# How to Calculate Solar Reflectance

• 1). Measure the radiation of a test material with a gonioreflectometer. This device directs a specific amount of radiation toward a control surface covered by the sample material. The gonioreflectometer then records the amount of radiation with the desired wavelength that was reflected by the control surface. This test is performed under carefully controlled laboratory conditions.

• 2). Calculate the solar reflectivity. This is given as p(y) = R(y)/I(y) where p(y) is the reflectivity of solar radiation with a wavelength of y. R(y) is the reflected radiation of wavelength y and I(y) is the incident radiation of wavelength y.

• 3). Determine the solar reflectance from the solar reflectivity. The solar reflectance is the square of the solar reflectivity, so q(y) = (R(y)/I(y))^2 where q is the solar reflectance.

• 4). Establish the units of measure for solar reflectance. The incident and reflected radiation must be expressed with the same units of measure for the reflectance value to be meaningful. Since the reflectance is a ratio of two values with the same units of measure, reflectance is a pure number without units.

• 5). Interpret the solar reflectance. The reflectance is simply the square of the percentage of radiation that an object reflects for a given wavelength of light. The reflectance will therefore be a value between 0 and 1. A reflectance of 0 indicates that the object does not reflect any light of the given wavelength. A reflectance of 1 indicates that the object reflects all the light of the given wavelength.