A couple days ago I made the mistake of putting my tomato seedlings out
on a windowsill for direct sunlight, and a few hours later I discovered
they were all withered. (We've had a lot of sunlight here lately in
Portland, Oregon.)

I immediately removed them, and enclosed them in a humidity-containing
package (it used to be a doughnut package but I filled it up halfway
with potting soil), and hid it away.

The next day, 10% of the seedlings began to perk up again.

If I buy a photometer, how much light should tomato seedlings be exposed
to?

Is the real problem loss of water instead of too much sunlight?