Why Light Dimmers Get Warm and Measuring Dimmers Efficiency

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Light dimmers often feel warm to the touch even when they are used with a standard light fixture. Light dimmers are over 95% efficient, with the remaining 5% dissipated as heat. Watch this video to see an example of a fixture that uses 200 Watts and dissipates 10 Watts of energy.
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[@SteelBlueVision]

Thank you for the wonderful presentation. Not only do these forward phase control based switches get hot from their (mild) inefficiency, but they also introduce a significant amount of noise (which you can see as the over and under-shoot in this video) back into the main lines. This is highest when the waveform is chopped at just over the 50% mark ( 2:26 ).
Sensitive audio equipment (e.g., power amplifiers and active monitors containing power amplifiers with poor line conditioning) will pick up this noise and make it audible (as a buzz at 120 Hz). There are ways to reduce this noise, such as in-line coils on the mains in series with the switch, as well as using a laminated core EI (and not toroidal, which would couple noise from primary to secondary and not handle DC offset, well) isolation transformer in front of sensitive audio equipment to smooth the noise peaks.

[@ManishKumarIITG]

You got it all wrong. Don't focus on efficiency. Focus on the input minus output power instead. This balance power is getting dissipated in the dimmer and causes the heating. When you reduce the brightness, the efficiency decreases but so does the input power. Dimmer should get hotter at higher brightness rather than at lower brightness.