Luminosity based black and white conversion methods convert images to grayscale based only on the color's brightness or luminosity. This is similar to the way black and white film captures only the scene's brightness. These methods are very restrictive in how they convert colors to black and white. They convert images to grayscale the same way, every time, with no input from the user.
Luminosity methods work best on images that have plenty of contrast, or images with little or no color to begin with. For example, this photograph has plenty of contrast and almost no color.