Light & Spectrum: What Do Lumens Mean, Why Do Indoor Lamps Use Blue/Red/Violet Instead of Sunlight & Does Under-Canopy Light Really Work?

Anyone shopping for a grow light for the first time quickly runs into three big questions: What does the lumen rating on a lamp actually mean – and why do plant scientists measure PPFD (µmol/m²/s) instead? Why do indoor grow rooms use blue, red or violet (“blurple”) lamps when outdoors in nature there is simply one sun shining all year round? And does the recent trend of shining LED bars upward from the pot rim (under-canopy lighting) make biological sense? Here is the complete physics and plant biology of light – guided by Flo’s motto: “Understand nature, then copy nature.”

1. What Do Lumens (and Lux) Mean – and Why Are They the Wrong Metric for Plants?

Household bulbs and older grow lamps prominently advertise lumens (lm) or lux (lx = lumens per m²). Yet judging a plant light by lumens is a classic physics trap:^[1][3] well established

• Lumens measure human eye brightness (photometry): Human daytime (photopic) vision follows the V(λ) luminosity function, which peaks sharply at 555 nm (green-yellow). To the human eye, deep blue (430 nm) and deep red (660 nm) appear relatively dim, and far-red (700–750 nm) is nearly invisible.^[3][4]
• How plants use light (McCree action spectrum & Stark–Einstein law): Photosynthesis is a quantum process driven by the number of absorbed photons, regardless of how bright they look to human eyes.^[1][3] In his landmark study, Keith J. McCree (1972, Agricultural Meteorology) demonstrated that while extracted chlorophyll a and b peak in the blue (430–450 nm) and red (640–660 nm) regions, intact crop leaves use accessory pigments (carotenoids) and internal light scattering to harvest the entire 400–700 nm (PAR) waveband – and together with far-red photons up to 750 nm (ePAR).^[1][4]

Hempey’s Physics Rule: Lumens Are for Humans, Micromoles (µmol) Are for Plants!

A green-yellow streetlamp boasts huge lumen numbers while starving a plant of key blue and red photons. Conversely, a fixture rich in 660 nm deep-red diodes looks dimmer to human eyes (fewer lumens) while delivering maximum photosynthetic power to the chloroplasts!^[1][3]

Comparison Table of All Key Light Metrics (Lumens to PPFD & DLI)

2. Why Are Indoor Lamps Blue, Red or Violet (“Blurple”) – While Outdoors Only the Sun Shines?

Why do indoor growers talk about blue veg lights, red bloom lights or violet LED panels when outdoors a single sun grows everything from seedling to harvest? The answer combines atmospheric physics with the history of lighting technology:^[2][3][4]

2.1 The Sun Is Dynamic: How Sunlight Shifts Across the Day and Seasons

Following Flo’s motto “Understand nature, then copy nature”, sunlight is not static: as the solar elevation angle changes from spring to autumn, the path length through Earth’s atmosphere (Air Mass) and Rayleigh scattering shift the spectrum reaching the plant:^[3][4]

• Spring & High Summer (high solar angle at midday – vegetative phase): Sunlight travels a short, steep path through the atmosphere and is rich in high-energy blue light (400–500 nm) and UV-A/UV-B. Blue light activates cryptochromes and phototropins, suppressing auxin-driven stem elongation so the plant stays compact with short internodes, thick leaves, sturdy stems and strong roots.^[3]
• Late Summer & Autumn (lower solar angle – flowering & ripening phase): As the sun sits lower in the sky, short-wave blue light is scattered more strongly, shifting the spectrum toward red (600–700 nm) and far-red (700–750 nm) alongside shortening day length. This shift in the Red : Far-Red ratio (R:FR) flips the plant’s phytochrome system (P_r ↔ P_fr), triggering flowering stretch and floral maturation.^[3][4]

2.2 History of Indoor Grow Lights & Why the Violet “Blurple” Era Failed

1. Gas-Discharge Lamps (MH for Veg, HPS for Bloom): Early indoor growers had to swap bulbs because Metal Halide (MH) lamps emitted blue-white light (good for compact veg) while High-Pressure Sodium (HPS/NDL) emitted orange-red light (good for bloom, but caused stretching in veg).
2. First-Generation “Blurple” LEDs (Blue + Red = Violet): Early white LEDs were inefficient, so manufacturers combined only narrow-band 450 nm blue and 660 nm red diodes matching the absorption peaks of extracted chlorophyll in a test tube – producing an unnatural violet/magenta (“blurple”) glow.
3. Why Green Light (500–600 nm) Is Essential (Terashima et al. 2009; Bugbee 2016): In intact whole leaves, 70–85% of green light IS absorbed via internal mesophyll scattering (the “detour effect”).^[2][3] While blue and red photons are absorbed almost entirely in the top chloroplast layers of the upper canopy, green light penetrates deep into the leaf interior AND deep into middle and lower canopy layers, driving photosynthesis more efficiently than red light alone under high PPFD!^[2] Moreover, white full-spectrum light lets growers immediately spot early nutrient deficiencies and pests that are invisible under purple light.
4. Modern Full-Spectrum LEDs (Copying the Sun!): Today’s best LEDs combine white full-spectrum diodes (3000K–5000K) with 660 nm deep red (and optional 730 nm far-red via the Emerson enhancement effect)^[3][4] – bringing true sunlight indoors in a single dimmable fixture!

3. Does Light from Below (Under-Canopy / Sub-Canopy Lighting) Really Help – or Is Skepticism Warranted?

Recently, manufacturers have marketed under-canopy LED bars placed on the pots to shine upward against the undersides of leaves. Flo and many experienced botanists are skeptical of this trend – and leaf anatomy explains why:^[2][6]

3.1 Leaf Anatomy: Why a Cannabis Leaf Has a Distinct Top and Bottom (Bifacial Leaf)

• Upper Leaf Surface (Adaxial side – the solar collector): Protected by a waxy cuticle and an epidermis with virtually no stomata, directly above tightly packed columnar palisade parenchyma cells rich in chloroplasts that act as optical light guides channeling light from above.^[2]
• Lower Leaf Surface (Abaxial side – the breathing lung): Contains loose spongy parenchyma with air spaces for CO₂ diffusion and the vast majority of stomata (guard-cell pores). Evolution placed stomata on the shaded, cooler underside so the plant can take in CO₂ without desiccating under direct solar radiation.
• What happens when strong light and heat hit the underside? Direct radiation warms the abaxial surface and raises local leaf-to-air VPD right at the stomatal pores, risking stomatal closure (halting CO₂ uptake!), lower internal quantum efficiency, and unnatural leaf twisting.^[2]

3.2 Sub-Canopy Studies (Hawley et al. 2018) vs. Smart Canopy Architecture in a Home Grow

Flo’s Experience & the 3 Tigers: “Understand nature, then copy nature”

Why My Light Comes from Above Like the Sun – and What Runtz Thinks About It 🐾

The first time I saw people laying LED bars on top of their pots to shine light upward at the undersides of leaves, my motto immediately came to mind: “Understand nature, then copy nature” (Kapier die Natur, dann kopier die Natur)!

Just ask yourself: In nature, does the sun ever shine upward out of the soil? Of course not! A leaf has its solar panels (palisade cells) on top and its sensitive breathing pores (stomata) in the cool shade underneath. Every morning, right after my full-spectrum LEDs switch on, I do my daily hand-watering check with good water, worm tea or diluted nettle manure (Brennnesseljauche) in my custom soil-worm-casting-sand-Seramis-Living-Soil mix – right when those stomata open to breathe. The last thing they want is a hot lamp blasting them from below!

Instead of wasting electricity on bottom lamps, my Half-Pot + Topping + Adventitious Root method builds two tiers of roots in the dark pot and 8 uniform main colas at the top catching full-spectrum light from above. And if you want proof of the world’s best spectrum, just watch my three Maine Coon tigers Cookie, Haze and Runtz: as soon as a real sunbeam hits the floor, Runtz is guaranteed to be napping right in the middle of it – 100% full spectrum at 0 Watts! 🐾

My personal lamp favourite from years of testing: I have tried out a lot of different grow lamps myself over the years, and my personal favourite is SANlight. I have owned and used almost all of their series myself – from the classic 30-watt single lamp right up to the Q-Series and EVO-Series – and can recommend every single one of them with a clear conscience. Instead of loud marketing gimmicks, they focus on real photobiology, rock-solid reliability, a natural broad full-spectrum modelled on the sun, and clean secondary optics that guide light deep and evenly into the canopy from above.

Note: personal experiences are subjective, unpaid, and do not constitute advertising or a buying recommendation (§ 6 KCanG).