Brightness enhancement seen through a tube

We report a fascinating phenomenon that emerges when a surface is viewed through a tube held close to one eye, with the other eye open. The disk-shaped area seen through the tube looks strikingly brighter and, when viewed on a textured background, also of higher spatial contrast than the same surface area viewed without a tube. The effect is reminiscent of a spotlight illuminating the area under consideration. We call this the 'tube effect'. The tube effect is one of the strongest contrast illusions known to us. It requires a matching luminance that is twice as high as the reference luminance seen through the tube. Brightness ratings increase linearly with the log of the background luminance. The effect (i) produces a dark afterimage, (ii) reverses in polarity with low ambient illumination, (iii) assumes the complementary colour of the illuminant, and (iv) persists with fully dilated pupils. Potential explanations include simultaneous contrast (due to the penumbra and dark inner walls of the tube) and veiling of the surround (due to local adaptation and a lower gain factor).