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X-ray fill level inspection           with one bridge

X-rays is the most performant of all of the existing Technologies for the Fill Level Inspection.   

The most effective allied of the Bottling Manager looking for maximum Quality and minimum losses (false rejects).

x-ray-fill-level-inspection 2048x1786@1x

  X-ray fill level inspection in a Brewery using it to detect the filling level of cans out of a Cans' Filler Machine.  This is not in contradiction with what explained above about foaming beverage. Cans' foaming out of the Can Filler Machine is never as much as in Bottles' Filler Machines, because there are not foamer hot-water spray jets. More, Quality is assured by a second fill level inspection after the Pasteurizer, at one-half of this speed. In the image, its is possible to see how the X-ray Generator radiates in direction opposite to the Operator's side.  The X-ray detector infeed window can be recognized by the white-grey coloured plastic circle, right in the centre of the figure 

X ray fill level inspector.  X-ray fill level inspection systems exists in versions with a single cabinet including all parts, like the unit visible above, and in versions where the different parts have to be separately fixed to the ground. These last requires many more installation and fitting activities but, offer the best metrological performances

  X-ray fill level inspection systems exists in versions with a single cabinet including all parts, like the unit visible above, and in versions where the different parts have to be separately fixed to the ground. These last requires many more installation and fitting activities but, offer the best metrological performances

X-rays Emission

Atoms’ nuclei are made up of quarks, neutrinos and other forces mediators, arranged in groups with different physical properties, groups named:

  • protons and neutrons (the atom's Nucleus); 
  • electrons, charged particles of the same family of the quarks.         

To help to form at least an intuitive image of something no one never saw, electrons can be imagined be moving around the nucleus. This image is however a false one, originating by the outdated (1911) ideas of Rutherford.  A scenario different than what new experiments and theories later shown.  Electrons form a cloud-like shape around the Filament lying into a metallic cylindric tube named Cathode.  In the X-ray Generators for analytical applications like the Electronic Inspection, Cathode itself is frequently grounded. The Cathode’s function is the equalization of the electrons’ spatial distribution.  

Distribution completely uneven around the Filament, as hinted by one of the videos in the following.  When there is a vast difference of potential in a vacuum glass tube, negatively-charged Electrons around the Cathode are attracted toward the positively-charged Anode.  Reaching the Anode, usually made of tungsten (Wolfram), they decelerate after colliding with the Anode’ atoms.  The atom object of the collision excites itself to a superior energetic level.  After a brief interval, it'll be emitted a photon whose frequency is in the band of the X-rays.   

The dynamic of the electron emission by the Cathode and the successive X-ray emission by the Anode visually hinted by the sketch in the image below on right side (photo credit: ORAU, Oak Ridge Associated Universities/

  An atom of the anode, releasing a X-ray after having absorbed an electron in one of its inner orbitals

  Dynamic of the electron emission by the Cathode and the successive X-ray emission by the Anode (abridged by image credit: ORAU, Oak Ridge Associated Universities,


X-ray emission, in detail

Electron emission by metals

Since the start, we promise you that after having read this section you'll look with new eyes to objects as common as the traditional incandescence bulb lamps.  Any metal is continuously emitting electrons.   Their number and the speed with which they are emitted increases very strongly with temperature but, as a matter of fact, emission takes place at anything above the absolute zero kelvin degrees (-273.15 °C).   

To fully understand the electron emission, we have to look at what happens inside the the metallic material.  In the metals, there are one or two electrons that can easily be detached from an atom, so that inside the solid metal at ambient temperature there is always a great amount of electrons floating around, independently of any particular atom.   

The latter are fixed in place inside a crystal structure and do not move about at all, rather they only vibrate in place.   This scenario suggesting a “sea of electrons” is common to all metals, their defining characteristic explaining many of their physical properties, and between these: 

  • electrical conductivity;
  • shiny aspect.

Since the electrons are not attached to any particular atom, they move constantly, like the molecules in a gas.  The average speed of the electrons obeys a statistical distribution law and increases with temperature.   If an electron happens to be going toward the surface of the metal, then it will naturally tend to fly right out through the surface.  

However, other forces tries to stop it, because of several other positively charged metal atoms inside and none outside.   This, because they have lost one or two electrons to the 'electron sea'.  Thus an electron approaching the surface is slowed down, and only those having enough energy shall escape. The amount of energy required is called the “work function”, and varies for different metals.

Electron energy

The energy of an electron corresponds directly to its speed.  This follows the same known physical law for Kinetic Energy:

                                                       E  =  ½  m v2    


  • E      energy, in Joule
  • m     mass of the electron, ~ 10-30 kg
  • v      velocity, in m/s

The energy in Joule is replaced by a smaller value named electron Volts and abbreviated eV.  1 eV being the energy an electron acquires after having been accelerated through a potential field of 1 volt, an energy whose equivalent to joule ~10-19

Dushmann equation

The work function W of a metal is expressed in eV.   It represents the difference between the potential energy of an electron in vacuum and the Fermi level, where this last is the thermodynamic work required to add one electron to the body.   As an example, for tungsten it is 4.5 eV.   Any electron having less energy than this will not be capable to escape.  The electric field close to the surface of the material shall attract it and the electron will return into the body of the metal.   In the graphic below, the work function W of some (pure) metals, metals of interest in the specific case of the Bottling Controls, compared to with their atomic number.

               Work function W of some metals, versus their atomic number Z

The electrons leaving the metal correspond to an electric current, and this current is given by the equation of Dushmann:

                                   I0  =  A T2 e-11600 W/T    


  • I        emitted current
  • A       constant, 120.4 A cm-2
  • T       temperature, K degrees
  • W     work function of the emitting metal
  • e       base of Napier logarithm

A relevant aspect of the Dushmann equation is the coefficient of the exponential: it means that emission increases very rapidly with the temperature T.  The graphic on right side above, shows the emission of a tungsten filament as a function of temperature.   Even a small percentage change in temperature results in a big change in the emitted current.  For an oxide-coated Cathode under typical operating conditions, a 10% increase in temperature increases the emission by about 3 times.  Escaping electrons follow a similar distribution of energy as they did inside the body of the metal.  Some of them leave the surface at low other at high velocity.  A behaviour showing its relevance when examining the function of the tube.   

The distribution p of energy and, as a consequence, of the speed, follows the relation:

                      p  =  e-Vqe/kT


  • p      proportion of all the electrons whose energy is > V
  • qe    charge of the electron, 1.6 x 10-19 C
  • k      Boltzmann’s constant, 1.3806488 x 10-23 m2 kg s-2 K-1
  • T      temperature, in kelvin degrees
  • e      base of Napier logarithms, 2.718…

The red-coloured line in the graphic above on right side, shows this distribution.  The great majority of electrons have low energy levels, and the average for an oxide Cathode is only about 0.1 eV.  No upper limit exists on the energy a single electron may have.  

Different kinds of Filaments

The X-ray tubes adopted into Bottling Line Controls use solid tungsten Filaments.  'Heater' is a synonimous of Filament.  Tungsten has a high work function, and there are other metals which are much more suitable in this respect, like Caesium, whose work function is 1.6 eV.   A tungsten Filament has to be operated at about 2700 °C, like in the common light bulbs visible in the video down.  The amount of heat thrown out by any hot object increases with the fourth power of its temperature, meaning that a huge power is required to replace this lost heat, so to leave the Filament at that particular temperature.   Adding a small amount of Thorium (~ 1%) to Tungsten,  reduces its work function to ~ 2.6 eV, allowing Filament operation at ~ 1900 °C, resulting in a reduced power by around four times.

Adding a surface coating of Barium oxide (or, a mixture of Strontium and Barium oxides) even better results can be obtained. In these cases, the oxide is no longer a metal, and the energetic electrons within the body of the Filament can escape through the oxide layer at much lower velocity.  As a matter of fact, the oxide layer is a n-type semiconductor (one having an excess of electrons).   Oxide Filaments have a work function of ~ 1.1 eV, and can be operated at around 700 °C.  It is this oxide coating which makes Filaments and Cathodes appear white.    Electrons within the Cathode travels in all possible directions and so are those emitted.  The average lateral velocity of the electrons is around the same as their average forward velocity showing how far from truth results the quite common idea of all of the electrons travelling along the shortest path, from the Cathode towards the Anode (the electrode also frequently named Plate).

Below:  a General Electric®-built X-ray tube.  In evidence, on left side the Cathode.  At the centre, the copper protruding Anode.   The grey colour area of the glass is the one interested to the passage of X-rays (photo credit: ORAU, Oak Ridge Associated Universities)

  X-rays' frequency spectrum. Soft X-rays the band adopted in Bottling Lines for analytical applic. in the level control            

The Filament is inside the Cathode and, since the Cathode itself is small, there is little room for it and even less room for electrical insulation. A thin layer of aluminium oxide is normally used, but at the high temperatures involved this is not a perfect insulator, and if the voltage difference is too high then it can break down altogether.    If plate voltage is applied to a cold tube, then an electric field is set up between the Anode and the Cathode.   The voltage in this field increases linearly in the distance between the two electrodes.  A relation shown by the red straight line in the following graphic below.

Once the cathode is heated up and starts its emission, the electrons intervene changing the situation, because carriers of negative charge.  Although this is small (1.602 · 10-19 Coulomb) there are many electrons and the emitted electrons themselves contribute to the electric field in the space between the electrodes, preventing the behaviour described by the red colour straight line.   The green line in the graphic above shows the effect, ignoring initial electron velocity (Child-Langmuir law), while the blue line shows the reality when the initial electron velocity is considered.    The term 'space charge' here used to refer to the charge due to the electrons occupying the space in between the electrodes.  

Child-Langmuir law of emission

The number of electrons is related to the current flow.  Higher the current,  greater the number of electrons and therefore the greater the charge. Since the Cathode feels the influence of the plate through the negatively-charged electrons between them, the increasing current reduces the attractive force of the plate until the two reach a balance. At this point of balance, the effective field at the surface of the cathode is reduced to zero.   Moving away from the cathode, the electrons accelerate.  The accelerating electrons spread out, just like cars on a freeway moving away from a traffic jam, reducing the space charge so that the field increases.   This is illustrated by the green line in the graphics above, showing what happens inside a tube when current is flowing under normal operating conditions.  Child-Langmuir law, as it applies to a diode, states that: 

                           I  =  2.335 * 10-6  A  d-2 V3/2                                    


  • V       anodic voltage
  • I        electric current
  • A      cathode area
  • d      distance cathode-anode, in cm

This equation and the green line function in the preceding figure are based on a simplification, that all of the electrons are emitted with zero residual energy when yet we saw they are in the reality emitted following a certain distribution of energy.

Child-Langmuir law applies rigorously only to infinite and flat electrodes: in the reality the electrodes are finite and the difference implied results negligible.  One implication of this law is that the current flowing depends only on the strength of the electric field due to the Anode in the immediate vicinity of the Cathode. The field have to be just strong enough to counter exactly the space charge set up by the resulting current.  The electric field elsewhere serves to accelerate the electron flow and plays no role in determining its amplitude.

Collisions and X-ray emission

Electrons are arranged on clearly defined shells, spins, orbitals and obeys Pauli's Exclusion Principle.  Upon colliding, the electron 'knocks out' an electron from the inner shell, which jumps to a higher energy level.   After a brief time interval of approximately 100 microseconds, the electron which jumped to a higher level decays to a lower one.   The energy the atom acquired before the collision of the first electron is then re-established by the emission of a photon.   The frequency of radiation is part of Planck's equation:

                                                 e  =  h v 


  • e      is the energy of the individual photon;
  • h      Planck's constant; 
  • v      (Greek-aphabet letter: "nu") is the frequency of the radiation.

In some way, it is not incorrect to say and differentiate

  • X-rays, produced 'mechanically';
  • Gamma-rays, result of nuclear decay or disintegration. 


Toshiba I-222 X-ray tube specifications 

In the following, as a practical example, we'll examine fundamentals specifications of an X-ray tube by Toshiba, today's their greatest Vendor.



High Voltage Generator                                       Constant potential HV generator

Grounding                                                            Cathode + X-Ray port grounded

Operating Tube Voltage                                       (20 - 75) kV 

Focal Spot                                                            1.0 mm     (*)

 (*) Permissible Value: refer to IEC60336/1982



Angle                                                                      20 degrees 

Material                                                                  Tungsten

Inherent Filtration                                                   Be 1.0 mm 

X-ray Coverage (Angle):

Tube Axis Direction                                               30 degrees

Nominal to Tube Axis Direction                             45 degrees 

Insulating Medium                                                  Insulation Oil                                                               

Oil Cooling Method                                                Oil immersed (60°C Max.) convection oil cooling

Absolute Maximum and Minimum Ratings

Maximum Tube Voltage                                         75 kV        

Minimum Tube Voltage                                          20 kV 

Maximum Tube Current                                         8 mA 

Maximum Filament Current                                   3.9 A 

Filament Frequency Limit                                      DC or AC (Sine)

                                                                               0 ~ 20 kHz 

Maximum Input Energy (continuously)                  350 W 


  The power cable BS Norms-compliant powering the FCI on left side.  It provides an impressive mechanical protection toward cut, reducing the electric hazard 


  The transparent glassy aspect of a legacy photomultiplier. Photons infeed by the upper side

Ionising radiations

An ion is a charged atom (or, molecule). The atom is neutral when the number of electrons equals and balance the charge due to the number of protons in the atom (or, molecule).  An atom can acquire a positive charge or a negative charge depending on whether the number of electrons in the atom is greater or less then the number of protons in the atom.   If the atom has more electrons than protons, it is a negative ion (or, 'anion').   

On the opposite, if it has more protons than electrons, it is a positive ion.  X-rays act on atoms so strictly to be ionising radiations.   They ionize by either of the three ways: 

  • photoelectric effect;
  • Compton effect; 
  • pairs production.                                                                                  

          The ionization process resulting in an atom losing an electron

In diagnostic and analytical applications corresponding to the low energies where are applied the Electronic Inspectors as fill level control, ranging (30 - 100) keV, the photoelectric effects mentioned above predominates.   Photoelectric effects are proportional to the cube of the atomic number Z3  that is exposed andf this explains why:

  • bones, containing Calcium, contrast so well with soft tissues in the images we all saw;
  • water, contrast so well with respect to the gas in the head space of bottles and cans.

At higher energies, as is employed in radiotherapy, Compton effect.   Here, the incident electron transfers some of its kinetic energy to the target electrons and the rest as a deflected, less energetic photon.   At still higher energy levels (> 1.02 MeV), the energetic electrons will form 'matter-antimatter' pairs in the exposed material.   A positron and an electron will form, which will annihilate later forming two photons which will fly ~ 180 degrees apart, in opposite directions.

Safety considerations referred to the 

Bottling Hall ambient

Minimum Safety Requirements

Today, have to be considered Minimum Safety Requirements for the X-ray Electronic Inspectors, whatever their scope of use, exclusively configurations similar to the one pictured down, including:

  • the long time established (a legal requirement) orange colour external light, signalling the generation of ionising radiations, an orange colour we all remember by the medical radiographies;
  • the orange colour signal above powered by a special electronic safety circuit, one which cannot at all be 'fooled' by any human-safety-bypass: in the case the luminous signal is not functioning (a common case of fault) power is not applied at all to the X-ray generator;
  • two limit switches enabling X-ray generation (applying the 60 kV to the Anode described above) but not the X-ray emission in the ambient, mechanically fixed in the front and in the rear of the Inspector.    The X-ray tube emission can only be started when manual switches signal a safety status, because connected in series.   This way, both of them must be in the switched on condition, in order to release the X-ray tube.
  • a shutter, automatically open shortly before the arrival of the first container (or crate, case, keg) in a row, and closing itself shortly after the last container (or crate, case, keg) of the row.    This way, there is never a permanent source of ionising radiations in the Bottling Hall, rather and only if and when there is passage of containers (or cases, clusters or kegs);
  • the automatic closing of the shutter, when conveyor stops;
  • the shutter described above, operated by two switches enabling X-ray  emission, mechanically fixed in the front and in the rear of the Electronic Inspector;
  • plate, carrying the international symbol of ionising radiations, applied on the front and rear of the Electronic Inspector.

                High voltage, shutter and emergency controls in an X-ray fill level inspector


Be wary of devices by Vendors who do not include all and each one of these minimum safeties.  In case of doubts about what is included and what is not, copy the list of features above in your request of quotation presented to the Vendor.  Also, this shall help you to define the real price for a new Inspector and compare different quotations.

Fill level inspectors

The most common application of the X-rays in the Bottling Line, the fill level inspection, implies the passage of the container thru an  bridge-like structured inspection device.  The bridge is dimensioned to let containers' necks pass thru: there is space to introduce a hand but no space at all to do something which could truly constitute a risk-factor for Production Operators.     To clear in a straight way the point: if human eyes' retina, a delicate neuronal area, could be exposed along time to the X-ray beam, this should surely and permanently harm the sight of the person.    But, it is impossible at all to introduce the head in that limited space.   The banal exposure of an hand to the passage of the X-rays, because of several reasons, like:

  • minimum power of the radiator;
  • choice of X-ray band of frequencies;
  • absence of external lobes of radiation external to the beam, a beam focused within a few millimetres out of the optic axe joining the generator and detector.     

does not harm anyone and no one, also if deliberately would like to do it, could keep along weeks, day after day, his hand in that space blocking the entire Bottling Line.

One of the cheapest, simplest, reliable and most successful X-rays Electronic Inspectors of the World, the model Compact Line-X by Stratec™ BBULL™ Company.   Image shot in a Mineral Water Bottling Plant in the region of Shiraz, Iran.                

Case, Clusters and Kegs Inspectors

Looking around in the Bottling Hal, they also exist other Electronic Inspectors like the Case, Clusters or the Kegs' X-ray fill level Controls, where the space between generator and detector, ranging (800 - 120) mm allows the introduction of the human head.    Case, clusters and kegs X-ray Inspectors, additionally require:

  • a closed tunnel, around the area of the beam.  A tunnel protruded out of the Electronic Inspector cabinet enough to prevent Operators' from introducing their hands in the beam, during the passage of cases and crates.   If the tunnel equipping the Inspector as a standard is not long enough, they have to be added metallic guards to extend its infeed and outfeed sides;
  • an inner red colour LASER light spot, illuminating the focusing area, so to signal its existence.

Peculiar Legislations

Some local Norms and legislations, like the BS enforced in the United Kingdom, prescribe additional measures of safety: 

  • around the X-ray inspection bridge has to be built a guard making impossible to Operators to reach the beam also with the fingers.    This, meaning an extension to fill level Controls of the logic followed for cases, clusters and kegs.  Such a box-like guard, in an area like and around an object like, a fill level Inspector, implies doors in the front and rear of the box; 
  • opening of these doors has to be mandatorily connected with switches to the electronic circuit enabling the X-ray generation.  An example visible in the figure down, where the red-coloured objects is the front door limit switch;
  • orange colour light, signalling the generation of ionising radiation is replaced by a red colour;
  • the power cord has to be steel wires armoured type, offering mechanical protection to the inner copper power conductors (see figure on right side, down);
  • delivery to the Customer, by the Service Technician who installs the system, of a Technical Relation about all techncial aspects and Norms respected during the installation, completed with drawings of the 'box' enclosing the system, electronic diagram carrying marked the introduction of the safety switches on box doors, etc.


Front view of a BS Norms-compliant Lexan cabinet completely embedding an Electronic Inspector controlling by mean of X-rays the filling level of cans.   It lies in the second largest AB-InBev™ Brewery in the United Kingdom, at Samlesbury, Lancashire, a Brewery producing 2.5 million hectolitres per year.  Part of these hectolitres canned in <1700000 cans-per-day passing thru the X-rays inspection bridge visible above.

 Details of the X-ray protective cabinet, as seen by by its rear side                  

X-ray inspection independence on

ambient conditions

Due to the high energy of the X-ray photons, the fill level inspection based on X-rays is not related to ambient condition: one more important advantage with respect to technologies like the High Frequency fill level inspection, where the difference between a humid and a dry ambient makes a different result.

X-ray fill level inspection and beverages

X-rays absorption is not dependant at all by the colour or transparency to visual light of the:

  • bottle;
  • beverage;

being this one more advantage with respect to alternative fill level control technologies, like the Infrared and the LASER.   Its only true real limit is that it can only be applied to still beverages.   This, means it can also be used with maximum profit in terms of:

  • Qualitative performances;
  • smallest possible false rejects 

on applications like fill level inspection, on:

  • common soft drinks, out of the Filler and of the Labeller Machines;
  • Beers, after the Pasteurizer.

It can always be applied to water, however carbonated, but it’s a fact it is not the best choice if applied after the Filler on foaming products. Further details about this subject here.

  X-ray fill level inspection is not the best choice on heavily foaming products, like Sprite® or Beers, inspected immediately after the Filler machine

X-ray fill level inspection performances

The  author  installed,  started  and  commissioned around  one  hundred  and  twenty  X-ray fill level inspections by different Vendors.  On the base of this direct experience, it is a fact based on a wide statistical sample of tests, that:

X-ray fill level inspection is the only one technology existing truly capable to allow to reject an under or overfilling > 1 mm (equivalent to ± 0.5 mm) with defects' detection ratio truly 99.9 % and associated false reject  < 0.01 %

More, they are not exaggeretely confident the performances estimations appearing on Vendors' Technical Guarantees: those digits are true.    But true, as much as the Guarantee about the Quality of the installation, startup and commissioning of these devices.   As an example: it is easy to understand that in a device measuring Heights, then assuming the conveyor surface as reference point, it is fundamental to care on extreme terms this installation factor and its stability along the years after the installation.   A contrain this, common to all Fill Level Inspectors, whatever their Brand and Technology.   Tests (and, production) performances conditioned to constancy of:

  • ambient temperature, preventing variations of liquid volume in the container;
  • containers' speed, preventing a tilted upper surface of the liquid in the container.

What above meaning that, e.g., false rejection ratio, for all existing technologies of fill level control, cannot be measured during: 

  • Filler or Labeller Machines, ramp-up or ramp-down phases;
  • time intervals so wide to have implied a change on the thermal conditions of the Filler Machine, itself in equilibrium with the ambient apart an unavoidable hysteresis implied by its huge metal mass.  Increases on liquid temperature, reduce artificially the false rejection ratio.   Decreases on liquid temperature, on the opposite increase it.

X-ray fill level inspection maintenance

Each approximately > 5 years, the X-ray generator could start to need replacement, a very simple maintenance operation whatever Maintenance Electric Technician is fully capable to carry out with success.     An activity implying to unscrew 4-6 screws and plug out and in again a common plug.    The true periodic maintenance is reduced to keep clean by accumulation of sugar deposits, the plastic windows front of radiator and detector where the X-rays have to pass thru.

X-ray detection

Photomultipliers detectors

Photomultipliers are vacuum tubes in which the optical photon is converted into an electron by the photoelectric effect.  The photo-electron is then amplified in a cascade of dynodes capable of generating a virtually noise free gain in excess of 1 x 106 with a bandwidth greater than 1 GHz.   The quantum efficiency of the photocathode is typically 10% to 20%, although some newer semiconductor photocathodes can exceed 40%.   The images down and on right side respectively illustrate the aspects of a modern version of these tubes and of a legacy model.

Photomultiplier and X-ray fill level inspection,

Electrons incoming by the multiplier are collected by the Anode in this modern metal-encased photomultiplier tube

Photomultiplier's spectral response can be adjusted in a wide range by selection of the photocathode.  Photomultiplication process, visible in the images on right side down, is sythesized by the following sequence of events chronologically ordered:

  1. the photocathode converts the flux of light in a flux of electrons;
  2. an electro-optical input system, focus and accelerates the electrons;
  3. after seconday emission in a serie of secondary-emission electrodes (commonly named dynodes) whose potentials are progressively increasing, electrons are multiplied;
  4. electrons incoming by the multiplier are collected by the Anode.

photomultiplier principle in X-ray fill level inspection,

Dynodes' secondary emission of electrons in a photomultiplier (animation credit Comsol, Inc.)

Photomultiplier principle,

Photon-electron conversion and power supply of a photomultiplier 

The output signal, referred to this last electrode, has a potential related to the energy of the incoming X-rays.   

Then, the now purely electric signal shall be:

  1. amplified;
  2. normalized in a desidered range;
  3. compared with programmed limits, so to evaluate the presence or absence of liquid at the centre of the X-ray beam;
  4. used to control the bottle rejection, if the measured property does not meet the requirements established during the Fill Level Inspection commissioning. 

Solid-state arrays of photodiodes

The traditional 16 kV operated photomultipliers, described in the prior section,  today are no more so frequently used as they were in past Bottling Control applications.    X-ray standard detectors are becoming solid-state detectors, where a linear array of photodiodes whose sensitivity is artificially extended to X-rays' band, converts the signal carried by photons in a signal carried by electrons.    Two example of these, down in the figure:

  Linear arrays of X-ray CMOS photodiodes are today's most common X-ray detectors

Linear detector arrays are based on CMOS silicon photodiode array detector chips.  These are mounted on a printed-circuit board like the couple visible above.    Todays' best models of X-ray detectors feature pixels extremely small, allowing height measurements extremely fine.   More, their imaging circuit is no more only an array of X-ray photodiodes.   Each detector may also include:

  • a contiguous linear array of photodiodes, 
  • an array of charge integrating amplifiers, 
  • sample-and-hold circuits, 
  • signal amplification chain.

In these applications, a scintillator material is customised over the specific application: attached to the surface of the detector array, so to convert X-ray photons into visible light for detection by the photodiode array. 

Resolutions' available range:                                                      (50  - 100) μm

The array lengths commercially available include models where the array is particularly wide, like:

  • 1.5 inches                                                                        768 pixels, at 50 μm
  • 2.0 inches                                                                      1024 pixels, at 50 μm

The availability of X-ray detectors like those here described, explains today's tendence to integrate in a single inspection bridge functions in the past requiring an expensive couple of them, namely:

  • under fill level inspection;
  • over fill level inspection. 

A single bridge, including a detector with many pixels and vertically arranged, allows today results only a few years ago reserved to a two-times more expensive couple of inspection bridges.

Signal processing

And finally, after this introduction to the process of emission and detection of the X-rays, we’ll apply them to a practical case.  The image on right side below, shows an example of X-ray fill level 'radiography' of the neck of a bottle of Coca-Cola® Regular, format 500 ml.   As visible, it is predefined an inspection window of 26 mm, corresponding to the distance between the couple of blue colour vertical lines.   Lines crossing the red colour radiographic profile corresponding to the opacity  to X radiation (absorption) of the medium interposed in the bridge at the height where the radiography is made.    'Medium' is the sum of components interested to the passage of the X-radiation:

  • container sidewalls, 
  • beverage,
  • gas, this last giving a negligible contribution,
  • air, interposed along the X-ray bridge and the container's sidewalls.

The correct rule to define the window amplitude is: 

40 % of the diameter of the column of liquid at the height where the inspection shall be made.  Then, the now purely electric signal  in the inspection window shall be:

  1. amplified;
  2. normalized in a desidered range;
  3. compared with programmed limits, so to evaluate the presence or absence of liquid at the centre of the X-ray beam;
  4. used to control the bottle rejection if the measured property does not me
  5. et the requirements established during the Fill Level Inspection commissioning. 

                                      'Radiography' of the fill level in the neck of a bottle of Coca-Cola® Regular format 500 ml

True Positive rejects. The wide range of kinds of beverages which can be fruitfully inspected by mean of X-rays in a Bottling Line having just a single Fill Level Inspection, includes foaming.  All those as foaming as CO2- and sugar-added soft-drinks.  Like these visible in an Asian 50000 bph PET Bottling Line

When desired a cheap solution with only one bridge and simultaneous inspection of under and over filling, linear array detectors are commonly arranged vertically.   In this case, the accuracy is negatively influenced by the fact the number of possible measurements on the container shall be reduced. 

The wide range of kinds of beverages which can be fruitfully inspected by mean of X-rays in a Bottling Line having just a single Fill Level Inspection, includes foaming.  All those as foaming as CO2- and sugar-added soft-drinks.  Like these visible in an Asian 50000 bph PET Bottling Line

Links to other fill level inspection technologies:


This website has no affiliation with, endorsement, sponsorship, or support of Heuft Systemtechnik GmbH, MingJia Packaging Inspection Tech Co., Pressco Technology Inc., miho Inspektionsysteme GmbH, Krones AG, KHS GmbH, Bbull Technology, Industrial Dynamics Co., FT System srl, Cognex Co., ICS Inex Inspection Systems, Mettler-Toledo Inc., Logics & Controls srl, Symplex Vision Systems GmbH, Teledyne Dalsa Inc., Microscan Systems Inc., Andor Technology plc, Newton Research Labs Inc., Basler AG, Datalogic SpA, Sidel AG, Matrox Electronics Systems Ltd. 

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