The first reference for takeoff above 150m RVR is in JAR 1.430 Table 1 note iii)
"The reported RVR/Visibility value
representative of the initial part of the
take-off run can be replaced by pilot
assessment."
The reference regarding takeoffs below 150m is in JAR 1.430
"(4) Exceptions to sub-paragraph (a)(3)(i)
above:
(i) Subject to the approval of the
Authority, and provided the requirements in
paragraphs (A) to (E) below have been
satisfied, an operator may reduce the takeoff
minima to 125 m RVR (Category A, B
and C aeroplanes) or 150 m RVR (Category
D aeroplanes) when:
(A) Low Visibility Procedures
are in force;
(B) High intensity runway
centreline lights spaced 15 m or less
and high intensity edge lights spaced
60 m or less are in operation;
(C) Flight crew members
have satisfactorily completed training
in a [Flight] Simulator [ ];
**(D) A 90 m visual segment is
available from the cockpit at the start
of the take-off run; and
(E) The required RVR value
has been achieved for all of the
relevant RVR reporting points." **
Reference
http://www.jaa.nl/section1/jars/443844.pdf