Passbands and calibration
Sources, detectors and filters have wavelength-dependent responses. A photometric result therefore needs a passband and calibration as well as a numerical flux or magnitude.
Systems such as UBV select different bands. Standardization enables comparison and preserves colour information; mixing unmatched systems can confuse instrumental differences with source properties.
Repeated measurements form a light curve
A brightness-versus-time record is a light curve. It preserves amplitudes, durations and recurrence intervals unavailable from a single image.
A transiting planet blocks part of its star’s light. Depth and repeated timing inform relative sizes and orbital period, while an absolute planet radius also requires the stellar size.
Measurements and physical inference
Multiband measurements form colour indices, and distance helps distinguish received from intrinsic brightness. Gaia combines such data with positions, motion and spectra for stellar and Galactic studies.
Photometry alone does not automatically specify composition, age or distance. Models and complementary observations remain necessary, while intervening dust can dim and change the colour of the signal.