Number of photons that would be detected by the Nancy Grace Roman and HabEX space telescopes (apertures 2.4 m and 4.0 m, respectively) from a white, Lambertian reflecting, Earth-sized planet in a 1 au orbit around a solar-type star at α = 0°, in optical spectral bands with widths of 50 nm for a 3.0 h observation. These lines can be regarded as upper bounds for the number of photons from an Earth-twin (the lines for the 500, 550, 600, 700, and 800 nm bands are very close together). The signal-to-noise ratio is proportional to .