Intensity Temporal Autocorrelation (G2)

Definition: Intensity Temporal Autocorrelation ( $G_2$ ) or its normalized form, “Normalized Intensity Temporal Autocorrelation ( $g_2$ )”, is the most commonly measured quantity in Diffuse Correlation Spectroscopy (DCS) measurements. Normalized Electric Field Autocorrelation function is often derived from $g_2$ using the so-called “Siegert Relation” which is used to fit DCS data. In Siegert relation, the relationship between $g_2(tau)$ and $g_1(tau)$ is; $g_2(tau) = 1 + beta |g_1 (tau) |^2$ . In this equation, $beta$ is an experimental factor which is equal to 1 for an ideal experimental design.Its formal definition is $G_2 ({bf{r},tau,t) = <I(bf{r}, t) I(bf{r}, t + tau)>$ where $bf{r}$ is the source-detector separation/position, $tau$ is the delay time in the autocorrelation, $t$ is time and $<...>$ indicate ensemble averages.

Alternative definition:

Synonym: Normalized Intensity Temporal Autocorrelation (g2)

References:

https://doi.org/10.1364/JOSAA.14.000192

https://doi.org/10.1103/PhysRevLett.75.1855

https://doi.org/10.1016/j.neuroimage.2013.06.017

https://doi.org/10.1088/0034-4885/73/7/076701

Related terms: Electric Field Temporal Autocorrelation Function, Diffuse Correlation Spectroscopy