Light intensity is the amount of light measured over a certain area. In essence, it is how much light is being provided. The lighting industry measures light intensity in lumens; however, we know lumens are for humans and not for plants. In horticulture, light intensity is measured in micromoles (μmol) which describe the amount of photons emitted from a light source. Those photons are what plants perceive.

There are a few different ways to measure umols from a light source:

  • Integrating Sphere Spectroradiometer
  • Goniophotometer
  • Light Meter

Integrating Sphere Spectroradiometer

One way to measure micromoles (μmols) is with an integrating sphere spectroradiometer. By definition, it is a spherical shell used to determine the total luminous flux of a spot of light through an aperture. The white interior of the sphere produces thorough diffusion of light from a source placed at its center. When the light is emitted, it bounces around the sphere and out through the aperture where the data is taken.

An integrating sphere is used to isolate a light source and capture all of the light coming out of it. A spectrometer is then used to split that light into its constituent parts and measure each part. In essence, this measures how much blue, yellow, red or any other color of light there is at each wavelength. At that point, math is completed to calculate the PPF and PBF.

An integrating sphere spectroradiometer will also provide information about the light source such as its spectrum, color temperature, color rendering index, power and more.

Though a sphere generates the data to obtain the PPF and PBF of a light source, this only shows how much light the product can generate given the nanometer range that is being measured. It will not provide the amount of light the plants will receive. Often the measurement of PPF is used to describe the intensity of a light source, however this measurement can be misleading. It does not include the beneficial light from UV and far-red, nor does it truly explain the amount of light the plant will actually receive. PBF is more inclusive of these two ranges, however it too does not fully describe how much light the plant will receive. This is because things like reflectors, optics and how far lights sources are placed from the plants play a factor in the actual light intensity the plant is getting. For that, PPFD and PBFD are used.

Goniophotometer

A goniophotometer is similar to an integrating sphere because it provides the same type of data, but it expands on that data and is captured by a different means. A big difference with a goniophotometer is that it can be used to obtain the efficacy of a light source and its spatial distribution. Goniophotometers use a mirror to reflect the light from a source at different angles. Measurements are taken at those angles and provide how the light projects from the source. More recently, these devices are used to measure LED products and HID fixtures because the spatial distribution of those products are directed and not congruent. The goniophotometer also generates an IES file which contains the light’s spatial distribution information. That file can then be used to create lighting layouts.

Light Meter

Another way to measure intensity is by using a light meter. This will measure the amount of μmols generated from a light source given the distance in which the meter is held from that source. In other words, this measures the amount of light output at a specific distance from the light. The key here is distance. While a sphere is capturing all of the light emitted, a light meter is capturing the amount of light at one particular spot or distance from the light. Evaluating the data from a light meter can provide the uniformity as well as the PPFD or PBFD of a fixture.

There are many types of light meters and new ones are being developed all the time. It is important to find a light meter from a reputable company that is inclusive of the entire spectrum.