What is blocking range and why does OD 4 vs OD 6 matter?

When selecting an optical filter, understanding the concepts of blocking range and optical density (OD) is crucial for ensuring your system performs at its best. These terms determine how effectively unwanted light is removed—vital for clarity in scientific and industrial imaging.

The blocking range is the specific band of wavelengths that an optical filter significantly attenuates. In a transmission graph, this appears as a flat region near 0% transmission. Different filters may block light over a narrow or wide range depending on the application:

• Narrow blocking range: Suitable for blocking a single laser line.
• Wide blocking range: Used for applications like LIDAR or fluorescence imaging, where broad background light must be suppressed.

Optical Density (OD) measures how strongly a filter blocks light, using a logarithmic scale:

OD = 2 − log₁₀(%T), where %T is percent transmission.

• OD 4: Blocks 99.99% of light (only 0.01% passes).
• OD 6: Blocks 99.9999% of light (only 0.0001% passes).

The right OD value depends on your application and system sensitivity.

• OD 4 is usually enough for general suppression in many settings.
• OD 6 is essential for applications with strong background light or highly sensitive detectors, such as laser blocking or fluorescence detection.

Combining both blocking range and OD lets you target unwanted light precisely. For example, in flow cytometry:

• Use OD 6 to block scattered excitation laser at a specific wavelength.
• Use OD 4 for moderate blocking at other wavelengths to reduce background autofluorescence.

• Blocking range tells you which wavelengths are blocked.
• Optical density tells you how well they are blocked.
• Choose OD 4 for general applications; OD 6 for higher sensitivity or laser-based setups.
• Specifying both ensures optimal light filtering and cost efficiency.