OSRAM SOLERIQ™ S 15, GW KAGMB9.KM

Abstract
White light is not the same as white light. When different light sources are used, color differences may become visible. To understand why this can happen, it is necessary to understand how people perceive color and light. Nevertheless, it is possible to reduce the color shifts by choosing suitable white LEDs combined with an appropriate system setup. This application note provides basic information on optical quantities, color spaces and CIE chromaticity diagrams. Furthermore, it describes how color consistency for white light applications can be achieved.

Optical quantities
Visible light is part of the electromagnetic spectrum, comprising all electromagnetic radiation within the wavelength range of 380 - 780 nm. The wavelength of this radiation defines the color perceived by the human eye. Certain colors, such as pink or purple, are absent from this part of the electromagnetic spectrum and can only be conceived via a mix of multiple wavelengths. As such, monochromatic colors with a single wavelength are known as spectral colors. A general knowledge of how the eye works is required to understand the idea of colorimetry. Light is focused onto the retina by the lens (see Figure 1). There are two types of photoreceptor cells on the retina which contribute to vision:

• The rods: Rods sense very low light levels and also contribute to peripheral vision.

• The cones: Cones are concentrated in the center of the retina; they function as color detectors and come in three types (short, medium and long). Each type senses a different wavelength range, and can roughly be thought of as red, green and blue detectors. The resulting perceived color is the combination of the stimuli that the brain receives from these three cone cells. A cone's level of reaction to a color stimulus is found by integrating
the cone spectral response with the emission spectrum of the incoming light.

Colorimetry is the science that describes the color perception of the human eye in terms of numbers. Starting with the spectral power distribution of the light, scaled to the response of the human eye, color space coordinates are calculated. Human response to color has been characterized as Color Matching Functions based on experiments by the CIE in 1931 with a 2° field of view. This represents the average human eye's chromatic response within a 2° arc inside the fovea (see Figure 2).

The following section provides a brief introduction to the basic terms and definitions in photometry and colorimetry. It is important to distinguish between radiometric and photometric quantities which describe the physical radiation properties and its effects on the human eye.

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