Photo-acoustics measure ink thickness

In an effort to add a little variety and step away from all the new biomedical applications out there that exploit the endless wonder of light, here's a paper describing the use of light to make measurements of ink thickness! Okay, that might seem a little mundane, but it’s a classic example of the way in which light is used to solve an everyday problem, in this case to measure the thickness of black ink, typically a few microns or so, whilst it is spinning on the roller of a printing press at 300 rpm¹.

The photo-acoustic effect was discovered by Alexander Graham Bell way back in 1880, as part of his pioneering work that led to the invention of the telephone. He used a rapidly spinning disk with slits in it, through which he exposed thin diaphragms to intense bursts of sunlight, causing sound to be produced. This is due to light being absorbed in the material and converted into heat and, because the effect is periodic, pressure waves are generated, resulting in sound. The effect is used to characterise materials and today we have much more sophisticated tools with which to apply it, thanks to high intensity light sources such as lasers, and sensitive microphones with which to detect the sound.

This paper describes a non-contact method for measuring the thickness of black ink during the printing process when the roller is spinning. Infra-red light absorption methods are available for measuring different coloured ink thicknesses, but they are not effective for optically opaque black ink. The paper reports on the experimental demonstration and theoretical analysis of a method with which black ink thickness can be measured using the photo-acoustic effect.

Halogen light was delivered to the roller surface by an optical fibre, exposing a 10 mm diameter spot size with a power of 0.8 W, and an optical chopper was used to interrupt the beam at a frequency of 1235 Hz. The sensing element consisted of a metal pipe, approximately 16 cm long and 14 mm inside diameter, closed at one end where there was condenser microphone and open at the other end, which was held within close proximity to the roller surface (ideally < 0.7 mm).

The principle by which this technique works is that light from the halogen source is periodically absorbed in the black ink, heating the air at its surface and thereby generating pressure waves. These waves resonate in the pipe and are detected as sound by the microphone. When the ink thickness is thinner than its thermal diffusion length, heat is lost to the brass roller by conduction, resulting in decreased acoustic signal strength, which varies according to thickness. At thicknesses beyond the thermal diffusion length, there is no discernible change in signal strength, so the method works only for thicknesses less than that. By selecting a chopper frequency that will produce a resonance in the pipe due to the photo-acoustic effect from the ink film, it is possible to detect small variations in the ink thickness. Direct, online measurements can be made by first correlating the thickness of the given ink with the resulting acoustic signal strength.

The chopper frequency of 1235 Hz was chosen for the high acoustic signal strengths produced at the typical ink thickness, which ranged between 1 and 12 μm, and the pipe's dimensions were chosen so that it produced a resonance at that frequency. The experiments demonstrated that the acoustic signal strength varied with respect to the thickness of the ink film, and a theoretical model of the process, based on the optical absorption coefficient of the ink and its heat transfer characteristics, showed good agreement with the results. The study also included experiments to study the effect of clearance distance between the pipe and the roller surface, the optimum distance being one below 0.7 mm.

Printing press operators may well be rubbing their inky hands with glee at the prospect of new sensors using the methods described in this paper, but for the rest of us… well perhaps it isn’t that interesting. A good innovator, however, would see even more promising opportunities for the application of this technique, which most of us would never see until they came along and thrust their million dollar ideas in our faces. At the very least, it's a neat little optical tool.

1) Kurita, K. (2008). A non-contact and online ink thickness sensor for printing machines using the photoacoustic effect. Measurement Science and Technology, 19(7), 075206. DOI: 10.1088/0957-0233/19/7/075206