"By introducing an 18% gray card into the scene and placing it in such a way that it is in the same lighting as your main subject that you are trying to expose, you can now take your spot meter reading from the gray card and you will have a “perfect” exposure for that subject in that lighting.  In doing this, white will be white, black will be black and all other tonal values will fall into place." - Steve Kozak, M.Photog., M.Cr., CPP