What is BVI Photometry and how is it used? I searched for data about Cepheids light curves (often used as standard candles for measuring distances), and found that all of the data comes in a form of "BVI Photometry" or "BVI Photoelectric Photometry"
For exmaple, the OGLE database gives data as "I band" and "V band" - a quick google search got me to the name BVI Photometry.
Unfortunately, I couldn't find any real explanation of what are does those I-V bands, or how the BVI data is collected and used.
I would love to get some help here: How is BVI data collected, and how does one use it correctly? Even a link to a reliable source would suffice.
Thank you, o kind people of the internet
 A: BVI photometry simply means that the brightness of the stars have been measured using three different filters - known as B, V and I filters.
These filters have band passes of order 100-200 nm wide and are centred at around 450 nm, 550 nm and 850 nm respectively. The exact values will depend on exactly what filters are being used.
The quoted stellar magnitudes in each of these filters are known as B, V and I magnitudes. They will have been calibrated onto a standard system by observing "standard stars" with the telescope/filter/detector setup being used.
Differences in magnitude (which is a logarithmic system and smaller magnitudes are brighter) correspond to flux ratios. Thus the B-V "colour" corresponds to the ratio of blue to (roughly) green light in the spectrum and tells us something about the temperature of a star.
A precise summary of astronomical photometric systems and some details about filters, calibration and zeropoints can be found in The Handbook of Space Astronomy and Astrophysics by M.V. Zombeck (1982).
