Photometric compliance of standard and digital infant acuity tests

Lock-in amplifiers (LIA) are currently used for signal recovery in the presence of high noise [1]. They are based on low - pass filtering the product of a sinusoidal signal by a reference sinusoid with the same frequency, effectively calculating the zero - time correlation between sinusoid and signal (Figure 1). To ensure that the frequencies of signal and reference are the same, in a LIA signal and reference are phase - locked. We note that LIAs can be sensitive to narrowband noise with a frequency closer to the reference frequency than the bandwidth of the low - pass filter. In this respect, generating a signal synchronous with a pseudo - random reference, and detecting it by correlation with the sequence itself, offers promise of a better noise immunity [2]. In this work we investigate a computer sound card as an acquisition system for the extraction of signal by correlation with a pseudorandom sequence.