No product can be guaranteed to stop all collisions in real world conditions. Glass can vary in appearance from one façade of a building to another or with time of day, depending on solar angle, reflected environment and other factors. The Threat Factor number is determined under standard conditions and does not capture that variety. Instead, Threat Factor is a relative index, and products with lower TFs are considered more effective at preventing collisions. A TF of 30 suggests that collisions will be reduced by at least 50 percent and is ABC's upper threshold for recommending a product. Products with UV patterns should be considered carefully for some applications: Materials with ultra-violet patterns will vary in effectiveness with the amount of UV light present. This means that when the UV index is low – under low light conditions in early morning or evening or if the glass is shaded, or when it is overcast – UV patterns will be less visible to birds. In addition, not all types of birds can see UV. For example, songbirds can see UV, raptors and pigeons cannot. Because Threat Factors can be effected by low-e coatings, published Threat Factors apply only to the glass assembly tested.