Light
A well lit room is roughly two hundred and fifty times darker than open tropical sun.
Your eye is the problem. It adapts across an enormous range and reports back on a compressed, comfortable scale, so a room lit well enough to read in feels bright. A light meter disagrees, and so does the plant. The middle of a well lit room sits near 8 micromol per square metre per second and open tropical sun near 2,000, which is the factor of about 250 in that headline3.
Light converter
The lux to photon conversion changes with the light source. Lux is weighted by human vision, which peaks in green, while plants use photons roughly equally across red and blue, exactly where the eye is least sensitive1. Any calculator offering one number for every light source is wrong. The per source factors here are the conventional trade values.
- Total output over that area
- 0
- Photons landing
- 0
- Daily light integral
- 0
- Compared with noon
- 0
Lumens are total light leaving a source. Lux is lumens landing on a square metre. PPFD counts only the photons a plant can use, from 400 to 700 nanometres. The daily light integral multiplies PPFD by time, and it is the one horticulture uses to predict crop response, because a plant adds up light over the whole day rather than sampling it once2.
Where your plants actually come from
The headline above is also the wrong comparison, and that is the more interesting half of it. Almost nothing sold as a houseplant evolved in open sun. These are understorey plants, living in a low single figure percentage of the light falling on the top of the forest, which is exactly why they survive a room at all. The ones that struggle indoors are usually the ones from canopy gaps, which are adapted to short bursts of much stronger light than a window ever delivers.
Read the whole table as a map of orders of magnitude rather than a set of readings, and when a decision matters, measure your own window. A cheap meter on the sill for one clear day will tell you more about that spot than any chart can.
- Walter, A. and Schoebel, H. (2023). Shed light on photosynthetic organisms: a physical perspective to correct light measurements. Photosynthesis Research 156:325 to 336. Why lux and lumens are human weighted units and the wrong ones for a plant, and the 400 to 700 nanometre photosynthetic band that PPFD counts. DOI 10.1007/s11120-023-01001-5
- Faust, J.E. and Logan, J. (2018). Daily light integral: a research review and high-resolution maps of the United States. HortScience 53(9):1250 to 1257. The review that established the daily light integral as the horticultural measure of light supply and mapped it from satellite cloud data. DOI 10.21273/HORTSCI13144-18
- Our own arithmetic on conventional reference values. Not a published result. The factor of about 250 is 2,000 divided by 8, using the open tropical sun and well lit room rows of the converter's own reference table, which are the conventional horticultural figures. The conversions the calculator performs are arithmetic on the stated inputs: lux divided by a source factor gives PPFD, PPFD multiplied by hours and by 3,600 and divided by a million gives the daily light integral. calculated, repeatable, and only as good as the reference values it runs on