6W -10W LED Dimming through SPI control

I need to slowly DIM and slowly make a LED brighter. The LEDs are ceiling lights may be in the range of 6W-12W. it is having a SMPS already there.
So I plan to introduce a MOSFET between the SMPS and LED which in turn will PWM'ed from a Microcontroller. This is my plan and I tried it, but the dimming is not smooth especially in the extreme low brightness area.

is there any commercial modules which can be interfaced to a micro controller for dimming leds?

1. is there any specific LED dimming ICs which can be controlled from a micro controller via SPI or IIC?
2.if we are doing PWM to dim and led using Micro controller what should be the optimum pwm period to get a smooth effect?

Thanks

edit: even if i tried the lowest possible pwm duty cycle, the led cannot be made to off in a linear fashion. is it because of the nature of LED lights or problem with MOSFET switching?

At what point does the dimming become undesireable?

Did you measure the DC along the way?

What is the frequency?

Note: the dimming might well be linear in lumens or other units BUT it can appear non-linear to the eye

In the extreme low brightness area, it doesn't appear slowly fading. frequency is around 1khz. the led suddenly seems off when duty cycle changes from lowest division 1 to 0. I doubt it may be because of the nature of LEDs emission characteristics.

that is the keyword.........................appears

your vision is non-linear

measure with a light-meter.

Try making the control non-linear. If you don't have a LOG function you can use look-up table and spline between points, else there is an equation you can use.