Scale for use with opto-electronic scale reading apparatus

DETAILED DESCRIPTION The object of the present invention is to produce a more diffuse secondary light source at the scale by providing a scatter mechanism for the incident light at the scale itself to break up the ordered diffraction at the grating structure defined by the scale grooves.
A proposal to avoid ordered diffraction from the scale has been made in International Publication No.
Wo 88/00332 in which it is disclosed that diffusion at the secondary light sources can be achieved by a variation in the shape or surface finish of the grooves.
The only example of shape variation given is a proposal to round off the profiles of the grooves, while an example of surface finish variation given is a proposal to choose an appropriately diffusing type of plastic for the protective coating on the scale.
However, both of these examples are applied to grooves with straight edges transverse to the scale which do not of themselves provide diffusion transverse to the scale, and in the second example the use of a diffusing coating gives rise to a loss of reflectivity of the scale.
It is known from International Publication No.
Wo 86/03833 to vary the spacing of the scale grooves along the scale for an entirely different reason, but this may also have some destructive effect on the ordered diffraction pattern produced by the scale.
We have found however, that since diffraction is basically an effect produced by the straightness of the grooves on the scale, such variations in the profile of the grooves or in their spacing are not totally effective in breaking up the ordered diffraction pattern.
One embodiment of the present invention achieves the effective break-up of the ordered diffraction pattern by varying the shape of the groove along its length, to produce a non-linear edge to the groove.
This causes destruction to the ordered diffraction pattern by scattering the reflected light, without any loss of reflectivity from the scale.
The non-linear edge may be obtained by varying the width of the groove along its length or by varying its direction to produce a zig-zag form