Leaf burns? With piccies

Here's the story...

Start out with two plants. Tried growing under an experimental LED light but LED (A freaky-deaky-pricey Luxeon V) eventually failed due to supply regulation problems. Grew for a little while in normal daylight. Tried growing for a little while with ~200 / 210W of fluorscents that were spare from refitting the lighting in our loft with halogens. Plants were kept in a cupboard which is about 80cm x 50cm x 7ft tall. The temperature was about 38C with the doors shut and 30 - 35 with them open. Went back to daylight for a little while because the fluorscents are going to be used for the lights in our garage. Preflowers began appearing and stems were elongating way too much for my liking.

Bought a 600W HPS light. Put HPS system in cupboard; worried about heat, rightly. Temperature with the doors shut went right over 55C within about 30mins and probably would have kept rising had I not been checking it every now and again. Moved cupboard to somewhere I could leave a door open without the light being obvious or getting my eyes all the time. With the door open I get about 40C and with a fan running it's more like 38 - 39C. I'm taking all my temperatures from the very top of the cupboard, where it's hottest, right next to the light. And the fan is blowing onto the thermometer, so the temperature elsewhere will be significantly lower. I know that even with just the door open it's cool enough for the plants, my main worry was that I'd set fire to the cupboard while I was out (I have been, and will be, putting a fire alarm in the cupboard with the light).

After the initial 55C the very top leaves had wilted a bit, so I watered the plants and opened the door. Fine, they were unwilted in a very short time. Absolutely zero problems with wilting remain. But soon after switching on the light, maybe a few hours or so, I noticed bright white patches appearing on some of the top leaves of one plant. They grew a bit and appeared on leaves slightly lower down. They're neon white under the lights, like you'd expect a variegated plant to look but without the stripes, they're just blobs. These grew a bit over time but seem to be slowing down now ~24 hours later after first lighting on a 12/12 cycle.

At first I thought these were obviously just burns from the light and that I'd have to right off the top of that plant. Which isn't a major thing because I'm only growing for myself and I think I'll easily get enough to be happy with for a first try.

But...

The very, very top growth point leaves of this plant don't have burn marks on them. Only the leaves below them do. And some of these 'burnt' leaves are lower than the top of the plant beside them, which is also unburnt. The pistils of the preflowers at the very top of this other plant have wilted a touch (not majorly bothered for reason stated above) but there's no burning on it's leaves. One explaination for this might be that the bulb holder is slightly 'bent' upwards, so that end of the bulb is about 1" max higher over the unburnt plant, the light was also positioned ever so slightly more over the burnt plant than the unburnt one. But it's a tiny difference, I would definitly have expected to see some of this burning effect of the unburnt plant if it was entirely from the light since the lowest burnt leaves on one plant are quite a lot lower down than the top of the unburnt one.

Also, these burn marks start in the body of the leaf, not at it's tips or edges.

The light is ~50cm (almost 2ft) above the tops of the plants and is now fan cooled (I know it'd be better to cool the tops of the plants but I can't arrange that at the moment). The plants are in soil. I've been feeding them a zinc rich plant food because I noticed slightly variegation on the leaves while I was growing them with the fluorscents, which signifies a zinc deficiency, but switched back to water two or three weeks ago. I don't folair feed the plants or use any kind of hydroponic nutrients.

The white patches have yet to turn brown. The leaves look a bit twisted and warped around them.

I've moved the light over towards the other plant a touch so's that I could hang the fan from the light bar. Have yet to see any of these patches on the unburnt plant (hopefully I won't).

I'm wondering if perhaps the burning effect is actually some reserve of minerals from the plant food that's reacting to the light or if it's just plain burning.

I've included some piccies below. Sorry some are a bit blurred but it was tricky taking them due to the brightness of the bulb. It was actually daytime while I was taking these, the HPS has just washed out a lot of the background light.

http://img371.imageshack.us/img371/2389/17fh2.jpg (wooooooo, I like chilling out sitting infront of the cupboard, it's like being on holiday)
http://img371.imageshack.us/img371/1575/29zo.jpg (you can see the big difference between the two, note that the lowest burnt leaf on the right plant is lower than the top of the left plant)
http://img371.imageshack.us/img371/3519/38ec.jpg (top of right hand, burnt, plant, noticed that very tops leaves are unburnt)
http://img228.imageshack.us/img228/1917/49zl.jpg (pistils of left hand, unburnt plant)

I also realise that 600w is a bit extreme for cupboard growing but I like growing other things as well and managed to buy this lamp second hand pretty cheap, so figured I may as well go for it and then I could scale things up to grow in the garage maybe.