Light and Colors



Spectral Colors

The human visual system is sensitive only to a small portion of the electromagnetic spectrum, the portion between wavelengths of about 400 to 700 nm. In this part of the spectrum the visual system reacts actively to received electromagnetic signals with three individual but slightly overlapping types of light sensitive cells in the retina within the human eye.

electromagnetic spectrum

This 'visible' range of the electromagnetic spectrum causes a sensation of 'colors' within our visual system but in reality electromagnetic rays have NO color! The color impression is solely created within us. How we see colors and what colors we see is a result of the evolutionary process. It started early in the evolution with 'primitive' animals.

That means a perceived red light is actually not red! It is just an electromagnetic signal with a certain frequency and wavelength that causes a 'red' sensation in our visual system. With a different type of visual system it could also be blue or could be any other 'internal' sensation!

Frequency (THz)

Wavelength (nm)


Color Circle

Wavelength

Color Nanometer
Ultraviolet 380-280
Violet 430-390
Indigo 450-440
Blue 480-460
Green 530-490
Yellow 580-550
Orange 640-590
Red 750-650
Infrared 1000-750




Additive and Subtractive Color Mixing

Received light is directly emitted or emitted light is reflected on surfaces. Arriving sun light is directly emitted by the sun but a illuminated surface only reflects this sun light. A luminous source produces light and an illuminated surface reflects light.
All colors that we perceive can be produced by combinations of three primary colors: red - green - blue. How these three colors mix depends on whether we are dealing with directly emitted luminance (the sun or a lamp) or reflected light (a illuminated surface). These two different color systems are referred to as the additive and the subtractive color system.
The additive color system pertains to self-luminous light sources and mixing equal amounts of red, green and blue light produces "white" light. The absence of light creates "black."
The subtractive color system pertains to an illuminated objects that reflect light but does not produce light itself. The color we see is the light reflected from the object. The colors we don't see are absorbed by the object. "Black" is the absorption of all light, hence all colors and "White" is the reflection of all colors.



Additive RGB Color Mixing


Color CirclesColorRGB ValueHex Value
red255, 000, 000FF0000
green000, 255, 00000FF00
blue000, 000, 2550000FF
white=red+green+blue255, 255, 255FFFFFF




Subtractive CMYK Color Mixing


Color CirclesColor
cyanCMYK Color Table
magenta
yellow
black (key)




White Surfaces - Colored Surfaces

White surfaces look 'white' because they reflect the incoming light in the entire visible spectral range with equal intensity. A surface that reflects the incoming white light differently at different wave lengths 'looks' colored. And a 'white' surface only illuminated with (for instance) red light, looks 'red'. But a surface that is illuminated with complete white light but reflects only red light also looks 'red'.

white cube
illuminated by white light
looking white
white cube
illuminated by red light
looking red
white cube
illuminated by blue light
looking blue
red cube
illuminated by white light
looking red
red cube
illuminated by red light
looking red
red cube
illuminated by blue light
looking black
blue cube
illuminated by white light
looking blue
blue cube
illuminated by red light
looking black
blue cube
illuminated by blue light
looking blue

If a surface reflects incoming white light only in a certain amount, the reflected light looks still 'white' even if it is grey. It will be seen as 'white' as long as no comparable reference surface is available. Compared with a 'brighter' white' it will be received as 'grey'.



Pantone Color Charts

Pantone color charts (online Pantone color charts) are probably the most detailed color defining samples available. Sample sheets must be purchased. See Pantone.com.




RAL Colors

RAL Color Table

The RAL Colors were defined by the 'Deutsches Institut fr Guetesicherung und Kennzeichnung' in Germany in 1927. RAL stands for 'Reichsausschuss fuer Lieferbedingungen', a control authority for trade standards. At this time 40 color tones were part of the RAL table. Over the time this number changed to 210.
The colors are described by a 4-digit number code. The first number defines the color range.

Every color can be defined as RGB value (hex or dec), Y Cb Cr value or CMYK value.



CMYK , RGB Color Table

CMYK , RGB Color Table



Electromagnetic Spectrum

1 mm - more than 10 km radio waves
0.5 mm - 1m micro waves
761 nm - 5 mm infrared radiation
760.8 nm visible light, dark red
686.7 nm visible light, red
656.3 nm visible light, red
589.3 nm visible light, yellow
527.0 nm visible light, green
486.1 nm visible light, blue green
430.8 nm visible light, blue
396.8 nm visible light, purple
393.4 nm visible light, purple
1nm - 393 nm ultraviolet radiation (UVA,UBD,UVC)
0.001nm - 1 nm X-rays
smaller than 0.000001 nm
- 0.1 nm
gamma-rays




Quantifying visible Light

The first standard chosen was a source that was familiar and common to everybody: a candle. It was a specifically sized candle with specific material and molded in a specific way. The amount of light emitted from such a candle became the first unit of 'brightness'. It was called 'one candlepower'.
Imagine such a candle at the center of an darkened room, the energy of this candle is radiating equally in all directions. The farther the distance from its flame, the less light it appears to be shedding.
Light from a point source (the candle) radiates outward in all directions such that it uniformly illuminates the surface of an ever expanding sphere. As the radius of that sphere gets larger, the surface area grows even more and thus the energy from the candle is spread ever thinner. This is the 'Inverse Square Law' (the radius of the sphere is R,
the surface of the sphere is 4x R2).





Basic Light Terms

Luminous Flux (lumen)
The total light output from electric sources is expressed in lumens.
Lumens = {Lux * Area} / {Screen Gain}

Luminance (NIT, cd/m2)
The specific light that comes off the surface of a lighting device, whether off a filament, light bulb, lens, louver, tabletop, etc. Luminance varies with the direction and its gloss characteristics. Luminance is measured in candela per square foot.

Illuminance (lux)
The density of luminous flux on a surface, is measured in lux (one lumen per square meter) or footcandles (one lumen per square foot).

Candela
The unit of the intensity of light at the source equal to the amount of light from the flame of a candle in any direction.

Footcandle
The measure for the density of light as it reaches a surface. One footcandle equals one lumen per square foot. Footcandle is sensitive to the inverse square law and the angle at which the light reaches the surface.

Color Temperature
The color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (K). The range of used color temperatures is about 2800K to 6000K.

Color Rendering Index (CRI)
The ability of a light source to produce color in objects. The CRI is expressed on a scale from 0-100, where 100 is the best in producing vibrant color in objects.

Luminous Efficiency
Describes the efficiency of a lamp and is expressed as the ratio of the emitted luminous flux in lumen and the power used in watts.
The theoretically maximum value at 555 nm would be 683 lm/W. Today's lamps vary between 10 and 250 lm/W.



Speed of Light in Vacuum

The speed of light in vacuum is 186,282 miles per second / nearly 300,000 kilometers per second. A light year is the distance light travels in one year at the speed of 186,282 miles per second, about 6 trillion miles.



Grey Scale

The grey scale for tv, display and projector calibration is measured in IRE (Institute of Radio Engineers), there black is 0 IRE and white is 100 IRE. IRE is used to measure composite video signals. The scale represents the color temperature of 6500K (Kelvin) without any color information in any of the grey steps between black and white. 6500K is the equivalent for daylight.



0 IRE represents the blanking level between frames (no picture). A pure black image on analogue NTSC video should have 7.5 IRE, a pure white image will have 100 IRE. In calibrated digital video, the black level is set to 7.5 IRE or 0 IRE as the black cutoff. The 7.5 IRE black level on NTSC video is also called "pedestal". Japanese NTSC is calibrated for 0 IRE as black since 1985 and PAL is also set as 0 IRE for black and 100 IRE for white! Grey scale problems arise when video is converted with incorrect black level.



Luminance and Illuminance Tables

Luminance
Luminance is the amount of visible light leaving a point on a surface in a given direction. It can be a physical an imaginary surface and the light that leaves the surface can be due to reflection, transmission, and/or emission.
standard unit is candela per square meter (cd/m2)  
20 - 90 Brightness of Plasma Screens
55 Cinema Screen
90 - 300 CRT, Computer Monitor, Video Display
31.800 Reflecting, Diffusing Surface in Sunlight
7.600 Moon, Clear Sky
3.100 - 6.900 Overcast Sky
2.100 - 6.000 Clear Sky
1.65 Giga Sun at Zenith


Illuminance
Illuminance is the total amount of visible light illuminating a point on a surface from all directions above the surface. The surface can be a physical or an imaginary surface. Illuminance is equivalent to irradiance weighted with the response curve of the human eye.
standard unit is Lux (lx) which is lumens per square meter (lm/m2)  
1 milli Starlight
0.1 Full Moon
10 Twilight
1.000 Overcast Day
108.000 Direct Sunlight