Matthew Montchalin wrote:
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?

those seedlings died because of dryness and heat, not too much light.
better start over, and keep them moist at all times.