What changes when you dim a light bulb?

[Lee M]

What happens when you dim a indecent or led light bulb/source? Does the lumens go down? The Kelvin? The wattage?

Any disadvantages to using a dimmer as opposed to a less bright bulb?

Newbie question here and a quick google search wasn't any help.

 

[Josiah Duncan]

Yes, as you lower the wattage, the lumens (brightness) and kelvin will go down. Makes it a warmer or more red source. I can't really speak on behalf of led lights because I haven't tested them all, but thats how tungsten fixtures work.

 

[Marc Wielage]

If it's an incandescent bulb, the color temperature usually goes down and the color values become a lot warmer/yellower. There are special bulbs made that hold their color temperature much more accurately across a wider range of voltage, but they all will change to some degree, high or low. Specific LEDs and HMIs designed for film use will hold their color temperature more accurately.

There's a lot of "it depends" to this question, and these questions are answered in any good book on cinematography and also the art of the gaffer. Gaffers are the unsung heros on a lot of shoots, and they do a ton of really important, difficult work.

 

[ericyoung]

When you dim an incandescent filament bulb like a tungsten lamp, you are sending less electrical current through it, so wattage (volts x amps) goes down, lumens go down, Kelvin colour temperature decreases (goes more red/orange).

When you dim an led light bulb/source, it depends on the method used to dim the LED. A simple dimmer that just reduces the voltage and hence the current going to the LED will affect it's colour temperature and spectrum to some extent (but not as much as for an incandescent light). A Pulse Width Modulated dimmer will not affect the colour temperature/spectrum, but will introduce flicker which may be visible to the naked eye or camera depending on its operating frequency. The higher the switching frequency the less the flicker will be visible. Wattage will go down, the lumens will go down.

Any disadvantages to using a dimmer as opposed to a less bright bulb?
Dimmers will change incandescent colour temperature, and may emit an audible hum/buzz, and may introduce flicker if not well designed.

 

[Lee M]

Thanks everyone, this clears up my question