Hi:
No, white balance has nothing to do with exposure. Rather it is a reference for the camera to know the light temperature so that white is recorded as white rather than yellow (incadenscent/warm) or blue (tungsten/cold). For example it means you can totally negate the "golden hour" effect of warm late afternoon sunlight or the coolness of early morning sunlight just by adjusting the whitebalance on a digital camera video or still.
Whatever light source your under, you just point the camera a a white object, such as a sheet of paper for example and hit the whitebalance control and from then on the camera makes the proper adjustments. It like having smart film in a camera if that makes it easier to understand. No need for daylight film, tungsten film and a lot of color temp filters.
This begs the question of how you get a slightly warmer color in your pics if the camera insists on balancing for pure white? You can do it through filters or by shooting a card that is something other than pure white. Years ago we used to use off color paper, but there is a product out there now I think called warm cards or something like that which is a set of colored cards you can use to whitebalance a camera for the effect you want.
Now the 18% reflectance card is just a handy reference for getting exposure right. It mainly gives you an average reference of reflectance when determining exposure settings. I think it loses a bit of its appeal with digital since you can see what is going on immediately but it still has use when setting up.
Hope this helps.
Rick
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