Color & Crı Mayıs, 2022

What is light color?

Light is made up of multiple wavelengths; each wavelength is defining a color. The combination of these reflected wavelengths gives different shade of colored light the eyes see.

Obtaining almost all visible colors could be achieved by additive mixing of three Primary additive colors: RED, BLUE and GREEN (RGB). The combination of the primary colors gives the subtractive colors: Magenta, Yellow and Cyan and
white color contains all three primary colors equally. As shown in figure 1

All perceived colors can be represented by three independent variables: Hue, Saturation and lightness (HSL).

Where:

red, greenness of green
Saturation: the purity of the color, or how much white is contained in the color.

Lightness: for measuring reflected light (luminance of a reflecting object). And Brightness: for measuring emitted light (luminance of a light source)

This three variables are represented using Munsell color system and chromaticity diagram.

 Hue: main color characterized by the dominant wavelength (λDOM), the redness of

Color appearance depends on other factors such as

• Illuminant Spectrum: Spectral distribution of light source (brightness), the color of object is tested under natural light source (D65 is preferred as reference light source)

• Surface Reflectance Spectrum: object illuminated (Lightness). Reflected color depends on reflection characteristic of surface or spectral reflectance curve (SRC). It means that same color may show same under one light source and differently under another, because of different SRC of objects this phenomenon called metamerism.

• Spectral Sensitivity of the Visual System: detector that recognizes color by the reflected light from the object. For non-imagining system human eyes is the main detector. Good measurement devices are the ones that has detector with adjustment function that allows match the human eye’s respond as specified by CIE.

Color Rendering Index (CRI)

Definition: is a measure of a light source’s ability to show object colors «realistically» or «naturally» compared to sunlight.

CRI value, in range Ra [0,100] shows how well or badly a test source would render colors compared to reference light source. Lamps have different CRI because of different spectral power distribution (SPD) of them.

The general color rendering index Ra represents the average of the first eight test color samples. Where LED lamps are concerned, color rendering index R9 for strong red is also frequently taken into consideration (for human skin), because white LEDs demonstrate weaknesses within this spectrum. New measurement devices use average of 14 test color samples. The color rendering index increases with increasing proportions of the visible spectrum covered by emissions. Also, CRI decreases with increasing lumen output. The higher the color rendering index, the less color shift or distortion occurs.

The CRI standard dictates that color temperatures 5000K and greater use a daylight

spectrum

spectrum as reference light source under test.

CRI values were classified into groups with different ranges and usages.

, but for color temperatures less than 5000K, use the Planckian radiation

1. 2A  70 - 79

2. 2B  60 - 69

3. 3  40 - 59

4. 4  20 - 39

Good Fairly good Acceptable Poor

Industry, offices, school Industry, offices, school Industry, sport halls Traffic lighting

Choosing the proper light source depends not only on life time of source or power consumed, but also on light pleasant, CRI and light efficiency. Taking tasked space into consideration.

Prepared by Mrs. Sacide Safadi