Relating to a device for imaging

A substrate for facilitating the measurement, using a Total Internal Reflection based optical imaging system, of the status of an assay;

• the substrate comprising:
• a first surface including a test site at which one or more reagents for the assay are immobilised;
• a second surface substantially parallel to the first surface;
• a third surface joining the first and second surfaces;
• and
• a fourth surface joining the first and second surfaces,
• wherein three characteristics of the surfaces are manipulated to ensure that light launched through the third surface impinges on the first surface substantially at the test site and then undergoes Total Internal Reflection, impinges on the fourth surface, and is partially reflected to impinge on the second surface,
• wherein the reflected light forms a closed loop reflection pattern comprising at least one diamond pattern, and
• wherein the three characteristics are:
• the length of the first and second surfaces between the third and fourth surfaces is 5 to 130 mm;
• the length of the third and fourth surfaces is 0.5 to 10 mm;
• and
• the angle subtended between the first and third surfaces is substantially 90°.