A halation is the result of relatively bright points of light projecting through the semi-transparent film and then bouncing off a film pressure plate through the layers. Halation occurs to some degree on all films, which is why manufacturers usually include an anti-halation layer with their film (famously, Kodak Vision 3’s remjet: analog.cafe/r/how-to-remov…). Most films don’t use remjet; instead, their anti-halation layer is either dissolvable or integrated deep into the emulsion.
Halations can occur on black-and-white and colour films. Colour films without the remjet layer, like CineStill 800T (analog.cafe/r/cinestill-80…), and, notably, Harman Phoenix.
Harman must have experience with anti-halation layers (since they produce all Ilford-branded black-and-white films, which use anti-halation), which suggests that excluding this feature is a deliberate choice.
The halation effect has been a hallmark of all CineStill’s colour-negative products, and it now appears to be a permanent feature of Phoenix films.
Curiously, the halos on Phoenix films are somewhat orange, whereas CineStill’s are red. This may have something to do with the colour of the film base (CS is orange, Phoenix is purple).
Dmitri