No-lens thermometers, like the IR Chill and TN2 infrared thermometers, use a reflective funnel design to focus infrared energy against the thermopile rather than a lens. Not having a lens at all offers its own distinct advantages:
Since there is no lens between the electromagnetic waves being emitted by a surface and the thermometer's thermopile, there are no significant contraction or expansion effects on no-lens thermometers. In most units, an internal sensor compensates for the ambient temperature effect on the electronics themselves, so you can literally walk from a warm room directly into a sub-zero freezer and begin taking measurements without waiting.
The important caveat with no-lens thermometers is that their distance to target ratio or DTR is always 1:1 or lower. That means that you should hold no-lens thermometers as close to the target surface as possible when taking measurements. No-lens thermometers are not as well suited for taking measurements at a distance.