Understanding and Measuring Visibility in Turbid Waters: Protocol and Results
In many underwater applications—such as infrastructure inspections, scientific observations, object detection, or simple visual documentation—operators face a major challenge: reduced visibility in turbid waters. This turbidity is caused by suspended particles, originating from stirred-up sediments, mud, currents, or human activities.
Understanding, measuring, and reproducing these low-visibility conditions is essential to reliably assess the performance of underwater imaging systems. To address this challenge, a dedicated experimental protocol was developed to quantify the impact of turbidity on visibility and to objectively compare different camera technologies.
How Is Turbidity Measured?
The most commonly used unit to characterize turbidity is the NTU (Nephelometric Turbidity Unit).
Historically, early measurements relied on simple visual methods, including the well-known Secchi disk, invented in 1865. This black-and-white disk was gradually lowered into the water until its pattern could no longer be distinguished by the naked eye; the corresponding depth served as an indicator of water clarity.
Today, measurements are far more precise thanks to turbidimeters, which operate by emitting light and analyzing the reflected signal.
Here are some typical turbidity levels:
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Tap water: below 1 NTU
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“Typical” marine environment: 1 to 4 NTU, with visibility greater than 1 m
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Harbors, rivers, estuaries: 4 to 8 NTU, where visibility is often reduced to around 50 cm
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Above 8 NTU: conditions become highly challenging for most optical systems, making inspections difficult. However, ORPHIE cameras are able to maintain sufficient visibility up to 16 NTU.
It is within this context that the i2S teams developed a protocol to measure, under controlled conditions, how visibility evolves with turbidity, by comparing an underwater camera with and without Orphie technology.
Creating Controlled Turbidity
The first step was to identify a reliable and reproducible method to increase water turbidity in a test tank.
The trials demonstrated that the gradual addition of muddy water made it possible to achieve controlled turbidity levels representative of real-world conditions.
A turbidimeter immersed in the tank ensured precise monitoring of turbidity throughout the tests.
Measuring Visibility
Visibility was evaluated by measuring the contrast obtained on a black-and-white target placed at various distances.
Key elements of the protocol include:
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A bar-pattern test chart used as the reference for contrast measurement
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The target included several bar sizes; contrast was measured on the pattern whose image size corresponded to 10 pixels per line pair
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A 10% contrast threshold was adopted as the visibility criterion, in line with industry practices: below this level, the distinction between black and white is no longer operationally usable
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In addition, visibility below 0.5 m was considered non-operational for safe inspection. In practice, approaching a submerged structure closer than 50 cm poses a risk both to the infrastructure and to the operator
Modeling: A Curve Derived from Secchi’s Work
The collected data were modeled using a curve proportional to that used to characterize human eye visibility, derived from Secchi’s work. This curve expresses visibility as a function of turbidity.
This modeling approach simplifies the interpretation of results: for a given NTU value, it enables a direct comparison between the visibility distance of a standard camera and that of a camera equipped with Orphie technology.
Measurement Campaign and Certification
The test campaign was conducted in September 2025 in a seawater tank measuring 5 meters in length and 1 meter in depth.
A vision expert appointed by Bureau Veritas was present to assess and certify the implemented test protocol. This oversight ensures that the measurements were obtained using a rigorous and reproducible methodology.
Results: Performance Doubled in Turbid Conditions
The results show that:
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A standard underwater camera enables safe inspection up to approximately 8 NTU
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The ORPHIE camera maintains usable visibility up to at least 16 NTU, i.e. in conditions that are twice as turbid
This ability to operate in severely degraded environments offers several advantages:
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Fewer mission postponements, with fewer cancellations due to poor visibility
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Time savings and reduced costs associated with vessels waiting offshore for improved conditions
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Safer inspections, thanks to better anticipation of obstacles and structures
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Extended operability in environments previously considered too turbid
A Step Forward for the Underwater Inspection Industry
The certification of this protocol and the resulting performance data opens new perspectives for the underwater imaging sector.
It now makes it possible to objectively quantify visibility in turbid conditions—previously assessed in a subjective or outdated manner—and to compare different technologies on a common, standardized basis.
This approach represents an important step toward the standardization of performance measurement for cameras operating in turbid waters, benefiting operators, manufacturers, and the overall safety of underwater interventions.