So I have two traditional PL desk lamps, both have one 11w PL-S bulb. (Lival Global & Airam Campus)
What I noticed today, is that whenever I turn on or off either one of them, my monitor tends to go black for a second. After that, it turns back to normal. My monitor is an external one (connected to my laptop with a cable), and nothing else is going off. What is causing this? Everything is in the same extension cord and grounded.

Here is another similar situation: once I was in my grandparents' home, I brought this Glamox light with me. Whenever I turned it off there, their television went completely black for a while! Only differences are that they were located in different rooms and not grounded.

Either the lamps or the TV or monitor aren't properly protected against (causing) RF interference.

Any device that creates an electric spark, creates radio waves. That is literally how the radio transmitter was invented. The first radio transmitters were giant transformers and coils, with a spark gap to create tremendously loud sparks. Little sparks always happen when turning on or off a fluorescent lamp with inductive ballasts, and the starter itself also can function as an RF impulse generator.

Any device sold in western countries has to meet quite strict regulations to avoid a simple desk lamp working as a radio transmitter. And conversely, any consumer device needs to be tolerant to RF interference up to a certain level. Cables, however, are not usually held to any standards.

If your desk lamp meets the regulations and only creates a small interference pulse, but your TV is overly sensitive, the TV is the problem.
IF both meet regulations, the cable might be the problem.
If it is 'grey' import from china, that device is highly unlikely to meet any RF interference specifications. That goes for almost everything you buy there, from LED bulbs to RF ballasts to phone chargers. Many aliexpress/temu/amazon products are extremely bad in that regard. Ironically, the charger of my Baofeng amateur transmitter interferes heavily on my AM radios. Meanwhile, i can use my laptop charger right next to them. Both are made in China - that's not the issue - it's the lack of proper engineering and quality control for the grey import ones.

And finally, over the course of decades of use, sometimes interference suppresion capacitors - even the ones that don't just violently burn out like a Wima or Rifa - fail. That has no effect on how well your lamp works, but now it does create significant interference pulses that can interfere with any kind of electronic device, if you're unlucky.

How to solve? Firstly, it's always best to tackle the root cause. Use an interference suppresion filter at the device that causes the interference. If that device already only barely interferes, the TV cable might have insufficient shielding. I've heard HDMI is particularly sensitive to interference, despite being a digital format.

Always make sure that devices that have a grounded plug, are also really plugged into a grounded outlet.

Yup, it's interference.
My monitor displays a horizontal line when turning on the fan, or when switching between the low and high settings, they're both plugged into the same UPS.
Then the desk lamp generates an insane amount of interference that will make the radio only play static, it's an older receiver and the lamp has a pretty crude external power supply, there might be better options that don't cause any interference but this was the only one I could find for the wattage output I need.

Try using different wall sockets.

To me it seems like a [unpublishable] cable with improper shielding or improper connection, the data transfer loses synchronization. The connection uses symmetrical lines internally, but just a LVDS, so with not that much common mode range and barely 200mV signal, so the signal gets corrupted if any spike larger than about 1V gets on the lines. The shielding and mainly laptop-monitor "ground" connection the shielding provides is quite essential there. Normally, without any external disturbance, the link uses to work even without the shield, but it is then extremely sensitive to any disturbance, so there are many cheepeese "cables" that are lacking that shield, as it still appears working normally when "tested".
Or the ground/shield connection could be bad within the connector assembly of the laptop or the monitor.

Thanks for the answers. Apparently, all of my other fluorescent lights make that too.

But now, I came across another interference thing. I recently installed some speakers to my room, and they run on AC power. Basically ALL of my fluorescent lights, INCLUDING the ceiling light, cause interference to the speakers and it can be heard as loud pops. Can that be reduced somehow? Also, once my fridge turns on, a small pop can be heard too.

First thing to try is to add a mains filter to your speakers' power supply. Something like this: https://elektrodump.nl/nl/lichtnetfilters/2442-lichtnet-rfi-filter-4a-250vac.html
This one is meant to build into a device. There are smaller ones too, just make sure they at least have a capacitor-coil-capacitor layout.
Make sure to hook the filter up to a grounded outlet, and if your speakers have it, to connect their ground too.
This will suppress the RF pulses that the various inductive loads create.

Before the filter, you want to put a surge protector. This is to short out any high voltage spikes that may be present on your mains, to extend the life time of the filter. They don't last forever.

I'm assuming active speakers with built in amplifiers here, not standard speakers with a separate amplifier.

In general, if literally all lights and the fridge do it, the issue is your speakers being too sensitive. But it's a fact of life that glowbottle starters and inductive ballasts puke out a ton of spikes and RF when starting. That's why many countries have strict RFI rules. If you have a ham radio operator nearby, you can ask them for help - they typically have a lot of experience with RFI suppression.

You also want to verify that the suppression capacitors in your lamps' starters are still in good condition, or that they're even just there.

There is one important note to make: All things interconnected by anything else than just fiber optic or an Ethernet cable must be supplied from one single point (socket). That means the computer, monitor, those speakers, printer, etc, needs to be all connected into one common extension cord strip fed from a single wall outlet. Otherwise loops formed via mains wiring, mainly if the grounding conductors are connected far away, is an "inviting" thing for all kinds of disturbances. And these "disturbances" not only means spurious glitches or cracks when lights are turned ON/OFF, but also destruction of the equipment if a lightning strikes anywhere near your home (the magnetic field induces high voltage into such loops, this high voltage then fries the equipment).
And the eventual filter must be on the common feed into that extension cord strip.
Definitely do not connect just some devices behind the filter and some somewhere else, that would make the situation even worse.

And make sure all cords have grounding wire in a good order, even when many power supplies do not actually use it for direct grounding, they use it to prevent Y capacitors to pass mains AC leakage to the secondary side.
And do not use questionable quality power supply boxes for the notebooks, printers and so on, they may be lacking the EMC filtering normally mandatory (it is practically impossible to pass EMC limits without them) in such devices, as those filters cost quite some money but are not directly "visible" in the main performance (power, efficiency,...)...

I remember a dentist office I went to in the past that was basically in a house that was repurposed and rezoned for commercial use.

Inside had the typical layout of a house with the standard ceiling heights and such, but modified to be a dentist office with some modifications.

The lighting was a number of 4 & 2 bulb T12 surface mount troffers and wall lights which were 5000K. I don't think the electrical system there was too happy because the lights seemed flickery and briefly dimmed whenever a dentist started up a machine, and all the LCD monitors would go black for a second. 

I haven't been there in over a decade, I imagine it has since been changed with T8 and probably LED by now. This wasn't some sketchy place either, the dentist is a well respected one. The area is mostly modern commercial shopping buildings with a handful of old houses here or there that were there when it was all farmland, and are now used for businesses.

Well, too many words and no concrete recommendations, as usual. i assume original poster uses HDMI cable connection to the monitor. HDMI is particularly sensitive to impulse interferences as it uses weak signals (~800mV) at rather high frequency. Especially with a cheap cable having thin shielding. My advice is just to ignore this (I had this too when connecting my monitor to a Blu-Ray player with a long cable) or to use a better cable, with a thicker shield and preferably interference-suppressing ferrite core molded in the middle. Ferrite core breaks ground loops at high frequency, working as a transformer, and so makes the life in that respect better. You may try to clip a ferrite core / tube on the cable yourself if you have a spare from some other equipment.

Thanks again. I will try to understand things you said here  But here is some more specific information about my devices:

My computer doesn't have an HDMI port, so I use a Mini DP to HDMI adapter. Then, I have a cable with HDMI on one end and then DVI on the monitor.

And the speakers have an external amplifier that seems to be homemade, at least partially. The cord in the right speaker was extended so it can reach the amplifier that is on the left side of my table.


Hope these would help with troubleshooting.

Well, too many words and no concrete recommendations, as usual.

I hate to break it to you, but this is 'real' RF engineering and that's simply not something that you can condense into a 150 character Twitter post or some dumbed down tiktok video.

Thanks again. I will try to understand things you said here  But here is some more specific information about my devices:

My computer doesn't have an HDMI port, so I use a Mini DP to HDMI adapter. Then, I have a cable with HDMI on one end and then DVI on the monitor.

And the speakers have an external amplifier that seems to be homemade, at least partially. The cord in the right speaker was extended so it can reach the amplifier that is on the left side of my table.

Hope these would help with troubleshooting.

The source of the problem is likely non-existent or failed mains filter caps in your lamps and fridge. They are not supposed to interfere that badly. I have a room full of vintage tech, and most of my fluorescents don't interfere on any of them - except for one AC/DC radio.

You can put snubber networks over the switch contacts of your lamps, and over the compressor relay contacts of your fridge: https://uk.farnell.com/roxburgh/re120033/cap-0-033-f-275vac-20/dp/2336107

If you cannot install them yourself, ask your local repair place or electrician. Usually it takes at most 10 minutes to install one, though it depends on how much space there actually is available.

I chose one by gut feel so no guarantees but here's a bit of info about how to 'properly' select them: https://www.digikey.com/en/articles/resistor-capacitor-rc-snubber-design-for-power-switches edit: disregard, that's about a whole different style of switch.

With regards to the speakers and amp - remove the input cable from the amp. Solved? Plug it back in, but unplug the signal source end. Solved? Likely a ground loop issue. Not solved? Bad shielding, or interference entering via the speaker or mains cable.

Still issues? Remove the long speaker cable. If it gets less, then your interference enters via the long section of unshielded speaker cable. You can suppress interference entering via that pathway by looping the wire through a ferrite core like this one: https://nl.aliexpress.com/item/1005004130655236.html
I typically use ferrite or iron powder cores scavenged from PC power supplies, it's not really important which one it is. Today it's not a common pathway for interference anymore, but in the CB radio era it happened quite regularly that interference entered via the speaker output.

No influence? Then add a mains filter like the one i posted before to your amplifier. Sometimes you can find them in power strip form for 'audiophile' use. It won't improve sound quality but it can reduce RF interference. But cheaper is to just use a filter module to build into the amp, if there's enough space.

With regards to the HDMI - ferrite clamps around both ends of the cable may work, but don't expect them to do wonders. They function only on the 'top' part of the RF spectrum, leaving the 'bottom' part unaffected. So if the interference is at the bottom, they won't help.
Better shielded cable might work, but again no guarantees, if the problem is the lamps that are unusually high interference.

I know, this is starting to get off-topic...

Anyways, now my monitor doesn't get any signal anymore! It turns out that the issue is either the Mini DP port on my laptop OR the Mini DP-HDMI adapter. Guessing the latter one.

But when I take the signal straight from another device that DOES have a HDMI port, so no "fancy" adapters are needed, the monitor no longer goes black with my fluorescent lights!

Then my guesses are bad contacts somewhere on the adapter. Makes sense - if that happens, all what the differential nature of the signalling in the HDMI brings, goes out of the window...