All light, no matter the source, is a group of photons. Photons are tiny particles with both mass and a wavelength. Depending on the wavelength, the photon radiates a certain light color. Blue, green, yellow, and red all have different wavelengths. Lux & lumen and PPFD & PAR are measurements of light quantity/intensity. Lumen and lux have been used for a long time and they work well to measure white light.

PFPD, or photosynthetic photon flux density, and PAR, or photosynthetic active radiation, is a bit more technical term but the theory is the same: it tells us how much light is emitted. PPFD can be used for all light but comes especially in handy for colored light, i.e. non-white light, which a large number of grow lights emit. 

Lumen and lux go hand in hand.

Lumen is used to describe how much light in total a lamp emits while lux is used to describe how much light hits a certain spot.

Let’s take an incandescent light bulb as an example. If we were to fix this bulb a certain distance, let’s say 6” (15 cm) above a plant, no matter in what angle, only a portion of the light would actually hit the plant. Some, or most, of the light would be projected in different directions.

If we’d want to describe how much the total light output from this bulb is, we’d use the term lumen. If we’d want to describe how much of the light that hits the plant, we’d make spot measurements in lux.

Same thing applies to PAR and PPFD where PAR = lumen and PPFD = lux. PAR describes the total photon output from a light source whereas PPFD describes how many photons land on a specific area.

The main difference between PPFD and lux as well as PAR and lumen is that PPFD and PAR measure not only white light, but a spectrum from 400 nm to 700 nm. This would be from violet (ultraviolet, UVA, ends at 400 nm) to deep rep (far red begins at ~700 nm).

In other words, PAR and PPFD is a more accurate way of measuring light of color. As many grow lights on the market today use only blue and red diodes, or some use dominantly blue and red diodes, PAR and PPFD gives a more accurate measurement of these specific colors.

PAR and PPFD are often confused where PAR is used instead of PPFD.

Regardless, the unit for both PAR and PPFD is umol/m2/s, micromoles per meter square per second. This tells us how many micromoles of photons hit a certain spot every second.

Using a quantum meter, an Apogee MQ-500 for instance, light intensity can easily be measured. There are other light meters or “knock-off” quantum meters widely available but these meters typically measure light in lux then convert the lux value into PPFD. Not very accurate or reliable.

A light meter that measures in lux is significantly cheaper and easier to manufacture than a quantum meter. A budget light meter sells for $10-30 whereas a proper quantum meter is several hundred dollars. 

For a small scale or hobby grow, it’s usually not worthwhile or economically justified to invest several hundred dollars in a quantum meter so a lux meter could be used as a substitute, but be aware that the result is not always accurate, especially with the low-end light meters.

There are various sites online that convert lux to PPFD if you’re in need. Here’s one:
https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator

We’ve tested converting lux to PPFD ourselves using a budget lux meters and one MQ-500.

We tested with two different LED strips and LED brands, Nichia and Samsung.

Here are the results:

Optisolis Nichia NF2W757GT-F1 5000K CRI 97 LEDs

Samsung LM301B 4000K CRI 80

We see clearly that a constant and universal ratio between PPFD and lux is difficult to conclude. PPFD 200 umol/m2/s and below the ratio to convert PPFD to lux is a factor of ~47, i.e. PPFD 100 umol/m2/s ~ 4700 lux. But as the PPFD levels increase above 300, the ratio increases to ~60, at least with our lux meter.

Anyhow, If you only have a lux meter at your disposal and want to estimate what the lux values would be in PPFD, these tables may be of help. Keep in mind that both LED light sources in our experiment are white light. If your lamp has multiple coloured diodes and emits a pink or red/blue light, then the ratio will most likely be different. The difference will vary depending on how dominant the coloured diodes are compared to the white diodes.