The optical fibre probe is constructed with centered illuminating fibre, receiving fibre A surrounding the illuminating fibre,and receiving fibre B surrounding receiving fibre A as shown in Fig.1. The light radiated from a laser diode enters the illuminating fibres and then radiates to a target. The light radiates in a diverging cone on the target surface.The light is reflected from the target as a function of g and led to photosensors.
The diameter of the light ring on the target surface is proportional to the gap g. Please note that PA and PB are also functions of Gap g. Figure 2 illustrates the relationship between the light powers PA and PB of respective receiving fibres. After an increase of PA starting from zero it decrease if g becomes larger. PB shows same behavior but delayed. (PA+PB) and (PB-PA) are results from these functions as shown in the figure. The range indicated as working range WR is the most linear range and therefore suitable for displacement measurement. Both (PA+PB) and (PB-PA) are directly propotional to the reflectivity of the target. Consequently, the effect of target reflectivity can be eliminated by dividing operation (PB-PA)/ (PA+PB). In practice, the effect of target reflectivity is eliminated by using high precision analog divider. In the case of an application for transparent glass target, the light reflected at back surface does not enter the receiving fibre if the inner diameter of the light ring is larger than 2XB(Fig.3) As the results,ATOPS can precisely work if transparent glass or other low reflective material is used as target.