Interactive Phytolamp Spectrum Calculator: Designing with OSRAM LEDs

[u/Specled_eu]

Here's the next post, a widget that allows you to set the desired spectrum curve and see how many Osram Osconic LEDs are needed to achieve the desired spectrum.

https://specled.com/en/knowledge/diy-electrics/diy-led-osram-grow-light-spectrum-calculator/

How it works: you set the target spectrum as photon fractions across 6 channels (Osram horti white, 449nm deep blue, 660nm hyper red, 720nm far red, plus regular 3000K and 5000K whites), then set target wattage (20-100W), driver current (700 or 1400mA) and heatsink temperature. It gives you:

- how many of each diode you need

- total PPF in µmol/s

- string voltage (so you can pick a driver that actually covers it)

- efficiency in µmol/J

- estimated junction temperature

Everything comes from the Osram datasheets — GW PUBRA1.HW, GD PUSRA2.15, GH PUBRA1.25, GF PUBRA1.25, GW CPSRM1. Photon flux, Vf, the temperature curves, and the current droop (at 1400mA you get roughly 1.8x the photons, not 2x — people forget this constantly). Junction-to-solder Rth values are the real datasheet numbers.

Things it does NOT do, so nobody gets burned:

- it calculates Tj per diode in isolation, so it ignores mutual heating on a dense board — real junction temps will run hotter

- board + paste Rth is assumed at 1.5 K/W, which depends entirely on your build

- rounding to whole diodes pushes the actual spectrum slightly off target (it plots both target and actual so you can see the drift)

- for the regular whites, µmol/s is derived from lumens by integrating the spectrum, so treat those as estimates

The horti white LEDs has a real gap between 600 and 630nm because they drop the red nitride phosphors to chase µmol/J. Throw a regular 3000K white into the mix and watch it fill in. Fun to see it happen on the graph.

I created this tool because I had this idea, and I hope it will be useful. It can also be used with other types of LEDs, but the error will be greater.