IR Filters for Aquaculture and Water Farming: Design Trends and Sourcing Notes

A camera on a fish cage, a raceway or a pond edge works through turbid water, low sun angles, salt spray and a surface that grows whatever the season grows. Under those conditions the infrared filter in front of the sensor is what decides whether the image still carries information at all. This note covers how the filter is specified for that environment, what the current sourcing practice looks like, and which requirements are genuinely changing as water farming moves towards longer-duration observation.

IR Filters in Aquaculture and Water Farming

The optical task in a water farm is to see something through a medium that changes continuously. Water absorbs strongly in the infrared beyond a short near-infrared window, turbidity scatters whatever survives, and the sun angle over a pen or a pond changes the amount of surface reflection that reaches the camera. The infrared filter is placed to make the imaging band reproducible so that a behaviour event, a biomass change or a surface condition can be compared between visits months apart. Because the camera rarely returns to exactly the same position and the water is never the same twice, the filter's job is less about maximum information and more about narrowing the variable set, so the same measured feature means the same thing over time. That is why a stable, well-blocked near-infrared pass band tends to beat a broader, more sensitive arrangement on this application.

What drives cost and lead time

For this industry the cost drivers are different from a laboratory filter. Size and shape dominate first, because a cage or raceway head is usually a heavy housing and the filter is cut to whatever aperture fits, and a non-standard aperture means a non-standard tooling cut. Blocking depth is second: a filter that blocks the visible band well is more expensive to make than one that only tints, because deep blocking means more layers and cleaner deposition. Third is the coating durability class, since a filter that must be cleaned with bleach or a permitted aquatic cleaner cannot use the softest stack that would otherwise be chosen. Lead time is driven by whether the filter is a catalogue item with a known band or a cut-to-size job from a custom coating run, and in this sector the practical step is usually to standardise the head design across a site so that one filter part number covers every camera.

How IR filters works in practice

A near-infrared filter is a dielectric stack whose layer periods set the pass band, and its blocking shoulders sit either side of that pass. The stack is deposited on a substrate that must transmit in the band, so the substrate choice and the coating choice are coupled: a filter built on a strongly coloured glass has its band shape partly defined by the substrate, while a filter built on a clear substrate relies entirely on the deposited layers. In practice a hybrid is common, where a substrate blocks the deep infrared and the coating sharpens the near-infrared edge. Angle shift is the property that bites hardest here, because a camera looking at a pen from a shallow angle through water sees the image off-axis, and a stack designed only for normal incidence will drift. The practical requirement is therefore to state the acceptance angle and the maximum incidence angle across the field, and to have the shift measured across that range rather than on axis.

Design rules that reduce cost

Five rules cover most of the savings. Keep the aperture at a standard size even if that means a slightly larger housing, because a standard cut is a stock operation and a custom one is not. Set the blocking depth to the ambient the camera actually works in rather than a maximum, since a higher optical density than the scene needs costs layers without buying contrast. Fix the pass band once the sensor and the illumination are chosen and then stop changing it, because re-specifying the band invalidates the recorded history. Choose a hard-coated part where the head will be cleaned chemically, even at a modest transmission penalty, rather than a soft stock that has to be replaced. And keep the filter flat and simply mounted rather than wedged or wedge-corrected, because a wedge brings extra metrology for a benefit the scattered-water application rarely uses.

Standards, documentation and traceability

Sourcing for this sector needs slightly different paperwork than a laboratory order. The pass band, the blocking band, the substrate and the coating durability should each be recorded with a lot number, because a cage operator replacing a filter months later needs to know whether the replacement is the same part as the one the historical images were taken with. Surface quality and dimensional tolerances are conventionally expressed under the optical drawing standard, so the figures should be taken from the current issue rather than from a catalogue. Where the installation is in a regulated food production chain, the material declarations for the filter assembly may also be relevant, and any environmental compliance statement about the coating materials should be confirmed against the current text of the relevant directive rather than accepted as a general claim. Traceability in practice means keeping one qualified filter as a reference part and recording which lot is fitted to which camera.

Requirements specific to Aquaculture and Water Farming

A water farm camera should specify the pass band together with the illumination and the sensor, then hold it fixed so that observations remain comparable between seasons. Depth of blocking is set by the ambient the head actually works in, the stack must be qualified across the real acceptance and incidence angles rather than on axis, and the coating must survive the permitted cleaner. One standard aperture and one filter part number across a site is the cheapest route to a maintainable installation.

• Pass band fixed with the sensor and illuminator, then not changed
• Blocking depth matched to actual ambient, not a maximum
• Angle shift measured across the real acceptance and field angles
• Hard coating qualified against the permitted cleaner
• One aperture and one part number standardised across the site
• Lot number recorded per camera for historical comparability

Framework references: pass band, blocking, surface quality and tolerances for optical filters are conventionally stated in ISO 10110 and the relevant filter standard for the wavelength region; confirm values in the current issue of those documents. Any environmental or materials compliance statement for the coating should be verified against the current text of the applicable regulation rather than taken from a supplier summary.

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