UVScan

Visualize the skin damaging and aging effects of ultraviolet light.
UVScan is designed to visualize the damage and aging effects of ultraviolet light on your skin by analyzing photos from your iPhone's or iPod's photo library or integrated camera.
Excessive exposure to sunlight and tanning booths can lead to premature skin aging and chronic skin damage, which may result in skin cancer. These damaging effects can be alleviated by staying in the shade, wearing light clothing, or by using sunscreen.
UVScan visualizes the damaged part of the skin as darker areas. Smaller wrinkles in the facial area, which are indicative of prematurly aged skin due to uv exposure, are easy to spot in the same way. The analysis works best about 24 to 48 hours after intensive exposure.
The analysis relies on images taken under good lighting conditions, without extreme dark or bright spots. For best results with an iPhone 4, use images taken with a distance of roughly 4 inches (10 centimeters). Shots of the cheek and nose should be taken from the side, while shots of the forehead should be taken directly facing the area to analyze. The integrated flash light of the iPhone 4 will further enhance the quality of the analysis.
UVScan resulted in a 2004 scientific paper:
Blum-A, Vollert-B, Schlagenhauff-B (2004)
Visualization Method Based on Digital Image Analysis Reveals Photodamage of the Skin
Arch Dermatol 2004;140 1173-1174