In recent years, MHs with accentuated red to orange parts of the spectrum have been used.
It seems that LEDs are going to dominate this domain.

Osram
Kingstar

I have no personal experience but I heard that even the spectrum is fine, it doesn't really work as the plants don't receive enough heat.
In case of MHs, the heat is radiated towards the plants. The LEDs produce less heat and it is absorbed by a heatsink and then it goes up, so the plants receive almost nothing and an additional heater must be employed.

My coworker has an experience with fluoerscents being way less efficient when rising tomato seedlings than MH's of the same power, we didn't had the explanation, but indeed the heat will be a strong factor: IR heating the leaves evaporate the water from them, so support nourishment transport from the roots...

And the required color depend on what you want to reach: For blooming you need the red part, but for green development you need more the bluish part of the spectrum...

By the way for the seedlings it seems, the cheepeese "auto xenon" MH's (from E-bay,...) work very good, mainly for their cost...

This is of interest to me as I grow chilli plants (for my own use, not commercially) and the previous plants I had died off due to lack of sunlight last spring due to the appalling weather - overcast and raining most of the time - plus insect infestation, mostly aphids and whitefly, which literally sucked the life out of them.

I've been thinking of supplementing them with an HPS lamp to provide the necessary light (and heat) if the seasons turn on us again this year as these plants can live for years producing excellent yields. I still have some Bird-Eye chilies in the freezer from a previous crop along with some remaining Scotch Bonnets. I use a lot of chilli in my food and with the price of bought chilies being so high per pound these days growing some plants of my own not only makes sense in this way but I just love growing stuff. The only issue is keeping the cats off as they like chewing the leaves!!

I currently have 2 Habanero plants, one Cayenne and a sweet red pepper plant and these are doing fantastically well on top of the TV in the loft (3rd/2nd floor depending on where you come from, my main living room) protected from moggy inquisitiveness by a wire cage and if the weather makes a U-turn (usual British crap summer, we've had 7 Sprautumns in a row so far) I will be looking at helping them with some extra light.

Heat is obviously an issue with the folks who grow naughty stuff in tents but not a problem for me as I can put a lantern in the window if necessary to help out, I think the neighbours and passers-by can tell the difference ;-) .. I think a 70W HPS will do the job here. LEDS don't help at all if it's cold enough to freeze the balls of a brass monkey as the plants will suffer from this .. as well as the additional heat being welcome in the loft or bedroom where I have the plants. LEDs are useless in this respect unless the heat from the drivers/heatsinks can somehow be contained in a tank or something to keep the cold at bay. I previously had a perspex sheet on the inside of the window to keep the cats off them and allow daylight in & me to see out with the last crop, but the lack of sunlight was the problem. This will obviously have to be positioned such that the lamp won't cause problems so maybe a fluorescent (if I can find a source of reasonably priced Gro-Lux tubes) with a ballast in the space occupied by the plants may be an option.


BG

I grow some chillies too. The last spring in our country was also pretty bad. I lost a complete window box of them but probably due to over watering. This winter I tried one of my MVs. I don't know if it really helped but the old plant has survived and now there are several pods.

I'm not sure about the HPS spectrum. It's not just about the light intensity but the light spectrum should meet the plant requirements (see Medved's post). I'd rather suggest HMI as it has proved working among growers.

Insects and mildew are a real pain. I heard that strong UV radiation (from a broken MV without the phosphor, for example) can kill them but you must be very careful not to overexpose plants because it can be harmful for them as well.

The HMI is a highly loaded short-arc MH lamp for stage lighting. It isn't intended for horticulture.

The HMI is a highly loaded short-arc MH lamp for stage lighting. It isn't intended for horticulture.

It is indeed not designed for that, but why should that matter? It does emit lines over the full spectrum, include some UV and IR, so basically what the plants need, so where is the problem?

The HMI requires a forced cooling and have a shorter life than a regular MH lamp, because of the high loading.
Beside, this is a bare arc tube lamp, so it produces dangerous amount of short wave UV, which can damage the eyes and skin, and manufactures ozone.

The HMI requires a forced cooling and have a shorter life than a regular MH lamp, because of the high loading.
Beside, this is a bare arc tube lamp, so it produces dangerous amount of short wave UV, which can damage the eyes and skin, and manufactures ozone.

Sorry, it was a sort of "typo" - I thought of regular MHs, of course. AFAIK, many growers use them and there are also special horticulture versions with modified phosphors.

I don't know if the HMI-produced UV would be dangerous or rather beneficial for the plants (direct sunlight can also burn the skin while various plants like it) but using this light source is at least not economical due to HMI prices.

The HMI:
Well, the lamp cost does not have to be always a problem, when someone get bunch of lamps for free, as the original owner does not need them anymore (it is hard to resell them, as no one really using them want to waste his expensive equipment by bursted lamp of unknown history). So adding the cooling is not as big deal either...

But yes, for horticulture application it is rather impractical light source...

In Canada I grow citrus in large pots. I overwinter them (over 5 months) indoors with normal spectrum M-H (~1000w total), but for early spring I use mixture of M-H & HPS. I don't change gear but I simply use a M-H conversion lamp that runs on HPS ballasts. I let my lemons & limes grow on the tree for nearly 2 years growing to about 4"x6" per fruit!