Some models are, some aren't. Check the packet before you buy.
Some LED lights are not dimmable under any circumstances. This is sometimes mentioned in fine print on the box. But if nothing is mentioned about dimming on the box, you should assume the light isn't dimmable.
If dimming is mentioned as a feature of the light, then there are often qualifications. The light is likely to only be compatible with particular types of dimmers. Often on the box, this is just an asterisk, but on the web sites for the reputable manufacturers you can find detailed tables about actual compatibility with specific dimmers and transformers (for the case of low voltage lights).
As a general rule dimmers of the 'leading edge' type are more compatible with magnetic transformers, while 'trailing edge' dimmers are recommended for use with electronic transformers. 'Universal' dimmers are also available, which can change operation from leading-edge to trailing-edge depending upon the load, thus taking a lot of guess work out of your installation.
Care should also be taken not to exceed the maximum load of the dimmer.
"LED lamps can be dimmed with a multitude of leading edge or trailing edge dimmers available on the market. OSRAM provides a list of suggested dimmers for each product"1. The implication being that all of Osram's lights can be dimmed, but this is not the case.
(On the packet). "Dimmable when using leading edge dimmers. Visit www.philips.com/beautifullight to find up to date dimmer compatibility information". This page on the Philips web site doesn't mention anything about dimming at all however. You need to dig deeper.
(Buried on the website). "Philips offers a range of dimmer-compatible LED lamps, with specially designed electronics so that the lamp works on most common dimmers. Given the vast diversity in dimmer types and dimmer quality, a 100% compatibility cannot be guaranteed. Dimmer compatibility issues can include flickering lamps at certain dim level, audible noise(hum), higher light output at low dim level. Also the dimming performance may vary when connected with different transformers"2
Here is an extract3 from the Philips dimming compatibility table for the MASTER LED 7W MR16 light for the Asia pacific region
"The CR Series LED Downlights is designed to be dimmed with standard incandescent type dimmers, and is also compatible with most Magnetic Low Voltage (MLV) and Electronic Low Voltage (ELV) type dimmers. Due to the custom electronic design and the minimum loading requirements of most dimmers, it should generally be treated as a 75W load when sizing dimmers. The recommended list is generated from laboratory testing, and the results can vary in certain field applications due to a number of factors. The absence of a dimmer from this chart does not necessarily imply incompatibility. Please reference the dimmer manufacturer’s instructions for installation"4.
1http://www.osram.com/DIM (Nov 2012).
2http://download.p4c.philips.com/files/8/871829112829800/871829112829800_col_aen.pdf (Nov 2012).
3http://www.usa.philips.com/consumerfiles/pageitems/master/categorypages/enhancinglife2011/assets/pdf/Philips%20Dimmer%20Compatibility%20Sheet.pdf (Nov 2012)
4http://www.cree.com/~/media/Files/Cree/Lighting/Dimming/CR_Downlights_Dimming_Compatibility.pdf (Nov 2012).
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