Depth
Sensor depth changes the amount of light reaching the measurement point. Compare corals at their actual positions rather than assuming the surface value applies everywhere.
Reef Lighting Measurement Guide
Measure What Your Corals Actually Receive.
Reef lighting is not one PAR number. Depth, horizontal position, rockwork, shadows and fixture settings can create very different light conditions across the same aquarium. Measure at coral level, map the tank, adjust the lighting, then measure again.
The measurement problem
Your reef does not receive one PAR number.
A fixture specification or one center reading cannot describe the light at every coral. Underwater light changes with position and setup, so useful reef measurements are taken where the corals actually live.
Horizontal position
The center, edges and corners of a tank can receive different intensities, even when they are at the same depth.
Rockwork & shadows
Aquascape structures, overhangs and neighboring corals can create localized shadows that a single open-water reading will miss.
Fixture settings
Intensity, mounting height and channel settings can change both the amount and distribution of light in the tank.
Step 1
Measure
Take readings where the corals actually sit.
Step 2
Map
Repeat across depths and positions to reveal the pattern.
Step 3
Adjust
Change fixture settings, mounting or placement based on evidence.
Step 4
Measure again
Verify the change at the same measurement points.
Understand the measurements
PAR is the starting point. Position and spectrum add context.
Reef-light data becomes more useful when you know what each measurement describes and when you keep the measurement position and lighting conditions consistent.
400–700 nm photon flux
Use underwater PAR to compare the photosynthetically active light reaching different coral positions inside the aquarium.
Additional UV information
UVA measurement adds visibility into ultraviolet output that is not described by the PAR value alone.
Channel balance
Separate red, green and blue information can help document how spectral balance changes as fixture channels are adjusted.
Distribution, not one spot
Multiple recorded positions help reveal high-light areas, lower-light zones and shadows across the reef structure.
Tank mapping
Turn scattered readings into a reef-light map.
The goal is not to chase a single impressive number. Build a repeatable pattern of measurements that helps you understand how light is distributed through the aquarium.
Measure consistent positions
Choose repeatable points across the tank—top, middle, lower areas, center, edges and important coral locations. Keep fixture settings and sensor orientation consistent when comparing results.
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Read the pattern, then make a decision
A light map can show where intensity falls off, where rockwork creates shadows, and where a coral's actual position differs from the tank's center reading.
Compare like with like: use the same lighting program, fixture height, water depth,
sensor angle and measurement positions before deciding whether a change improved the tank.
AquaHorti reef workflow
Measure the reef. Control the light.
AH-Reef and MegaReef 100 play different roles. One gives you underwater measurement data; the other gives you a controllable reef-light source. Use them as separate tools or as parts of the same measure-adjust-verify workflow.
Measure
AH-Reef
Underwater PAR and UVA measurement at the locations where your corals actually live.
- PAR measurement from 400–700 nm
- UVA irradiance measurement
- Live R / G / B spectral balance
- IECF underwater correction when selected
- HOLD and recording for up to 99 points
- Waterproof probe for in-tank measurement
Best for: coral-position measurements, reef-light mapping,
shadow detection, fixture comparison and verifying changes after adjustment.
View AH-Reef →
Control
MegaReef 100
A seven-channel Bluetooth-controlled reef LED light for building and adjusting a reef-lighting program.
- Seven independently controlled lighting channels
- Bluetooth app control
- Real-time spectral visualization
- Sunrise and sunset scheduling
- Master-slave synchronization for multi-light setups
Best for: reef keepers who want to adjust intensity,
channel balance and daily lighting behavior, then verify the result with actual measurements.
View MegaReef 100 →
Reef lighting knowledge
Learn the lighting question before changing the tank.
Use the AquaHorti reef-lighting articles to understand PAR, spectrum, coral placement and measurement workflows before interpreting a single number.
Reef lighting archive
PAR, spectrum & coral placement
Browse AquaHorti reef-lighting articles in one focused knowledge path.
Start with PARFinding the PAR Sweet Spot for Corals
Understand why visual brightness and actual underwater PAR are different questions.
AquaHorti BlogMeasurement before guesswork
Continue into reef lighting, measurement methods and other AquaHorti technical guides.
Quick answers
Reef lighting measurement FAQ
Why can PAR vary so much across one reef tank?
Underwater PAR changes with depth, horizontal position, rockwork, shadows, fixture height, channel settings and sensor position. A single center reading cannot describe every coral location.
Where should I measure reef-tank PAR?
Measure at the actual locations and depths where your corals live. When comparing readings, keep the fixture settings, sensor position and other relevant conditions as consistent as practical.
Why measure UVA together with PAR?
PAR describes photosynthetically active light from 400–700 nm. UVA measurement adds another layer of information about ultraviolet output that is not represented by the PAR number alone.
How should I use a reef-light map?
Record consistent positions across different depths and horizontal locations, compare the distribution pattern, make the lighting or placement change, then measure the same positions again.
Measure → Map → Adjust → Verify
Build reef-lighting decisions from measurements inside the tank.
Start with AH-Reef when you need underwater PAR, UVA and tank mapping. Use MegaReef 100 when you need a controllable seven-channel reef-light source, then measure again to verify what changed at coral level.