Hydroponic tower light management in Bangkok: EC consumption, spectrum, and what the sensor data shows

Key takeaways

Above 4,200 lux at canopy level, daily EC drop in Bangkok hydroponic towers tracked light intensity within a consistent band: every 1,000 lux increase above that threshold correlated with an additional 0.06 to 0.09 mS/cm of EC drop per 24-hour period. Below 4,200 lux, the correlation broke entirely and nutrient consumption became erratic. (Plant Mood grow log data, 2024)

A single overhead LED on a 5-tier tower produces a lux drop of 61 to 74 percent from tier 1 to tier 5. At a 16-hour photoperiod, the DLI gap between the top and bottom tier can exceed the difference between a 12-hour and 18-hour timer schedule. A feeding schedule calibrated to the top tiers is chronically over-supplying the bottom tiers while appearing correct in the shared reservoir log.

Adding supplemental grow light increased daily EC depletion by 31 to 47 percent -- but only from day 8 onward. Across lettuce, basil, kale, and pak choi, the first seven days showed under 0.03 mS/cm difference between lit and unlit cycles. Growers who check in week one and see no difference are not wrong. They are just early. (Plant Mood grow log data, 2024)

Bangkok wet season overcast days reduced EC consumption 28 to 41 percent compared to sunny days in the same crop week. Fresh weight growth dropped only 9 to 18 percent over the same period. The plant slows nutrient uptake much faster than it slows growth when light drops -- and a fixed feeding schedule adds nutrient to a reservoir the plant is not consuming at the expected rate.

AS7341 spectrum readings show the red-to-far-red ratio at the bottom row of a four-tier tower averages 0.91 versus 2.34 at the top -- below the threshold that activates shade-avoidance responses in most leafy crops. A crop at that row position is receiving adequate photon quantity and a shade signal simultaneously. PPFD alone cannot detect this. (Plant Mood grow log data, 2024)

The correct response to light-driven EC variability in Bangkok is not to adjust nutrient concentration -- it is to adjust top-up frequency based on measured lux at canopy level. Raising starting EC to buffer a faster drawdown causes tip burn in sensitive crops. Lowering EC on overcast days causes nutrient stress. Both adjustments treat the wrong variable.

The feeding schedule that worked until it did not

January through March, the lettuce tower ran cleanly. EC dropped predictably, top-ups happened on schedule, harvests came in on weight. I had a protocol and it was working. April arrived and the same protocol started producing different results. Some weeks the reservoir barely moved between checks. Other weeks it crashed faster than the schedule anticipated. The plants looked fine through most of it -- full canopy, good colour, standing upright. The harvest weights started drifting. Not catastrophically. Just inconsistently, in a way that did not point at any single obvious cause.

I ran through the usual suspects. Probe calibration. Root zone temperature. Reservoir volume. Seeding density. Nothing explained the pattern. The inconsistency was not random by week -- it tracked with something I had not been logging against the EC data. When I finally overlaid the BH1750 canopy readings against the EC drop column for the same days, the correlation was immediate and obvious. The EC was not being driven by anything in the nutrient system. It was being driven by the light. On high-lux days the reservoir dropped fast. On overcast days it barely moved. The feeding schedule had been treating those as equivalent days and they were not.

That was the moment the tower stopped being a nutrient management problem and became a light management problem. Every unexplained inconsistency -- the wet season accumulation, the bottom-tier yield gap, the week-two crash after adding supplemental light, the basil that looked healthy and tasted watered down -- was the same problem from a different angle. The light was the variable driving all of it, and I had not been measuring it where it actually mattered: at canopy level, at each row position, across seasons.

The numbers that change how you read your reservoir

28 to 41 percent lower EC consumption on overcast days versus sunny days in the same crop week

Measured across 14 wet season hydroponic cycles in Bangkok, with BH1750 sensor at canopy level and EC logged each morning before top-up. Overcast classified as days below 3,500 lux at canopy throughout the light period. Sunny classified as above 6,000 lux average. The same crop, same reservoir, same week of its cycle -- consuming at rates up to 41 percent apart based purely on what the sky was doing outside. A fixed feeding schedule calibrated to sunny-day consumption will over-supply on every overcast day in Bangkok wet season. (Plant Mood grow log data, 2023 to 2024)

4,200 lux: the threshold below which lux-to-EC correlation broke down entirely

Below this level, daily EC drop ranged from 0.04 to 0.19 mS/cm with no consistent relationship to measured lux. The most plausible explanation is a shift from photosynthesis-driven to stress-driven ion uptake at sub-threshold light. Above 4,200 lux, every 1,000 lux increase above the threshold correlated with 0.06 to 0.09 additional mS/cm of EC drop per 24 hours. The relationship is linear above the threshold and unpredictable below it. (Plant Mood grow log data, 2024)

7,400 lux at tier 1 versus 2,200 lux at tier 5 under the same overhead 45W LED

BH1750 mapped at each planting site of a 5-tier tower, oriented as the leaf surface faces the light source, canopy at week two. Tier 3 sat at 4,100 lux -- at the edge of the reliable correlation zone. Tiers 4 and 5 were below it. A shared reservoir feeding all five tiers simultaneously is calibrated to the consumption behaviour of tiers 1 through 3 while tiers 4 and 5 are operating in a regime where more nutrients will not increase their uptake. (Plant Mood grow log data, 2024)

R:FR ratio of 0.91 at row 4 versus 2.34 at row 1 in a four-tier tower

Measured with AS7341 sensor at canopy height at each row, supplemented with a 730nm photodiode for accurate far-red resolution. The shade-avoidance activation threshold for most leafy crops is cited between 1.0 and 1.2 in plant photobiology literature. Row 4 is below it. A crop at that position is receiving adequate PPFD while simultaneously reading a shade signal that drives internode elongation, reduces anthocyanin production, and shortens shelf life -- none of which appears in a PAR meter reading. (Plant Mood grow log data, 2024)

31 to 47 percent increase in daily EC depletion under supplemental grow light, from day 8 onward

Measured across paired lit and unlit cycles for lettuce, basil, kale, and pak choi. Supplemental light was a 45W full-spectrum LED strip at 14 hours per day. In days 1 through 7, the difference was under 0.03 mS/cm. Basil showed the largest increase at 47 percent, pak choi the smallest at 31 percent. The difference in crop response reflects secondary metabolite synthesis demand in basil -- terpene and phenolic production is light-dependent and nutrient-hungry in a way that vegetative growth alone does not account for. (Plant Mood grow log data, 2024)

0.18 mS/cm average EC accumulation above target during a 3-day Bangkok overcast period with unchanged feeding schedule

Logged across 8 separate 3-day overcast events during the 2024 wet season. Range was 0.11 to 0.27 mS/cm. The accumulation was highest in weeks two and three of the crop cycle -- peak canopy development -- when the gap between consumption under full light and consumption under cloud was at its maximum. The accumulated EC does not correct itself. It stays elevated until the first clear day arrives and the plant pulls it down hard. (Plant Mood grow log data, 2024)

The plant that looks healthy during overcast weather is building the problem for the next sunny day

Fresh weight growth on an overcast day slows by 9 to 18 percent. EC consumption on the same day drops 28 to 41 percent. The plant looks close to normal because cell expansion -- the process that adds leaf area and fresh weight -- runs on water pressure and stored photosynthate, not on active nutrient uptake. As long as the hydroponic system is supplying water (which it always is), the cells keep expanding. What does not keep pace is new tissue synthesis: proteins, chlorophyll, enzymes. That process requires nitrogen, phosphorus, potassium, and active photosynthesis. It slows much faster than cell expansion.

The visual health signal during an overcast stretch is real. The plant is not lying to you. But the reservoir is accumulating nutrients that the plant has not consumed because the feeding schedule was designed for a higher-consumption day. Three consecutive overcast days at unchanged top-up frequency can push EC 0.18 to 0.27 mS/cm above target. Then the sky clears.

When the clear day arrives, stomata respond to light within minutes. Nutrient uptake through the root system follows within hours. The plant is now pulling from a reservoir that is already above target. The daily EC drop on that first clear day in our Bangkok wet season data averaged 0.31 to 0.52 mS/cm -- the single largest drawdown of any 24-hour window in 9 of 14 logged wet season cycles. If EC was 0.2 above target at the start of that day, the combination of elevated starting EC and the uptake surge is when tip burn and edge burn appear on lettuce and basil.

The grower sees tip burn on a sunny day and blames the light. The cause was the three overcast days that came before it.

In three basil cycles run specifically to test this sequence, we held EC 0.3 above target for three days (simulating what an unchanged feeding schedule produces during overcast) then restored normal irradiance. Tip burn appeared in two of three cycles on days 2 to 3 after the light returned. In control cycles where EC was held at target through the same period, tip burn did not appear. The light did not cause it. The overcast-period feeding did. (Plant Mood grow log data, 2024)

If the plants look healthy during a rainy stretch -- full canopy, no yellowing, no wilting -- there is no management problem. The plant is signalling that it has what it needs. The reservoir can be topped up on schedule and the system will self-correct when the sun returns.

The reservoir problems that are actually light problems

'My EC drops fast some weeks and barely moves other weeks on the same schedule. I cannot figure out what is driving it.'

This is almost always lux variability that looks like an EC mystery because only one variable is being logged. In Bangkok, week-to-week variation in canopy lux driven by cloud cover and seasonal sun angle is large enough to shift daily EC drop by 0.1 to 0.2 mS/cm on the same nutrient concentration and the same crop stage. Log BH1750 daily averages at canopy level alongside the EC column for two weeks. The correlation will be visible in the data before the two weeks are up.

'My top tiers grow fast and my bottom tiers are always slow no matter what I do with nutrients or watering.'

Almost certainly light, and specifically spectrum quality at the lower rows. EC and pH at lower reservoir zones can be optimised correctly while the R:FR signal at the bottom row is below 0.91. The crop is responding to a light quality signal, not a nutrient signal. The AS7341 reading at each row position will show whether the bottom tiers are receiving a shade signal, and the BH1750 will show whether they are above or below the 4,200 lux threshold where EC consumption tracks lux reliably. Both are required for a complete diagnosis.

'I added a grow light and week one looked fine, but week two the reservoir crashed and the roots went pale.'

The light effect on EC depletion does not activate until day 8. In days 1 through 7, the supplemental light produces under 0.03 mS/cm additional daily drawdown -- not enough to register as a change. By day 8, root mass has expanded enough to act on the higher photosynthetic demand and depletion accelerates by 31 to 47 percent depending on crop. The week-one stability was real. It did not predict week two. The feeding schedule should have increased check frequency from day 6 onward, not after the crash.

'I get tip burn right after a sunny day following a rainy stretch. I assumed the light was too intense after the dim days.'

The light is not the cause. During the preceding overcast days, the reservoir accumulated above target because the feeding schedule was not adjusted for lower consumption. When the clear day arrived and the plant ramped back to full photosynthetic activity, it pulled from an already-elevated EC. Calcium delivery to the fastest-growing tissues -- leaf margins, shoot tips -- becomes locally deficient during the surge. The tip burn is a calcium delivery problem caused by overcast-period feeding, not by the light intensity of the following day.

'I upgraded to a higher-wattage fixture and the bottom tiers still underperform the top.'

Adding more light of the same spectrum does not change the R:FR ratio at lower rows if canopy geometry has not changed. In testing a 45W versus 90W overhead LED on the same tower, the tier 1-to-5 lux ratio was 3.36:1 at 45W and 3.40:1 at 90W -- functionally identical. The spectrum problem at the bottom rows is driven by canopy interception, not fixture wattage. A secondary light source positioned at mid-tower height aimed at tiers 3 through 5 is more effective and produces less heat load in a Bangkok grow room. (Plant Mood grow log data, 2024)

'My basil loses flavour and aroma by harvest even though it looks healthy early in the cycle.'

Essential oil production in basil is light-dependent and photosynthetically driven. On extended overcast periods, fresh weight growth continues but dry weight and secondary metabolite production fall behind. In Plant Mood wet season cycles, basil dry weight at harvest was 22 percent lower than matched dry season cycles despite fresh weight being within 8 percent. The plant kept its visual appearance while its biochemical output declined. For a buyer purchasing on flavour, the wet season product is a different product from the dry season one regardless of what the scale says.

What five years of Bangkok tower logs show that guides written elsewhere do not

In 9 of 14 logged wet season cycles, the single largest daily EC drop of the entire cycle occurred on the first clear day after a 3-day or longer overcast period. The average drop on those days was 0.31 to 0.52 mS/cm in 24 hours -- compared to a cycle average of 0.18 to 0.26 mS/cm on normal sunny days. The plant was catching up on uptake from the low-light period while the reservoir was running at or above target from unchanged top-ups during the overcast stretch. Weeks two and three of the cycle produced the most severe accumulation-crash events because canopy is fully developed and the gap between consumption under light and consumption under cloud is at its maximum. (Plant Mood grow log data, 2023 to 2024)

The first-clear-day EC crash is the most predictable failure point in a Bangkok wet season crop cycle

The highest-risk management day in a Bangkok wet season cycle is not a high-lux day. It is the transition day. Checking EC at the start of the first clear day after any sustained overcast period -- and diluting slightly if EC is above target before the uptake surge begins -- prevents the most common wet season quality failure without changing any other part of the feeding protocol.

AS7341 readings taken at each row at day 7 and day 14 of the same cycles showed that the R:FR gradient from top to bottom was consistently worse at day 14 than at day 7. At day 7, row 4 R:FR averaged 1.12 -- above the shade-avoidance activation threshold. At day 14, the same row averaged 0.91 -- below it. The canopy filter had developed across week two as upper-row plants established full leaf area. A grower who maps lux at transplant and uses those readings to assess whether lower rows have adequate light is working with a measurement taken before the problem exists. The reading that matters is the one taken after the canopy fills. (Plant Mood grow log data, 2024)

The spectrum gradient across tower rows builds across the crop cycle, not just with distance

Light management on a tower is not a set-and-forget calibration. The light environment at lower rows actively worsens as the crop matures. Selectively removing outer leaves from upper-row plants at day 10 to 14 raised row 4 R:FR from 0.89 to 1.21 within 48 hours in Plant Mood trials -- above the activation threshold -- without changing the fixture or the nutrient programme. Canopy management is spectrum management on a tower.

Across our logged cycles, 0.1 mS/cm EC drop in the 18-litre tower reservoir represents a specific absolute nutrient removal. The same 0.1 mS/cm drop in a 5-litre system represents a fraction of that removal. Growers who compare their lux-to-EC-drop numbers against other growers' reported figures without knowing the reservoir volume of the other system are comparing incompatible data. The lux-to-EC correlation we report is calibrated to a specific reservoir volume -- the pattern holds across systems but the absolute mS/cm numbers require recalibration for different volumes. This is rarely stated in any hydroponic grow data that gets shared in community forums.

Reservoir volume normalisation is the most commonly missing step when growers compare EC management data across systems

Before applying any lux-EC correlation from an external source to your own system, run 10 to 14 days of paired BH1750 and EC data in your own setup. Your system-specific coefficient will differ from ours by crop type, reservoir volume, ambient humidity, and canopy density. The methodology transfers. The numbers do not.

Plant Mood grow log data, 2023 to 2024

More light above a tower solves the bottom-tier problem

The solution to insufficient light at lower tower tiers is a more powerful overhead light. If tiers 4 and 5 are reading 2,200 lux and the crops need 4,000, doubling the wattage brings the bottom tiers into range. More power above means more light everywhere, and the gradient is just a brightness problem that intensity solves.

Light intensity from a point or linear source follows the inverse square law. Doubling wattage doubles absolute lux at every tier, but the ratio between tiers stays identical. In direct testing, a 45W overhead LED produced a tier 1-to-5 ratio of 3.36:1. A 90W overhead LED on the same tower produced a ratio of 3.40:1. The bottom tiers doubled in absolute lux and remained proportionally just as disadvantaged as before. What changed was the ambient room temperature increased 2.1 degrees Celsius from the higher-wattage unit -- which in Bangkok is a shift toward reservoir heat stress and accelerated root zone problems. The spectrum problem at lower rows is not an intensity problem. It is a canopy geometry problem. The filter between the fixture and the lower rows is the canopy of the upper rows, and adding more light to the top of that filter does not change what reaches the bottom.

A 25W secondary LED positioned at tier 3 height aimed downward at a 30-degree angle toward tiers 3 through 5 reduced the tier 1-to-5 lux ratio from 3.36:1 to 1.9:1, bringing tier 5 from 2,200 to 4,800 lux. Total wattage increased 56 percent. The gradient reduction was 70 percent. The same total wattage spent on a single larger overhead unit produced a ratio of 3.40:1 -- no practical improvement. The position of the secondary light mattered more than its wattage. (Plant Mood grow log data, 2024)

How light drives nutrient consumption and why the mechanism gets complicated in Bangkok

The base relationship is not complicated: photosynthesis drives transpiration, transpiration drives water uptake through roots, and water uptake carries dissolved nutrients into the plant via mass flow. Higher light intensity means higher photosynthetic rate, higher transpiration, faster water and nutrient movement through the root system. EC drops because the plant is removing ions from the reservoir faster than they are replaced.

Bangkok complicates this in three distinct ways that most hydroponic guides written for controlled environments never have to address.

First, humidity. Indoor ambient humidity in Bangkok without dehumidification runs 75 to 85 percent for much of the year. Lower vapour pressure deficit means the stomata do not need to open as wide to maintain gas exchange, which partially suppresses transpiration even at high lux. The practical result is that the lux-to-EC-drop ratio in Bangkok is compressed compared to what the same crop at the same lux in a drier climate would produce. A lux-EC coefficient imported from a European or North American grow guide will overpredict consumption in Bangkok because it was calibrated in lower humidity. Calibrate in your own system.

Second, seasonal lux variability. Bangkok sits at 13 degrees north latitude. The difference in noon solar elevation between December and May is approximately 25 degrees. For a grow room with a south- or east-facing window, this changes the ambient light contribution to canopy lux meaningfully across the year. A tower running on a feeding schedule calibrated in November will drift out of accuracy by April not because the crop changed but because the ambient light component of canopy lux changed. Monthly lux measurement at canopy level catches this before it affects feeding accuracy.

Third, the spectrum problem that PPFD cannot see. As light passes through successive canopy layers on a tower, leaf tissue absorbs red (660 to 680nm) and blue (440 to 480nm) preferentially -- those are the wavelengths that drive photosynthesis. Far-red (700 to 750nm) transmits at higher rates. Each layer of canopy the light passes through strips the spectrum of red and blue while passing far-red forward. By the time the light fingerprint reaches row 3 or 4, the red-to-far-red ratio has crossed the threshold where phytochrome shifts from its active Pfr form toward the inactive Pr form. The plant reads this as shade competition. It responds with the shade-avoidance response: longer internodes, increased leaf area relative to stem mass, reduced anthocyanin production. None of this is visible in a PPFD reading. The PAR meter is counting photons. The crop is reading their colour.

In a cycle 11 from the Bangkok wet season logs, a 4-day overcast sequence in week three coincided with ambient relative humidity running 87 to 91 percent. Daily EC drop during those four days averaged 0.08 mS/cm. The week-two sunny-day baseline for the same crop and reservoir was 0.26 mS/cm. The combined effect of low lux and high humidity -- both suppressing transpiration through different mechanisms -- produced nutrient consumption that was 69 percent below the clear-day baseline. Fresh weight gain that same week was 13 percent below the sunny-week average. The plant looked like it was barely slowing down. The reservoir disagreed by a factor of five. (Plant Mood grow log data, 2024)

The below-threshold lux behaviour warrants specific attention because it is not documented adequately in most hydroponic guides. Below 4,200 lux at canopy, EC drop in our data sometimes increased rather than decreased relative to the linear prediction -- erratic consumption at low light that may reflect a shift from photosynthesis-driven to stress-driven ion uptake. This is a pattern, not a confirmed mechanism. The practical implication is that sub-threshold lux periods need their own management logic: more frequent EC checks, smaller top-ups, and no attempt to apply the above-threshold lux-EC coefficient to predict consumption.

Fixed schedule feeding versus lux-adjusted management: what the difference looks like across a Bangkok wet season cycle

Fixed schedule top-up regardless of light conditions

Does not account for lux variability. On overcast days, EC accumulates above target because top-ups add nutrient solution the plant is not consuming at the anticipated rate. On the first clear day after an overcast stretch, the reservoir is above target when the plant ramps to peak consumption, producing tip burn and edge burn in sensitive crops. Over a full Bangkok wet season cycle, EC can swing 0.4 mS/cm above and below target on the same nominal concentration. The crop alternates between mild excess on overcast days and sudden deficit during the first-clear-day crash. Neither problem is dramatic enough to flag as a crisis. Together they accumulate as quality and yield inconsistency across the cycle.

Works acceptably in dry season when lux is relatively consistent day to day. Produces systematic errors in wet season when cloud cover creates 3-day overcast stretches followed by clear transitions. The grower who runs this protocol year-round gets good dry season results and cannot reliably replicate them in wet season, and typically blames crop variety, seed quality, or something in the root zone rather than the feeding schedule.

Daily EC check with top-up triggered by drop threshold

Better than fixed scheduling. Still reactive. The grower is responding to EC that has already dropped rather than anticipating the drop based on incoming light. On a high-lux day in weeks two and three, EC can drop 0.3 mS/cm before the daily check catches it. The check frequency is also constant regardless of season -- daily in wet season provides adequate resolution on some days and insufficient resolution on the first-clear-day transition when EC can move 0.5 mS/cm in a single 24-hour window. Without lux data alongside EC data, the grower cannot distinguish genuine consumption-driven drops from dilution, root pattern changes, or probe drift.

The minimum viable protocol for year-round Bangkok tower growing. Produces reasonable results with consistent daily EC checks. Becomes substantially more reliable when BH1750 data is logged alongside EC so the two variables can be read against each other rather than the EC alone.

BH1750 daily lux average at canopy level correlated against rolling EC drop baseline, with pre-emptive top-up on high-lux days and check-before-top-up rule on overcast days

Requires setting up the sensor at canopy level -- not above it, not at the fixture -- and logging both variables at the same time each morning. After 10 to 14 days of paired data in the specific system, the lux-EC coefficient becomes a reliable predictor 12 to 18 hours in advance. Morning lux readings on clear days trigger a pre-emptive partial top-up before the drawdown happens. Overcast days trigger a check-before-top-up rule that prevents accumulation. The first-clear-day transition gets flagged automatically when the lux column shows a high reading after a run of low readings. Total EC oscillation across a full cycle stays within 0.1 to 0.15 mS/cm of target instead of swinging 0.3 to 0.4 mS/cm.

The reservoir runs at the intended EC on the days the crop most needs it -- high-light, high-growth days in weeks two and three. Tip burn incidence drops because EC is not accumulating above target during overcast periods. Quality is more consistent across seasons because the feeding protocol is responding to actual crop demand rather than a calendar interval. The setup cost is a BH1750 sensor and 10 to 14 days of paired logging. The ongoing cost is checking the lux reading each morning alongside the EC reading.

The sensor that most Bangkok tower growers are missing is not a PAR meter

The most common recommendation for improving hydroponic light management is to buy a quantum PAR sensor. It is not wrong advice -- PPFD is more relevant to plant physiology than lux. But for a Bangkok tower grower whose primary problems are seasonal EC inconsistency, bottom-tier yield gaps, and wet season feeding errors, a PAR sensor solves the wrong problem first. The PAR meter tells you how many photons are arriving at a row. It does not tell you whether the plant is reading a shade signal from those photons. It does not tell you that the overcast day you are on is going to produce a first-clear-day EC crash in 72 hours. It does not tell you whether your feeding schedule is adding nutrients to a reservoir that the plant stopped consuming yesterday.

A BH1750 at canopy level, a logbook that pairs lux and EC readings at the same time each morning, and an AS7341 run once per cycle at each row position give a more complete picture of what is actually limiting performance in a Bangkok tower than a PAR meter alone. The BH1750 costs under 150 baht. The AS7341 module is under 400 baht. Neither requires calibration hardware or a software subscription. Both produce data that is immediately actionable in a feeding decision or a crop assignment decision.

The PAR meter is valuable and I am not arguing against it. I am arguing that the order of operations matters. Fix the lux-EC correlation problem first. Fix the spectrum gradient problem second. A PAR meter is the right tool for the third problem, which is knowing whether your absolute photon delivery is within the range the crop needs for a given growth stage.

This is true and it is the right counterargument. The BH1750 limitation is real. But the specific failures that Bangkok tower growers experience most frequently -- seasonal EC inconsistency, wet season accumulation-crash cycles, week-two drawdown spikes after adding supplemental light -- are driven by light intensity variation, not by spectrum measurement error. A spectrally imperfect sensor that is in the right position and being logged daily against EC data solves those problems. A PAR meter that is read occasionally above the canopy solves none of them. Use the right tool for the actual problem, then upgrade when the actual problem changes.

PAR meters give a single measurement that integrates spectral quality into a plant-relevant unit. A BH1750 is calibrated to human vision and cannot tell you whether red and blue photons are reaching the canopy at adequate levels for photosynthesis. A grower who trusts lux as a proxy for plant-available light is working with a spectrally incorrect measurement that can mislead them when light source spectrum changes.