# Light, spectrum and DLI

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Spectral composition, PPFD, DLI, photoperiod, plant position and the distinction between electrical power and photon exposure.

In brief

Spectral composition, PPFD, DLI, photoperiod, plant position and the distinction between electrical power and photon exposure.

**Central optical module.** Original v2.0 design image. Plant-level photon exposure must be established by a measured PPFD and spectral map. [I02]

## Required metrics

 |
 Metric | Definition

 | Spectrum | photon distribution by wavelength at plant position
 | PPFD | instantaneous 400–700 nm photon flux density
 | DLI | daily integrated photosynthetic photon exposure
 | Photoperiod | scheduled light duration
 | Uniformity | spatial distribution across plant positions
 | Leaf temperature | thermal outcome at the tissue, not only room temperature

 L-1 D L I = 1 10 6 ∫ 0 86400 P P F D ( t )   d t \mathrm{DLI}=\frac{1}{10^6}\int_{0}^{86400}\mathrm{PPFD}(t)\,\mathrm{d}t DLI = 1 0 6 1 ​ ∫ 0 86400 ​ PPFD ( t ) d t D L I \mathrm{DLI} DLI in m o l   m − 2   d − 1 \mathrm{mol\,m^{-2}\,d^{-1}} mol m − 2 d − 1 for constant P P F D \mathrm{PPFD} PPFD and photoperiod t h t_h t h ​ in hours.

**Explanation**Daily light is the time integral of photon flux at plant position. For constant PPFD, this reduces to PPFD multiplied by photoperiod in seconds, divided by one million.

## Published basil study

In hydroponic Italian Large Leaf basil, controlled supplemental-light spectra altered key aroma volatiles under a defined experimental environment. [R01] The transferable conclusion is that spectrum is a valid treatment variable. The exact result requires matching cultivar, DLI and other conditions.

## Why wattage is insufficient

A 50 W electrical rating and manufacturer efficacy estimate cannot define PPFD inside a cylindrical system. Optical distribution, distance, angle, reflection, obstruction, thermal state and plant position must be measured.

## References

- [I02] FlavoRotor project team (2026). FlavoRotor v2.0 system architecture. *Internal engineering design report*.
- [R01] Hammock, Hunter A.; Sams, Carl E. (2023). Variation in supplemental lighting quality influences key aroma volatiles in hydroponically grown 'Italian Large Leaf' basil. *Frontiers in Plant Science*. https://doi.org/10.3389/fpls.2023.1184664