Know your IREDs. Many are designed to be driven at 50mA or more continuously, unlike LEDs of the same size, which usually max out at 20mA. You must read your data sheet. This will also give you information on the dispersion angle.
Know your sensor/lens. Some are designed for visible wavelengths only and have an IR cut filter built in. Commonly available IREDs come in two peak wavelength flavors, 880 nm and 940 nm. Your sensor will be more sensitive to one than the other.
My apologies for covering some information that was included in prior posts.