Basics of color theory. Part 3. Light reflection and lighting

How Light Interacts with a Surface

When a light flux falls on any object, part of this flux is reflected from the surface and part is absorbed by the material.
The ability of a surface to reflect or absorb light is characterized by the reflection coefficient.

Surfaces of different colors reflect rays of only certain wavelengths. The ratio of the light flux of a specific wavelength reflected by a body to the light flux falling on the surface is called the spectral reflectance coefficient. It is usually measured as a percentage.

Selective Reflection of Light
All colored surfaces have selective reflectance. This means they reflect only part of the light spectrum.

For example:

• if a surface mainly reflects red rays, it will be perceived as red;
  
• if it reflects blue rays, it will appear blue.

This is why objects have different colors.

Achromatic Colors
A special group is formed by achromatic colors:

• white
• black
• gray

They do not have their own color hue.

White Surface
A white surface reflects light of all wavelengths approximately equally. Its reflection curve is almost horizontal. If the reflectance coefficient of a surface exceeds approximately 60%, such a surface is perceived as white.

Black Surface
A black surface, on the contrary, absorbs almost all light that falls on it. Surfaces with a reflectance coefficient below 10% are perceived as black.

Gray Color
Gray is intermediate between white and black and also belongs to achromatic colors.

All achromatic colors reflect light non-selectively. The higher the reflectance coefficient of such a surface, the lighter it appears.

Chromatic Colors

Colors that have at least a slight color hue are called chromatic.

These include all colors of the spectrum:

• red
  
• orange
  
• yellow
• green
  
• light blue
  
• blue
  
• violet

Colorless Materials
There are also colorless media - materials through which light passes without changing its spectral composition.

Examples:
transparent glass
colorless varnish

Such materials do not give light their own color.

Why Color Depends on Lighting

The color of objects depends not only on their ability to reflect light, but also on the spectral composition of the light source.

The same object can look different under different lighting conditions.

For example:

• in daylight
  
• under an incandescent lamp
  
• under gas discharge lamps

Spectrum of Different Light Sources

Different light sources have different spectra.

Daylight
The spectrum of daylight is close to uniform. It contains almost all wavelengths of the visible spectrum.

Incandescent Lamp

• In the spectrum of an incandescent lamp:
  
• the maximum lies in the red and yellow region
  
• the minimum lies in the blue and violet region

Sodium Lamp
A sodium lamp emits almost only yellow light.

Mercury Lamp
A high-pressure mercury lamp emits light in the

• violet
  
• blue
• green
  
• yellow parts of the spectrum.

How Lighting Changes Colors

When lighting changes, the colors of objects can change significantly.

For example, under incandescent lighting:

• red colors appear more saturated
  
• orange colors become more reddish
  
• light blue colors acquire a greenish tint
  
• blue and violet colors become reddish and turn purple
  
• dark colors become even darker

General Rule of Color Change Under Different Lighting

The following rule can be formulated:

• colors close to the color of the light source become more saturated;
  
• colors opposite in tone lose saturation or darken;
  
• other colors acquire a tint of the lighting.

At the same time:

• colors close to the color of the light source become lighter;
  
• colors close to the opposite tone become darker.

Conclusion

The color of the objects that surround us is determined by two main factors:

The ability of a surface to reflect or absorb light.
The spectral composition of the light source illuminating the object.

It is the combination of these two factors that forms our perception of color.