In order to model the optics correctly, we must understand the reflectivity (and transmissivity) of the various components. The behaviour of the photocathode material is complicated and will be described elsewhere. The other reflective surfaces of interest in the phototube are the glass envelope, which is well described by Fresnel's laws, the reflective back hemisphere and the dynode stack. The back hemisphere surface is a thick layer of aluminium, and so can also be modelled by Fresnel's laws; the stack is more difficult. The former will be discussed below, whilst the latter is taken from [12], but first, we will consider the reflectivity of the petals.