Birefringence is an app that calculates an Michél-Levy interference color chart. As soon as the app is opened, the chart is calculated and displayed.
The app will work on both iPads and iPhones, I recommend iPads in order to get the best of the app. The app runs on iOS 10.3 or higher.
Tap on the point that corresponds to your observed color and sample thickness. The app will calculate the birefringence and suggest candidate minerals.
To clear selections, tap in the area outside the chart.
Given that the chart is interactive, you may find it clearer to hide the radial lines. Click "simple" to do this.
You can change the range of the chart, and offset by + or - lambda. In the shot below the range has been set to 551 nm (lambda), and offset by +1 lambda. The second order colors are shown.