Understanding the Absorption Factor in Lighting

In lighting systems, when light is emitted by a source, it travels through a medium and interacts with surfaces. Some of this light is reflected, some is transmitted, and some is absorbed by the surfaces or the medium. The absorption factor is a measure related to how much light is lost due to absorption.

The absorption factor quantifies the reduction in the amount of light (measured in lumens) available after it interacts with surfaces or a medium that absorbs light. It is typically defined as the ratio of the light available after absorption to the initial light emitted by the source.

Let's look at the given options and analyze which one correctly defines the absorption factor for lighting purposes:

• Option 1: "It is defined as the ratio of total lumens available after absorption to the total lumens emitted by the source of light." This statement aligns with the standard definition of an absorption factor, which represents the fraction of the initial light that remains after absorption. It compares the outgoing light (after absorption) to the incoming or emitted light.
• Option 2: "It is defined as the ratio of the candle power before and after absorption of light." Candle power (measured in candela) is a measure of luminous intensity in a particular direction, not the total light output (lumens). While absorption affects intensity, the primary definition of the absorption factor for overall lighting efficiency relates to the total flux (lumens).
• Option 3: "It is defined as the ratio of utilisation factor to the depreciation factor." The utilisation factor describes how efficiently the light emitted reaches the working plane, and the depreciation factor accounts for the reduction in light output over time due to lamp aging or dirt accumulation. Neither of these factors directly defines the absorption factor of a surface or medium.
• Option 4: "It is defined as the ratio of the change in the brightness after the light has been absorbed by the surface." Brightness (luminance) is perceived light coming from a surface. While absorption affects the luminance of a surface under illumination, the absorption factor itself is a property related to the amount of light absorbed from the incident light, not the resulting change in brightness.

Based on the definitions used in lighting calculations and photometry, the absorption factor is correctly defined as the ratio of the light flux remaining after absorption to the initial light flux emitted or incident.

Therefore, the statement that correctly defines the absorption factor in lighting purposes is the one that relates the total lumens available after absorption to the total lumens emitted by the source.