15.1 General

It is impossible to cover such a field in a few lines yet it has to be mentioned in order to draw attention to the problems.

The development of instruments up to the mid 1960s was relatively straightforward. Practically all aspects of the instruments were analogue. One aim of these instruments was to provide analogue circuitry, which when acted on the roughness data, provided a measure according to a specified parameter. For example, to simulate R _a, the mean line was established by an analogue circuit. In the early days, this was a 2CR filter. The actual data was subtracted continuously from this mean with this line and the rectified result averaged over the assessment length to give R _a. In all this, the investigator had no access to the raw data emanating from the pick-up and amplifier. What the investigator saw was either a meter reading or a chart. The integrity of the signal was maintained from pick-up to chart. In fact, the frequency response of the recorder was matched to the frequency equivalent of the stylus tip at the tracking speed. The fact that there were many approximations made in terms of filter and parameter circuitry did not seriously jeopardize the resultant roughness value. The investigator simply used what was made available by the instrument maker: the user had no say about the way in which parameters were measured. This all changed with the advent of digital in-surface metrology methods in 1965 [15.1]. Earlier...