Thanks to everyone for chiming in. I am still doing some tests but getting nowhere, I see the thin black lines even when trying both fluorescent and LED tubes at same time. If I turn off LED tubes the lines disappear. So kind of sure the LED tubes are causing the issue. It is a bit tough to do testing because the brightness from the fluorescent and LED tubes are about the same. So not 100% sure if that test is worthy.

 

I wonder if it's some kind of electrical interference from the LED tubes that's messing with the scanner sensor? Maybe try some metal shielding mesh (grounded). Or run the LED tubes on isolated DC power.

 

I will give that a try.

 

it seemed weird about the 'flicker' detection. If you bother to get a microcontroller, e.g. Arduino, and all the rest Rpi Pico, RPi, offerings from Adafruit, stm32duino (ST's Nucleo boards, or those on Aliexpress) etc, you can 'flicker' your leds at a software driven rate.

Otherwise, there is the plain old 555, which you can use variable resistors to vary the rate.
https://www.ti.com/lit/ds/symlink/lm555.pdf
https://www.allaboutcircuits.com/tools/555-timer-astable-circuit/
https://www.allaboutcircuits.com/textbook/experiments/chpt-8/555-ic/
the 555s are cheap these days
https://www.aliexpress.com/wholesale?catId=0&SearchText=lm555
https://www.aliexpress.com/wholesale?catId=0&SearchText=ne555
on online flea markets, just be wary it could be a gamble (but it is rather likely to get the real ones, get from the more popular vendors, check the sales and reviews). otherwise, they are there from the common sources, digikey, farnell (element14.com etc), mouser etc.

you would likely need to couple the output of the 555 (or microcontroller) with some transistor driving the leds if the currents are large. if the currents are below 1A, i'd guess bi-polar (e.g. npn transistor) is likely adequate, for higher currents, mosfets could do a better job. Bipolar ones is easier to drive, Vbe is normally as little as 0.7v to turn on the transistor. For mosfets, you'd need to find ones (e.g. logic level ones) that your signal voltages can turn on the transistor. I'd think 555 has less problem with mosfet, it can be powered with voltages up to 15v for LM555.