How to read optical curves

PHOSLENS selection notes · Updated 10 October 2026

An optical curve is useful only when you know what was measured and under which conditions. Before using a plot to select a filter, lens or coated component, identify the model, quantity on each axis, wavelength interval and test geometry.

Use four checks before comparing curves

1. Identify the quantity. Transmission and reflectance describe different outputs. Read the vertical-axis unit and scale; do not treat a percentage axis and an optical-density axis as equivalent.
2. Read the complete wavelength interval. Check both the useful band and the regions you need to reject. One favorable point is not evidence that the whole operating range meets your requirement.
3. Identify the sample or model. A substrate curve, a coating curve and a finished-component measurement have different scopes. Record the stated thickness, material and coating variant.
4. Determine the status of the data. Distinguish an illustrative or typical curve from an acceptance specification or an individual measurement report. Ask which applies if the page does not say.

Understand the filter labels you are comparing

CWL — center wavelength

The stated center of a bandpass region. It does not, by itself, describe the width or the rejection outside that region.

FWHM — full width at half maximum

The width between the half-maximum transmission points. Half maximum is relative to the peak, not necessarily 50% absolute transmission. Compare bandwidth values using the same definition and stated conditions.

Blocking range and optical density

Blocking needs both a wavelength interval and a transmission limit or optical-density requirement. Higher optical density means lower transmission. An OD value without its applicable band is incomplete for selection.

Longpass and shortpass edge

Confirm the specified transition criterion and the useful transmission and blocking bands; do not assume a step-like boundary at every wavelength.

Browse bandpass filters, longpass filters or shortpass filters to review the published series and model fields.

Check angle, polarization and the incident beam

Interference-filter spectral response depends on incidence angle. Increasing the angle from normal generally moves spectral features toward shorter wavelengths. A converging or diverging beam also presents a range of angles. Use the model’s specified conditions rather than applying a normal-incidence curve unchanged to another geometry.

Record whether the curve describes unpolarized light or a particular polarization. Follow the model’s orientation guidance where supplied; markings and preferred orientation should not be inferred from another manufacturer’s product.

For a material-transmission plot, retain the sample thickness. A plot measured on one thickness is not a guaranteed transmission specification for an optic of a different thickness. Similarly, an average reflectance statement does not specify reflectance separately at every wavelength.

A useful comparison does not need an invented performance value

If your application uses a filter at an angle that is not stated on its page, save the candidate model and ask for its response under your conditions. Do not estimate a guaranteed shift from an illustration or treat an unavailable curve as evidence of performance. For a changed band or geometry, use the custom optics route.

Include these questions in the inquiry

• Which exact model and coating does the curve cover?
• What wavelength band, incidence angle, beam geometry and polarization apply?
• Is the curve typical, a stated acceptance limit or a measured result for an individual component?
• What transmission and blocking are required across your complete operating range?

The Curves tab is shown where a verified series curve is available. Printable catalogue summaries are not manufacturing drawings or measured test reports. Add the model to your quote list, then use the inquiry preparation guide to state any unanswered conditions.

Technical references: Edmund Optics — optical filter terminology; Edmund Optics — orientation, incidence angle and cone angle. These explain general principles, not PHOSLENS model guarantees.