Posted on by Alex Russell

Light Conversions and Smart Phone Light Meters

When growing plants, light provides the initial energy source for growth. The quantity and spectrum of the light is important for development of the plant, just as we measure the quantity of nutrients we feed to our plants, we must also control the quantity of light we deliver to our plants, this can be measured with various 'Light meters'.

Light meters come in various forms, from Lux meters to Quantum Meters, the most popular and commonly found is the Lux Meter, which measures Lumens exclusively.

Apogee Instruments have a range of amazing light meters, most famously known for their quantum meters range, but due to the nature of the technology they also come at a high price, not everyone will have the budget to afford one. So what other solutions are there to measure the quantity of light in their grow room?

For those growers without the budget for a proper Quantum Meter, we have found a few 'Light Meter' apps that will allow you to get a rough ball-park number measurement of the quantity of light your grow light is omitting in the grow room. 

These Light Meter apps use your phone's camera to capture the light in the form of a Lux or Foot-candle reading, which can then be converted to other metrics, such as PAR, for a very rough measurement of the quantity of light your plants may be receiving.

Alternatively, the better version of these apps use a lux meter adapter for your phone to take measurements instead of your phone's camera.

Keep in mind that Lux meter measurements and phone cameras cannot measure all PAR light or light that sits slightly out of the 400-700nm range that plants can still utilise, therefor measuring light with your phone using these apps will only be able to give you a very rough, not precise, measurement compared to an accurate reading from a quality Quantum sensor.

For the best budget app based light meter, we recommend you use a phone lux meter adapter for more accurate readings, as the phone camera can only do so much.

Below is a graph taken from the Apogee Instruments website which will help you do the math when converting the Lux (lumens) or Foot Candle (FC) reading that your phone gives back to PPFD.

The PPFD is the measurement of the quantity (umol) of light at plant canopy per square meter, per second.

PPFD (µmol m-2 s-1) to Foot-candles

Light Source Conversion Factor
Sunlight 5.01
Cool White Fluorescent Lamps 6.87
Mogul Base High Pressure Sodium Lamps 7.62
Dual-Ended High Pressure Sodium (DEHPS): ePapillion 1000 W 7.11
Metal Halide 6.60
Ceramic Metal Halide (CMH942): standard 4200 K color temperature 6.01
Ceramic Metal Halide (CMH930-Agro): 3100 K color temperature, spectrum shifted to red wavelengths 5.45
Low Pressure Sodium Lamp (monochromatic at 589 nm) 9.85
Multiply the PPFD by the conversion factor to get foot-candles. For example, full sunlight is 2000 μmol m-2 s-1 or 10,020 foot-candles (2000 ∗ 5.01).

 

Foot-candles to PPFD (µmol m-2 s-1)

Light Source Calibration Factor
Sunlight 0.200
Cool White Fluorescent Lamps 0.146
Mogul Base High Pressure Sodium Lamps 0.131
Dual-Ended High Pressure Sodium (DEHPS): ePapillion 1000 W 0.141
Metal Halide 0.152
Ceramic Metal Halide (CMH942): standard 4200 K color temperature 0.167
Ceramic Metal Halide (CMH930-Agro): 3100 K color temperature, spectrum shifted to red wavelengths 0.184
Low Pressure Sodium Lamp (monochromatic at 589 nm) 0.102
Multiply the foot-candles by the conversion factor to get PPFD. For example, full sunlight is 10,020 foot-candles or 2000 μmol m-2 s-1 (10,020 ∗ 0.2).

 

Table source: https://www.apogeeinstruments.com/conversion-ppfd-to-foot-candles/