Using Simulation and Liquid Lenses for Early Melanoma Detection

Engineering Solutions 

To test the different lens requirements, Optotune turned to Ansys Zemax OpticStudio software. When using a liquid lens in an optical design, it needs to be dynamic. In this case, Optotune needed to adjust the distance between the patient and the camera module (called the working distance) from 350 to 650 mm, with an intermediate distance of 420 mm. Unlike other lenses, a liquid lens does not need to undergo a large change in focal power (+1 diopter to -1.7 diopters) to adjust the focus according to these different working distances. The change of the curvature of the liquid lens to adjust the focal strength is minimal compared to the adjustment a fixed lens would make in its own operating environment.

The performance of the lens required distortion of less than 1.5% and relative illumination greater than 70%. At all three distances, the lens achieved a distortion rate less than 1% and a relative illumination of 95%, even in the corners. In the image below, the orange dots represent the nominal quality Otoptune wanted to achieve. The red dots represent the minimum production quality. The nominal design is well above the target specifications. However, they were tricky to meet because the required optical quality of the iToBoS is so high.

After determining the nominal design, Optotune simulated the real performance of the lenses. Since liquid lenses have liquid inside, Optotune had to account for the effect gravity would have on the lens. When a lens is held vertically, the liquid inside the lens pools at the bottom, creating what’s known as coma. In human eye, coma is small enough to not affect our vision, but optics are so sensitive that coma can affect resulting images. In most cases, this type of error is negligible, but it does matter to doctors trying to determine if a mole or pigmentation is a melanoma.

Optotune simulated the effect of gravity on their lenses in OpticStudio software. Below, you can see the nominal design results on the left and the impact of coma on the performance on the right. The results are no longer within the accepted limits indicated by the blue boxes.  

To compensate for the effect of gravity on extremely sensitive optics like the iToBoS, Optotune created gravity-compensated (GC) lenses. These lenses use the same technology as regular liquid lens, but they contain two membranes and two liquids instead of one.   

After implementing the GC lens for the iToBoS, Optotune simulated maximum residual coma caused by gravity and maximum allowed astigmatism of the lenses. While the GC lenses compensate for nearly all gravity-caused coma, there will always be some left over due to manufacturing tolerances. Optotune lenses have incredibly low high-order aberrations, but some magnitude of low-order aberrations like astigmatism will appear. After simulating this worst-case scenario in OpticStudio software, the Optotune team determined that the lenses still meet the minimum acceptable parameters for all three distances, as seen below. 

Benefits

• Accurately simulated working distances for three different parameters
• Determined the effect of gravity comma on the liquid lens and how to compensate for it to meet the required specifications
• Compared the simulated results from OpticStudio software and actual results from scanner and the results were comparable