Actually matey your exactly right.......though it depends on what type of bulb your using, something that people don't tell you is there are several different types of mh bulbs, but to lower it down there are o type and s type, basically o type are open rated meaning you can use without a shield or protective layer like a cool shade - this type can be used in a basic euro reflector, the s type need a cool tube or cover reflector so that the additional layer of glass filters the uv.
What this means is that an open rated mh has a double layer of glass, basically a bulb within a bulb and this second bulb filters the uv, I discovered this myself, got myself a new venture mh enclosed but used it without a cool tube, and my plants started fading in colour, knew it wasn't a nutes problem so started looking into whether the plants could suffer light poisoning, no such thing as 'too much light' but each bulb emits uv, what I discovered is that while the mh bulbs are starting up, the lit they emit is mainly uv light, this poisons your plants.
Next what you have to realise with mh bulbs is whether they have been made for horizontal or vertical use, and next when you get a mh bulb it says vertical 15 degrees - what this mean the filament inside the bulb has to be positioned vertical within + or - 15 degrees, this can effect both lumens output emitted uv and also lamp life.
The last thing is fire safety, an s rated bulb does not have any 'blast protection' around the filament and so is prone to exploding, if used without an enclosure like a cool tube these can explode and actually send chords of glass piercing through a grow tent. And that glass can be extremely hot.
Best place to read up on mh bulbs is aquarium websites as they use mh bulbs in aquariums for the uv given off as different types of coral react to uv etc.
The same Way as different strains of Mj react to uv light, in general an indica hates uv during growth but loves it During flowering, and sativa love it during growth but hate it during flower, try experimenting and be amazed at what Effect uv has?
Get a cool tube and then when you screw the bulb in adjust the filament lying position, so if a guide says use that bulb within 15degres, set it at 90 degrees etc. And see the difference it makes.
People often wonder how same setup,same strain can get different results, it's basically down to the bulb.
And for all those who are going to waffle about uv being harmful to plants, the suns gives off tremendous amounts of uv, now in a normal climateuv is filtered by Cloud and the water droplets contained within, this is one of the reason why humidity can be beneficial to plants as the moisture in the air within a grow room actually filters some of the uv from your light,remeMber humidity should not be good for mj as it restricts the plants ability to perspirate yet plants can thrive with added humidity, again experiment?
To close basically mh does give off harmful uv, depends on bulb type but most of this can be filtered, but if you are going to use mh then either high humidity or cool tube it, even if you don't have the cool tube attached to a fan.
Also NEVER look at either a mh or hps bulb directly during bulb start up as this will end up blinding you or causing cancer.
