Choosing The Correct Transmission Sphere

T-sphere Calculator

This tool helps you determine the most suitable transmission sphere (t-sphere) for testing spherical optical surfaces using a Fizeau interferometer. By entering your sample’s outer diameter (OD) and radius of curvature (ROC), and selecting the surface type and aperture size, the calculator evaluates all available t-sphere options and recommends the best match based on fill factor, focus compatibility, and physical constraints.

How It Works

Fill Factor: Measures how well the t-sphere beam fills your sample’s aperture. The ideal fill factor is as close to 100% as possible without exceeding it.

Focus: Calculated using the thin lens equation. A t-sphere is considered “Good” if the focus falls within the interferometer’s acceptable range.

Physical Constraint: Ensures the sample’s ROC does not exceed 80% of the t-sphere’s working distance.

Instructions

1. Enter OD (mm): The outer diameter of your optical sample.
2. Enter ROC (mm): The radius of curvature of your sample. Positive numbers only.
3. Select Surface Type: Choose whether your sample is convex or concave.
4. Select Aperture Size: 6″ (150mm) for larger 6-inch interferometers & 4″ (100mm) for standard 4-inch interferometers
5. The R# (ROC ÷ OD) will update automatically.
6. Click Calculate to view a comparison of all compatible t-spheres.
7. The best option for you will be highlighted in green.