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3.2 Consideration of retro-reflective targets.
3M have been manufacturing retro-reflective sheeting for many years. The main volume use of this material is for road traffic signs,
many of which are a few square metres in size. A minor use, by comparison, is for safety clothing, and an even smaller use is for
photogrammetric measurement targets. These materials are optimised for the main market, but requirements for photogrammetry and
other uses appear to be very similar, all involving a light source that is close to the viewer (CCD or film cameras in the case of
photogrammetry). Although photogrammetrists have largely used just one material (of a type used for projection screens) for
targeting there are three different materials used for traffic signs, each with a number of variants which could be useful, but little
appears to be known about their photogrammetric characteristics.
In the environment of the centrifuge it was initially thought that it would be difficult, if not impossible to use retro-reflective targets.
This was because the surface of the 3M 7610 series material cannot be wetted - a likely occurrence in the centrifuge. However, the
advantages of these targets are such that an investigation of other retro-reflective materials was started. The main benefit in this case
is the ability to use materials of differing reflective characteristics i.e. black or white, with the same set-up and targets.
3M products use two means of obtaining the retro-reflective effect - miniature balls and miniature prisms. The prismatic material is
divided into two types, Diamond Grade and VIP Diamond Grade, while the balls type are divided into High Intensity and
Engineering grade. For use in traffic signs the least effective of these materials is the Engineering grade and the most effective is the
Diamond grade. This investigation concerns an evaluation of what is most useful for photogrammetry and what in particular can be
used in the centrifuge. The information that is required for each material is: surface type; angular reflectivity characteristics;
orientation characteristics; and physical properties. The angle of return is of importance in defining the camera angle of view to avoid
light fall-off and the angular reflectivity must be known to determine the location of the illumination with respect to the camera. As
the camera cannot easily be mounted far from the test specimen, a knowledge of these parameters is of paramount importance.
An apparatus was designed to enable a comparison to be made between materials and, if possible, between other surfaces such as
mirrors or plain paper. Measurement over a number of orders of magnitude is required so the methodology used in previous work
(Clarke, 1994) could not be used. The first requirement was for a light source of suitable power and stability. A tungsten filament
system used for microscope illumination and a high intensity fibre-optic system were investigated, but considered inappropriate due
to mains electricity intensity modulation and insufficient output. A SmW. HeNe. laser (633 nm.) was used. The laser provided the
necessary intensity of illumination whilst guaranteeing the delivery of the light to a known location on the material. The output
stability of the laser is also very high. The other reason for using the laser was the avoidance of the requirement for a phase lock
amplifier and light chopper to separate the illumination of the target from other sources of illumination. The disadvantages are the
use of coherent illumination and an investigation at a single wavelength. However, as a first approach to characterising the
performance of the materials it was considered adequate.
The apparatus consisted of the laser which was directed via an aperture through a beam splitter. One part of the beam was reflected
inside the beam splitter through 90° and then reflected again by another mirror to avoid any further influence on the experiment. The
remaining beam passed through the splitter to impinge on the surface of the retro-reflective material. The returned illumination from
the retro-reflective material was then partially reflected, in the opposite direction to the useless beam, to a detector. The test material
orientation was adjusted by rotation and by a further three degrees of freedom to allow correct alignment. The beamsplitter itself was
also mounted on a similar rotation and alignment stage. The materials tested are given in Table 1.
Type 3970 series 5870 series 3990 series 2290 series 7610 series
Name Diamond grade High Intensity grade | VIP Diamond grade | Engineer grade High Intensity grade
Construction Prisms Spherical balls Prisms Spherical balls Spherical balls
Table 1. Retro-reflective materials tested.
To gauge the relative response of the various materials used the apparatus was set up for maximum light return to the detector and the
light returned by all of the materials measured. In one set of measurements all of the returned light was collected by a lens and
focussed onto the detector (row 2) while in another set of measurements (row 3) a small aperture was used. Care was taken to
establish the level of returned light, with no material in the field of the test beam, due to imperfect surfaces on the mirror and beam
splitter. The responses are given in table 2. It is interesting to note the difference between a front surface silvered mirror and the best
material and the differences between the materials themselves. The material usually used in photogrammetry is the 7610 series which
is mainly used for projector screens.
Type Silvered 3970 5870 3990 2290 7610
mirror series series series series series
Relative total response 380 385 175 310 60 126
Relative sampled Not 6.04 1.58 39 0.5 2.57
response measured
Table 2. Relative peak responses of each material.
IAPRS, Vol 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995
