The subject of pinholes can be contentious as it can depend on where you look and under what conditions you look as to how many and how big they appear. Pinholes can be produced by a number of different methods; coating debris that was present on the film surface that is later moved away leaving behind an uncoated area, pickoff of the coating where the coatings is not well adhered and is removed sometime later by hard contact with the back surface of the film whilst rolled up, prevention of coating adhesion by oligomer evaporating off the film surface or by spits from the evaporating source that prevent normal aluminium deposition. The oligomer vaporising or the spits may allow partial deposition and so may not be a pinhole with zero coating in the hole but may be a region of thinner aluminium deposition that shows a pronounced visible contrast when viewed in transmission.
I have seen many different ways of evaluating the level of pinholes. This includes simply holding the metalized film up and using the sky as the background illumination and the person holding the film then estimates the number of pinholes and decides if it is better or worse than normal. Obviously this is poor quality control and subject to large errors.
More typically a light box is used and a specific area of film is examined with the number of pinholes in the viewed area counted. This number is then recorded, compared and the quality of the coating judged against a reference number of pinholes per unit area. Fewer than this number is good and more than this number is poor. Although this is a common method there can still be differences between measurements taken at different sites unless the design and construction of the light boxes is identical.
The basic construction is for a box to contain a light with a flat top surface on which the metalized sample can be placed for evaluation. Where there can be differences are the light source, both in number, power and type of lamps. The lamps may have a white diffusing glass envelope and the box may have reflectors below or may just have all surfaces white. Above the lamps the top plate may be clear glass or a white diffuser material. The differences in lamps can change the light intensity as filament lamps are more of appoint source of light whereas the fluorescent lamps already have a more diffuse output. The number and position of the lamps can also change the uniformity of the light across the top surface plate. The aim is to have as uniform illumination as possible over the whole surface rather than bright spots directly over each lamp.
If you get a film sample and increase the light intensity it is noticeable that there are fewer pinholes visible at low intensity that at a higher light intensity. Hence it is important to have identical light boxes if you want to be able to compare results. The higher intensity illumination makes it easier to see the finer pinholes and also the contrast differences where there are areas of reduced coating.
It is important to use a number of samples when assessing a roll of metalized film as the distribution of pinholes may be random although sometimes there are some differences that can be associated with other converting processes. Slitting is a dirty operation and even with vacuum extraction it can often be seen that there are more pinholes towards the slit edges that there are towards the centre of the film. Thus to get a full picture of the distribution of pinholes it may be necessary to compare several samples taken from the edges to another set taken from the centre of the web.
As many pinholes are derived from debris that is moved after the metallization it will also depend on how the film has been handled following metallization. The web will have been rewound in the vacuum system and then the samples taken either by cutting into the roll or by unwinding a length of material and cutting out the samples on a cutting table. This may be more gentle handling than the film may see in other downstream converting. What can be done is to measure the sample for pinholes and then to brush the surface to evaluate if this produces more pinholes as more debris is moved. This can be revealing as to how much debris is on the film prior to metallization. This does add another variable as the type and pressure used when brushing can affect how much of the debris is moved around. A lens brush used for cleaning camera optics is a very soft brush that if used in a single pass across the surface is probably as gentle as it is possible to get. However this may not be representative of the handling received in other converting processing. Using a second piece of film and dragging it across the surface may be a more representative and harsher treatment. I am sure there are many other options that people have used. Again it is important to be consistent from sample to sample and try to keep it independent of whoever does the test.
I hope this highlights why evaluating pinholes can be so difficult.