Indoor visible light communication (VLC) is considered secure against attackers outside the confined area where the light propagates, but it is still susceptible to interception from inside the coverage area. A new technology, intelligent reflecting surfaces (IRS), has been recently introduced, offering a way to enhance physical layer security (PLS). Most research on IRS-assisted VLC assumes the same time of arrival from all reflecting elements and overlooks the effect of time delay and the associated intersymbol interference. This paper tackles, for the first time, the effect of time delay on the secrecy rate in VLC systems. Our results show that, at a fixed light-emitting diode (LED) power of 3W, the secrecy rate can be enhanced by up to 253\% at random positions for the legitimate user when the eavesdropper is located within a 1-meter radius of the LED. Our results also show that careful allocation of the IRS elements can lead to enhanced PLS even when the eavesdropper has a more favourable position and, thus, a better channel gain than the legitimate user.

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