Focus Shift on Long-Range Thermal Lenses
Infiniti WhitepaperLong-range thermal lenses can experience focus shifts over time due to several factors related to thermal optics and environmental conditions. This is why Infiniti’s thermal cores have a built-in autofocus timer to correct any shifts without the need for manual user intervention. This document explains the factors beyond our control that contribute to focus shifts, particularly in larger long-range lenses.
Thermal Expansion and Contraction
Material Properties: Thermal lenses are typically made from materials that expand and contract with temperature changes. When the lens heats up or cools down, its shape and size can change, causing the focal length to shift. Larger lenses will experience a greater shift in size due to thermal expansion, which can alter the focus point more than what is experienced with more compact lenses. Our uncooled 310mm f/1.3 thermal lens is very large, for example it will experience up to 5–10 times the focus shift of a smaller 105mm f/1.6 lens. Unfortunately one of the tradeoffs of longer-range lenses is that larger apertures are required to maintain a clear image, which does make focus shift more of an issue.
Temperature Variations: Over time, environmental temperature fluctuations cause the lens to continually expand and contract. This can lead to a gradual but significant shift in focus, especially in long-range applications where precision is critical.
Depth of Field and Aperture Size
Depth of Field (DoF): Depth of field refers to the range of distances within a scene that appear acceptably sharp in an image. A larger depth of field means more of the scene will be in focus, while a smaller depth of field means only a narrow plane of the scene will be in focus.
Aperture Size: Lenses with larger apertures (smaller f‑numbers) have a shallower depth of field. In long-range thermal lenses, a large aperture is used to gather more thermal radiation which improves image detail by reducing the amount of noise. This is critical with uncooled sensors at long ranges, due to the smaller amount of light/thermal energy collected with a narrower field of view.
Impact on Focus Shift: With a shallower depth of field, any slight focus shift due to thermal expansion or environmental changes becomes much more noticeable. Small changes in the lens shape or position can cause the focus to move out of the critical plane, leading to a blurred image. Again, this issue is more present the more powerful the lens is (longer focal length) and especially the larger the aperture (smaller f‑number).
Air Density Fluctuations
Refractive Index Changes: The refractive index of air changes with temperature. Warmer air is less dense and has a different refractive index than cooler air. This variation can alter the path of light rays passing through the thermal lens, affecting the focus.
Thermal Gradients: In long-range applications, there may be significant thermal gradients (temperature differences) in the atmosphere between the lens and the target. These gradients can bend light rays (a phenomenon known as atmospheric refraction), further contributing to focus shifts.
Heat Dissipation
Internal Heating: Thermal lenses will absorb some of the infrared radiation they are designed to focus. This absorption can cause internal heating, leading to non-uniform temperature distribution within the lens. Uneven heating can cause localized expansion, distorting the lens shape and shifting the focus. This effect is harder to manage with zoom lenses as opposed to a fixed lens, since zoom lenses require more complex designs that incorporate more lens elements than simpler fixed focal length lenses.