Can you create a GrowLight from an existing Downlight?

Using Existing Downlights to Create a DIY Grow Light: PPFD Calculations for Bridgelux BXRC-40G10K0-D-73-SE with 10°, 35° & 55° Optics

Lets take a random example so we can at the very least understand the process and approach you can take to achieve this

It’s surprisingly straightforward to repurpose architectural downlights into functional grow-lights—provided you understand how light intensity and beam spread affect plant performance. In this post, we’ll walk through a real-world example: mounting Bridgelux BXRC-40G10K0-D-73-SE COB LEDs 2.275 m above a 7.7 × 8.1 m planter, calculating the photosynthetic photon flux density (PPFD) with three common optics (10°, 35° and 55°), and identifying the optimal configuration to hit a target of 45 µmol/m²/s across the entire area.

WARNING AHEAD is a smidge of mathiness, ok maybe a smidge and a half

To start with we need to convert our usable light into a metric that plants are interested in

1. From Lumens to Photons: Converting Flux to PPFD

Bridgelux’s BXRC-40G10K0-D-73-SE chips (4000 K, CRI ≈ 90) output around 9 600 lumens each at 2.1 A (25 °C). But plants don’t “see” lumens—they respond to photons in the 400–700 nm band (PAR). A 4000 K white LED typically yields about 0.015 µmol/s per lumen, so each module delivers roughly:

PPF = 9600 Lumens X 0.015 umoles/s per Lumen = 144umol/s of Par Photons

This is the total photon flux an LED produces, ready to be spread over the crop. In more detail.

So most down lights will come with a standard optic, that may be a TIR or a reflector. So given we know or can find out the beam type we can make some calculations based on those constants. Lets consider some random but very likely beam angles you may encounter

2. Beam Spread & Footprint: 10°, 35° & 55° Optics

Secondary optics shape the light into cones with full angles of 10°, 35° or 55°. At 2.275 m height, the spot diameters become:

• 10° lens: ~0.40 m diameter (tiny, tightly focused)
• 35° lens: ~1.43 m diameter (medium spread)
• 55° lens: ~2.37 m diameter (wide flood)

Assuming a uniform photon distribution inside each cone (a conservative simplification), average PPFD ≈ PPF / footprint area:

• 10°: 144 µmol/s ÷ 0.125 m² ≈ 1 150 µmol/m²/s
• 35°: 144 µmol/s ÷ 1.62 m² ≈ 89 µmol/m²/s
• 55°: 144 µmol/s ÷ 4.40 m² ≈ 33 µmol/m²/s

Beam Geometry & Footprint Calculation

A beam of full-angle θ from height h illuminates a circular spot of diameter D on the plane below:

D = 2htan(0/2)

Here, h = 2.275 m. Compute for each optic:

Spot area A (m²) = π·(D/2)²:

• 10°: A ≈ π·(0.20)² ≈ 0.125 m²

• 35°: A ≈ π·(0.72)² ≈ 1.63 m²

• 55°: A ≈ π·(1.18)² ≈ 4.37 m²

Unsurprisingly, the 10° beam is far too intense and narrow for broad coverage, while the 55° beam alone doesn’t quite reach our 45 µmol/m²/s target.

We have our beam angle data, how do we work that to coverage?

3. PPFD at the Target Plane

Assuming uniform distribution (idealised), average PPFD in μmol·m⁻²·s⁻¹ = Φphoton ÷ A:

PPFD=144μmol/s / A(m²)​

Caveat: Real beams follow a cosine-corrected intensity profile (Lambertian-like fall-off). Centre PPFD will exceed the average; edges will be lower. Our uniform-spot model gives a conservative “average” PPFD—actual centre values could be ~1.2–1.5× higher.

Lets lay this across our grow area

4. Scaling to a 7.7 × 8.1 m Area (62.37 m²)

4.1 Total PPF required

To achieve a uniform 45 μmol·m⁻²·s⁻¹:

4.2 Module count (ideal)

So you need at least 20 modules’ worth of photon output, but beam overlap and uniformity drive the exact count upward or downward.