When specifying an optical filter, you will encounter a critical parameter: Optical Density (OD) . It determines how effectively the filter blocks unwanted light.
Selecting the wrong OD level can ruin your experiment — too low and stray light drowns your signal; unnecessarily high and you pay for performance you don't need.
This guide explains what OD means, provides a practical selection table, and helps you choose the right OD for fluorescence microscopy, laser systems, and spectroscopy.
Optical Density (OD) is a logarithmic measure of how much light a filter blocks at a specific wavelength.
The formula is:
OD = −log₁₀(T)
Where T = transmission (as a decimal, e.g., 1% = 0.01)
This table shows exactly how much light passes through at each OD level. The blocking efficiency tells you what percentage is removed.
| Optical Density (OD) | Transmission (T) | Blocking Efficiency | Plain English Description |
| OD1 | 10% | 90% | Very weak blocking |
| OD2 | 1% | 99% | Basic blocking |
| OD3 | 0.10% | 99.90% | Moderate blocking |
| OD4 | 0.01% | 99.99% | Strong blocking |
| OD5 | 0.00% | 100.00% | Very strong blocking |
| OD6 | 0.00% | 100.00% | Extreme blocking |
| OD7+ | <0.00001% | >99.99999% | Military/aerospace grade |
The correct OD depends on three factors:
Stray light intensity (how much unwanted light is present)
Detector sensitivity (how well your sensor sees weak signals)
Signal strength (how bright your desired wavelength is)
| Application | Recommended OD | Why |
| Basic photography / machine vision | OD2 – OD3 | Ambient light reduction only |
| General fluorescence microscopy | OD4 – OD5 | Blocks excitation light while passing weak emission |
| Raman spectroscopy | OD6 – OD8 | Raman signals are extremely weak (10⁻⁶ of laser power) |
| Laser safety eyewear | OD4 – OD7 | Direct or scattered laser exposure |
| Single-molecule detection | OD6 – OD9 | Maximum stray light rejection |
| Infrared gas sensing | OD3 – OD4 | Moderate background IR rejection |
| Astronomical narrowband imaging | OD5 – OD6 | Blocks moonlight / light pollution |
You illuminate a sample with a 488nm laser (100mW power). The fluorescence emission is at 520nm but only 0.1% as bright as the laser.
Without a filter: Laser light overwhelms the detector
With OD4 filter at 488nm: Only 0.01% of laser passes = 0.01mW leakage
Result: Fluorescence signal (0.1mW equivalent) is now 10x stronger than laser leakage
A 785nm laser (500mW) excites a Raman signal that is 10⁻⁶ times weaker (0.0005mW).
With OD6 filter at 785nm: Only 0.0001% of laser passes = 0.0005mW leakage
Result: Raman signal and laser leakage are equal – barely detectable
With OD8 filter: Laser leakage drops to 0.000005mW → Raman signal is 100x stronger
Conclusion: Weak signals require higher OD.
Different filter types have different OD priorities:
| Filter Type | Typical OD Requirement | Critical Wavelengths |
| Long Pass Filter | OD4 – OD6 | Below cut-on (blocking zone) |
| Short Pass Filter | OD4 – OD6 | Above cut-off (blocking zone) |
| Bandpass Filter | OD4 – OD6 | Both sides of passband |
| Notch Filter | OD6 – OD8 | Center wavelength (rejection band) |
| Neutral Density (ND) Filter | OD0.3 – OD4 | Uniform across all wavelengths |
For long pass filters specifically (our focus here), the OD specification applies to the blocking band – the wavelengths shorter than the cut-on.
Answer these questions to determine your required OD:
| Question | Low OD (OD2-OD3) | Medium OD (OD4) | High OD (OD5-OD6) |
| Signal strength | Strong (e.g., sunlight) | Moderate (e.g., fluorescence) | Very weak (e.g., Raman) |
| Unwanted light level | Low | Moderate | High |
| Detector sensitivity | Low (photodiode) | Medium (CCD) | High (PMT, cooled camera) |
| Consequence of stray light | Minor noise | Reduced contrast | False data / no signal |
At Bena Optics, we manufacture long pass filters with guaranteed minimum OD specifications – not averages.
Our standard offerings:
OD4 – For general fluorescence and imaging
OD5 – For demanding microscopy
OD6 – For Raman and single-molecule detection
OD7+ – Custom orders for military/aerospace
Additional specifications:
Passband transmission: >90% with AR coating
Blocking range: UV to 1200nm (extended options available)
Substrates: Fused silica, B270, N-BK7, colored glass
Custom cut-on wavelengths: 350nm to 2000nm
