Does led work this way? I have tested two LED light bulbs by capturing them through phone's camera(Galaxy Note 8)and both seems to flicker/flash very fast. Is this a problem with my table desk lamps? Why is it not cosntant light? It is direct current desk lamp
There can be several reasons for LED lights to flicker. Most were mentioned in comments above. I’ll summarize them in one answer. (Note that the one actual answer, “All LEDS flicker” is wrong.)
LED brightness is controlled by the amount of current that feeds them. Usually that is controlled with supply voltage.
A cheap power supply can have voltage fluctuations, which will cause the brightness to flicker.
A REALLY cheap power supply might use half-wave rectification, which clips off the negative half-wave of the AC supply, causing the LED to flash on and off with the positive half of the AC sine wave (60 Hz in the US, 50 Hz in much of the rest of the world.) It’s also possible the power supply is low voltage AC, and the LED just blocks the negative half of each cycle. The effect of either of these would be for the LED to be on half the time and off half the time, probably at the frequency of your AC power.
Some LEDs lights control their brightness using pulse-width modulation (PWM). This varies the percentage of on time to off time to change the average brightness. Usually this is done quite fast, and and our eyes perceive the average brightness. If the PWM frequency is high enough (10s or 100s of kHz) it takes a very fast camera shutter-speed for it to show up at all.
Other LED setups like calculator LED displays and other grids of LEDs multiplex a large number of LEDs by lighting them in groups and cycling between those groups rapidly. At any instant only a fraction of the LEDs that appear lit actually are lit.
All LEDs flicker.
Some flicker slow enough that the naked eye can see it, some flicker fast enough that the naked eye cannot see it, but a camera, shooting at 30-60 frames per second will capture some frames when the lights are on and some where the lights are off.
If you or a friend/family member have a newer car with LED lighting (of any sort) film it with your camera, you'll see the same thing.
I ran into the same problem a number of years ago shooting pictures of my son's gymnastics meets. I used a very fast shutter speed (a 1/1000th of a second or so) to stop the action of his routines, then would turn to take a picture of his score displayed on an LED scoreboard. Looking at the scoreboard picture, often less than 1/2 the LEDs would be lit, and I couldn't make out the score. I would have to turn the shutter speed down to 1/10 - 1/20th of a second to get a picture of the scoreboard so the shutter was open long enough to capture all the LEDs lit up to get the complete score.