Short Answer
High Optical Density (OD) values—such as OD 7 or OD 8—are not typically established through direct measurement alone.
Instead, they are supported by:
Measured transmission data (within reliable instrument limits)
Known material behavior and scaling
OD is supported by material science—but applied through safety standards.
The Misconception
A common assumption in the laser safety industry is that OD values—especially higher ratings like OD 6+—are directly measured using spectrophotometers.
In reality:
Most spectrophotometers cannot reliably measure transmission at very high OD levels.
This doesn’t mean OD values are arbitrary—it means they are established using a more complete and practical methodology.
Why High OD Becomes Difficult to Measure
Optical Density is defined as:
As OD increases, transmission becomes extremely small:
OD 5 → 0.001% transmission → 99.999% blocked
OD 7 → 0.00001% transmission → 99.99999% blocked
At these levels:
Instrument noise begins to dominate
Stray light affects readings
Measurement uncertainty increases significantly
So while lower OD values can be directly measured with confidence, higher OD values become increasingly difficult to verify through transmission measurement alone.
What the Industry Actually Does
Because of these limitations, the laser safety industry follows a consistent and practical approach:
1. Measure What Can Be Measured
Spectrophotometers are used to:
Capture transmission across relevant wavelengths
Establish material behavior within reliable OD ranges
2. Use Material Behavior to Extend Understanding
For many laser safety materials—especially absorptive ones:
Transmission follows predictable attenuation behavior
Optical Density increases as attenuation increases
This is where the idea that “OD scales with thickness” comes from.
However, this behavior depends heavily on the material:
The following are general examples—not universal rules. Different materials (including coated or multi-layer systems) may behave very differently.
Two common cases:
1. Some uniform, absorptive materials (e.g., certain types of glass):
Increasing thickness can increase attenuation in a predictable way
Thickness may appear to be the primary driver of OD
2. Engineered laser safety plastics (acrylic and polycarbonate):
The formulation (dye type and concentration) determines how strongly the material absorbs
Thickness does not define it
Why this matters
OD scaling works when materials are uniform and well-characterized
It does not universally apply to all material types
Especially in systems involving coatings, layers, or reflective components
3. Align With Recognized Safety Standards
Laser safety products are designed to meet the requirements outlined in standards such as ANSI Z136.
These standards emphasize:
Matching protection to the specific laser wavelength(s)
Ensuring sufficient attenuation (OD) for the application
Evaluating exposure conditions such as power, beam characteristics, and use case
This means:
Optical Density is a critical parameter—but on its own, it does not define whether a laser safety product is appropriate or safe for a given application.
Why This Approach Exists
This methodology reflects three practical realities:
Instrumentation has limits at very low transmission levels
Materials behave predictably within known ranges
Safety must be evaluated in the context of real laser use—not just a single measurement
Rather than relying on a single method, the industry uses a combination of measurement and material understanding to support OD ratings.
What This Means for Laser Safety
Understanding how OD values are established helps clarify what actually matters:
OD is a useful indicator, but not the full story
High OD values are supported, not always directly measured
Proper protection depends on matching the product to the laser system
LSO Note
Laser safety products should always be selected based on the specific laser system in use. Optical Density values and wavelength coverage must be verified by a qualified Laser Safety Officer (LSO) in accordance with ANSI Z136 guidelines.
Bottom Line
High OD values are not typically established through direct measurement alone.
Instead, they are supported through a combination of measurement within instrument limits, material behavior, and alignment with recognized safety standards.
This approach reflects how laser safety is applied in real-world conditions.