for a system that could search for that colour to add further uniqueness a spot which fluoresced under ultraviolet or x ray stimulation could be employed in keeping with the requirement to achieve the highest possible signal level the use of retroreflective material was selected as the optimum means of providing both the location making means 20 and the code contrast unlike coloured or fluorescent patches the light emanating from the retrorefective material is not uniformly scattered and is always directed back along the path of the illuminating source 26 the other techniques cause the light to be emitted with generally uniform intensity in all directions and so the achievable gain in returned light intensity for retroreflection can be orders of magnitude greater than for non specular reflecting systems over large distances directed reflectivity from specular reflection can also achieve this but can only happen when the object surface is in a particular alignment with respect to source and imager any attempt at relaxing the source to imager alignment requirements for a specular reflector would result in drastic reflectivity drops compared to retroreflection the reader will generally look for the bright retroreflective target 10 in a much larger field of.