I would think it would be better and easier to first put them inside a closed window. The glass will block most of the UV but the plant will still get strong light.

Yeah that would be ideal but in my case it's not possible.
I live in small appartment.

If it's the UV that's hurting them then a blacklight might work. You could also get a germicide bulb http://www.1000bulbs.com/products.php?cat=T8-Germicidal-Tube-Lamps,-Medium-Bi-Pin

those put out almost pure UV, but it may be too much UV depending on how close the plants are and the power.

I wonder if it's infra red that's hurting them and not UV? Just a thought.

Here is a link that might be useful: germicide lamps

Do you have a place outdoors that is somewhat shaded where you could harden them off? I am lucky to have a somewhat shaded east facing porch. I start the plants at the shadiest end and move them down the porch towards the sunnier end gradually over a week or so.

You can also rig up some temporary shade using deck chairs, beach umbrellas, or whatever is handy.

Another alternative (quite a nuisance though) is to put the plants outside for an hour the first day, then bring them back in, then gradually increase the time each day.

It also helps if you watch the weather forecast and try to plant outdoors at the beginning of a stream of cloudy, overcast days.

I hope one of these options works for you.

I'm afraid hardening them off the natural way is not possible(or practical).
My balcony is facing East (sunny from 7am to 2pm).
Most of the plant survived (about 80%)but they were set back a good 3 weeks.
Since I have a short summer and peppers and heirloom tomatoes have a long growing season these 3 lost weeks can mean the difference between a poor crop and a good crop.

Good point mkiker maybe it's the infra red that causes damage.
I think I'll try a little experiment.
Expose some seedlings to UV with a small black light and some to infra red with an incandescent light bulb. Then put them outside in full sun and see which do best.

If you want to experiment with IR you should get a cheap heat lamp. You can get heat bulbs for regular sockets but I don't know the implications of just putting one in a lamp.

The heat coming off my 400 watts HPS is just too much during summer.
So two weeks ago I put my herbs outside. (Basil Genovese, Basil lettuce, Greek Oregano, French thyme, rosemary, cilantro, camomile)
I was expecting to lose everything to sunscald since none of these plants had been hardened.
And the leaves are much more delicate then tomatoes or peppers that I transplanted a few weeks ago.
The only thing different is I did not transplant the herbs, I just buried them with the hydro pots in earth.
None of these plants showed any signs of sunscald or stress and they look very happy.
So my guess is that I planted my tomatoes and peppers outside when it was too cold. The cold did the damage.
All my plants are facing East with sun from 6am to 1pm.

Funny thing is all my herbs are now bolting all of them!
My basil Genovese are over 6 months old and under the HPS never once did they bolt.
My brother in law who is also growing some basil Genovese that he got from the nursery are also bolting.

I really doubt any type or combination of indoor lamps will prevent scalding/shock when plants go outside into sun - the intensity is just too much of a change. Balconies are often somewhat windy, increasing the likelihood of shock when plants first go outdoors.

You'd be better off figuring out some way of gradually hardening off plants, using a temporary shading device, coldframe or similar means.

I agree with Eric. Nothing is going to duplicate being put outside and exposing the plants to natural elements. It's not that hard to rig up some shading (a piece of cheesecloth, a few wire hangers, and some rocks would do it,or, as suggested, prop up misc. furniture etc. around them) and it would help your plants adjust. The gradual exposure suggested by watergal works well, too, if you're home to do it.

It's hard to compare herbs to tomatoes and peppers -- they don't have the same genes/requirements/hardiness/etc. Cold would most likely only stunt the growth of heat loving plants -- not cause them to turn yellow quickly. I doubt that setting them out in the cold was the main problem, although it doesn't help. They like nights above 50F but can be planted out much sooner with some protection (I used to plant my tomatoes out in April when the nights were still going down in the 30's, but I used wall-o'waters and things like that to keep them warm at night, with tremendous success. Caging them and wrapping the cage in plastic, with a cap at the top, has worked well for me, too.) Anyway, I digress. Hardening off is very difficult to avoid unless you want to put the plants through extra shock. They're just more tender when grown indoors.

Good luck!

Warning! Warning! Warning!

Germicidal lamps (shortwave UV) are extremely dangerous.
Please do your research carefully first before using!

dcarch