A new non-destructive method is presented for obtaining true resistivity (ρ), mobility(μ), and electron concentration(n) topography on photoexcited, semi-insulating GaAs. The method is based on the use of two perpendicular light slits, which join four removable In contacts on the periphery of the wafer to form a classical Greek-cross configuration. By placing contacts all around the periphery the whole wafer can be mapped. We give results for 1.1 μm photoexcitation on a 3′ low-pressure, liquid-encapsulated Czochralski wafer and compare with EL2 results on the same wafer. A by-product of the analysis is the determination of electron lifetime. Finally, the possibility of nondestructive dark electrical topography is discussed.