The glass filter in the plasma light is to filter out UVC which is dangerous for the eyes when looked in at short distance, the special glass transmits UVA and UVB which is good for the plants. Though all HID
arc tubes indeed emit some or a lot of UV light (specifically MH and mercury lamps, every lamp with mercury in it emits UV - this is how CFLs work!) most lamps have a UV shielding outer balloon. The lamps that do emit UV light or can cause dangerous UV radiation when broken (specifically MH!!!) are not rated suitable for an open fixture and need a glass shield that protects you from UV and specifically UVC when a lamps outer balloon breaks. Actually they need a completely closed fixture, not one with large flanges for air cooling, as a breaking lamp of exploding lamp can cause very hot particles and dangerous heavy metals to be distributed through your ducting (therefor always use the fan to push air through a air cooled fixture to prevent fan damage by hot air and particles in your ducting). A HPS lamp which is broken should also be replaced immediately, as it will radiate harmful UV light, but not to the extent of a MH of coure, which contains much more mercury. Many HPS lamps fail when imploding.
When it comes to HPS light: There are supposed to be shielded for UV light. A normal HPS lamp emits very little if any UV. That is the reason why it is allowed to use them in non-shielded hoods. When broken many fail anyways, but yes, they can cause some UV radiation as well. A MH when the outer balloon is broken is really dangerous, even at greater distances, as they do emit enormous amounts of UV light.
Let's take a commercial greenhouse as an example: They have thousands of HPS lights in open reflectors. If this would create a hazard for the personnel everyone in a greenhouse would be using protective clothing and glasses. That is not the case. Actually some greenhouses invest in special UVB transmitting greenhouse glass to allow UVB enter the greenhouse, as there is none whatsoever coming from the HID lamps and their glass roof does not transmit UVB. This is why plants need to harden under a shield after coming from a greenhouse before they go under direct sunlight. Try it with a cutting grown under high intensity indoor HPS lighting, put it directly in the sun: it will burn because of the UVB radiation, specifically in high summer when UVB levels are highest.
Bottom line:
- HPS lamps are safe to use in an open reflector. When broken, replace. You instantly know when a HPS lamp is broken: it implodes and may times the arc tube just fails when broken (but not always!). A failing HPS arc tube seldom explodes. We had 250W HPS lamps put to a stress test on a 1000W ballast. This causes the lamp to overheat and burn up fast, but they did not explode, though we encountered implosions when the wire frame bends and the hot wire frame touches the outer balloon. In many cases the implosion takes the arc tube with it. What you sometimes see with failing HPS lamps is leaking arc tubes. They do not cause the outer balloon to fail (rare footage:
example on video)
- MH requires a lot more caution. First of all, check if they are suitable for an open fixture. Open fixture lamps have a special construction to prevent the arc tube from disintegrating when a lamp fails by a wire that holds it together, or an extra inner balloon, to prevent the outer balloon to scatter. With MH you see mostly a failure of the arc tube when things go wrong. In any case switch them off immediately if they fail. When behind glass most (if not all) of the UVB is already filtered out.
For your reference I included the spectrum of a HPS lamp. It is not a low-res manufacturer cut-off SPD to just show 400-700nm, but it also shows you the high IR peak and the spectrum below 400 nm (UVA, UVB). Now the sensor is less sensitive for anything below 300 nm, but it will give you a good idea. There is mostly sensor noise in this area, no reading. This is lab measurement.
Are sunglasses required in a grow room? With MH and open reflectors I would say they are an extra safety, they will provide protection for your eyes, as some MH do emit UV radiation and for sure when they break, that is a very dangerous situation. Buy your MH lamps form a reliable manufacturer who has taken precautions to limit UV radiation and block UVC, and always follow manufacturers instructions as for horizontal or universal position, and specifically open or closed fixtures. A glass shielded hood is safer with any MH or CMH lamp.
HPS lamps from all major manufacturers have UV blocking glass.