Understanding optical parameters

PHOSLENS selection notes · Updated 10 October 2026

Read an optical specification as a complete set of requirements: geometry, optical function, material, coating and operating conditions. Matching a diameter or a focal length is a useful first filter; it is not enough to establish that two components will work in the same assembly.

Read the label, unit and reference together

Dia. — outside diameter

The outside size of a circular optic. Clear aperture is the area over which the optical specification applies; do not assume that it extends to the edge. Check the holder fit separately from the usable optical area.

CT and ET — center and edge thickness

Thickness at the center and edge. Both can matter when reviewing mechanical space, seating and mounting. A nominal dimension does not state its permitted variation unless a tolerance is also given.

Focal length, EFL and BFL

Effective focal length is referenced to a principal plane. Back focal length is referenced to the last optical surface vertex and the rear paraxial focal point. They need not be equal. Preserve the sign of a stated focal length and check its reference wavelength and conditions.

Radius of curvature

Read the radius for the identified surface and retain the drawing’s sign convention. Do not assign one stated radius to every surface of a component.

Material and coating

The substrate and surface treatment are separate fields. Record the material designation exactly and identify the coating variant and operating band. A similar material name or an overlapping coating range does not establish interchangeability.

Compare models in a practical order

1. Define the task. State what the component should do, the working wavelength or band, and the available mechanical space. Start with the appropriate lens category or optical window category.
2. Narrow the published dimensions. Use the lens finder for published scalar diameter and focal-length values in mm. Models described by other geometry may require review on their series page.
3. Expand the individual model. The compact comparison table shows selected fields. Open the details to review thicknesses, surfaces, material, coating and model-specific notes before adding a quantity.
4. Record the remaining conditions. If a requirement is absent from the page, carry it into the inquiry rather than assuming that a general series description covers it.

A matching pair of numbers can still describe different choices

Suppose two catalogue rows have the diameter and focal length you need, but show different coating labels. Keep both as candidates until the coating conditions have been checked against your operating band. Then review thickness and mounting requirements. This is a comparison method, not a claim that either row is suitable for your system.

Separate published values from requirements to confirm

A blank field or dash means that the value is not provided; it does not mean zero. A typical value, a nominal value and a guaranteed acceptance limit also serve different purposes. Keep a short list of the information needed to make the selection:

• Exact model, quantity and any acceptable alternatives.
• Dimensions, clear aperture, mounting constraints and required tolerances.
• Wavelength or band, incidence angle, polarization and relevant environmental conditions.
• Any inspection results or documentation required with delivery.

Use the optical inquiry checklist to turn that list into a request. If the catalogue geometry or coating needs to change, start a custom optics discussion.

Check the curve before completing the selection

Where a verified curve is available, check that its material, coating and measurement conditions apply to your candidate. The optical curve guide explains what to look for. Review your chosen models in the quote list before submitting.

Technical references: Edmund Optics — optical specifications; RP Photonics — principal planes and focal-length references. These references explain general principles. PHOSLENS model values and agreed quotation requirements determine the scope of a supply.