Light Calculator

How Light Calculator calculates its result

The calculator models the fixture as a set of point emitters with a Lambertian angular distribution, then sums their contributions at each sample point on the canopy.

1. Beam shaping: the Lambertian exponent comes from the beam angle, n = ln(0.5) / ln(cos(beam angle / 2)). A 120 degree beam gives n = 1.
2. Emitter intensity: the hemispherical integral of cos^n is 2 pi / (n + 1), so intensity at nadir is I0 = PPF / (2 pi / (n + 1)). Skipping this normalization would report a PPFD above the physical ceiling.
3. Point contribution: PPFD = I0 x cos^n(theta) x cos(theta) / d squared, where theta is the angle from vertical and d is the distance in meters.
4. Emitter layout: a multi-bar fixture is split into points along each bar; a single panel or COB is one point. Effective PPF is scaled by the dimmer, with driver efficiency reduced below 50 percent.
5. Canopy average: the model samples a 7 by 7 grid across the canopy and averages the result.
6. Wall reflection: reflective walls add a fixed uplift per sample position, 1 + (reflectivity x factor), where the factor is 0.08 for the canopy average, 0.05 at the center, 0.15 at the edge, and 0.12 at the corner.
7. Uniformity: Michelson contrast, (1 - (max - min) / (max + min)) x 100, expressed as a percentage.
8. Daily light integral: DLI = average PPFD x photoperiod hours x 3600 / 1,000,000.

PPF is in micromoles per second. PPFD is in micromoles per square meter per second (SI canonical: mol/m^2/s). DLI is in moles per square meter per day. Distances convert to meters at 0.0254 m per inch.

Worked example

Running the calculator at its own default inputs:

A single fixture over a 60 x 60 in canopy concentrates light in the center, which is why uniformity is low here. Raising the fixture or adding a second one spreads the distribution.

Reference values