Vedhathiri
Author
Forrest M. Mims III has done a DIY twilight photometer that estimates the height of the ozone layer by measuring changes in the brightness of the sky before sunrise or after sunset. The project was inspired by the need for a simpler ground-based method of studying the atmosphere. Mims has worked with professional ozone-measuring instruments and wanted to create a smaller and less complicated device that amateur scientists could build and use. The instrument relies on Chappuis absorption, a process in which ozone absorbs certain wavelengths of visible light, especially in the yellow and orange part of the spectrum. By observing how the twilight signal changes as Earth’s shadow moves through the atmosphere, the device can estimate the altitude of the ozone layer’s peak.
The core of the instrument is a small printed circuit board carrying an Analog Devices ADA4530 ultra-low-leakage operational amplifier, a 620 nm red LED, a 5 GΩ or 10 GΩ feedback resistor, and a small feedback capacitor of around 10 pF to 20 pF. In this design, the LED works as a light detector rather than as a light source. The ADA4530 amplifies the extremely small photocurrent produced by the LED, while an Onset UX120 16-bit data logger records the output. The circuit is mounted at the focus of a small telescope or lens system with a narrow field of view. Mims used a high-quality Thorlabs lens and lens tubes for the advanced version, although he says that a low-cost Fresnel lens can also be used. After collecting data for an hour or more during twilight, the readings are processed in spreadsheet software such as Excel or LibreOffice. The maker reported an average ozone peak altitude of 25.13 kilometres from 11 measurements made while preparing the article, close to the 25-kilometre average reported by NASA for his location.
The photometer must also be placed away from artificial lights and carefully levelled before measurements.The device is therefore best understood as an experimental scientific instrument for estimating atmospheric-layer heights, not as a replacement for professional ozone-monitoring systems.
