LED Grow Light

Understanding electromagnetic spectrum, solar spectrum, photons, microns and micrometres (µm), nanometres (nm), PAR and beyond, photosynthesis, chlorophyll. Measuring LED grow lights with PPFD and PPF.

This is the core information LED horticulturalists should know. We have tried to keep this brief, factual and easy to read – here goes…

Solar radiation and PAR

Energy emitted by the Sun is known as solar radiation and includes the solar spectrum covering ultraviolet, visible and infrared. Solar radiation from the Sun is the primary source of light and energy for all life on Earth. Plants use some of this energy to grow. Solar radiation is one part of the broader electromagnetic radiation (EMR) spectrum which also includes radio waves, microwaves, x-ray and visible light.

EM spectrum comparison


All EMR is simply a stream of rays (mass-less particles) travelling at the speed of light, called photons. The full range of the EMR spectrum (including visible light and PAR) is measured in both wavelength and frequency. All energy has a wavelength consisting of high and low troughs like waves in the sea. While the wavelengths flow at a certain frequency measured in cycles per second or hertz (Hz).

Wavelengths can be measured in either ‘microns or micrometres’ (µm) which are equal to 10-6 meters or measured in nanometres (nm) which are equal to 10-9 meters. Either µm or nm can be used as a wavelength measurement, just like 1,000 metres can also be 1 kilometre so too can 400 nm = 0.4 µm and 800 nm = 0.8 µm.

The spectrum of the Suns solar radiation predominantly extends from ultraviolet 290 nm to 3,200 nm  which is past far red and in to infrared. However plants only use a portion of this spectrum. Plants absorb violet, blue, green, yellow, orange, and red light for photosynthesis. These wavelengths are between 400 and 700 nanometers and this portion of the spectrum is known as PAR (Photosynthetically Active Radiation).

However, PAR is a broad term and plenty of academic research shows plants absorb and use more than just the PAR spectrum. Plants absorb energy from at least 380 nm to 780nm. However, most LED grow lights barely cover PAR let alone harnessing the benefits of small amounts of UVA and far red.

We have specifically designed our LED grow lights to include UVA and far red within our spectrum. The UVA 380 nm helps with disease and pest resistance while the far red from 750 to 780 nm helps keep plants squat and dense. When plants can’t absorb much fa red if they get none, they assume they are being shaded and grow taller to find light known as shade-avoidance syndrome (SAS). So w had some far red in to keep your plants squat, dense and high yielding.

Left LED with limited spectrum illustrates shade avoidance response, (SAR) growing tall stretched out plants. Compared to the squat and dense plants produced by full spectrum LED with far red included.

Chlorophyll absorption

Chlorophyll is a molecule. Molecules can be simple or complex, simple examples are H2O (water) and N2 (nitrogen) while more complex molecules include chlorophyll.

Chlorophyll is the green pigment required for plants to convert carbon dioxide and water, using sunlight, into oxygen and glucose through the process of photosynthesis.

Different molecules absorb different sections of EMR energy. Water absorbs microwaves and it’s why  we use  it in cooking, while calcium found in bones absorbs x-rays far more than the surrounding tissue. This enables us to generate images of our skeleton. There are three key pigments in photosynthesis each with different absorption peaks.

(1) Chlorophyll a absorbs mainly violet-blue and orange-red light and peaks at wavelength circa 430 nm and circa 660 nm.

(2) Chlorophyll b primarily absorbs blue light with peaks at circa 460 nm and 660 nm.

(3) Carotenoids a key group of pigments (lycopene red of a tomato, zeaxanthin yellow in bananas and β-carotene orange peel color) absorption wavelength is 460-550 nm in violet to green light.

Absorption spectra of chlorophyll and carotenoids. University of New Hampshire

Absorption spectrum of chlorophyll versus the solar spectrum. Carnegie Mellon University.

** There are actually many types of chlorophyll; a, b, c and d however a and b are the main ones.

Green light is broadly reflected by plants and generally not absorbed. Therefore our LED grow lights minimise the access to green light which saves you electricity. However we include some green for organic pest control and to maintain a broad spectrum required for healthy high yielding plants.

LED Grow Light Measurement – PPFD and PPF

Lumens – Measured either by LUX (lumen/m2) or foot candle (lumen/ft2). Beware as LUX only measures the Photopic Response Curve which is visible light to the human eye. However we know plants ‘see’ absorb more than that just the visible spectrum. So…never use Lumens, LUX and foot candle when comparing LED grow lights.


PARPhotosynthetically Active Radiation are the wavelength of energy (light) that plants ‘see’ and absorb between 400 – 700 nm. In reality plants usage spans 380 to 780 nm. PAR is not a measurement like miles or inches, so we don’t use PAR to measure LED grow lights power just what spectrum in nanometres they cover.


PPF – Photosynthetic Photon Flux measure all photons within the range 400 to 700 nm equally. Measured in micromoles per second (μmol/s).

Think of a garden hose and assume the water is PAR spectrum pouring out. PPF would  simply show ‘PAR’ anywhere the water is found… it’s all PAR. However, PPF doesn’t outline the number of photons (density or flow) landing – how strong is the water flow. PPF just tells you it’s PAR and not some other part of the EM spectrum, but doesn’t outline how much is flowing out.

**Using the PPF number and the LEDs input wattage you can work out how efficient an LED grow light is at converting electrical energy into PAR. Measuring Watts is in Joule per second (J/s). Therefore μmol / J = X and the higher X is, the more efficient your LED grow light is.


PPFD – Photosynthetic Photon Flux Density measures the amount of PAR that ‘rains down’ on the plant. The number of photosynthetically active photons that fall on a given surface each second measured with micromoles per second per sq. m. (μmol/m2/s).

Think about the hose with water ‘PAR’ spraying out onto a wall 10 feet away. Up close to the nozzle, in the middle, there is loads of ‘PAR’  (so high PPFD) but further away it disperses and is less intense, so lower PPFD A PPFD meter is a spot measurement of any point under a light. A PPFD meter will show the highest numbers directly under the light, closets, in the middle, and this number will decrease as you move further away down or outwards.

LED grow light manufacturers should publish the distance a PPFD measurement is taken both vertically and horizontally and how many measurements they took to calculate averages.

Here at Super Grow LED we use an UPRtek spectrometer. Note, it has been designed to specifically cover beyond PAR at 380 to 780 nm. The sensor head conforms with the JIS AA Class and DIN B Class Illumination requirements.


How do you compare one LED grow light with another?

  • The spectrum should be beyond PAR from a balanced and considered mix of LEDs. The FATPHARM250 has been specifically designed by our team to offer full spectrum lighting from 380 nm to 780 nm;
  • Build quality – Is it fan-less with aluminium heat sinks? Is the LED driver an industrial grade brand like MEAN WELL;
  • LEDs from known brands like EpiLED and Bridgelux;
  • What is the true draw in Watts, not just the advertised. Calculate it here with volts / amps and Watts;
  • What is the PPFD on average at certain heights.

Take a look at the UPRtek FATPHARM250 Spectrometer Report

We have sold these lights since 2011, mainly in the USA and since then refined the modular construction, driver, the LED type and mix for the indoor UK market. We deliver full spectrum LED grow lights within an industrial grade horticultural fixture designed to be quiet and maximise yields.