No-lens thermometers (like the Mini-IR and the CZ-IR) 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. No-lens thermometers...
- typically cost the least
- can be even more durable and survive drops even better than fresnel lens thermometers
- are usually smaller and easier to handle than other types of infrared thermometers
- are the most accurate in cold spaces
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 (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.