Theory of operation

SPECTRAVUE is a powerful spectrometer paired with a leaf probe attachment, on-board operating software, and display screen. Two broadband light sources are attached inside the device. One is positioned in the leaf clamp for transmissive measurements, and one is placed inside the case for reflective measurements. The spectrometer module takes the light from the leaf probe attachment and projects the wavelength dispersed light onto a CCD array. Each pixel of the CCD array thus corresponds to a specific wavelength of light. The operating software displays the light intensity of each pixel of the CCD array.

Three spectroscopic measurements can be performed by SPECTRAVUE:

Transmittance (T) is the fraction of the incident light passing through a sample, such as a leaf. When the intensity of the incident light is Io, and the intensity of light passing through the sample is It, the transmittance, T, is then expressed as T = It / Io. The CI-710s software suite can automatically calculate and plot the transmission spectrum with the reference spectrum (the intensity profile of the light source) stored.

Absorbance (A) is related to the transmittance as A = – log T = -log(It / Io). Absorbance is commonly denoted in optical density (OD) as it is linearly proportional to the amount of light absorbing sample in the light path length (Beer-Lambert law). For example, in a standard 1-cm cuvette, a sample with an OD of 2 is twice as concentrated as a sample with an OD of 1. But note that the intensity of light transmitted from a sample with an OD of 2 is only 1/10 of that from a sample with an OD of 1. Transmission and absorption spectrum measurements are the most common applications of spectrometers. The spectrum can be used for the quantification of chemical concentrations, color analysis, the study of photochemical reactions such as photosynthesis, and the quantification of physical or optical properties such as film thickness, index of refraction, and extinction coefficient.

Reflectivity (R) is the fraction of incident light that is reflected from a sample. Depending on the purposes of the study, reflectivity can be measured for reflection at a specific angle from the sample, or the entire reflected hemisphere. The leaf probe attachment of CI-710s is not designed to measure the directional reflectivity. Nonetheless, it can be used to measure the absorption spectrum of surface substance of reflective or opaque samples.