We are used to units of illumination that involve human vision, such as footcandle, lumen, and lux. But plant biologists of various kinds have been interested in measuring Photosynthetically Active Radiation (PAR) from 400-700 nanometers (nm).

In 1972 Keith McCree showed that photosynthesis was better predicted by counting photons than using footcandle and lux measurement meters, leading to the development of devices called quantum meters.

All this lead to the designation of the "Einstein" as a unit of light emission..

A whole Einstein is defined as 1 mole of photons, in which one mole is 6.022x10²³ (Avogadro's number) photons. The unit used for PAR is defined as a millionth of an Einstein per second, or 6.022x10¹⁷ photons per second, limited between 400 and 700nm, i.e: [align=center]The microEinstein (µE)[/align]

Note, however, that the Einstein is not an official SI unit of measure.

For comparison: The power in Watt of a single photon equals h.c/λ/s (in which h= Planck' s constant (6.63×10⁻³⁴ m².kg/s), c = Velocity of Light (299792458 m/s) and λ = The Wavelength in m, which at 555nm works out to: 3.58x10⁻¹⁹ Watt per photon. The PAR value for that very same photon would be: 1.66x10⁻¹⁸

Now, this may be fine for those who are comfortable with the theory and have enough money to buy quantum meters, frankly in my opinion:

[align=center]This does not suit us at all.[/align]

- None of our lights are characterised in PAR. They are all graded in Watt, and I don’t like complicated conversions when I’m in a shop.
- I (and many more like me) will never buy a PAR-meter in a hurry.

So where will this leave us?

The answer is that we should return to Watt and assume that you get about:

[align=center]one PAR for every Watt installed.[/align]The true relation varies by ± 10% for full spectrum tubes, so just remember that most “quality” lights you buy may give you 10% more or 10% less from that assumption. Hi Kelvin lights can give very different ratios.

Now that is not a bad estimate for our use. All we can reasonably estimate is how much light actually hits the water surface (that is if you know your reflector efficiency). As for the rest:

We are completely in the dark about how much light reaches the bottom of our tanks, it depends on the wavelength, the colour of the water, the clarity, and how many objects and plants inhibit the light propagation, not considering how much light actually enters the water after the reflection loss at the surface interface, and never mind the variation in the power of the tube, which is a rated number and NOT the actual number. It can be assumed that the actual power is probably somewhere between +5% and -5% of the rated power.

So, in my opinion we shouldn’t be too pedantic about the true relationship between PAR and Watt. I’m sure we can manage well with this simple rule of thumb.

Just remember that measured PAR values are in PAR/m², because that is the only way the quantum meter can measure the light density in a particular spot. So when Tom Barr reports that most plants will grow in 30 – 40 PAR, he actually means: 30 – 40 PAR/m² (or according to our little rule: 30 – 40 Watt/m²).

For info: 1m² = 1550 sq inches or 10.8 sq foot

Furthermore:

- Densities < 10 W/m² is considered Low Light.
- Densities between 10 and 30 W/m² is considered Medium/Low Light,
- Densities between 30 and 50 W/m² is considered Medium Light.
- Densities between 50 and 100 W/m² is considered Medium/Hi Light.
- Densities > 100 W/m² is considered Hi Light.