Abstract
Photometry is the most fundamental of all the observation tools used in astronomy. It measures the brightness of stars (or others objects) as well as other strong features in the energy distribution output. One can and does use these measures of magnitude, color index, color differences, and other parameters to estimate surface temperatures, true luminosities, metallicity, strengths of strong lines, interstellar reddening and absorption, distances, and other important details. All these give us important information about the star or other object, useful for studies of stellar structure and evolution, galactic structure as well as for the properties of the interstellar matter, in fact, for almost all of any observational work in astronomy. However, as with all techniques, great care must be paid if one is to obtain precise and accurate data. It is only too easy to make accidental or systematic errors that can lead to serious misinterpretations of the data. Several examples are given, relative to photometry of open clusters and of the interstellar reddening properties. A good unde.rstanding of stars, gas, and dust in the galaxy demands the very best from photometrists.
