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What Are Reflectance Curves?

Optical systems monitor everything from nuclear power plants to crop health to internal organs during operations. To do their jobs, the lenses and mirrors within the systems must manipulate light very precisely. Part of the precision is ensured by machining the shape of the surfaces to very tight tolerances to make sure the light is bent in the right way. But to make certain the right amount of light gets where it needs to go, the reflectance curves of the surfaces must be characterized.

All the Light
- Imagine a factory inspection at a bottle factory. The bottles are supposed to be a certain green color, and a camera looks at the color. If the camera lens transmits more yellow light than blue-green light, then the apparent color of the bottles will be shifted, making the bottles appear more yellow than they actually are. So an important factor in evaluating the quality of the lens is how it transmits light of different colors. That's where reflectance curves come in.
Reflection Absorption &Transmission
- Whenever light interacts with matter, one of three things happens. It gets reflected, absorbed, or transmitted. For well-designed optical systems, absorption can usually be neglected. That means that when light strikes against a lens, for example, it either reflects back or it transmits through. So if you measure either reflectance or transmittance you can figure out the other. That is, reflectance --- often measured as a percent --- is 100 percent -- transmittance.
Reflectance and Color
- Reflectance is a measure of the ratio of the energy of light reflected from a surface divided by the total amount of light incident on the surface. The reflectance is a function of wavelength. That is, the reflectance of the surface at the long-wavelength red color will not be the same as the reflectance at shorter yellow, green, or blue wavelengths. The basic reflectance curve is a measure of the ratio of reflected to incident light over all the wavelengths at which the optical system will be used.
Reflectance and Angle
- Reflectance is not only a function of wavelength, it is also a function of angle. That is, if light strikes a surface at an angle of zero degrees --- incident perpendicular to the surface --- it will have one value of reflectance. At 10, 35 or 45 degrees --- whatever angle --- the reflectance will be different. So another reflectance curve can be created by measuring the ratio of reflected to incident light over a range of angles of incidence. In practice, a reflectance measurement will usually consist of several curves of reflectance vs. wavelength collected at several different angles of incidence.

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