Lumens are for Humans but PAR is for Plants!

Prism Splitting Light

It blew my ten-year-old mind when my “all knowing” grandmother told me that the Blue Jay we were watching was in fact not blue. She explained that light is composed of many colors, and it is the colors that are reflected, not absorbed that our eyes perceive as the color of an object. This is a necessary reminder that what is perceived might not be what it appears to be. For decades the indoor gardening community has used Lumens as the standard increment for the measurement of light. Lumens were unfortunately a poor choice, here’s why.

Diagram of How a Prism Works
coutesey of freedigitalphotos.net

Lumens are essentially a measure of brightness based on human perception. Precisely, a lumen is equal to the light emitted by one candle falling on one square foot of surface located one foot away. This however presumes a human as the perceiver of the light. Plants “perceive” light differently; from a plant’s perspective, light that is useful for photosynthesis is not necessarily bright. Light, or more specifically, visible light is made up of wavelengths of energy on the electromagnetic spectrum ranging from 380-770 nm (nanometers). Plants utilize wavelengths from 400-700nm for photosynthesis. Brightness does not accurately describe if the light will be more or less useful to a plant.

Light can be characterized in other ways when discussing its benefit to plants. Color temperature is often referred to in the horticultural industry on lamp boxes to describe the color of the light emitted by the lamp. Does 4,000K grow a plant better than 7,500K? Color temperature is listed in Kelvin (K), which is a measurement of temperature. The temperature of what you might ask? It is a description of the relative whiteness of a piece of tungsten steel when heated to that particular temperature in degrees Kelvin. This accurately characterizes the color of the light as we perceive it, but color temperature again fails to address how effective a particular light source will be at providing the energy necessary to drive photosynthesis.

Don’t get frustrated by this inadequate information. There is in fact a measurement that precisely describes how effective a particular light will be for growing plants; PAR (Photosynthetic Active Radiation). PAR spectrum accounts only for light or more precisely photons emitted between 400-700nm. Scientists have concluded that it requires about 9 photons to bind one CO2 molecule in photosynthesis [6CO₂ + 6H₂O (+ light energy) C₆H₁₂O₆ + 6O₂]. Even though blue photons have more energy it has also been found that there is little difference between the effectiveness of red versus blue photons at driving photosynthesis as long as the photons are within the 400-700nm range. This leads to a direct correlation between the number of photons produced in the PAR spectrum by a given light, and the photosynthetic potential of that light.

Photons are emitted by light sources in very large numbers so we do not talk about billions or quadrillions of photons, instead we refer to them using the multiplier moles (which stand for 6.0221415 x 10²³) To make the numbers even more accessible, the number of moles is often divided by 1 million resulting in micro-moles (μmol). Light sources emit photons continuously over time so the number of micro-moles is more accurately described as μmol/per unit of time (most commonly seconds).

When trying to quantify how effective a light source is beyond the total output of μmol/per second, you must consider one last piece of information… the size or area of your garden. Inevitably some of the photons produced will not reach your garden. So the most accurate representation of a light source’s ability to drive photosynthesis will take into account the area being lit and how many photons reach that given area per second; usually a square meters. That representation which actually summates the effectiveness of a light source for photosynthesis is written as μmol/m²/s. This descriptor is actually referred to as Photosynthetic Photon Flux Density or PPFD for short.

So in light of all of the information above, let’s remember that lumens are not a useful descriptor of a light’s ability to drive photosynthesis. I think I will sit back with a drink, and digest all of the information about PAR & PPFD while I watch the not blue Blue Jay outside my window.

Indoor Garden Lighting (Part 2): Choices, Choices, Choices!

Photo courtesy of Freedigitalphotos.net

In my prior blog I discussed the obstacles that indoor gardeners must contend with when using horticultural lighting as the primary source of light for their gardens. With those challenges of indoor garden lighting in mind, lets review what options indoor gardeners have when selecting lighting.

Fluorescent lighting comes in a variety of sizes, shapes, wattages and styles. The old standard 4’ shop light T12 bulbs (40 watt) produce about 2,600 Lumens. The newer 4’ T8 bulbs (40 watts) produce about 3,200 lumens. The preferred horticultural fluorescent lamps are T5 bulbs (54 watt) and produce about 5,000 lumens per 4’ bulb. There are also compact fluorescent lamps (or CFL) that have built-in ballasts in either 125, 200, or 250 watt options that produce about 45 lumens per watt.

Lamp Type	4’ T12	4’ T8	4’ T5	Compact Fluorescent
Lumens Per Watt	65	80	92.6	Roughly  45

The pros of fluorescent lighting are that they have relatively low wattage, can be utilized in all sorts of size areas and configurations, and they are more affordable than some of the other options. However, fluorescent lighting does not deliver the intensity of light to provide good penetration through the canopy. There are also issues with  fluorescents lacking the intensity necessary to produce fruit on high light plants. Lastly fluorecent lights give off a good bit of heat. They are an excellent choice for mother plants, clones, and young seedlings.

HID or high intensity discharge lighting has been the standard lighting for horticulture for decades. Although they produce an enormous amount of heat, they are able to provide good penetration of light through the canopy to about 3 feet. Utilizing switchable ballasts & different bulbs you can choose a spectrum heavy in blue light (Metal Halide) for vegetative growth, or heavier in red light (High Pressure Sodium) for flowering and fruiting.  HID lights are reasonably priced, they are proven as a primary or sole light source, and they able to produce enough intensity of light to allow high light plants to produce excellent crops. For more information about HID lighting, check out this helpful HID lighting guide.

Plasma Lights are a type of electrodeless lamp energized by radio frequency or microwaves. The interest in this type of lighting is driven by two factors: spectrum and the potential for financial savings (based on lower electrical consumption). The spectral output of a plasma light is almost identical to the light spectrum of the sun making it ideal for horticultural applications. Plasma lights are also capable of producing large amounts of light from relatively small amounts of electricity yet they still produce large amounts of heat. Currently there are Plasma Grow Lights available for sale made by Gavita Lighting in Holland but they are still in their respective infancy.  Based on my personal testing I will say they hold a LOT of promise.

LEDs or Light Emitting Diodes are starting to draw a lot of attention, not just from indoor gardeners but also from the general public. There are LED bulbs to replace your standard incandescent household light bulbs, LED flashlights, and even LED wallpaper. Why this explosion of LEDs, and can we as gardeners benefit from LED Grow Lights? Stay tuned to future blogs to find out.

Indoor Garden Lighting (Part 1): Obstacles to Overcome!

Image courtesy of freedigitalphotos.net

“Fiat Lux,” Latin for “let there be light,” is the famous quote that begins the third verse of Genesis.  We all know that plants need light to carry out photosynthesis and the best (and cheapest) way to get it is to use natural sunlight.  Alas, that is a luxury many of us don’t have.  We are relegated to growing our plants indoors; in closets and cubbies, in basements and bedrooms.  So for us, what is the best way to light our gardens?  We have several options: Fluorescents, HID (Metal Halide or High Pressure Sodium), Plasmas, and LEDs.  What basic grow light information do we need to know when we choose how to light our garden(s)?  First we need to know what our obstacles are…

One of the biggest problems we as indoor gardeners face is that most of our lighting options produce heat, a lot of heat; approximately 3.4 BTU (British Thermal Units) per hour/per watt.  That means that a single 1,000 watt HPS light system generates 3,400 BTU of heat every hour.  A BTU is the amount of energy required to raise 1 pound of water 1 degree Fahrenheit.  To give you an idea how much heat that is; each 1,000 watt HPS light radiates enough heat into a grow room to boil 3 gallons of water an hour!

Another obstacle for indoor lighting is the penetration of light through the plant canopy.  Light diminishes from its source with distance.  The relationship of light emitted from a point source (a bulb) and distance is known as the inverse square law.  The law states that the intensity of light changes in inverse proportion to the square of the distance or I (intensity) = L (light) / D (distance.)  That means that the intensity of light 2 feet away from the source is 25% of the intensity 1 foot from the same source.

The last challenge of indoor grow lights is that most lighting requires huge amounts of electricity.  In order to cover a 10’ x 10’ room high light plants require 4,000 watts of lighting or more.  The average American household uses 14,000 watts of electricity per day according to the Department of Energy so the above mentioned garden would account for 28.6% of their total electrical consumption.  Use this handy electrical usage guide to find out how much your electricty will cost per month.  With the cost of electricity ever on the rise the search for more efficient horticultural lighting has LED down several roads…  Check out part 2 of this blog series for a review of different types of horticultural lighting and how they stack up.