Treat the subject and background as two separate lighting jobs.

First, turn off the background lights and expose the subject correctly using only the subject lights. Adjust aperture and shutter speed until the subject looks right at ISO 100.

Then add the background lights. To make the background go white, it needs to be brighter than the subject, but that extra light must not spill onto the subject. To do that:

• place the background lights as close to the background as possible
• move the subject farther away from the background
• keep the subject lights farther from the subject than the background lights are from the background, so you can control the ratio
• use flags or other blockers if needed to stop background light hitting the subject

If you are working in a small white room, spill and reflections can make this difficult.

For very small objects, raising the object on a white block can help separate it from the floor/background so each can be lit independently. If the background is translucent, lighting it from behind can also help.

In short: set the subject exposure first, then brighten only the background until it clips to white without affecting the subject.