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Elliptical Reflector and Focus

The optical efficiency of the lamp reflector/system is the amount of radiant energy collected and reflected versus the total emitted in any spectral range. The larger the total angle that the reflector subtends around the bulb, and the higher the reflectivity, the higher the optical efficiency. The typical elliptical reflector "wraps" about the bulb, including an angle of about 270°, so collects approximately 75% of the light emitted by the bulb.

The radiant energy from the bulb focused by the reflector is affected substantially by the optical efficiency over the range of wavelengths of interest and can be described by:

where wλ is the spectral radiance of the bulb per unit length, R is the spectral reflectance, θ is the angle subtended by the reflector, a is the sector of reflector that is obscured by the bulb itself.

The “quality” of focus is observed in the peak irradiance (W/cm²) compared to the input watts of the bulb.