Hello MicaToolbox team! I’m part of a group using multispectral photography to match bird plumage to the avian visual spectrum. We want to make a UV-capable chart with artist pastels, as has been detailed on these forums. However, on the page describing chart-based cone-catch models, it’s stated that the “measurement needs to be incredibly accurate, and a standard spectrometer probe is not sufficient.” Is this an issue of optical resolution? Do you have a sense of what probe specifications would be sufficient to get accurate measurement of pastels for use as a color chart? I understand that you used a Jeti Specbos 1211 UV spectroradiometer. Is it worthwhile to get access to one of these machines if an Ocean Insight spectrometer isn’t sufficient? Thank you!
An Ocean Optics spectrometer will be fine. Ultimately the key here is to measure the radiance reflected from each pastel in the same way as the camera itself will be measuring the same radiance. This means:
- Using the same light source between spec and camera (ideally the sun on a clear day, failing that some kind of very stable broadband light source a decent distance from the surface so that each pastel receives equal irradiance). e.g. light source coming in from an angle, and spec and camera measuring from directly above. Doing this ensures you’re controlling for not-perfectly-diffuse (Lambertian) surfaces, and any fluorescence.
- Ideally the spec probe should be far enough away from the pastel surface that it doesn’t interfere with surface lighting. A collimating lens would make this easier and more accurate than a bare probe. The Jeti Specbos is nice because it measure light from a very narrow, easily defined angle. Adding a collimating lens to an Ocean optics spec will do the same, but you don’t get the nice laser pointer to show measurement angle. Using an integrating sphere might work well, but actually this isn’t how the camera would measure the same surface, so a collimating lens is actually best.
- Work fast! No lighting is perfectly stable, and ocean optics spectrometers are awful at maintaining their dark point in my experience. So take regular white standard and dark point measurements. Ensure the spectrometer is “warmed up” to a stable temperature. Tell it to take multiple exposures (many tens ideally, until there’s no noise).
- Take repeat measurements and get an idea of repeatability error in the spectra. If there’s much difference then take more measurements until you’re confident in the setup. Unlike normal reflectance spectrometery in this case you really need to ensure the absolute intensity is correct (you’re not just measuring the shape of the curve, but also the height of the curve as accurately as possible).
- Photographing the pastel chart is much easier than spec measurements, but make sure you use the same white/grey standard(s) between spec and camera, without anything moving ideally.
Hope these tips help!