A lightbulb in a 60Hz mains frequency country will pulse at 120Hz, these discs are intended for use with a handheld strobe light that flashes at a 'true' 60Hz.
There are various mobile phone apps available that allow your phone to be used as a strobe light. You may have better results with a dedicated hand-held strobe light like the one sold by David W Harris here
Why no 50Hz version? 50Hz strobe discs will not give an accurate result at 45rpm.
PDF files of the disc are available here