Incremental Index and Comparing different Scores from different Index

Hi,

I have some questions about the scoring function and about how different
scores can be compared.

I use Lucene for indexing an archive of the web.

/archive/day1/differentsites => indexday1
/archive/day2/differentsites => indexday2
/archive/day3/differentsites => indexday3
/archive/day4/differentsites => indexday4
/archive/day5/differentsites => indexday5

No I have a continious query for a certain topic.

Querying indexday1 gives me some Hits with the best having score a
Querying indexday2 gives me some Hits with the best having score b
and so on .....

Now how can I compare those scores, since the regular idf will be
different in the formular of every index?! Am I right here?

I ve read about the incremental idf. Is there a possibility to normalize
the scores so that different indexes/scores can be compared

My Goal is to plot the movement of the score a query has over the time. 
So when I ll look at the Hits on a certain day, I ll must be sure that
all the Hits are out of the day's archive.

I hope you understand my problem and can help me.

Thanks 

Nils