How to Read an Optical Filter Transmission Chart

A filter chart is a map of how much light passes through at each wavelength. Once you know what to look for, it becomes much easier to compare products without relying on their names alone.

Keep one question in mind as you read: does this filter pass the light I need while sufficiently reducing the light I do not want?

Read the two axes first

The horizontal axis shows wavelength, usually in nanometers (nm). Moving right means a longer wavelength. The vertical axis on a transmission plot shows the percentage of incident light that passes through the filter at that wavelength.

For example, 80% transmission at 600 nm means approximately 80% of the light at 600 nm passes through under the measurement conditions. It does not mean the filter transmits 80% of every lamp's total output.

Try a measured KUPO spectrum

The interactive plot below shows the measured BBP-626 sample. Tap or point at the curve, or enter a wavelength, to inspect the value. Notice both the main transmission band and the response outside it.

This is a measured sample curve, not a guarantee for every order or a prediction at every angle. Use the product's stated measurement information and agree the required production specification with KUPO.

Find the useful band, not just the highest point

The peak is the highest transmission on the curve. Your light source may cover a much wider range. A filter with an impressive peak can still remove a substantial part of a broad source's output if their spectra do not overlap well.

Look at the width and shape of the main band. Then inspect the rest of the plot. A secondary band is important when your detector can see it, even if it is far from the wavelength printed in the product name.

Understand what “half maximum” means

Full width at half maximum (FWHM) is the distance between the two band-edge wavelengths at half the peak transmission. In a simple example with an 80% peak, the half-maximum level is 40% transmission. It is not automatically the 50% line on the vertical axis.

A cut-on or cut-off specification may instead use an absolute transmission level, often 50%. Read the stated definition before comparing two numbers. Center wavelength, peak wavelength and a wavelength midpoint are also not interchangeable without a definition.

A line near zero cannot prove deep blocking

On a 0–100% scale, very small transmissions become hard to distinguish. A visually flat baseline does not establish a particular optical density. Deep-blocking assessment needs an appropriate scale and measurements with sufficient sensitivity.

Likewise, blank space beyond the plotted interval is unknown performance, not zero transmission. Always compare the measured range with the full wavelength response of your detector.

Compare like with like

• Use the same wavelength and transmission scales.
• Check the angle of incidence and any stated polarization or temperature conditions.
• Distinguish a measured sample from a specification limit or calculated design.
• Ask whether a quoted transmission is a peak, minimum or band average.

Explore KUPO broad bandpass designs. If a curve looks suitable, send us the product code and your useful wavelength range; we can discuss whether it is an appropriate starting point.