The 3 Costliest Optical Filter Spec Mistakes

Feb 27, 2026 by IDEX Health & Science

Mistake 1: Specifying Deeper OD (Optical Density) than Needed

Deep OD blocking drives costs. Use deeper OD only in critical regions and limit the blocking range to what your system requires.

• The Problem: Over specifying blocking occurs when you request deep OD (like OD6 or OD8) over an extended wavelength range far from the passband (e.g., 200 nm to 1200 nm).
• The Cost Driver: Blocking depth is directly tied to the number of coating layers. Increasing blocking from OD4 to OD6 can require 50% more coating layers. Furthermore, blocking in the Infrared (IR) range requires much thicker layers than the visible range; moving from 600nm to 1200nm doubles the layer thickness required.
• The Fix: Target your deep blocking where it matters most. For example, specify OD6 only at your specific laser line or excitation source, and allow less blocking (OD3 or OD4) in the regions where your detector is less sensitive or there is no light source present.

Mistake 2: Specifying Tight Transitions & Steep Edges

Tighter transitions increase coating layers and cost. Wider transition widths and less steep edges reduce cost.

• The Problem: Edge steepness refers to how quickly a filter moves from blocking (OD > 6) to transmitting (T > 50%). Engineers may specify ultra-steep edges to get as close to a spectral point of interest as possible.
• The Cost Driver: Achieving a tight transition width is a high-cost driver because it requires additional coating layers and leaves little room for natural variations in the manufacturing process.
• The Fix: Be realistic about your transition width. If your fluorophore has a large Stokes’ shift, you can afford a wider transition width. Every extra nanometer of transition you allow between the blocking region and the transmission region increases yield and reduces the complexity of the filter design.

Mistake 3: Specifying T_abs (Absolute Transmission) When T_avg (Average Transmission) Would Work

T_abs can raise costs. Use T_avg when system tolerances allow.

• The Problem: T_abs requires every single wavelength within a passband to meet a minimum transmission level (e.g., "minimum 95% from 500–550nm"). Natural variations in the manufacturing process can cause a miniscule encroachment into the passband.
• The Cost Driver: Even an encroachment as small as 0.1 nm would fail a T_abs specification, lowering yield.
• The Fix: Use T_avg (Average Transmission) instead. Most systems—especially fluorescence and imaging systems—integrate light over a range of wavelengths. If your system can tolerate a tiny clip of the passband - as long as the total light throughput is high- T_avg will lower your bill-of-materials (BOM) cost.