I found this on the CAA website
http://www.caa.co.uk/docs/33/AIL0168.PDF
For the purpose of improving the conspicuity of helicopters in daylight conditions, the minimum intensity of a white strobe light for it to be effective is considered to be 2000 candela. Anything less powerful will not provide an effective and easily discernible contrast against ambient sunlight and light coloured backgrounds. Therefore in the context of helicopters undertaking pipe and power line survey operations, HISLs are considered to be those of 2000 candela or more. The intensity of other HISLs, fitted as optional equipment for other types of operations, may be chosen at the discretion of the applicant.
Hope it helps you