The following is based on
Personal experience and extensive reading of the relevant literature after my wife burned her eyes with a UV germicidal lamp after following my instructions.
Test advice from Nichia re blue light hazard from visible white LEDs
You'd want a proper datasheet and the following is definitely on an "all care no responsibility" & this is my semi-informed opinion only basis, but:
EXPECTATIONS
(1) You'd want to exercise due care when using it on the bench - staring directly at the LED from close range would almost certainly produce a slow but noticeable amount of damage BUT this would probably take hours of exposure to manifest and I'd say almost certainly not less than minutes AND it would manifest as itchy or somewhat sore eyes and would self heal in about a week with no permanent damage.
(2) My real expectation is that you could play around with one of these as much as you wished while setting it up including looking into it across a model from the far side occasionally - and have no noticeable effect whatsoever.
(3) Even worst case you could probably not cause any sort of significant long term damage with anything you would sensibly do with it.
(4) If illuminating a largish area compared to the LED (eg a model) and having it fluoresce and reradiate visibly I'd expect the amount of UV to be at a very safe level.
Reasons to follow
Brief update - more to follow:
REASONS
I understand Kortuk's concern.
I believe that my advice as given is OK (read carefully please) BUT erring on the safe side is always wise.
(1) Background: UV and "arc-eye" & "snow blindness":
People exposed to bright high altitude sunlight for many hours risk being burned by the UV component of the sunlight. A skier at say 6000 feet above sea level who skis all day on a bright day without using eye protection has a good chance of experiencing a degree of "snow blindness". The eyes become itchy and somewhat inflamed. The UV has caused burning if the eye interior - mainly towards the surface. Even people who are exposed so badly that they are literally "blinded" and lose the practical use of their eyes due to soreness and inflammation, will essentially always recover without any permanent effects. Recovery is usually in under a week in typical cases. I've personally experienced minor "snow blindness" on a few past occasions either while skiing or in other higher altitude situations. Wearing eye protection makes much sense.
An identical effect occurs from looking at the arc when arc welding. Large quantities of emitted UV cause "arc-eye" - same result, different name.
Germicidal lamps which emit short wavelength UV
In my workshop I have a ~= 20 Watt UV germicidal lamp - in the form of a "fluorescent tube" BUT with no phosphor so no fluorescence just hard short wavelength UV. This kills germs with abandon and will happily burn the insides of your eyes if you look directly at it for reasonably short periods. eg 1 minute would be far far far too long.
Long long ago I used this lamp for bulk erasing windowed eproms (some of us are that old :-) ) and more recently it has been used for etch resist exposure or materials testing.
I have used this light reasonably extensively over many years with no obvious harmful effects. As harmful effects are quite easy to acquire (see below) I assume that this means that taking quite basic precautions goes a long way towards reducing the hazard level.
Diagram: Germicidal versus wavelength of interest. Almost
A few years ago I set up some plastic samples under this lamp, placed a cover over the lamp and samples and left it running. Test time was expected to be many weeks. Some time during the "run" I went to China on business. The UV test results were relevant to what I was doing in China and I asked my wife to report on results to date. I provided detailed & careful written instructions on how to uncover the lamp, how to inspect the samples, how to recover the lamp and, very importantly, how to avoid looking at the light in the process, complete with very clear instructions re the hazards. My wife is a competent and careful science professional (in another field) so I anticipated no problems.
Prescript for the queasy: End result excellent AND they swear that while the UV obviously caused the "arc eye" / "snow blindness", the deeper eye damage was unrelated and the UV damage caused it to be found.
Within a day of checking my test samples my wife's eyes became itchy and sore. After a few days she went to the Doctor who confirmed the obvious. She ended up needing both eyes bandaged due to the severity of soreness and burning. Due to the apparent severity she visited the local hospital eye clinic. The routinely interested examiner suddenly leaped up and ran from the room to get a second opinion. In one eye only she had partial retinal pulling causing a subsurface void near the optic nerve.which would have lead to retinal tearing and separation if left unrepaired. This was subsequently repaired by an epi-retinal peel by a suitably skilled man using very very very small sharp things and lots of experience. (Cut hole in eye ...). It subsequently developed a cataract which was removed with a YAG LASER - shine VERY bright focused light into eye ...).
As one does, I did lots of related reading. The professionals are without exception adamant that the retinal problem was unrelated to the UV. They say macular degeneration of this sort happens with age and that the UV event was a fortunate means of showing them that this was happening. My reading showed that in the very very very large percentage of cases (probably 99.99%+) experience is as they report. UV exposure, even very severe and with short wavelength, does NOT lead to premanent damage or to retinal damage of any sort. However, it was also apparent that in extremely extreme cases (eg welding without a mask long term) then retinal damage almost certainly can occur. This is so rare as to be hotly contested by all experts. I'm also aware that if lensing did cause the arc-eye effect (see below) then the focusing MAY have resulted in a focused retinal spot - but the experts all say NO.
WHY DID IT HAPPEN: Uncertain. She is certain that she followed my instructions correctly and there is every reason to think that she did. But, my wife wears glasses for close vision (or did before her eye sight was corrected as a result of the above processes). I theorise that the very short wavelength UV was refracted substantially differently by the lenses and as she looked across and not at the light, the UV was bent into her eye.
(2) Nichia carried out tests on some lowish power (150 mW in max) white "phosphor" LEDs. In the blue region the output was high enough to make them of potential regulatory interest. I'd guesstimate that you could stare at one of these all day long from 100mm and only get bored.
BUT given this result, an LED designed to operate at about 400 nM at about 60 mW input can be expected to output substantially more "somewhat blue" light than my sample and so would technically at least be of potential interest.
BUT the above example with short wavelength UV and the expert opinion re the lack of permanent effect at even high doses suggests that you are unlikely to have any problems if very basic and simple precautions are taken.
That supply could very easily kill you.
That said, I've (long ago) had a number of shocks at that level with no long term ill effects - BUT some people manage to die on the first encounter. Try not to ever find out which category you will be in. (I've also had numerous 230 VAC mains shocks and a few at around 1000 VDC with in all cases never more than a nasty experience at the time.The last was long ago and I aim not to repeat them if at all possible.) (230 VAC is probably the most "disturbing")
A DC supply has a "can't let go" effect clamping the muscles. You don't want to EVER experience this.
HV is not something to worry about overly much - but must not be ignored. You may be able to receive say 100 shocks in quick succession from such a system and suffer no consequesnces apart from nightmares and a lifelong aversion to digital clocks of any sort. BUT it could kill you along the way.
Anything in the area outlines in red in the diagram below will be potentially lethal and anything outside it MAY be :-).
Historical advice is to keep one hand in your pocket while testing to prevent accidentally closing a hand to hand circuit via you chest/heart. That has some merit BUT slow deliberate thought out actions are at least as valuable.
Rubber dish washing gloves would offer very substantial protection if dry and not punctured. Puncturing can happen on a small wire end. They can be used as an added safety feature and PROBABLY make things much safer BUT act as if you are not wearing them.
If I "MUST" work near live conductors I try to keep fingers curled inwards so that a hand clench triggered by current will not cause grasping of a live conductor. Brushing the back of a hand against a live HV conductor is liable to cause a muscle contraction towards your body BUT do not ever rely on this.
Best method is to have HV turned off until it is needed for testing.
Think what you are going to do, and have tools, meters etc ready.
If you can attach a meter with a test clip with power off, so much the better.
If a test clip will not safely and reliably attach to the HV target you can solder a wire to HV when power is off (of course) and connect that to the meter probe. After you have experience with such things you are liable to think nothing of measuring say 230 VAC mains or 500 VDC with two probes with power on - but resist the urge to leap in an do what will be safe enough wit experience until you have some experience - or you may never acquire any :-(. It's really rather safe most of the time. But it's better to be safe than sorry as a beginner.
Power on, think, test, think, power off.
Be SURE power is off.
Be SURE power is off.
Be SURE power is off.
I have seen power on when it was thought to be off happen often enough over the years that I am quite obsessive about checking. If a mains cord is involved I am liable to turn power off, pull out plug, wave the cord to be sure the plug is the correct one, place plug near gear being worked on as an indication that it is safe. That's obsessive. I'm alive.
I recently installed a new domestic stove in place of an old one for a friend, with only a wall mains switch between me and mains. Fuse still in for various reasons. "Safe enough" but potentially lethal. Tested with meter. Shorted all leads (PNE) together to ensure no mains on.
Treated all wiring as if alive as much as possible throughout job. Obsessive. Alive.
Think carefully. Act slowly. Being very safe is easy.
Best Answer
Well the most important thing is to actually think about everything you're about to do when connecting a circuit.
Check if the oscilloscope probe's ground pin is shorted to the power supply's ground pin (in most cases it will be). Check if output pins of the power supplies are connected to ground pin. They might be, but good quality supplies will have separate ground pin available on front. Also check if the PSU case if connected to ground (it probably will be) and if it is, take care not to have a positive wire touch it.
Every time you connect the scope to something powered by the power supply, think what's going to happen. I've seen cases where people shorted-out their power supplies using oscilloscope probe ground pin and couldn't realize why that happened.
Next, check how the power supplies are going to react in overcurrent situation. Is it going to shut down or will it drop voltage or something else? In general, familiarize yourself with equipment you'll be using.
Do keep in mind that a multimeter in current measurement mode is basically a short-circuit and in voltage mode is basically open circuit. Take care how you connect it! Do read the manual and try to understand what happens in other modes, if there are any. Take note of maximum voltage the multimeter can take with each of them.
For the end, once again, read the equipment manuals and try to plan out each situation in which you can find yourself in as far as the equipment is concerned and think about what will happen if you make a short somewhere.
ABOUT THE COMMENT:
The problem has to do with "floating" and "ground-referenced" power supplies. When a power supply is said to be floating, it means that you can't make a current loop which goes back to ground. An example of this is a battery-operated device. Current goes from one terminal of the battery to another and if you connect one side of the battery to the ground, no current will pass through the wire, because there isn't a closed loop for current to go through. Take a look at this simulation where the ground is connected to the positive output of the battery and no current goes through it.
Same thing happens when you have a transformer separating the mains side of a power supply from the low voltage side of the power supply. All current going out of the secondary side of the transformer needs to go back into the secondary side of the transformer and if you touch a wire along the path, no current should go through you since there isn't a loop for it to go through. Take a look at this simulation. Here too we have ground on the secondary side of the power supply and co current goes through it.
Now to get back to measurement instruments. A hand-held multimeter is often battery powered and is therefore isolated from the circuit it is measuring. This allows you to for example connect the negative probe of the multimeter to the positive pin of the power supply and positive probe of the multimeter to the negative pin of the power supply and the measurement will work, but you'll get negative voltage.
On the other hand, most oscilloscopes are connected to mains power and the ground pin of the probe will usually be connected to the ground pin of the oscilloscope's power supply. On some bench-top power supplies, the negative side of the secondary is connected (or can be connected) to ground. If you for some reason connect the ground pin of the scope probe to the positive pin of the power supply, you can create a short. It will look something like this.