A spectrum tells you how an optic responds to different wavelengths. Think of wavelength as the address of a color or infrared signal, and transmission as the fraction of that light allowed through.
01
Start with the axes
The horizontal axis is wavelength, usually in nanometers (nm). The vertical axis may show transmission or reflection in percent. A transmission value of 80% means that about four fifths of the incident light at that wavelength passed through the measured sample. It does not mean the filter passes 80% of every light source.
02
Read the whole curve
A bandpass filter selects an interval; a longpass filter favors longer wavelengths; a shortpass filter favors shorter wavelengths. A notch rejects a selected band. Check secondary passbands as well as the main feature, because a detector may respond outside the band you intended to measure.
03
For the optical designer
Specify center wavelength, bandwidth and the blocking interval separately. Full width at half maximum (FWHM) describes the width at half the peak response; it is not the same as the entire useful passband. Optical density is OD = −log10(T), with T expressed as a fraction: OD 3 corresponds to 0.1% transmission. A low-resolution plot alone cannot establish a deep-blocking specification.
04
Measurement is not an acceptance limit
A sample curve describes a measured sample under stated conditions. Production tolerances, instrument uncertainty, angle, polarization and temperature still matter. On this site, interactive readouts use the nearest measured point and do not extrapolate beyond the supplied range.
05
A practical example
For a camera illuminated by a narrowband source, first locate the source wavelength, then check transmission there. Next review blocking across the rest of the sensor’s response. Finally check whether the actual light cone changes the filter behavior.
Editorial review: 17 September 2026 · Educational principles support product selection; model-specific performance is established by the product data and agreed specification.