Now my doubt is that if I install this circuit for switching my outdoor light, should the relay fluctuates in the transition time? I have a doubt that relay may toggle abnormally for some time in transition period, because I think the illuminance doesn't suddenly decrease at point of transition.
There's a fairly easy software fix called [u]hysteresis[/u].
Here's a made-up example of how it works...
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Let's say your light level is 0-10 where 0 is dark and 10 is full-brightness.
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You program the light to come-on when the light level falls to 4, but it doesn't switch-off until the light level comes back to 6.
Heaters & air conditioning systems all use hysteresis and sometimes they call it "swing" (i.e. 1 degree of swing).
You could also add some delay so that every time it switches, nothing can change for some period of time.