Hi davidcolonies,

TSER = Total Solar Energy Rejection

IRR= Infrared Rejection Rate

TSER is a more accurate reading to measure the heat rejection because it takes in a few parameters into consideration. Whereas IRR just takes amount of Infrared rejected into account. Infrared is the main component from the invisible sunlight that contributes to heat. That's why most of the window films are developed in a way to cut down the infrared transmitted. The more infrared the film blocks, the better comfort it is.

Hope my answer helps