Photosynthetically active radiation

PAR stands for "Photo-Synthetically Active Radiation". PAR is a classification of light (Micro Einsteins) also known as Micro Moles per second, per square meter.

PAR is used to determine the capability of a light to drive photosynthesis. In the past, we have seen light measured solely in lumens. Lumens measure the power of light perceived by the human eye. Our eyes peak somewhere in the green-yellow spectrum, but a plant has far less sensitivity. PAR is the range within the light spectrum between 400nm-700nm that plants absorb and are able to use for the integral stages of photosynthesis. Micromoles are the measurement we use to express a PAR reading which. A micromole is the photosynthetic photon flux density of light, per square meter, per second.

Most LED light manufacturers use Lumens as the unit to measure bulb brightness. This means very little for the growth of your plants. Micro moles give a more accurate measurement of what your plants are absorbing the because the plant requires different wavelengths.

Here are our product Par data (tested by Li-Cor 250A Quatum sensor)

Note: If you give a plant too much light, or “saturate” it, the growth may be stunted and they may even die from shock

About Par meter:

An ideal quantum sensor would give equal emphasis to all photons between 400 and 700 nm and would exclude photons above and below these wavelengths.

There are two popular quatum par meters available in the market, Apogee and Li-Cor

Below are review of the quantum meters from two different manufacturers:

Apogee Quantum meter:

Quantum Sensors and Quantum Meters measure Photosynthetic Photon Flux (PPF) in μmol m^-2 s^-1.

The spectral response of the Apogee Sensor used in Quantum Meters and the Quantum Sensor is shown at right. As the figure indicates, the sensor underestimates the 400 to 500 nm wavelengths (blue light), overestimates the 550-650 wavelengths (yellow and orange light), and has little sensitivity above 650 nm (red light).

Fig. Apogee quantum sensor/meter response (blue line) compared to defined quantum response (black line) of equal sensitivity at all wavelengths between 400 nm and 700 nm

The Good: Portable, easy to use, cheaper

The Bad: Not accurate for LEDs

The bottom line: If you are not a professional, a $300 meter is affordable if you are looking for a rough estimate, but you need to understand that 400-500nm is underestimated and 550-650nm is overestimated.

Li-Cor Light meter with sensor Li-Cor 190

Accurate measurements are obtained under all natural and artificial lighting conditions because of the computer-tailored spectral response of the LI-190. Colored glass filters are used to tailor the silicon photodiode response to the desired quantum response. An interference filter provides a sharp cutoff at 700 nm, which is critical for measurements under vegetation where the ratio of infrared to visible light may be high. A small response in the infrared region can cause an appreciable measurement error. This sensor, developed from earlier work (1), was pioneered by LI-COR and has become the standard for PPFD measurement in most photosynthesis-related studies.

Li-Cor Light Meter

Typical spectral response of LI-COR Quantum Sensors vs. Wavelength and the Ideal Quantum Response (equal response to all photons in the 400-700 nm waveband).

The Good: Accurate, multiple use meter

The Bad: Expensive

LED Grow light and Par meter:

Since two main wavelength (440-470nm and 630-660nm) are used in LED grow lights, the quantum sensor must be sensitive to their wavelengths (refer to the manufacturer spectral diagram). Li-Cor is the best for LED grow lights par testing. All of our data is tested using a Li-Cor Quantum Sensor.