Heatsink 6 LED's at 350mA

I have been wondering maybe someone can help me out.. I how can you cool 6 LED's but getting the heat away from the LED's in a enclosed fixture that's mounted on a wall outdoors? I would assume it needs to be passive cooled?

The heat needs to go somewhere.

A heatsink inside a typical sealed fixture will simply prevent your LED's from burning up immediately, with the LED temperature ultimately dependent upon how much heat is transmitted to the outside environment. Generally, this temperature will exceed guidelines for your LED's and they may live very short lives.

Thermally coupling the LED's to the fixture would be better, but most fixtures designed for incandescent bulbs by design are thermal insulators since the exterior of the fixture ideally remains cool - and doesn't transmit any significant heat to the oftentimes plastic J-box in the wall or ceiling. As such, I would not assume that the typical fixture has any appreciable heatsinking capabilities of itself.

Six LEDs at 350 mA isn't a lot of heat, only 6 watts or so. For a small plastic enclosure, that might be a lot, but if the enclosure is metal, or larger, it's probably doable by mounting the LEDs on a heatsink inside the box. Of course, it depends on the outside temp. If it's going to be on when the sun is beating on it, and might get to 150 or 200F, nothing is going to work very well.

I would recommend you get a meter with a thermocouple and mount close the the leds and outer housing, place the entire fixture in a bucket of sand and run for 5-10 hours, you will get a very good sense of the thermal performance, but as mentioned you need a thermal path to the outer body

I'm curious, what's the bucket of sand for?