Iris Recognition Camera Module Using Holographic Optical Element

The optical system is divided into multiple lens groups (first lens group, second lens group, third lens group) with different functions. The first lens group handles initial light convergence, the second lens group provides negative refractive power for compactness, and the third lens group fine-tunes the optical path. This segmentation allows achieving long focal length without proportionally increasing the overall optical path length and camera volume.

The patent employs a nested arrangement where lens groups are positioned within a compact lens barrel structure. The multiple lens groups are arranged concentrically along the optical axis within the confined space of the lens barrel, allowing the long focal length optical system to be housed in a relatively small volume camera module.

The patent uses materials with different thermal expansion coefficients for different lens groups. The second lens group, made of material with higher thermal expansion coefficient, compensates for the thermal contraction of the first and third lens groups when temperature decreases. This parameter change approach maintains the overall focal length stability across temperature variations without requiring active adjustment mechanisms that could degrade image quality.

The patent converts the harmful effect of thermal expansion and contraction into a beneficial compensation mechanism. The differential thermal expansion of the second lens group (with higher expansion coefficient) counteracts the contraction of other lens groups, transforming temperature-induced dimensional changes from a source of error into a self-compensating mechanism that maintains focal length stability and image quality.