in impeded dark adaptation when the intensifier system is taken/turned off.

From a tube standpoint, standard Gen 3 tube performance parameters are unaffected by large excursions of luminance

gain (10,000-85,000). As gain begins to approach 100,000, however, bright noise scintillations begin to noticeably

increase. These scintillations increase in such a gradual manner that there is no easy way to characterize the point at

which they assume objectionable proportions. At gains below 100,000, they have only a minor (< 20 percent) effect on

tube SNR, yet they can be highly distracting to the user. One experienced intensifier user has likened the effect to

“looking through the noise” - if one concentrates, all the scene detail can still be discerned, though this effort may be

tiring and undesirable in the long run.

2.2 EQUIVALENT BACKGROUND INPUT (EBI)

Very few treatments2

of the intensifier’s signal/noise characteristics address the intensifier’s additive dark noise, because

it is in fact negligible in many operational environments. EBI characterizes the “dark” output of an image intensifier

tube. To arrive at an intensifier tube’s EBI, one typically first measures the tube’s output luminance for the no-input

condition. This value of output luminance is then divided by the tube’s luminance gain to arrive at a corresponding

value of input illumination. EBI therefore includes the dark-noise contributions of the photocathode, MCP, and

phosphor screen. It is well known, however, that thermionic emission from the photocathode contributes at least 90% of

the total EBI, with the MCP contributing the remainder. The EBI specification has remained remarkably unchanged

over the past forty years: the typical specification allows an EBI no higher than 2.32E-08 footcandles (FC), or

equivalently no higher than 2.5E-11 lumens/cm2 (or "phots"). EBI is typically measured at a photocathode temperature

of 21.0C. The maximum allowable EBI level is approximately 200x lower than a typical photocathode illumination

under starlight conditions; moreover, it is 20x lower than that typically produced under overcast starlight, which is the

lowest scene illumination for which typical image intensifiers are designed to operate in passive mode.

One can argue that this EBI specification has changed little over the years because it properly reflects the operational

environment, despite appearances to the contrary. This is because EBI has been consistently demonstrated to vary

exponentially with increasing photocathode temperature. RDECOM CERDEC NVESD measurements4 have indicated

that Gen 3 tube EBI doubles for every 5C increase in photocathode temperature, while Gen 2 tube EBI doubles for

every 6C increase in photocathode temperature. Although EBI may not be much of a factor in cool to moderate

ambient temperatures, summer nights in Southwest Asia can approach 45C, where specification-compliant EBI could

still induce a tube output that is nearly 50% higher than that produced by overcast starlight.

2.3 SIGNAL-TO-NOISE RATIO (SNR)

Tube SNR is a measure of the output luminance's temporal mean-to-standard-deviation ratio for a highly specific input

condition. SNR reflects a host of arbitrary conditions that are unique to the image intensifier field. First, tube SNR

utilizes a small input spot of highly arbitrary size (0.2mm diameter) and illumination (1.0 x 10–5

FC). The spot size

relates in no direct way to any important tube or system parameter, and its illumination corresponds merely to a high

extreme of clear no-moon conditions. In fact, both values were chosen more for test expediency and consistency than

for any intrinsic operational significance. In contrast to the SNR conventions of other electro-optical devices, tube SNR

is not a single-pixel measurement, and it is not a straightforward measure of typical field performance.



Interestingly, the size of the input spot loosely correlates with the lowest spatial frequency (2.5 line pairs/mm) for which

tube MTF is specified. One might therefore presume that this spot size was adopted to capture only the signal associated

with useful imaging, as opposed to that associated with the tube’s halo artifact (see section 2.5 below), which manifests

itself as a diffuse bright disk surrounding bright points of light focused onto the tube’s photocathode. Halo is a basic

consequence of the tube’s proximity focus design, and results from photoelectrons that do not initially enter a

microchannel, but are instead initially reflected elastically off the MCP’s webbing. Because such photoelectrons can

travel up to twice the proximity spacing (almost 0.3mm) before they again strike the MCP, they obviously do not

contribute to useful imaging. Close inspection of the above SNR test requirement, however, indicates that the halo

signal was not completely excluded from the SNR computation. The output measurement aperture was allowed to be

larger than 0.2mm, and in fact the aperture utilized at RDECOM CERDEC NVESD is 0.3mm, presumably chosen many

years ago in an effort to collect most of the reflected halo photoelectrons for the SNR measurement.

The SNR measurement also conventionally utilizes a 10 Hz noise-equivalent bandwidth in computing the signal/noise

statistics for the tube’s amplification of the extremely dim input spot. The underlying assumption of the SNR

measurement is that the eye-brain system of the intensifier user perceives brightness fluctuations primarily over this 10

Hz temporal-frequency bandwidth.

An additional important nuance is that the intensifier tube’s principal background source is the photocathode’s

thermionic emission (i.e., its dark current), which obeys the same Poisson statistics as the photocathode signal current

itself. This means that this background dark-current component would always increase the computed tube SNR;

accordingly, its effects are specifically removed in accordance with longstanding measurement conventions.

The U.S. has historically followed a SNR measurement convention, however, that other countries may not necessarily

observe. The U.S. considers SNR to be a quality metric addressing only the photocathode and the MCP. Consequently

the temporal effects of the phosphor screen (i.e., its “noise-smoothing”) are specifically removed from SNR via a “k-

factor” correction, which reduces the “uncorrected” SNR by 15-20%. The k-factor of the type P-43 phosphor found in

virtually every U.S. Gen 3 tube has an associated k-factor of 1.15, while that of the somewhat slower P-22 phosphor

used elsewhere in the world has been measured to have a 1.21 k-factor5.

The U.S. Government has found it necessary to conduct periodic correlation efforts with the U.S. intensifier industry, to

maintain conformance with the above very challenging test requirements. For all the reasons described above,

comparison of international marketing information regarding SNR can be misleading.

2.4 "LIMITING" RESOLUTION VS. "BRIGHT-LIGHT" RESOLUTION

Resolution, conventionally known as limiting resolution or optimum resolution in the industry, characterizes image

sharpness. The convention is to project a 1951 U.S. Air Force Resolving Power Test Target onto the tube input, and to

then determine the smallest element for which an observer can completely discern both the horizontal and vertical triple-

bar sets on the tube output. The conventional format is black (≈100% contrast) bars on a bright background. An

illustration of the 1951 USAF Resolving Power Test Target and its salient characteristics is provided in Figure 2. This

parameter is a measure of the finest detail which can be provided by the tube, when an input image has very high

contrast. As such, the input illumination is generally varied until image quality (i.e., bar sharpness) is optimized. One

should also note that resolution is now generally measured with a microscope operating at a substantially higher

magnification than the 10x normally utilized by direct-view system eyepieces; the rationale is that resolution data should

reflect the tube’s resolution limit, not the resolution degradations associated with test equipment or the observer’s eye.

Up through the 1990s, tube power supplies protected the tube from damage under bright-light conditions via a Bright

Source Protection (BSP) mechanism, which greatly lowered the voltage placed between the photocathode and MCP.

They were informally called "DC" power supplies, even though this voltage had some temporal modulation. This

approach provided effective protection, but the tradeoffs were degraded tube resolution and image quality in twilight and

higher scene illuminations. “Auto-gated” power supplies, in contrast, provide this protection by performing

autonomous duty-cycle gating; the "ON" level is a much higher fraction of the full photocathode-to-MCP voltage, while

the "OFF" level is set at a reversed-bias level that shuts off all photocathode current. This ON-OFF switching is

performed at a frequency sufficiently high to be imperceptible to the user. The tube’s effective “ON” time is thereby



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reduced to prevent damage, but the tube can still retain optimum image quality and 75% or more of its optimum

resolution, even in daytime conditions. This has proven to be an important benefit, given that many nighttime operations

commence at dusk, when the light levels can drop by 2x for every five minutes after sunset.

The bright-light (also known as "high-light") resolution parameter characterizes the resolution of the I2 tube for

illuminations substantially higher than full-moon. The tube's input illumination for this measurement is specified to be

no lower than 20fc, which roughly falls within dusk/dawn conditions. Under this test, tubes with auto-gated power

supplies can yield double the resolution of tubes with the original "DC" power supplies.

Figure 2. 1951 USAF Resolving Power Test Target (shaded areas denoting various bar-element groups).

2.5 HALO

“Halo” refers to the bright diffuse disk of light which surrounds bright light sources in the imagery of a proximity-

focused image intensifier. It is an intrinsic artifact of all wafer image intensifier tubes, and results from the elastic and

inelastic backscattering of photoelectrons from the amplification element, be it the MCP or the phosphor screen. Halo is

not blooming. Blooming occurred in Gen 1 devices, where bright point sources could actually burn holes in the output

phosphor screen, not to mention washing out a large portion of the imagery. The amplification controls provided by the

MCP, however, effectively eliminated the blooming effect for wafer tubes. Halo size conventionally refers to the

diameter of this disk, when the input to the intensifier consists of a bright illuminated spot focused upon the

photocathode that has a diameter of 0.35mm.

In direct-view systems, the halo’s appearance is that of a diffuse bright disk which surrounds bright radiation sources. In

most cases, this disk is not opaque to scene information - rather, it appears more like a veil through which significant

scene detail can still be discerned. Only an extremely small area around the light source (much less than 10 percent of

the halo’s total area) is typically opaque.



It can be shown that the diameter of the halo is determined by those electrons that are elastically reflected off the

amplification element at an angle of 45-degrees to the normal. The physics also show us that halo size is a function

solely of the relevant proximity spacing; for a point source, the halo diameter is simply 4 times the proximity spacing.

This makes the halo measurement an excellent nondestructive method for measuring the input spacing of a wafer tube.

The Gen 2 tube design is potentially capable of a smaller input spacing than its standard Gen 3 tube counterpart, because

of the latter's ion-barrier film, which requires a higher photocathode-to-MCP voltage to ensure effective photoelectron

penetration of this film. The Gen 2 tube design is consequently capable of smaller halos than the conventional Gen 3

tube design (i.e., based upon an MCP with an ion-barrier film).

2.6 MODULATION TRANSFER FUNCTION (MTF)

Modulation Transfer Function (MTF) characterizes the amount of contrast reduction occurring from tube input to output,

when a real image of given sinusoidal spatial frequency is focused upon the input. At lower spatial frequencies, back-

scattered electrons and scattered light are the predominant MTF limiters. As the spatial frequency increases, the tube’s

proximity focusing and the MCP channel size become increasingly important. Performance specifications and models

for image-intensifiers and other VNIR sensors typically hinge upon parameters related to (or derived from) SNR and

MTF. This is not surprising, given that MTF is the principal measure of system performance when optimum signal

levels are present, while SNR is the principal determinant of performance degradation as signal levels decrease from

optimum levels.

Several fundamentally different MTF measurement techniques have been employed at various times by RDECOM

CERDEC NVESD and the intensifier industry. One distinct approach is to project an illuminated slit upon the tube's

input, measure the tube’s output line spread, and then compute the modulus of its Fourier Transform to determine the

tube's MTF. This approach generally entails demanding techniques to maintain overall tube linearity while maximizing

the useful dynamic range of the MTF analyzer. Determination of the tube’s true “black” level is arguably the most

challenging aspect of this approach. A unique benefit is that this approach generates a large sample-limited set of MTF

values, rather than the relatively few discrete values generated by discrete bar-pattern approaches.

Some MTF testers developed in the late 1990s projected discrete Ronchi patterns of various spatial frequencies onto the

tube’s photocathode, rather than a single bright-slit pattern. Using a Fourier transform methodology, such MTF testers

measured the amplitude of the fundamental and harmonic frequencies of the Ronchi patterns. These values were then

divided by the corresponding system response without the tube in place, thereby providing a definitive measure of the

tube’s MTF response. The present RDECOM CERDEC NVESD MTF tester (the Automated Image Intensifier

Measurement System or "AIMS II") developed by the Pennsylvania State University Electro-Optics Center (PSU EOC)

utilizes this approach for MTF measurement.

The reader should note some important nuances of MTF measurement. First, the test system calibration (or

normalization) frequency should be 0.25lp/mm or less, in order to suitably capture the MTF effects of both veiling glare

and halo, which can be as large as 1.12mm for point source. Second, the input illumination of the photocathode must be

low enough to ensure that the MCP does not undergo gain saturation (and resulting nonlinear gain response) during the

measurement process.

It can be difficult for the layperson to gain an intuitive feel for how MTF relates to general image quality. To assist the

reader, Figure 3 below presents corresponding Ronchi-pattern images for two tubes which were measured to have MTF

at the high end and the low end of the tube groups measured in this paper. The reader should note the substantially

higher bar contrast for the tube measured to be at the high end of MTF.



Ronchi Image Comparisons at Various Spatial Frequencies:

15 Ip/mm 25 Ip/mm 40 Ip/mm

Low MTF Tube High MTF Tube Low MTF Tube High MTF Tube Low MTF Tube High MTF Tube

-

Figure 3. Illustration of MTF results for low MTF and high MTF tubes.

3. DESCRIPTION OF THE MEASURED DOMESTIC AND FOREIGN I2 TUBES

The tested image intensifier tubes comprised four major groups:

1. Two (2) each non-gated Gen 3 tubes purchased on the commercial market in 2011 from an Asian manufacturer;

2. Four (4) each auto-gated Gen 2 tubes purchased on the commercial market in 2012 from a European

manufacturer;

3. Ten (10) each domestic auto-gated Gen 3 tubes purchased by the U.S. Government within the past five years,

each having active or recently expired warranties;

4. Three (3) each domestic non-gated Gen 3 tubes purchased by the U.S. Government in the late 1990s.

The late-1990s U.S. Gen 3 tubes were chosen primarily to assess historical trends in MTF performance. Additionally, a

group of recently-fabricated Generation 3 tubes of a U.S. manufacturer were assessed, in order to gain insight into the

performance of approaches for providing bright-light resolution that do not entail auto-gating per se.

All of the tubes generally conformed to the MX-10160 package dimensions, albeit with some minor variations (see

Figure 4 below). All non-U.S. tubes utilized twin battery contacts, as did the tubes of the original Aviator's Night Vision

Imaging System of the 1980s. The Asian tubes utilized a black plastic housing, while the European tubes utilized a

golden, metalized housing. The U.S. tubes, in contrast, utilized a single positive battery contact electrically isolated

from the remainder of the silver-colored metalized housing (i.e., the "silver bullet" configuration).

Figure 4. Left to right: Asian Gen 3, European Gen 2, Recent U.S. Gen 3, Late 1990s U.S. Gen 3 tubes.



4. MEASUREMENT RESULTS - COMMON TUBE PERFORMANCE PARAMETERS

Table 1 below presents a summary of the measurements for all tube parameters, with the exception of bright-light

resolution:

Table 1. Summary of tube measurements for common performance parameters.

All of the above data was measured at RDECOM CERDEC NVESD. The tube types and individual unit numbers are

listed in the leftmost column of this table, while the performance parameters are indicated in the remaining columns.

The parametric data for each individual tube occupy a row, while the average for a tube group is indicated in the row

with the light-gray highlighting. The rows immediately beneath the average data contain the manufacturer's "typical"

value, the manufacturer's specification value, or the U.S. Government requirement, whichever was applicable.

Table 1 Note: Phosphor spectral measurements performed at RDECOM CERDEC NVESD confirmed that all tube

groups utilized a Type P-43 phosphor, having an associated SNR k-factor of 1.15 (as indicated in table).

Tube Saturation Luminance Gain EBI (x10-11) Resolution SNR FOM** Halo

Number (FL) (FL/FC) (lum/cm²) (lp/mm) (Kf = 1.15)* (Resolution x SNR) (mm) 2.5 7.5 15 25

1 1.39 30,466 2.26 57 19.6 1117 0.86 77.0 65.0 46.0 27.0

2 1.39 28,055 1.86 57 21.6 1231 0.89 78.0 66.0 46.0 27.0

Asian Non-Auto-Gated Gen 3 Avg (2ea) 1.39 29,260 2.06 57 20.6 1174 0.88 77.5 65.5 46.0 27.0

Mfr "Typical" ------ 40,000 2.0 64 ------ ------ 1.00 ------ ------ ------ ------

Mfr Spec ------ > 10,000 < 5 > 50 ------ ------ ------ ------ ------ ------ ------

1 1.47 37,833 2.52 57 21.4 1220 0.80 76.0 64.0 49.0 36.0

2 1.48 37,017 2.38 61 20.4 1244 0.78 78.0 69.0 53.0 38.0

3 0.94 53,642 2.42 61 20.4 1244 0.77 83.0 70.0 54.0 38.0

4 1.24 36,351 2.49 64 20.8 1331 0.72 78.0 67.0 51.0 37.0

European Auto-Gated Gen 2 Avg (4ea) 1.28 41,210 2.45 60.8 20.8 1261 0.77 78.8 67.5 51.8 37.3

Mfr "Typical" ------ ------ 1.5 72 28 2016 0.80 94 85 70 50

Mfr Spec > 1.17 > 30,000 < 2.5 > 64 > 23 > 1472** ------ ------ ------ ------ ------

1 3.19 54,250 1.73 64 26.2 1677 0.77 76.0 68.0 53.0 37.0

2 3.45 58,558 0.54 64 26.5 1696 0.80 73.0 66.0 51.0 35.0

3 3.02 55,438 3.65 57 24.5 1397 0.83 72.0 64.0 47.0 31.0

4 3.24 59,823 1.89 64 24.2 1549 0.87 73.0 66.0 51.0 35.0

5 3.24 61,119 2.39 61 23.3 1421 0.81 73.0 66.0 51.0 36.0

6 3.16 55,221 2.45 64 25.2 1613 0.83 74.0 68.0 51.0 35.0

7 2.99 59,342 2.81 64 25.2 1613 0.85 73.0 67.0 52.0 36.0

8 3.14 56,521 0.39 57 25.4 1448 0.83 73.0 67.0 52.0 36.0

9 3.08 57,178 3.42 64 26.9 1722 0.83 73.0 66.0 51.0 36.0

10 3.01 56,399 2.89 64 25.6 1638 0.87 72.0 64.0 49.0 34.0

Recent U.S. Auto-Gated Gen 3 Avg (10ea) 3.15 57,385 2.22 62.3 25.3 1576 0.83 73.2 66.2 50.8 35.1

Mfr "Typical" ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------

U.S. Gov't Requirement > 2.5 > 50,000 < 2.5 > 64 > 28 > 1792** ------ ------ ------ 61 38

1 2.67 34,316 0.22 61 21.7 1324 1.10 75.0 65.0 52.0 37.0

2 2.75 45,445 0.26 64 22.7 1453 1.07 77.0 69.0 54.0 39.0

3 2.78 45,186 3.40 61 22.8 1391 1.10 77.0 69.0 54.0 39.0

Late 1990s Non-Auto-Gated U.S. Gen 3 Avg (3ea) 2.73 41,649 1.29 62.0 22.4 1389 1.09 76.3 67.7 53.3 38.3

Mfr "Typical" ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------

U.S. Gov't Requirement > 2.0 > 45,000 < 2.5 > 64 > 21 > 1344** ------ 92 80 61 38

* P-43 phosphor temporal response. ** Figure of Merit (FOM) is generally not a spec per se. ------ denotes either "Not Applicable" or "Not Available"

MTF % @ (lp/mm)



4.1 OBSERVATIONS

The average data of the three recent tube groups appeared similar for the parameter of halo, while all four groups were

similar for the parameters of resolution, EBI and MTF. The U.S. Gen 3 tube groups were measured to be significantly

higher than the other groups for the parameters of saturation, luminance gain, and SNR. The measured SNR advantage

of the recent U.S. Gen 3 tube group led to its significantly higher value for the derived parameter "FOM," which is

commonly defined as the product of the limiting resolution and SNR. The European Gen 2 tube group had the best

average halo, but it also had the highest (worst) average EBI. Reproducibility of the measurement averages was

demonstrated to be 3% or less for all parameters including EBI, for which all tubes were pre-incubated within a tightly

controlled temperature range of 20.5 - 21.0C.

Whereas the measured-data averages generally appeared to be at least marginally compliant with the applicable

specifications/requirements, the manufacturer-indicated "typical" levels were often substantially higher than the

measured averages. Notable examples can be found for the parameters of luminance gain, resolution, SNR and MTF.

4.2 DISCUSSION

It is acknowledged that the tube groups represented both commercially-obtained tubes and tubes purchased by the U.S.

Government. To a degree this was unavoidable, since very few foreign tubes have ever been purchased by the U.S.

Government. Exclusively comparing commercially available tubes, however, was considered to be incompatible with a

primary objective of this specific exercise, which was to measure and compare tubes which had not been pre-selected by

their manufacturers.

5. MEASUREMENT RESULTS - BRIGHT-LIGHT RESOLUTION

Table 2 below presents a summary of the measurements for the parameter of bright-light resolution. As before, the tube

types and individual unit numbers are listed in the left column of this table, while the bright-light resolution is indicated

in the right column. The parametric datum for each individual tube occupies a row, while the average for a tube group is

indicated in the row with the light-gray highlighting. The rows immediately beneath the average data contain the

manufacturer's specification value or the U.S. Government requirement, whichever was applicable.

Since RDECOM CERDEC NVESD did not have operational equipment for measuring bright-light resolution during this

period, the above measurements were performed by Government personnel at ITT Exelis, Roanoke, VA. Government

personnel spot-checked the ITT Exelis bright-light resolution equipment for both input illumination and target size prior

to these measurements, in order to confirm compliance with Government requirements.

The tube groups utilized for this measurement differed somewhat from those utilized for the other parameters. The ten

(10) each MX-10160-type tubes purchased by the U.S. Government (which were ultimately utilized for the other

parameters) had not yet been identified at the time of these measurements, so ITT Exelis offered the opportunity for

Government personnel to measure five (5) each auto-gated tubes (Model F9800VG) they had recently fabricated in

conjunction with anticipated MX-10160 requirements of the U.S. Government. ITT Exelis also offered Government

personnel the opportunity to measure three (3) each tubes they had recently fabricated that employed a proprietary

alternate approach to auto-gating (i.e., "High-Light Control" or "HLC") for providing high bright-light resolution (Model

F9800HLC). The final two tube groups (European Gen 2, Late 1990s U.S. Gen 3) were the same as used in Table 1.

The non-gated Asian Gen 3 tubes were not measured in this exercise.



5.1 OBSERVATIONS/DISCUSSION

For all tube groups, the measured averages for bright-light resolution exceeded the applicable specification/requirement.

Not only were the European Gen 2 tubes measured to have significantly higher bright-light resolution than all the other

tube groups, this value was only 7% lower than their corresponding limiting resolution average (i.e., less than a single

increment of the 1951 USAF Resolving Power Test Target). In contrast, the bright-light resolution average for the non-

gated, late 1990s U.S. Gen 3 tube group was about half that of all the other groups.

It is noted that the ITT Exelis bright-light resolution average for their HLC group was 93% that of their auto-gated

group. This aptly illustrates that the auto-gating approach is not unique in its ability to provide high bright-light

resolution.

Table 2. Measurement summary for bright-light resolution.

6. CONCLUSIONS

1. The auto-gated European Gen 2 tubes and recent auto-gated U.S. Gen 3 tubes (MX-10160 type) arguably represent

the overall highest performance tubes on the worldwide market today. There was little measured difference between the

two regarding the parameters of resolution and MTF. The European Gen 2 tubes were measured to have somewhat

smaller (i.e., better) halo, while the recent U.S. Gen 3 tubes were measured to have somewhat lower (i.e., better) EBI,

and substantially higher luminance gain, saturation, and SNR.

2. The auto-gated European Gen 2 tubes were measured to have substantially higher bright-light resolution than the auto-

gated U.S. Gen 3 tubes. Both auto-gated tube groups were measured to have approximately two-times the bright-light

resolution of the non-gated U.S. Gen 3 tubes of the late 1990s.

Tube Number Resolution at 20FC

1 52.8

2 58.7

3 53.8

4 60.4

Recent Auto-Gated European Gen 2 Average (4ea) 56.4

Mfr Spec (min) 55

1 46.6

2 45.3

3 48.0

4 47.9

5 47.9

Recent Auto-Gated U.S. Gen 3 Average (5ea) 47.2

U.S. Gov't Requirement (min) 45

1 45.3

2 46.6

3 40.3

Recent Non-Auto-Gated U.S. Gen 3 Average (3ea) 44.1

(Exelis High-Light Control)

U.S. Gov't Requirement - N/A ------

1 24.1

2 25.4

3 24.7

Late 1990s Non-Auto-Gated U.S. Gen 3 Average (3ea) 24.7

U.S. Gov't Requirement (min) 12

------ denotes "Not Applicable"

Bright-Light Resolution

Proc. of SPIE Vol. 8706 870607-10


3. Recently fabricated ITT Exelis Gen 3 tubes with non-auto-gated power supplies (i.e., utilizing their proprietary High-

Light Control) were measured to yield nearly the same bright-light resolution as tubes utilizing the auto-gating

technique.

4. Measurement data provided herein indicates that U.S. Gen 3 tube performance has substantially improved since the

late 1990s for the parameters of luminance gain, saturation, SNR, halo, and bright-light resolution. It has not

appreciably improved, however, for the parameters of resolution and MTF.

ACKNOWLEDGEMENTS The authors would like to thank ITT Exelis, Roanoke, VA for providing access to tubes and equipment utilized in the

reported bright-light resolution measurements at their facility.

REFERENCES

1. Biberman (editor), Electro-Optical Imaging: System Performance and Modeling, chapter 5, pp. 21-31, Ontar

Corporation/SPIE Press, Bellingham, WA, 2000.

2. Csorba, Illes P., Image Tubes, pp. 255-263, Howard W. Sams & Co. Inc., Indianapolis, IN, 1985.

3. Burle, “Electro-Optics Handbook”, Technical Series EOH-11 (1974).

4. E. Bender, M. Wood, S. Hart, G. Heim, J. Torgerson, “Characterization of photocathode dark current vs.

temperature in image intensifier tube modules and intensified televisions,” Proc. SPIE, 5563, pp. 205-215, 2004.

5. A. Hastings, E. Bender, J. Hixson, and M. Wood, “Temporal Modulation Effects of Various Phosphors in Night

Vision Goggles,” Military Sensing Symposia, Passive Sensors (2006).

Proc. of SPIE Vol. 8706 870607-11


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