Fast and accurate detection of possible traces is of vital importance. Many biological fluids are fluorescent in nature, when such traces are illuminated with light of the right wavelength, they fluoresce and are detectable to the investigator. Don't rate 1 star before you read the description!!! When to use UV light to detect blood: • Reduce the risk of collecting unnecessary stains • Detect blood on dark, red or violet surfaces • Find blood stains that are concealed by paint Change frequency to get different tones and better visibility of a stain! This app might not work on all devices because not all devices can produce UV light properly.