Handykam (the birdy box camera people) are advertising a "Bird box window", it's a circular clear reflector without the reflective capacity, just opaque. Wouldn't this extra light frighten the birds or make them think the box was unsuitable?
It would depend on the particular garden environment Tony, e.g. do you have cats visiting on a regular basis? If so birds will be unlikely to do nest in such a place, it also depends on the birds themselves since some pairs are more tolerant then others. Nevertheless, the nest box needs to be placed away from the position of the mid-day sun, but in answer to your question some will take to a window box disregarding the light from the room. (See link)
Having cover in the form of shrubbery in the vicinity of the window / box will also help, but again birds will build and raise young in the most unlikely of places e.g. blackbirds, robins, and wrens, often build and raise their young on my archery bosses nesting between the gaps of the long wooden stakes that prevent the boss from tipping over; and in the wooden frame on the top of the boss completely ignoring arrows that strike the front of the boss.
However, just to avoid confusion to the reader mine aren’t the common round straw type, since I make my own archery bosses that vary in size from five x five feet and 8 feet x 10 feet made from layers of sheet foam strapped together within a wooden frame, similar to the one shown in the (link), the only difference being my bosses are made for a fraction of the cost since I have thirty of these placed between 20 and 100 yards apart. These are placed within a woodland sitting and remain in situ all year round, since those purchased from archery centres made from straw aren’t weather proof and simply rot and fall to pieces. Nevertheless, mine do prove to be very attractive to small nesting birds since they completely ignore natural nesting sites and most of the nest boxes I’ve provided.
You’ve probably heard of birds nesting inside the front of traffic lights completely ignoring the noise from the traffic so I suppose it’s a bit like that example; plus the fact traffic lights change colour and brightness.