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Light and Matter
Light interacts with matter in three ways: reflection, absorption and transmission. Reflection is light energy that bounces off a material. Transmission is light energy that travels through a material. Absorption is energy that transfers from light into a material.
Reflectance, transmittance and absorbance are those same characteristics, but they're specific measurements for a sample. So, the total light energy that hits a solar cell is divided into the reflected, transmitted and absorbed portions. (ref. 6)
Energy Transfer
The electrons in atoms most frequently absorb light, but not as simply as pouring water into a bucket. Unlike an normal empty bucket -- which can take any amount of water up to its capacity -- an electron can only absorb a specific amount of energy, either an integral number of whole buckets full or none, never in between. The details depend upon the wavelength of the light, the type of atom and the arrangement of atoms in a material. Each solar cell has separate layers, different materials and varying sorts of structures, so calculating the absorbance of any given solar cell is very complicated.
Calculating Absorbance
Instead of trying to calculate absorbance by doing detailed calculations with light and atoms, you can make a simpler calculations after taking a few measurements. You start by collecting an incident light beam, the light reflected from a solar cell and the light transmitted through the cell. You make three separate measurements with a radiometer or other power meter to get the reflected and transmitted energy.
The first step in the calculation is to find the initial energy by subtracting the reflected energy from the total energy. Then take the logarithm of the transmitted energy divided by the initial energy. The absorbance is traditionally stated as a number, but you can multiply it by 100 to convert it to a percentage.
Solar Energy
A more useful measurement for solar cells is the efficiency. That's a measure of how much of the absorbed sunlight energy the solar cell converts to electricity. To calculate the efficiency you need to know the energy in the sunlight -- on a clear, sunny summer day at noon, this will be about 1,000 watts per square meter, but you can also measure this with a meter. Multiply that by the area of your solar cell, in square meters.
Then measure the power the solar cell is generating. You'll measure this by connecting the solar cell to a variable resistor. Change the resistance and find the maximum power -- the maximum product of current and voltage. Divide the maximum electrical power by the power in the sunlight and multiply by 100. That's the percentage efficiency of the solar cell.