Lighting and environmental feedback

One reading, two benches

Two benches under the same greenhouse roof can receive different light because of structural shade or their position beneath lamps. A reading from the controller on the wall does not establish what either crop receives. Begin with the crop and the question the measurement should answer.

If seedlings are stretching, inadequate light is one possible explanation. Their variety, temperature, spacing and other conditions also matter. Measure the environment and retain alternative explanations rather than immediately increasing lamp hours.

Three light questions

Photosynthetic photon flux density, or PPFD, measures the rate of incident photons in a specified photosynthetically active wavelength range per area. Its conventional unit is µmol/m²/s. It is an instantaneous rate, not a daily total or a measure of carbon fixed by the plant.

Daily light integral, DLI, accumulates those photons per area over a day, commonly in mol/m²/day. Day length describes the light–dark timing. Two schedules can have the same DLI while having different durations or intensities. Photoperiodic flowering responses and photosynthetic light supply are related but distinct questions, with crop-specific requirements.

Measure at the crop

Use a suitable light sensor near the crop canopy, recording its position, height, units and measurement interval. Keep it clean and correctly oriented. A single reading of variable sunlight does not represent the whole day; repeated readings or an integrating instrument are needed.

For constant PPFD, multiply the rate by the seconds of exposure, then divide by 1000000 to convert µmol to mol. For changing PPFD, sum the exposure across measured intervals with their actual durations. Lux weights light for human vision and is not a universal substitute for plant photon measurements across different lamp spectra.

Close the feedback loop

Feedback means using observations to adjust an operating choice, then measuring again. The diagram follows a hypothetical low-light bench through measurement, crop-specific interpretation, lighting or shading action and a new observation. A sensor signal alone is not a crop requirement.

An automatic control can compare a reading with a chosen setpoint, but the setpoint needs evidence for the crop and stage. Sensors may drift or sample an unrepresentative location; actuators may fail or have insufficient capacity. Check that the intended action actually changes conditions at the plant. Detailed sensor and control design has its primary home in Agricultural Engineering & Automation.

Interacting conditions

Supplemental lighting adds photons but also requires electricity and can affect heat loads. Shading reduces exposure and may help manage excessive light or heat, while also reducing the crop’s light supply. Neither action has an unlimited benefit. A growth response depends on water, nutrition, temperature and other constraints as well as light.

Observe several relevant positions and stages. If lamp hours increase while daytime sunlight falls, total exposure may not increase as expected. Retain the measured daily record rather than inferring DLI from a timer setting alone. Keep the dark-period and photoperiod requirements identifiable.

Compare value and cost

Evaluate a change using useful output, quality, timing and included costs. Added photons are not automatically added marketable harvest. Electricity price, fixture performance, crop response, maintenance and utilisation affect the decision. Consider the same crop, stage and comparison period.

Begin with a bounded trial and a clear stopping or review condition. A controller that keeps its setpoint can be functioning correctly while the crop still performs poorly for another reason. The goal is adequate, measured growing conditions, with an operating choice whose benefit can be checked.

Worked examples

Check your understanding

1. Can the same DLI imply the same day length?

   Show answer and reasoning

   No.

   Different rates and durations can produce the same integrated photon exposure; flowering timing can still differ.

2. Why may a ceiling-mounted sensor misrepresent a shaded bench?

   Show answer and reasoning

   Its location samples a different light environment.

   Check conditions near the crop rather than assuming one location represents every plant.

3. A controller reports its setpoint was maintained. Does that prove that extra light increased harvest?

   Show answer and reasoning

   No.

   The environmental record and crop outcome are different evidence; other constraints and costs remain.

Key ideas

• Keep rate, daily exposure and timing distinct.
• Measure at representative crop positions.
• Feedback needs a crop-specific interpretation and a checked response.