After 1999, they were made available by Vendors for Bottling Fill Level Control applications, the first fill level inspection devices based on commercially available systems of IR LED and IR detector.   Devices operating around the wavelength of 1μm, strictly optimized for price, rather than performances, devoted to liquid based on water into non-labelled nor foiled, transparent containers, adopting detectors sensible in the infra-red portion of the electromagnetic spectra.

Typical lifetimes quoted by the Vendors of these electronic components range:

                                                     (25,000 - 100,000) hours

and it is a fact that are still functioning LEDs built 40 years ago.  

However, heat and current settings can shorten this time significantly.


(Right) Simplified diagram showing the extents of all the emission cones out coming by a LED wafer.  The light emission cones of a real LED wafer.  Light emission zone is commonly a bidimensional plane between the wafers.  Every atom across this plane has its own set of emission cones



Vendors guarantee: 

  • differences on filling height in the range: (2 – 3) mm rejected with probability 99.9 %;
  • false reject <0.1 %. 

Comparing these digits with those given for the X-ray bridge, it becomes evident the difference on inspection performances.

 Relative response % of an infrared LED and of the phototransistor adopted as detector, ~1 μm.

Also, and relevant, the fact that performances of the Infra Red fill level inspection are extremely close to those of the HF fill level inspection, with the true unique ‘ minus' being the fact that containers have to be transparent to IR radiation when, on the opposite, also non transparent (or, ceramic) containers can be isneocted by mean of High Frequencies.   This inspection encounters today still minimum applications, a fact in conflict with its economy and the however wide spread range of possible and fruitful applications in the majority of the Bottling Lines.

energy-factor-of-different med-2

  Shown above the energy factor of different widespread fill level inspection technologies, referred to the High Frequency (HF).  Its reciprocal ( 1/e ) is an indicator of the independance of the results of any technology by the ambient fluctuations, or the container or beverage conditions. The correspondance between energy and wave length derives by Special Relativity and Uncertainty Principle. Larger energies correspond to smaller distances: a more energetic wave is sensitive to interactions occurring over shorter distance scales.  Physical Laws introduce in the Food and Beverage Packaging process constraints stronger than whatever commercially-oriented suggestions, smiles and accompanying handshakes

Infra Red Fill Level Inspection relative efficiency

The liquid in the bottles is mainly a vast collection of molecules of water, arranged in groups kept joint by the Hydrogen bond.  At right side, an example with five participating molecules.  A single molecule of water has a mass of 2.992 x 10-26 kg.    At a first sight 2.992 x 10-26 kg looks insignificant: an effect of the erroneous perspective of our viewpoint.   Readjusting to the proper perspective in terms of energy, size and duration of what we are trying to measure, the energy equivalent of a single molecule of water is huge.    The energy equivalent of a single molecule of water may be represented in a way homogeneous to the electromagnetic spectrum, frequencies and energies seen before, by its de Broglie wavelength 0.035933842 nm.   

As visible by the spectra below, the wavelength ~1 μm of the Infra Red photons used as interactant, is far from the de Broglie wavelength <0.01 nm of the molecules of water (further details here) in the neck of the bottles, with whom we want to to interact.   

Hydrogen bonds between five molecuels of water (  /Qwerter/CC-BY-SA 3.0)

In this regard, comparing the Infra Red photons' size and energy to all the other fill level inspection technologies, we see that the Gamma-rays, X-Rays and the visible photons of the Optic inspection with camera, they all results superior to the Infra Red solution.

But, the wavelength of the IR interactant results however closer to water than other widespread solutions.  Seven orders of magnitudes closer than the High Frequency at 21 MHz (wavelength ~10 m) outcoming by a RF Generator, part of an High Frequency Fill Level Inspection. What above is just an intuitive explanation. The deeper explanation starts by the standpoint that a measurement, whatever measurement, happens always and only after a single correlated state of two physical systems is established.   

In our present case, to definetely perceive a correlated Liquid-IR Inspection State, they are necessary:

  1. Timeto transform the previous state, in which all possible kinds of correlation of the IR Inspection solid state detector coexist, in a following state in which the Inspection is “aware” to be correlated to a Liquid, because having recorded eigenvalues for the eigenfunction ΦiS1 describing a certain level of a Liquid. The correlation between the two systems, Inspection and Liquid, is progressively established during interaction and proportional to the natural logarithm (ln t) of the interaction time t.  An ideal correlation, one corresponding to a maximised information of the Inspection about the level of the Liquid, can only be reached allowing an infinite time.  The fact we cannot wait for an infinite time causes the measurements’ fluctuations, a synonimous of the spectrum of the eigenvalues, resulting in the Electronic Inspector's false positives (false rejects).  Time, for what?  To transform the previous state, in which all possible kinds of correlation (superpositions) of the Inspection coexist, in a following state in which the solid state IR detector of the Inspection is aware to be correlated to a Liquid, because having recorded eigenvalues for the eigenfunction ΦiS1 describing the macromolecular Hydrogen-bond we name “Liquid”.   

  1. Interaction between the systems such that the Information in the marginal distribution of the object inspected is never decreased.    In a probability distribution deriving by two random variables, we remember that marginal distribution is where we are only interested in one of them. Otherwise, we’d have forced a reduction in the sample space of one of the random variables and then, we could not have any more repeatability of the measurements.  As an example, this should be the case if we’d erroneously try to use a beam of high energy neutrons rather than IR photons, to interact with the beverage. The neutrons should modify the molecular structure of the liquid, modifying its eigenstates and then the eigenvalues we expected to derive by the measurement.


  • extremely extended lifetime
  • extremely cheap price;
  • inspection performances close to those of the much more expensive (and, heavily ambient conditions-dependant) High Frequency Fill Level Inspection;
  • mechanical robustness;


  • poor inspection performances;
  • the container has to be non-labelled;
  • the container has to be non-foilled;
  • the container has to be transparent to IR radiation.

 Infrared photons have today a widespread  use.   In the figure, an home IR heater

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