For the most part, when we see light intensity requirements for plants, they’re listed as PPFD or 150-250 µmol/m²/s, but what does it mean? I know most lights come with other values like kelvin and lumen(lm), but how can I know how much PPFD my light will give my plant?
I can say as much as this: If you need to know precisely how much PPFD your light will give your plants, you will need to buy the equipment, but those are kind of expensive, starting at around $250 for the cheapest, not sure how precise these are, or if it’s a lux meter with a PPDF/PAR sticker..
So I’m going to do the next best thing and create a calculator that takes you from lumen to lux and from lux to estimated PPFD. An exact calculation would not be possible without the exact light specter. I’ve tried a few of the free “PPFD”-apps, but those rearly give anything else than lux and lumen anyway.
PS: let me know if you find the calculator helpful with a comment below!
Normal light values explained
What is a watt?
A watt is a unit of power in the International System of Units that measures the rate of energy is used (or created). 1W(watt) = 1 J/s (joule per second), so it represents the energy consumed per unit of time. However when a light is given a value it's given in W/h (Watt per hour), i.e., 60W/h, but in many cases it's just mentioned as "60W" on the packaging, which is actually misleading.
What is lumen(lm)?
Lumen is the value most normal lights(i.e., non-grow lights) come supplied with. It's the amount of light created at the source that it can give off. Lumen is defined in the International System of Units (SI) and measures luminox flux, which means the total perceived brightness of light to the human eye.
What is Kelvin(K)?
Kelvin is another unit of measurement from the International System of Units, it is a measurement of temperature on the light, where 0K aka absolute zero represents the theoretical point where all motion of molecules stops and no thermal energy is left. Most lights come with a value between 2700K(warm, more red light) to 6500K(cold, more blue light). 4000K is considered neutral white light and is often used in offices, kitchens and bathrooms.
What is LUX?
Lux is the calculated average light value of the area the light source have to shine on with it's given lumen rating. In short lumen divided by area in m². If you have a light with 4lm (lumen) that shines on 1m² (1m * 1m) the result would be 4 lux. But if the light shines on a room that is 2m * 2m the area is 4m², and the result would be 1 lux. Examples of lux levels: Bright sunlight 100.000 lux, Overcast day ~1000 lux, Office lighting/kitchen 250-500 lux, living room 100-250 lux
PAR vs PPFD
PAR refers to the range of light wavelengths (400-700 nm) that plants use for photosynthesis. It is a measure of the light spectrum that plants can absorb and use for growth.
PPFD is a measure of the amount of PAR that actually reaches a surface, like a plant canopy. It represents the intensity of photons within the PAR range that hits a square meter every second and is measured in µmol/m²/s.
Myths
Incandescent light is just as good as any other light.
Yes, in fact the light from an incandescent light source does work, it still gives of light a plant can use. However, this type of light is based on heating a filament to the given kelvin rating, i.e., 2700k = 2426°C / 4399°F so this type of light gives of much more heat(infrared) than light, and is a poor choice for a grow light. As much as 90% of the power used is given off as heat instead of light, this is about the opposite of LED that turns up to 90% of the energy used into usable visible light and only 10% as heat, additionally LED can be produced to give off certain wavelengths of light.
RED and BLUE (blurple) LED light is best
You might have heard that blue and red LED(light emitting diode) in specific spectrum is all you need, but tests have shown, especially for taller plant that the green spectrum seems to penetrate further down in the canopy of plants, so that using only these red and blue(blurple) LED is not as good as full daylight, especially for taller plants. Not to mention you get totally weird in the head by looking at them.
However, having extra blue and red LED can indeed increase the PAR(photosyntetic active radiation) light used by plants, while including warm(3000K) and/or cold(6000K) LED would further increase the result for your plants. This is why most modern grow lights include some white LED additional to red and blue LED.
Here is a summary of different papers:
White LEDs with Extra Blue and Red: When plants like lettuce are grown under white LEDs with added blue and red light, the broader spectrum typically results in more balanced growth and better photosynthetic efficiency. White light includes a wider range of wavelengths, which can help plants develop more naturally. Studies found that white LEDs, especially when enhanced with extra blue or red light, promote compact growth and improve chlorophyll production(SpringerLink)(ASHS). In many cases, this combination leads to plants having higher nutritional content, such as increased chlorophyll fluorescence and leaf mass(ASHS).
Red and Blue LED Combinations (Blurple): Using only red and blue LEDs (the typical blurple lights) is efficient in driving photosynthesis because these colors align with the peak absorption of chlorophyll. However, exclusive use of red and blue lights can sometimes cause abnormalities in plant morphology, such as excessive elongation under red light or leaf thickening under blue light alone. Mixing them in optimal ratios can balance these effects, leading to better biomass production. For example, studies on cucumber and lettuce showed that a red-to-blue ratio of 9:1 produced the best growth outcomes(SpringerLink)(MDPI).
Both lighting approaches have their strengths, but using white light supplemented with red and blue tends to offer more consistent results in terms of balanced growth, higher yields, and better plant health.
UV light
Some tests I read showed that adding some UV LED increased some of the oils in the plant used to protect themselves against the UV, resulting in better taste and smell. I Believe some herbs like basil and oregano was used for the test. Some red plants don't turn red indoors or in greenhouses with UV protection, but does turn red in full sunlight, it was long believed that UV light was the cause.
Here is a summary of different papers:
UV-A (315–400 nm) has been found to stimulate biomass production and enhance the accumulation of secondary metabolites like anthocyanins and flavonoids, which can improve plant quality. For example, experiments on indoor-grown lettuce showed that adding UV-A increased leaf size and shoot dry weight, making it a useful tool for increasing yield in controlled environments(MDPI)(Frontiers).
UV-B (280–315 nm), while a small component of sunlight, can induce the production of protective compounds in plants, such as phenolics and flavonoids, which help plants cope with stress and can improve their resistance to pests and diseases(PLOS). However, excessive exposure to UV-B can cause damage to DNA, impair photosynthesis, and reduce reproductive success in some species(Frontiers).
UV-C (<280 nm) is generally harmful to plants and is not used in horticultural settings for growth purposes, as it can damage cellular structures and cause significant stress(MDPI).