LEDs do not dim linearly: when dimmed to a linear 50%, they appear only slightly less bright than at 100%.  The level must drop to 25% to appear at half brightness.  To make LEDs appear linear, they must be controlled with the Inverse Square Law curve.  All good quality LED dimmers use this technique.  RC4 also provides Digital Persistence (DP) to soften the rise and fall of LEDs when they dim.  ISL Slow adds this feature to basic ISL response, making LEDs behave like traditional incandescent lamps.