What is BVRI or Four Color Photometry?
BVRI is a measurement procedure in which the magnitude is measured in four colors (Blue, Visual, Red, & Infra-Red), and then transformed to a standard system that observers from different sites using different cameras, ccds, telescopes, and software can merge results to high accuracy (much better that 0.1 magnitude). To transform data one must determine their transformation coefficients by taking images of a standard field like M67 to determine a correction factor to make results agree with the standard system. Results in the standard system are usually capitalized (BVRI) while results before transformation, also called instrumental magnitudes, are in lower case (bvri).
