When I was learning to paint icons I was taught that the halo is not simply an arbitrary symbol, but rather a direct representation, albeit stylised, of the uncreated light shining from the saint.This immediately raised the question in my mind as to the validity of some halos I had that were in the form of a detached floating hoop, as we might see in a Raphael or a Leonardo (whose painting is shown below). Although clearly derived from this original idea, it's form had drifted so that it could no longer be seen as uncreated light, but rather an abstract symbol. Initially, my reaction was to argue that this form indicated a lack of understanding of what the halo really is and should not be used. Then it occured to me that given that the art of the High Renaissance and Baroque is aiming to portray historical man (and not as with the icon eschatological man united with God in heaven), what the artists are doing might in fact be consistent with this. One might propose that because the aura of uncreated light, the nimbus, would not be as visible (to the same degree at any rate) in fallen man, even if that man is a saint. So it would seem that the artist might choose not to portray a halo very feintly, as a slight glow, or even not at all; or else to indicate sanctity with a symbol derived from the heavenly sign. We see each of these possible avenues in the art of the 16th and 17th centuries. As a complication to this, recently I became aware of different shapes of icons in both Eastern and Western traditions. I was giving a lecture at Thomas More College about the portrayal of the Trinity in art and one of the students asked about the triangular halo in this example of an iconographic fresco fo the Trinity at a monastery in Mt Athos. I hadn't really thought about this before and guessed that it was an indication of the Trinity but couldn't really account for it with any certainty. Then, the next slide up in the lecture there was a Velazquez with the same triangular halo portrayed as a detached floating triangle on the same person of the Trinity, God the Father.
Later , when digging around a bit to find an explanation I found this site, which gave lists of many different halos, here. This listed quite a number of traditional halo shapes, most of which I had not been aware of. While not always showing a clear understanding of the Catholic view of things, this is good resource, I think, not to say unusually attractively presented for a website.
So there are two different considerations that come out of this. First, in more naturalistic traditions, should it be retained. And second, should we change the shape of the halo in different situations?
My opinion on the first is that we can happily follow the example set by the Masters of the Baroque tradition and employ whichever solution of the three list the artist prefers, for each, it seems to me, is consistent with the theology.
In regard to the second point: for me the debate is similar to that in regard to all the traditional symbols. Symbolism is only useful if it helps to communicate truth. If only a few understand it, it does the opposite, it mystifies. We have to consider this when considering whether or not to resurrect a symbolic language of the past. So if the symbolism is intuitively obvious then it might be worth using; otherwise we would need a huge job of education just to get people to recogniseit. This effort would be to great to make it worthwhile, I suggest, except where that symbolism is drawn directly from scripture.
In regard to triangular halos: it is not drawn from scripture - I am not aware, for example, of Ezekiel describing visions of triangular halos; but you might say that when placing a triangle over God the Father, in these examples shown, because the known symbolism of three, that it is to large degree intuitively obvious what it is saying, so for this reason might be worth using.
In my own case, while I would not object to any other artist using a triangular halo for the reasons given above, I think am going to keep it simple stay pretty simple on halos: a gold disc for eschatological man, and no halo for historical man. This is just a personal choice based upon what I feel looks best.
From top: A triangular nimbus in an iconographic portrayal of the Trinity at a monastery in Mt Athos, Greece; the halo represented as 'floating' triangle and disc in Velazquez and Leonardo in more naturalistic styles of Baroque and High Renaissance respectively; and the 17th century baroque approach in Guido Reni's St Matthew portrayed without halo of any form.