Where did the light go?
An idealized learning tool. These shapes and values are not KUPO product measurements.
A narrower band keeps less of a broad source
Source: uniform 830–870 nmFilter: ideal 100% passband
50.0% of source light retained20.0 nm overlap out of a 40 nm source band.
The source is deliberately rectangular to make the calculation clear. Real LEDs have shaped spectra. This model omits detector response, non-ideal edges, polarization, angle and background light; it does not calculate image quality or signal-to-noise ratio.
Optical density sets the remaining fraction
25.119% transmissionTotal OD 0.60 · 1.99 stops · 3.98× exposure factor
T = 10−OD. Densities add for this simple same-wavelength model. Real filters vary with wavelength; polarization and multiple reflections can alter a stack's behavior. The exposure factor assumes an otherwise unchanged, linear system.