LEDs are diodes driven by current. When connecting a small LED you chose a series resistor for limiting its current flow - without it, current would rise due to no resistance and eventually burn up. There are cases in which battery powered LEDs are directly connected to cell, e.g. in a UV-LED light of mine for curing UV-glue - it is assumed that no magic smoke is released when not exceeding assumed current flow by battery (dirty).
As the series resistor is burning current to heat, higher power LEDs are powered by a dedicated driver. This can either be a single IC or a full blown PSU with logic for driving LEDs. Constant current drivers provide power to LEDs connected in series or multiple strings.

lygte-info.dk: LED driver module index
lygte-info.dk: LED flashlight reviews

Constant current drivers

Incandescent lights are easy to dim, as they are slow-acting flow wire/filament (dimming is done whatever you do to the VAC-wave). There are two ways to dim an LED:

1. lower constant current, aka analog dimming
2. let LED flicker fast (with PWM)
3. over voltage No! I said two ways! Look at the curves of voltage vs current of LEDs overlapping. Temperature will even shift the curves slightly. Dimming LEDs via voltage is a side effect, no method!
   

If the LED driver can lower the constant current, it is called analog dimming. When lowering the brightness of an LED via current, the color temperature of the LED will shift (not color correct anymore).
The LED can only be dimmed to a certain point via lowering current before turning off. Analog dimming is not often to find in products as it is more complex, pricier and more restrained than dimming over PWM.

In contrast to analog dimming, the LED current is always at 100% with PWM - or 0% on. The light is turned on and off so fast that our eye does not see it as a flickering strobe anymore. The eye integrates the brightness of ~16ms, which is ~60FPS (frames per second). So letting it flicker on+off (to equal proportions) in that time will result in 50% brightness.
Pulse Width Modulation (PWM) describes the on and off frequency intervals.

Test as a tip: Hold your hand before your head when looking to light source. Spread your fingers and wave fast. If you can see your hand move like in a 90s strobe light party disco, then you should definitely get new lights!

It puts a strain eyes and brain having a low PWM flickering light source, especially if the spectrum is bad too - causing headaches and dry eyes. Epileptics are (even more) sensitive to fast&bright light changes - no wonder there are warnings on games and movies for a reason. Keep aware of what light sources you use. If no analog dimming: The faster the PWM-flickering the better. See https://notebookcheck.com for LCD panel reviews in notebooks with frequency measurements of PWM backlight (don't forget to filer out TN panels (they suck), go for IPS or OLED). For LCD Panels themselves search panel model on https://www.panelook.com/.

RPWM is Random PWM, means that the frequency intervals for dimming have random patters now. It is used with cameras to reduce possibility of flickering on video.

Hybrid dimming combines the goods of both worlds of analog and PWM dimming with the bad of the price.
Example: Dim from 100% down to 12.5% analog, dim with PWM from 12.5% down to 0.5%.
PWM is used for low brightness dimming, which would not be possible so low with analog dimming (also no color shift). For higher brightness, flicker-free analog dimming is used by limiting current.

FYI: See Infineon ILD8150, a hybrid dimming constant current LED driver IC.

- nodimming vs dimming MCU
- dimming in driver vs dimming on signal for lightbulb-MCU to translate it to PWM level
- analogue dimming vs PWM + Infineon 8150X + frequency + strobe-waving-test + slowmo-cam
- forward voltage /current in LED data sheet
- RPWM + “flicker free”
- notebookcheck.de lcd backlight flicker tests
- white LEDs = phosphor gelbe lackschicht auf blauer LED
- color temp + CRI + blue light filter
- light (full) spectrum + defraction grade gitter + CD-Lichtbrechung
- iR: RC, motion tracking, iR camera filter, lichtschranken, ToF sensors
- UV plants + glue curing + schwarzlicht neon colors, UV-LED as cheap UV sensor
- form factors + COB package + alu PCB + creeLED brand marketing names
- RGB + ARGB WS2812 NeoPixel + single-wire-adressing led-string + ambilight
- matrix LED + LED curtains + 7 segment + OLED + HUD-projector
- LED beamforming for projectors or bike lamps: reflector vs lens | flat vs round vs nohead cap | laserZz
- monitor light bar, bathroom mirror light
- diffusor: LCD-backlight panels (and matrix LED sector backlights) | homogene indirect light
- structured panel/diffusor (3D-printed moonlightstructure) + woodoptic display + LED light fibre tubes
- polarizer + stereoscopic 3D - solar powered lights: outdoor | keychain lights | schubladenlicht/schranklicht | Leselampeclip
- history of blue LED: japanese | chronically list RGB LED colors
- DMX | smart home wifi,zigbee whatever funk

- other: VCF-Display MR-303
- how to build a monochrome LCD
- how to chose/upgrade to IPS LCD
- Wackelbilder 3D + infinity-mirror + holographic 3D
- Xenon flash | LED ring-light