Figure 2.

The six test stimuli which were placed on a background of 25% reflectance (75% grey). (a) White circles: nominal reflectance 100%. (b) Gradient A: shaded from white to black with 50% grey lying in the centre and a mean of 50% grey. (c) Gradient B: same as gradient A but with 75% grey in the centre and a mean of 75% grey. (d) Low-contrast circles: with reflectance of 50%. (e) Circles: circles composed of half white and half black. (f) Three-dimensional objects: hemispheres made of white Plasticine. All stimuli were 24 mm in diameter. A negative control of uniform grey of 25% reflectance is not shown.