As optical systems increase in complexity, and optical components companies move upstream in their capabilities, metrology must keep pace. The ability to accurately measure the air gaps between lenses in an optical stack is crucial for ensuring the performance and quality of optical systems. A typical lens stack consists of multiple lenses separated by air gaps, and configurations vary widely, to include simple or achromatic doublets and triplets. Optical spacing or air gaps are carefully chosen and play a significant role in the system's optical performance, affecting factors such as focal length and image quality.
This guide provides a step-by-step approach to measuring air gaps within a lens stack using low-coherence interferometry. Lumetrics’ low coherence interferometers, OptiGauge II and OptiGauge II-EMS, are capable of measuring both lens thickness and distance between lenses with sub-micron precision. The OptiGauge II and OptiGauge II-EMS simplify the complex challenge of measuring multi-element optical stacks.
Lumetrics will demonstrate measurement of a lens stack at Photonics West 2025 in booth 266—lens stack courtesy of ThorLabs.
Step 1: Preparing the Lens Stack
Begin by assembling the lens stack on a stable, vibration-free surface. Ensure that each lens is clean and free of dust or debris, as these can affect the accuracy of the measurements. Use an optical flat to provide a reference surface for the measurements.
Step 2: Setting Up the Measurement System
For this guide, we will use a low-coherence interferometer, which is ideal for measuring the thickness of transparent and semi-transparent objects. The interferometer operates at a wavelength of 1310nm, making it suitable for most optical materials.
Step 3: Calibrating the System
Calibrate the interferometer according to the manufacturer's instructions. This typically involves setting the reference arm length and ensuring the system is correctly aligned. Proper calibration is crucial for obtaining accurate measurements.
Step 4: Measuring the Center Thickness (CT) and Sagittal Height (SAG)
Place the first lens of the stack on the optical flat. Measure the center thickness (CT) and sagittal height (SAG) of the lens. These measurements help evaluate the incoming lenses and ensure they meet the required specifications. Record the CT and SAG values for each lens in the stack.
How to Measure Air Gaps in Optical Stacks with Lumetrics’ OptiGauge
Step 5: Measuring Multiple Lens Thicknesses and Air Gaps
Measure the entire lens stack with the interferometer calibrated and the first lens measured. The interferometer can measure the thickness of each lens and the air gaps between them without needing to adjust the probe's height. This is particularly useful for quality control processes.
For example, a lens stack might include three lenses with air gaps between them. The interferometer will provide precise measurements for each lens and air gap, allowing you to profile the entire stack. Record the thickness and air gap values for each layer.
Step 6: Handling Thicker Lens Stacks
If the lens stack exceeds the measurement range of the interferometer, use a precision height adjustment stage to move the measurement probe up and down. This allows you to scan through the height of the lens stack and obtain accurate measurements for thicker stacks.
Analyzing Your Optical Metrology Data
Step 7: Analyzing the Metrology Data
Once you have measured the lens thicknesses and air gaps, analyze the data to ensure that the stack meets the required specifications. Look for any deviations from the expected values and make any necessary adjustments to the assembly process.
Step 8: Implementing Optical Assembly Process Changes
Based on the measurements, you may need to implement process changes to correct for out-of-spec air gaps. This could involve adjusting the spacing between lenses or making changes to the lens manufacturing process.
Improving Accuracy and Streamlining QC for Multi-element Lens Stacks
Measuring air gaps in optical stacks is a critical task for optical engineers and quality engineers. By following these steps and using a low-coherence interferometer, you can ensure that your lens stacks meet the highest standards of precision and quality. Accurate measurements lead to better optical performance and ultimately, higher-quality optical systems.
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