Why don't "98% reflective" HID aluminum reflectors actually reflect 98% of the light toward your plants?

Many HID (HPS, MH, LEC / CMH) lighting fixture manufacturers advertise that their aluminum reflectors are 98% reflective, and therefore reflect 98% of the light their bulbs aim at the ceiling back toward your plants. They are technically correct in that for a single wavelength (color) of light, 98% of it is reflected by the special aluminum, but this is not the case with all wavelengths of light in the PAR region of the spectrum.

material reflectance versus wavelength

This graph of material reflectivity by wavelength is courtesy of Orca Grow Film, which makes the most-reflective film you can hang in your growing area- we use it in all of our grows to direct more light back to our plants.

For some PAR wavelengths, only 88% of the light is reflected from the highest-end aluminum. And this is only if the HID light reflector is perfectly clean - dust and even oil from fingerprints on the reflector can significantly reduce reflectivity

Furthermore, many photons need to be reflected multiple times inside the reflector before finally being directed toward your plants. With 12% or more of the light getting lost with every reflection, a lot less of that upward-directed light is getting to your plants than you think.

Our LEDs only aim light downward toward your plants. There is no need for a reflector that gets dirty over time and reflects less light.

If you look at the actual PPFD averages we can deliver to your plants rather than the total light output of the "bare bulb", you'll see that our lights really can deliver more light per watt to your plants than HID.