Less common or future field measurement techniques One technique that is frequently examined for use in measurement of road

surface texture is that of profile projection. Essentially, light, in a straight line, is projected onto a surface at an angle, and then photographed. The resulting image of the straight line can be used to measure surface profile directly (Forster, 1989), although the resolution of this technique depends on the image resolution and the method of measurement. The method employed by Forster is as follows: the surface is coated with white paint and a semi circle of light is projected onto the surface at an angle of 45° so that a straight line is sharply focussed on the surface. Viewed from above, the straight line traces the vertical profile of the surface which can then be photographed and measured using a microscope or by projection onto a screen with a calibrated scale.

A study to assess laser triangulation under structured lighting as a potential method for determination of road surface microtexture was carried out by Demeyere and Eugène (2004). The method described,

Target surface Reference

distance

Laser diode

CCD

which also had applications in several areas of general metrology, used a laser to cast a line onto a road surface under test, which is observed using a digital camera. The 3D coordinates of each point on the laser curve were calculated leading to determination of the road microprofile with a claimed accuracy of approximately 10 µm.

Another commonly investigated technique is stereopsis, which is a field of photogrammetry, also referred to as stereo-photogrammetry or stereovision. Sabey and Lupton (1967) describe the technique and prototype equipment used to capture texture information about asphalt surfaces. Figure 2.12 shows a copy of the simple ray diagram they used to explain the principle. If a pencil, standing on its end, is photographed from directly above and from a position slightly displaced, the two images created when they are superimposed will also be slightly displaced. The top (T) and bottom (B) of the pencil will superpose in an image taken from directly above, at position M, when the camera lens is at position A. If the camera lens is moved a distance b to the right, to position A’, the images of the top and bottom of the pencil appear at T’ and B’ respectively. The displacement of the lens to the right is equivalent to a displacement of the pencil the same distance, b, to the left; the dashed lines show how the images of the ends of the pencil would appear at T’’ and B’’ if this were the case. When the second image is superimposed on the first, its displacement, to the right, is called its parallax.

If the parallax of the top of the pencil, the distance, MT’’, is called p then

2.7 where b is the distance between the two lens position, f is the distance between the lens and the photographic plate (or CCD sensor in a modern camera) and h is the vertical distance between the lens and the object.

Figure 2.12 Schematic for stereovision

From the ray diagram, it’s clear that this follows from observations of similar triangles. By finding the equivalent relationship for the bottom of the pencil, or by differentiation with respect to p,

2.8 where h is the height of the pencil and p is the difference in parallax, which is negative because points further away from the lens will create a displaced image closer to the original.

Millar and Woodward (2010) used a stereo vision technique to generate three-dimensional models of various road surfaces on a trip between Torino and Calais. Road surface texture measured using the models compares well with measurements made of the same surface using the volumetric patch technique. The same researchers went on to use the technique, and three-dimensional models of asphalt surfaces, to investigate tyre contact and its effect on wear and asphalt durability (2011).

A similar technique was used effectively by researchers at the SIC laboratory of the University of Poitiers and the LCPC through development of a transportable system for capturing pavement images (Slimane, Koudeir, Brochard, & Do, 2008). In this case, rather than moving the camera a known distance and taking two photographs, a camera in a fixed position was surrounded by three separate light sources. Three photographs, lit from different angles, were taken and used to calculate surface relief. A brief description of the system, and calculations, follows.

The amount of light reflected by a surface element in the direction of the

and its colour. Colour digital images contain information about hue, saturation and intensity for each individual pixel - this is the HSI model (Wyszecki, 1982). All of this information can be used for analysis of the surface being photographed.

The assumption was made that, for surfaces of uniform colour, variations in grey-level in images were equivalent to variations in relief. This is an important assumption that will be revisited in Chapter 4. The technique using stereovision with three lighting angles was developed to improve performance on surfaces that were not uniform in colour. The images, with resolution of approximately 50 µm per pixel, were taken in turn with lighting positioned at 120° angles around the camera and focused on the image area so that the light incident on the surface was at the same angle from vertical.

It was assumed that the surface was Lambertian – light falling on the surface is diffusely reflected in such a way that the apparent brightness of the surface to an observer is the same regardless of the observer’s angle of view – and that the intensity of light reaching the camera lens is expressed by:

2.9 where (x,y) is the incidence angle of light related to the reflecting facets of the surface, r is the distance between the lighting source and the reflecting facet and L(x,y) describes the colour properties of the surface at any point.

It is then shown that the angle of incidence of light related to the reflecting facet, (x,y), can be expressed in terms of the angle of incidence of light related to the surface plane, i (for i 1, 2 or 3 for each light source), the angle of incidence of light related to the x-axis, _i (0°, 120° and 240° as described), and the gradients of the surface facets.

Since there is one such equation, I1(x,y), I2(x,y) and I3(x,y), for each of the three lighting angles, they can be solved to cancel the colour information L(x,y) and yield the gradients of the surface facets for each

2.3.3 Laboratory techniques for measurement of surface