Light energy is an extremely complex property to measure. In order to better understand light energy and its relation to our world, we have developed many ways to quantify light.

Light measuring terminology

Luminous Intensity: a measurement of the light power emitted from a point source within a solid angle of one steradian (light is emitted in a spherical shape and a steradian is a cone section of the sphere used as a standard unit of measurement, stemming from the point of luminance).

Luminous Flux: the measurement of the perceived power of light. The unit of measurement for luminous flux is the lumen (lm).

Lumens: Units of measure for luminous flux. One lumen represents the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.

Candela: Unit of luminous intensity based on the standard luminous intensity per square centimeter of a blackbody radiating at the temperature of solidification of platinum (2,046ºK). One candela equals 1/60th of the luminous intensity as the standard blackbody intensity.

Lumens are for humans: The luminosity function

For me, and many others, the common terminology used for light measurement is just downright confusing. The good news is that only a basic understanding of lumen-based measurement needs to be had to evaluate horticultural lighting sources for their effectiveness on photosynthesis.

The most important concept to understand is the concept of the luminosity function, which is the standardized model of the human eye’s sensitivity to different wavelengths. Lumens, candela, luminous flux, and luminous intensity are all intertwined and weighted by the luminosity function. What this means is all lumen-based measurements are structured around a human’s ability to perceive light.

What is most interesting about luminosity function, however, is that a human’s perception of light differs greatly from a plant’s perception of light. The human eye is extremely sensitive to green and yellow wavelengths. In fact, the way we perceive the brightness of a light source depends on the amount of green and yellow light emitted from that source. Plants, on the other hand, reflect most green light and use only a small portion of yellow light for photosynthesis.

Wavelengths and color

The most common way we define light color is by its wavelength and the most common way to express light wavelength measurements is in nanometers (nm), which is one billionth of a meter. Humans have a visible spectrum of 380 to 780 nm, which accounts for all of the colors we see. Light wavelengths measuring below 380 nm are considered ultraviolet and light wavelengths that measure above 780 nm are considered infrared. The human eye is most sensitive to 550 nm (green light).

PAR is for plants

Most plants appear green to our eyes because they are reflecting green light, not absorbing it. Plants possess a special molecule known as chlorophyll, which absorbs sunlight and uses its energy to synthesis CO2 and water (using the process known as photosynthesis). Interestingly, chlorophyll—which actually gives the plant its green color—uses a relatively small portion of the visible light spectrum. The two types of chlorophyll (a and b) absorb light in the 410 to 460 nm range (blue light) and the 630 to 670 nm range (red light). These two ranges of absorbable wavelengths of light are known as photosynthetically active radiation, or PAR.

Measuring light for horticultural purposes

When measuring the amount of usable light for photosynthesis, we need to measure a light source’s PAR output. Thankfully the indoor horticultural industry has answered the call for plant-specific light measuring devices and has started to introduce PAR-specific light meters. Most of these meters give a measurement of the total amount of light energy being produced between the 400 to 600 nm range.

These meters are not perfect, however; they fail to exclude the 170 nm of mostly unusable light between 460 and 630 nm and they don’t take into consideration the peak nanometers of absorption for chlorophyll a and b. Nonetheless, these meters bring us toward a new era of light measurement devices for the hobbyist horticulturist and give the consumer the ability to compare available lighting technologies to each other.

When using a PAR meter, a grower can compare bulbs by their spectral output instead of just a perceived brightness. Eventually, even more accurate measuring devices (like nanometer-specific sensors) will be available at a reasonable cost and offer indoor gardeners a comprehensive analysis of their lighting in terms of PAR output.

Also, don’t throw away that old candela or luminous flux meter. Although these meters won’t tell you the amount of usable PAR energy emitted from a light source, they can still be used as a comparative tool in your indoor garden. For example, a luminous flux/candela meter could be valuable in determining beneficial cross patterns from reflectors, determining the best light footprint, measuring the effectiveness of reflective materials for light diffusion, etc.

Color temperature—Kelvin

Kelvin is actually a unit of temperature measurement, but it is also used to represent color in regards to lighting applications. The Kelvin color temperature scale is determined by corresponding temperature with the color emitted by a blackbody object as it is heated. This scale is a common method of rating horticultural bulbs for their peak color output, and many horticultural bulbs are intended for a specific purpose that can be represented by their Kelvin rating.

For example, high pressure sodium with a 3,000 K (red light) is commonly used in a fruiting or blooming room where a peak in red light would enhance ripening. Another example would be a specialty metal halide bulb with a 7,000 K (blue light) rating, which is used specifically for vigorous vegetative growth. You’ll find most horticultural bulbs have a Kelvin rating in the blue or red spectral range.

Again, this is to target the specific wavelengths used for photosynthesis. However, it’s important to note that these bulbs emit a full spectrum of colors, not just the color represented by the Kelvin scale. Although the Kelvin color temperature scale is used to represent color output, it does not define a specific wavelength; therefore, it is not interchangeable or convertible with nanometers.

Ultraviolet (UV)

Although plants only use specific wavelengths for photosynthesis, scientists believe that other light spectrums could also benefit plants. In fact, there have already been many discoveries that link certain light wavelengths to beneficial organisms and hormonal responses within plants.

Ultraviolet light, for example, can cause hormonal changes that directly affect the structural growth of many plants. Some plants have even evolved specific oils for protection against UV lighting or to reverse the light’s negative effects. Some scientists also believe ultraviolet light is key for young plants to build resistances to harmful bacteria and other pathogens.

Over millions and millions of years, plants have not only evolved their shape, but also the way they absorb and are altered by light energy. One example is the chlorophyll molecules that allow them to absorb particular wavelengths for photosynthesis.

Although we have yet to fully understand the way plant evolution is linked with light energy, we do know that a plant’s perception of light cannot be broken down to a single denominator or measured by the perceived brightness of a light source. Our science to measure light is evolving and as our knowledge of light energy in relation to plant functions expands and evolves, so will the technology used to measure it.