Lighting white backgrounds with film has always been a simple procedure. Make sure you give plenty of exposure and you’re basically OK. While 1 stop more than the foreground has generally been the rule, lots of guys give more and get away with it. This is not true of CCD photography, edges adjacent to the high valued background can transition poorly if too much exposure is given. How much is enough? How much is too much?
Digital photography allows the exact measurement of your scene, down to a single percentage point of accuracy. One percent. That’s less than the tenth your flash meter reads (and your lens cannot accurately be set to). Once the value will render as pure white on the paper target how much more light do you need? None.
Shoot a test and render the image to the paper target. Include with your test a gray scale that has file densities of 0, 2, 3, 4, 5, 6, 7, 8, 9, and 10 percent. Evaluate the printed image for separation. If the paper target will render 4 percent and no lighter as separable, 3 percent is white. There is no need to give more exposure. You can give more exposure but do so carefully. If reflectances adjacent to the white areas look jagged, you’ve given too much exposure.
The reason for testing to the minimum dot renderable is because you can measure low, single digit densities. It’s easy to verify the right exposure for white. If the value is 0 then verification is more difficult. 1 percent more light than 0 reads zero. So does 10 stops more light. 1 percent past zero is OK and 10 stops is not OK although both read the same, zero. This recommends striving for the single digit report that still renders white.
Do make sure you enter the results of your minimum dot test into the Color Coded Light Meter’s Range Colors. That will help to accurately and rapidly generate the proper exposure for the rendering you’re looking to achieve.