Theory Builder Title

Surface Evaluation - Reflection Lines & Iso-Angles

What we are attempting to achieve with surface evaluation is a prediction of what the design will look and feel like when it is manufactured. There is no single evaluation technique that can do this for us, and so we use a range of techniques to build up a picture of the surface quality, and to give us confidence that the data we hand over will create the quality of product that we are expecting.

All of the tools, in one way or another, measure the changing surface normals aross the surfaces. The difficulty is how to display this analysis in a way that is meaningful to the designer.

The first method is to display evaluation 'stripes' (or 'reflection lines'), and there are a number of tools for this in Diagnostic Shade. They are often thought of in the same way, but there are important differences in the way the stripes are calculated:

Reflection Lines

Calculation method for Reflection Lines

  • Calculates the angle of the surface normals with respect to the viewpoint.
  • Tumbling the view moves the reflection lines across the surface.
  • Quick and easy to use.


Calculation method for Iso-Angle

  • Calculates the angle of the surface normals with respect to a vector.
  • Tumbling the view has no effect on the lines displayed on the surface.
  • More accurate and reproducible.

Reflection Lines

The surface reflects a set of constant width lines, and this reflection can be analysed for breaks in the lines, or uneven variations that may indicate problems with the surface.

There are two reflection line tools which work in slightly different ways:

Reflection Lines Evaluation

Light Tunnel

Horizontal/Vertical (Zebra stripes)

Iso-Angle and Iso-Curves

Iso-Angle and Iso-phote mean "a line of constant angle". The angle is calculated between the surface normals and a single parallel direction (a vector). Areas of the surface with the same angle to the vector are displayed with a connected iso-line.

It is a more accurate analysis of the reflective properties of the surface than the reflection lines, so should be used when creating high quality production data.

One of the main benefits is that the 'direction' stays in the same orientation to the model, and doesn't change as you tumble the view. This makes evaluations easier to compare and to view in detail.

Again, there are two tools that calculate the evaluation using iso-lines:

Iso Angle Evaluation

Evaluating Continuity between Surface Patches

When evaluating surface joins, the continuity levels are shown by the flow of the stripes:

Interpreting the zebra stripes on boundaries between surfaces

Circle examples