Campbell and Robson found that, as their contrasts were increased, a low contrast square-wave grating could be discriminated from a low contrast sine-wave grating when:

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Campbell and Robson's research showed that the ability to discriminate between a square-wave grating and a sine-wave grating at low contrast levels depends on the visibility of the harmonics of the square-wave grating. Specifically, they found that discrimination becomes possible when the third harmonic of the square-wave grating reaches its contrast threshold. This is because the square-wave grating contains multiple harmonics (odd multiples of the fundamental frequency), and the visibility of these higher harmonics (especially the third) contributes to the perception of its square shape.

Campbell and Robson Found That, as Their Contrasts Were Increased