Measuring Spectra of PAR, Reflectance and


The Miniature Spectrometer MINI-SPEC is a compact and robust outdoor instrument for gathering spectral information of the light environment and the sample.

Figures 1 to 4 illustrate types and signal quality of PAR spectra as well as of reflectance and fluorescence spectra.

Figure 1

Absorption of light by water affects the spectral properties of radiation available for aquatic photosynthesizers. The spectra in Figure 1 were recorded above the water surface and at various depths down to 30 m. With increasing depth, intensities in the red spectral range decreased more than in the blue spectral range requiring that photosynthesis acclimates to blue-enriched low light at greater depths. Measurements performed by Sabrina Walz and Jonathan Richir at La STARESO (Station de Recherche Océanographiques et sous-marines), Corsica, France.

Figure 2

The spectral properties of radiation available for individual plants can be influenced by neighboring plants. The spectra in Figure 2 are recorded above and below canopy and demonstrate how markedly leaf absorption affects the intensity ratios visible/far red. Specifically, the red/far red ratio acts as environmental cue which is sensed by phytochrome photoreceptors.

Figure 3

Reflectance spectra contain information on light absorbed by a sample. Typically, troughs in reflectance spectra correspond to absorption peaks of photosynthetic pigment-protein complexes. Reflectance spectra can also reveal the screening function of non-photosynthetic pigments. Figure 3 shows how light screening anthocyanins mask the reflectance peak around 550 nm observed in a green leaf.

Figure 4

The shape of fluorescence emission spectra is affected by the chlorophyll content of a leaf (Buschmann C (2007) Photosynth Res 92, 261–271). Reabsorption of fluorescence at wavelength < 700 is the major factor determining the ratio of short to long-wavelength emission peaks. Figure 4 illustrates how this ratio decreases with increasing chlorophyll content in ivy leaves.