In mini-spectrometers, gratings – the wavelength dispersive element – generate wavelengths at higher orders than the primary wavelengths to be detected. These higher order wavelengths are fractions of the primary wavelength, and, if the spectrometer covers less than one octave, are spatially separated by angle. If the ratio of upper to lower wavelength is larger than two, the orders begin to overlap and must be removed or sorted out before they reach the detector.   Read More

This short presentation wants to clarify the notion of optical density (OD) and put it into context with signal to noise ratio (SNR). In our daily work as filter manufacturer we often talk to customers with an electrical engineering or imaging background. In these and other fields SNR is an important quantity to describe system or detector performance. Filters on the other hand are described by transmission and rejection performance. The former often on a scale from 0 to 1 (preferably) or on a %-scale. The latter is often described as optical density.   Read More