How the planner estimates light

The Window Light Planner is a physics model, not a measurement. This page lists every step and assumption, so you can judge how far to trust a result.

What it calculates

For each month it takes one representative day and works out the daily light integral (DLI): the photosynthetic light, in mol/m²/day, reaching a leaf that faces the window, at the distance you chose, horizontally centered on the window and level with the sill. It gives a range from a cloudier and a clearer month for the weather setting you pick, with a typical value between them. (Usually the clearer month is brighter; for a north window a hazy sky can give slightly more light, because haze spreads light into the northern sky.)

Step 1: where the sun is

Solar declination, Earth–sun distance and day length come from the FAO-56 equations (eqs. 23–25), every 5 minutes from sunrise to sunset on the month's mean day. The model reproduces FAO-56's worked example 8 (extraterrestrial radiation 32.2 MJ/m²/day at 20°S on 3 September), which is one of the automated tests. Its day lengths match FAO-56's table: about 15.0 hours at 40°N in June and 9.3 hours in December.

Step 2: how much reaches the ground outside

Sunlight above the atmosphere is multiplied by a clearness index, the share that gets through clouds and haze. Without a weather database the planner uses three presets, each a range from a dull month to a bright one:

Often cloudy: 0.3 to 0.45
Mixed: 0.4 to 0.55
Mostly sunny: 0.55 to 0.68

FAO-56 puts a cloudless day at about 0.75. The light is split into direct beam and diffuse sky light with the Erbs, Klein and Duffie (1982) correlation. As a check, the outdoor totals land inside the published US DLI maps of Korczynski, Logan and Faust (2002): 5 to 10 mol/m²/day across the northern US in December, up to 55 to 60 in the Southwest in summer.

Step 3: through the window

Direct sun counts only while the sun is in front of the window, above whatever blocks the view, and at an angle where its rays pass through the opening and reach the plant. A high summer sun shining on a south window lights the sill but not a plant 3 feet back; a low winter sun reaches deep into the room.
Sky light is treated as coming evenly from the whole sky (the isotropic model used for tilted surfaces since Liu and Jordan). The model adds up every direction the plant can see through the window, weighted by angle, using the same geometry as the radiation configuration factors in Howell's catalog. At the glass with an open view, a window-facing leaf sees half the sky, which is the standard result for a vertical surface.
Buildings and trees are modelled as a long obstruction across the view, rising to the angle you choose (open sky 0°, low trees or houses across the street 15°, buildings or big trees nearby 30°, tall building close by, or deep balcony overhead 45°). Above it the plant sees sky; below it, a surface reflecting 20% of the light falling on the ground.
Glass passes the visible transmittance of a typical whole window (single pane, clear 0.9, double pane, clear 0.81, double pane, low-e 0.72, triple pane, low-e 0.51; Efficient Windows Collaborative figures), reduced at steep angles with the ASHRAE incidence-angle modifier (b₀ = 0.1).
Window size sets how much sky a plant away from the glass can see: small (2 x 3 ft); standard (3 x 4 ft); large or patio door (6 x 6.5 ft).

Step 4: energy to plant light

42.9% of solar energy is photosynthetically active radiation (400–700 nm), and each joule of it carries 4.57 µmol of photons (Thimijan and Heins, 1983). Together that is 1.96 mol of PAR photons per MJ of sunlight, the conversion used for the US DLI maps.

Step 5: matching plants

Every plant in the database has an ideal light category and the lowest category it copes with, taken from extension services, the RHS or the Missouri Botanical Garden (each plant page lists its sources). The categories are turned into DLI like this:

Category	Grows well from	Holds on down to
Low light	3 mol/m²/day	0.54 mol/m²/day
Medium light	6 mol/m²/day	1.08 mol/m²/day
Bright indirect	10 mol/m²/day	2.15 mol/m²/day
Direct sun	12 mol/m²/day	10.75 mol/m²/day

Growth levels follow Iowa State Extension's DLI classes for indoor plants: low 3–6, medium 6–10, high 12–16. "Bright indirect" sits at the top of the medium class (10) and "direct sun" at the start of the high class (12), where Iowa State places succulents and leafy herbs.
Minimum levels come from Illinois Extension's minimum light for houseplants: 75, 150, 300 and 1,500 foot-candles. These are converted to DLI assuming 10 hours a day at that level in sunlight (1 foot-candle = 10.764 lux; 0.0185 µmol/m²/s per lux of daylight).
A plant grows well if the typical value reaches its ideal level every month; is dim in the darkest months if it misses it in up to four months; survives if it misses it longer but never drops below the minimum of the lowest light it tolerates; otherwise the spot is too dim. Plants that need several hours of direct sun are only offered if the spot gets at least 2 hours of it in some month.
The grow-light advice compares each month with medium light (6 mol/m²/day), the level most common houseplants need to keep growing.

What it does not know

Your real weather. The presets are broad; a foggy coast or a snowy, bright winter can differ a lot.
Curtains, blinds, screens, dirty glass, deep window reveals, overhangs and balconies (beyond what you choose as an obstruction).
Light bounced off pale walls and ceilings, which can add a little at the back of a bright room.
Leaves on deciduous trees, snow on the ground and seasonal shading.
Exactly where on the windowsill the plant sits. The model assumes the plant is centered and level with the sill; a plant at the side of a wide window, or well above the sill, gets a different share of sky and sun.
Plants are not light meters: temperature, water, humidity and the plant itself all matter too.

The model makes no claims beyond these steps. Every equation is in the site's source code, with unit tests that check it against FAO-56 and the closed-form geometry.