Until a few years ago X-ray sources with focal spot sizes less than 50
mm (0.050 mm) were commercially unavailable. However, today there are
several microfocus X-ray machines on the market that one can choose
from. Microfocus X-ray machines with focal spots as small as 1m m
(0.001 mm) and capable of delivering the required electron current ( 8
mA) over an energy range of 0 160 kV are commercially available. The
focal spots are usually continually adjustable up to 0.5 mm or more.
The tube head should be water cooled in order to remove the heat
generated at these high power densities. All of these "new"
systems are continuously pumped which gives them a certain technical
modular flexibility. The tube head can be opened, allowing to replace
the X-ray target (anode) by another metal in order to modify the X-ray
spectrum to meet special inspection requirements, change the tube head
window, and even remove the target module itself and replace it with a
rod-anode tube. These rod anode tube heads range in diameters from 18
mm down to as small as 4 mm and come in various lengths up to 1500 mm.
With rod anode tubes this small one can often perform inside out
single wall radiography on components with a flaw detection
sensitivity that was previously unattainable.
Another important consideration in selecting a microfocus X-ray source is the field
diameter of the X-ray beam or exit cone angle of the beam. The beam exit cone angle or
field coverage is important when one needs to perform microfocus radiography at short
source to film distances. For example, a tube head with a 40° exit cone will require
considerable fewer exposure to inspect the same area than if one were to use a tube head
with a 20° exit cone angle.
One should also consider the minimum distance between the focal spot and the tube head
face which determines just how close one can place an object to the X-ray source. This
parameter is important because it determines the maximum magnification that is attainable
for a microfocus system. Since facility limitations and/or the radiation inverse square
law (I/R2 ) prevent one from placing the detector at extremely long distances from the
source the minimum object to X-ray source distance determines the upper limit of
magnification. For example, a minimum source to object distance of 0.5" (12.7 mm) and
the X-ray imaging system placed at 6' (1.8 m) limits one to maximum geometrical
enlargement of 142.
Focal Spot Size of Microfocus X-ray Tubes
The advertised minimum focal spot sizes of microfocus X-ray tubes vary from 1 m to 50 m
and questions arise on how to measure them. The commonly used pinhole imaging techniques
employed with larger sized sources do not work because the pinhole has to be at least 1/10
the size of the focal spot one wants to measure. The smallest commercially available
pinhole camera is 0.030 mm in diameter and can be used to measure focal spots as small a
0.3 mm accurately. Alternate techniques such as converging lead star patterns or parallel
resolution test girds often used in the medical profession fail to work with focal spots
0.05 mm or smaller.
A method that has been developed to subjectively measure the focal spots of microfocus
X-ray tubes is to radiograph an X-ray lithography mask on film at a high magnification
(20-200 X) and determine the smallest line pairs still visible. The mask consists of
various sized etched gold parallel line pairs down to 2 m (0.002 mm). Experience has shown
that this simple method will measure the focal spot with a 30% accuracy. If a more
accurate measurement is desired these patterns can be scanned with a scanning
microdensitometer and the data analyzed by means of Fourier Transforms.
In practice, the real proof of fine focus X-ray tubes is in the quality of the image
(contrast and definition. A quick was of checking the performance of a focal spot is to
radiograph a fine wire mesh (i.e. 500 mesh) at high magnification or a covering star
pattern placed on top of an absorber. This will show up any asymmetries in the focal spot.
By nature of the tube head design many of the microfocal spots provide a better spatial
resolution in one direction than in the other. These "resolution" test patterns
are also very helpful in adjusting the focus to minimum size prior to radiography.
Since the purpose of most radiographic inspections is to examine an object for flaws, a
basic understanding of the underlying fundamental principles that affect the visibility of
details in the radiographic image is important. The quality of a radiographic image can be
described in terms of three factors. These are contrast, definition (unsharpness) and
image graininess. All three of these important factors affect defect detectability.
Radiographic contrast is the density difference between areas of a radiograph. Obviously
an image becomes more discernible when contrast is increased. Contrast is dependent on
X-ray energy, radiation scatter conditions, and on the type of film used, processing and
Definition refers to sharpness or unsharpness of the image. In general one can assume that
a sharp image is of higher quality than a less sharp image. However, at the limit of
detection the quality of the image depends on both contrast and unsharpness. Definition is
dependent on the geometric condition of the radiographic set-up, the focal spot size,
radiographic energy, the film and its development or radiation imaging system used.
Another qualitative term used by radiographers to describe radiographic quality is
"sensitivity" . It is a general term used to describe the ability of a
radiograph to show details in the image. It is a reference to the amount of information or
detail in the image. For example, if very small flaws can be seen in the radiograph, it is
said to have high or good sensitivity. Radiographic sensitivity depends on image contrast,
definition and graininess.
Radiographic Factors Affecting Image Quality
A successful radiograph depends on the control of many factors. These factors can be
usefully classified into several general areas geometry, X-ray machine, image detector and
specimen, and exposure factors.
The important geometry factors affecting image quality are:
1. X-ray source or
focal spot size - F
2. Source to object
distance - D1
3. Object to image
plane distance - D2
The most important of these factors is the focal spot size, because it not only influences
the geometry of the inspection but also the resolution and image definition limits. All
three of these variables are tied together in the equation for geometrical unsharpness
(Ug). Geometrical unsharpness is defined as the focal spot size as seen from the image
plane multiplied by the ratio of object to detector distance (D2) divided by the source to
object distance (D1).
Ug = F x D2 / D1
Since the X-ray source always has a finite size, geometric unsharpness or image blur will
always occur. And the only way to reduce the image blur for a fixed radiographic set-up is
to use a machine with a smaller focal spot. This is well illustrated in figure 1 where the
Image effect of different size focal spots and source to film distances is shown. When the
radiation emanates from a focal point a shadow occurs. This shadow or image blur is called
the geometric unsharpness. The magnitude of the geometrical unsharpness is directly
proportional to the X-ray source size and can vary widely depending on the industrial
X-ray machine used.
Now, when the radiographic specimen, for a fixed source to detector plane distance, is
moved closer to the source its image becomes magnified (Figure 2). The size of the
radiographic image varies with its position relative to the source and the detector. This
is known as geometrical image magnification (M). The magnification due to the geometry of
the radiographic set-up (figure 3) is expressed by
M = O2 / O1 = (D1 + D2) / D1
Where O1 is the object diameter and O2 is the image diameter and D1, D2 are the same as
defined above. The magnification (M) is simply the source to image distance divided by the
source to object distance.
Combining equations (1) and (2) one can arrive at another simple formula that relates the
image blur (Ug) to the magnification (M) and the focal spot size (F). That is
Ug = F (M-1)
This relationship is more clearly illustrated in figure 4 where the image blur is plotted
as a function of radiographic magnification for various sized focal spots. Conventional
industrial X-ray machines have focal spots 1- 4 mm in size, so called minifocus X-ray tube
focal spots are 0.2- 0.5 mm in size, while industrial microfocus X-ray tube focal range in
size from 0.050 mm (50m m) to as small as 0.001 mm (1m m).
Positive advantages of radiographing with very small X-ray sources are less image blur and
that one can use higher image magnification than what is attainable with conventional
The importance of radiographing with microfocus X-ray sources becomes obvious when one
considers the effect geometric unsharpness has on the radiographic image of small but
often critical defects. The effects are:
To increase the width of the image due to blurring
When the geometrical unsharpness is greater in size than the flaw to be detected there is
a loss of image contrast.
When the defect is smaller in size than the focal spot there is also a reduction in image
All three of these effects can easily result in a flaw not being imaged at all! Consider
the case of a defect which, under ideal conditions, would just be image with a minimum
detectable contrast, any factor producing radiographic unsharpness can also cause the
image of such a defect to disappear.
Although the contrast and resolution effects on defect detection have been discussed for
geometric unsharpness, it is obvious that they are also produced by any other source of
radiographic unsharpness. These are the film unsharpness, fluorescent screen unsharpness,
and motion unsharpness. According to Klasens the total radiographic unsharpness can be
expressed as the cube root of the sum of the cubed values of unsharpness due to each
source of unsharpness:
Utotal = [(Ufilm)3 + (Uscreen)3 + (Umotion)3 + (Ug)3] 1/3
Representative values of X-ray film unsharpness are 0.02 mm to 0.65 mm for X-ray at 10 keV
and 10 MeV respectively. The fluorescent screen unsharpness depends on both energy and
screen construction and can vary from 0.2 to 1.6 mm. The motion unsharpness is the
distance the object moved during the exposure interval. And the geometric unsharpness was
already discussed above (equation 1). Typically one of the unsharpness sources
predominates in any given factors very small unless the other factors can be reduced to
Projection radiography (image magnification) occurs when the specimen is moved away from
the image plane (figure 3). Conventional industrial techniques generally make little or no
use of direct image magnification. This is because the image unsharpness and potential
contrast loss caused by the large focal spot of the X-ray source and graininess of the
film or screen mottle limit the amount of additional information that can be obtained.
However, the application of fine focus X-ray sources to difficult or critical industrial
inspection problems has shown that considerable more details can be image projection
By significantly reducing the size of the focal spot in an X-ray tube a number of
advantages can be identified. These are:
The sample does not have to be close to the film. By positioning the radiographic object
close to the source and away from the film, an enlarged primary image is obtained on the
film or X-ray imaging system. This facilitates the detection of fine structural detail. It
also increases the size of the original detail that is lost in the background image noise
produced by screen mottle or film granularity and grain clumping. The enlargement of fine
detail makes it easier to detect an interpret by the radiographer and also reduces
The use of geometrical enlargement radiography also results in an increased signal (image
carrying radiation) to noise ratio reaching the detector. This results in a very desirable
increase in image contrast. By moving the radiographic object away form the film, most of
the forward scattered radiation (noise) misses the detector due to the geometry of the
set-up and is also attenuated in the air (figure 4). The forward scatered radiation
reaching the image detector is called "build-up
Faster films of film / fluorescent screen combination can be used with this technique. The
use of faster film results in shorter exposure times without jeopardizing image quality.
Therefore, geometric magnification improves image contrast, resolution and reduces
operator fatigue. All of which translates directly into enhanced defect detection. The
ability of Microfucus X-ray machines to magnify image with an improved spatial resolution
and a reduction in radiation build-up makes them ideal sources for industrial computerized
Other Benefits of Microfocus Radiography
If primary magnification is not required, the source to film distance can be significantly
reduced without a loss of image definition. This means that the exposure time can be
reduced and also that radiographs can be obtained in cases where access to the are to be
inspected is limited. Small diameter (4 mm) microfocus rod anode tubes may be very helpful
in these cases.
A very small focal spot also provides a better "depth of focus" (see figure 5)
and hence gives a more uniform definition of object details through the thickness of the
radiographic object. In conventional radiography, indications near the source side of the
object are more blurred (reduced flaw sensitivity) than indications nearer to the image
plane. Because of this, thicker samples can be radiographed with better
"throughout" flaw detection sensitivity than it is possible with conventional
radiography with the same energy. This greater "depth of focus" also makes it
easier to fix thick or irregular shaped objects and moving or rotating objects for
radiography because they do not have to be placed right next to the film.
Other Radiography Factors
The primary film factors that affect radiography are speed and grain size. Microfocus
radiography permits the use of fast films or film-fluorescent screen combination that are
many times faster than conventional X-ray films. These fast films are not normally
effective with conventional contact radiography because of poor definition and high
quantum mottle. The anticipated increase in "quantum mottle" or noise is reduced
proportional to the second power of the magnification (~M2).
X-ray source factors that affect radiography are focal spot size, the X-ray energy
spectrum and the source intensity. The advantages of fine focal spots were discussed
above. Microfocus X-ray machines with finely focussed beam currents as high as 8 mA over a
wide energy range are commercially available. Unlike conventional X-ray machines, the
energy spectrum of feinfocus X-ray machines is easily modified by either changing the tube
head window (i.e 0.1 mm Be) and / or the anode material (i.e. Ti, Cu, Mo). This may be
advantageous for very low energy radiography, or for special techniques such as K-edge
subtraction radiography. Specimen factors such as size, shape, composition and density
dictate the radiographic energy and technique to be used. While the exposure time controls
the film density and hence the contrast of the radiographic image.
Mircrofocus Real-Time Radiography
Microfocus real-time radiography utilizes a microfocus radiation source, a multi-axis part
positioner holding the radiographic object and an X-ray image intensifier coupled to a
Microfocul real-time radiography are:
- Instantaneous image
- The ability to see motion
- Improved crack / gap detection
- Better flaw sensitivity when compared to conventional realtime radiography
- 100% inspection is possible
- Save processing and film costs
- The possibility of computerized automatic defect recognition and instantaneous accept /
With all these desirable advantages going for it is surprising that not everybody uses
real-time radiography. One of the main draw backs of real time radiography with
conventional large focus X-ray machines is the relatively low flaw detections sensitivity
that can be obtained. In general with film a 2% thickness sensitivity is normal and 1%
sensitivity can be obtained with optimized radiographic techniques over wide material
thickness range. In comparision with conventional real-time radiography, a 2% sensitivity
is achieved over a limited thickness and energy range. Flaw detection sensitivities of 4%
- 10% are more typical.
For example, using a typical modern 9" X-ray image intensifier with a 2.5 line
pairs/mm resolution capability and no geometric magnification, a 2% sensitivity can be
achieved in aluminium form 1.75" to 6" with radiographic energies ranging from
80 320 kV, a 3% sensitivity at 1" and a 5% sensitivity at 0.5". For steel things
look worse. A 2% sensitivity can be achieved approximately over a thickness range of
0.65" to 0.9" of steel a 3% sensitivity from 0.5" to 1.5". Image
processing techniques such as recursive filtering (frame averaging) can only improve these
numbers very slightly (< 05%) although it makes the "real time" image easier
With conventional radiography the flaw detection sensitivity gets dramatically worse when
the material thickness fall out of the ranges quoted. It should also be noted that these
sensitivity numbers and thickness ranges quoted will vary slightly for different
intensifiers and different radiographic techniques. With real time, radiography just like
in film radiography one should always use good radiographic practice like tight beam
collimation, fixturing, beam filters, and even anti-scatter girds.
The poor sensitivity of conventional real time radiography is primarily caused by two
reasons. The first is the poor inherent spatial resolution of X-ray imaging system (0.5 to
4 line pairs / mm), while the other reason is caused by the physics of radiography.
It is not possible to electronically image the 0.010" hole in a 2% ASTM penetrameter
(IQI) in 0.5" of aluminium when it is radiographed with conventional radiographic
techniques (magnification < 1.2). With a standard TV line rate 525 on a 12"
monitor the width between the horizontal lines is already 0.020" which implies that
the penetrameter hole will probably not be imaged. Doubling the TV-line rate does not help
much. To be barely detectable under ideal conditions the hole must intercept at least one
TV-line. However, it is unlikely that the NDT inspector in a production environment will
With a feinfocus X-ray system one can considerably improve the flaw detection sensitivity
of a real-time radiographic image by employing the projection magnification techniques
discussed previously. Image magnification results in an improved spatial resolutiion and
an increase in image contrast as a result of less object scattered radiation being
detected. By spreading the hole image across 5 or more TV-lines, the image is not fuzzy
but well defined. A penetrameter sensitivity of 1% can be achieved at a 15 or more
diameter enlargement under well controlled conditions.
It is good radiographic practice to magnify the image in microfocus real-time radiography.
One an keep on increasing the image size until the geometric sharpness (equation 1) is
almost equal to the inherent unsharpness of the real time imaging system. Then the total
effective unsharpness is calculated with equation (4). This value is then substituted for
the geometic unsharpness in equation (3) and solved for the near optimum magnification.
M = ( Utotal /F) + 1
This equation yields the geometric magnification where the geometric unsharpness is equal
to the unsharpness of the imaging system. This method of setting Ug equal to U Screen is
somewhat of an over simplification. A more accurate formula for the optimum magnification
where the total effective unsharpness is a minimum is derived with differential calculus:
Mopt. = 1 + (Uscreen /F ) 3/2
It is evident from equation (6) that the focal spot size (F) is an important parameter in
defining the optimum magnification as well as the unsharpness.
To realize the full potential of microfocal real-time radiography one should employ a
5-aixs manipulator. Movement of the object through the X-ray beam enhances the
detectability of defects. A manipulator can also be used to optimize part position for
real time inspection (i.e to align a crack with the radiation beam). In some applications
merely rotating the object will give the desired effect. The image of such a moving object
on a TV-monitor will give a 3-dimensional impression which can be very helpful in
interpretation. Further on, the manipulator can be used to dynamically zoom in and out on
small indications to assist the interpreter in identifying the radiographic indication. In
all cases the image remains sharp and well defined whether it is close to the image plane
or far away because of the great depth of focus of feinfocus X-ray equipment.
Applications of Microfocus Radiography
The applications for microfocus radiography are fairly self evident form the above
discussions. Microfocus radiography can be used wherever conventional radiography is
employed provided that the application falls within the kV range of the microfoucs tube.
The application of microfocus radiography is also obvious for areas where conventional
radiography cannot be applied because of access problems. The include geometrics such as
jet engines, weapon components, complex assemblies, and castings and forgings. Inside-out
radiography (e.g, single wall radiography) often results in more than doubling the flaw
detection sensitivity attainable with previously used conventional radiographic
Feinfocus radiography should also be used to radiographically inspect components where
even a small chance of failure is unacceptable. Most of these applications fall within the
realm of the aircraft, aerospace, electronic industries.
Some applications where microfocus radiography has been successfully used include the
detection of microvoids in ceramics, critical castings (e.g, turbine blades), stress
corrosion cracking and fatigue cracks in aircraft structures, and the detection of voids,
cracks and porosity in all types of other metal components. Another area where microfocus
radiography is being applied is or the inspection of composites, plastics, and light
metals. Of course the electronic industry with its critical inspection requirements of
many small parts and assemblies is also a growing user of microfocus radiography.
Another unique application of microfocal projection radiography is the accurate
dimensional gauging of tubes, gaps, drilled holes, and other components that are
inaccessible, fragile, or too small (e.g. laser fusion targets) for mechanical gauges. The
technique is also being successfully applied to the inspection of radioactive components.
There is also a wide variety of applications, besides the ones mentioned above, where
microfocus real-time radiography is being used. Some of these are the real time
observation of dynamic component functioning, powder mixing studies, liquid flow in tubes
or passages, dynamic material studies, and precision part alignment prior to film
radiography. The number of applications of Microfocus Radiography is still growing.
Microfocus radiography offers a new dimension in industrial radiography. It extends the
flaw detectability of film radiographic techniques and brings real time radiographic
inspection into both the NDT laboratory and the factory for continuous on-line inspection.
It is a powerful new NDT tool that can be successfully used to meet many of today's
ciritical inspection requirements. Its application to radiographic inspection problems
will result in an enhanced flaw detection sensitivity and reliability for the user and his