Basics of light and lighting

 

Below terms apply to various light sources

1. What is Light?
 

   Light is a form of energy manifesting itself as electromagnetic wave radiation within a particular range of wavelength called visible spectrum to which human eyes are sensitive, other form of electromagnetic radiation such as Radio, Radar, Microwave, infrared, UV and X-ray are with different range of wavelength.

visible spectrum

Visible Spectrum

    The visible spectrum ranges in wavelengths from about 390 to 750 nm or 400 to 780 nm depending on which source is used. In fact it also depends on the sensitivity of a specific persons eyes. Human eye generally has its highest sensitivity at around 555 nm.
    White light is mixture of visible wavelengths radiation, a color light is a particular narrow range of visible wavelengths radiation, for example, 495 nm to 570 nm wavelength range radiation is defined as green color light.
    Light can be derived by various energy transforation: sunlight (nuclear fusion reaction), thunderbolt (discharging), chemical reaction light (including fire and explosion), biological light (such as firefly), and artificial light.

2. Artificial light sources
 
Incandescent bulb

1) Incandescent Lamp and halogen lamp, an electric current passes through a tungsten filament which is incandesced and emitting light. At the same time, incandesced tungsten atoms evaporate fast and makes the filament thinner until it eventually breaks at its thinnest point, ending the life of the lamp. later developed tungsten-halogen lamp can slow down the evaporation of tungsten, but still consider short life span.

CFL bulb

2) Electrical gas discharging Lamp: including fluorescent lamp or compact fluorescent lamp (CFL), sodium vapor lamps, High intensity discharge lamp (HID lamp), Metal halide lamp etc. In a gas discharge lamp, an electric current passes through a gas or metal vapor between two electrodes at the opposite ends of a closed glass tube, free electrons colliding with an atom in the vapor momentarily knock an electron into a higher orbit of the atom. When the displaced electron falls back to its former level, a quantum of radiation is emitted.

LED chips

3) Soli state lamp (LED lamp): LED stands for Light Emitting Diode. LED is a type of p-n junction semiconductor, a color (wavelength) of LED light is determined by the energy band gap of the semiconductor materials forming the p-n junction. Therefore, LED is very good monochrome light source.
    White light LED chip: most of white light LED chip today are Phosphor-based LED chip, however, LED light chips don’t have a tungsten filament to burn out or break, also no mercury to be evaporated, therefore, LED lamp last much longer than fluorescent lamp. Read this post to know more about LED and white light LED chip.

3. What is CRI-Color Rendering Index of white light?
 
Continuous SpectrumDiscontinuous Spectrum

Continuous Spectrum Discontinuous Spectrum

1) Continuous and discontinuous spectrum: A light spectrum in which all wavelengths are present is called a continuous spectrum, ranging from red through orange, yellow, green, blue to violet. For example, daylight or light from a candle and an incandescent halogen bulb.
White light, however, can also be achieved by two or more selected wavelengths with rest of wavelengths being totally absent. For example by mixing red, green and blue, or merely blue and yellow. Light sources with selected wavelengths such as most of gas discharge lamps have discontinuous spectrum.

Color Rendering

Color Rendering

2) Color of an object is response of human eyes to reflection of the object under white light. A white light with continuous spectrum will enable you to see true color of an object (every color existing on the object). However, under discontinuous spectrum white light, you will not see a color which is missing from the white light spectrum even the color does exist on the object, because of no reflection of the color from the object. Extreme example is that you will see every thing either red or black in a pure red narrow spectrum light environment.
A illustration shows a piece of red cloth appearing true red when seen illuminated by white light produced by a continuous visible spectrum, but under a white looking which is mixture of yellow and blue wavelength light, it will look greyish brown due to absence of red wavelength light, because there is no red wavelength radiation for the cloth to reflect into the eye.

3) In order to measure how well a white light source color rendering property is, Color Rendering Index has been introduced.

CRI = 90 - 100, Excellent colour rendering properties.
CRI = 80 - 90, Good colour rendering properties.
CRI = 60 - 80, Moderate colour rendering properties.
CRI < 60, Poor colour rendering properties.

4. What is CCT-Correlated Color Temperature?
 

1)  Black body: A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. A white body is one with a "rough surface" that reflects all incident rays completely and uniformly in all directions.

CCT

2) A black-Body radiator will become a luminary when it is heated up and emitting different color at different temperature (in kevin).
3) A color of a light source is a temperature of the heated black-body radiator having "matching" color to the light source. Therefore, the Color of the light source is correlated with the temperature of the black body, so its full name should be Correlated Color Temperature (CCT).
4) Usually, a light with 2700K-3200K CCT is described as warm white, a light with 4500K-5000K CCT is described as natural white, a light with 6500K-7000K CCT is described as cool white. But, again, it is human eyes dependent.
Note: Daylight CCT at noon time is about 5000K-5500K.

5) For white light LED chip to achieve different CCT, different layer of phosphor will be coated, as a result, different CCT LED will have slightly different spectrum output as below illustrations.

Warm white spectrumNatural white spectrumCool white spectrum
5. What are Luminance and Illuminance?
 
Luminous intensity Steradian

1) Luminous intensity and Candela: Luminous intensity is measure of magnitude or brightness of a light source in a particular direction per unit solid angle. To quanitfy the intensity of light, Candela has been defined. The candela is one of seven base international system of units (SI) with "cd" as symbol, it measures the amount of light passing through in the range of a three-dimensional angular span such a sphere space. The three dimensions angular span is expressed in steradian with "sr" as symbol. One steradian is defined as a unit surface area with solid angle subtended at the center of a unit sphere. That means a 1 meter radius sphere encloses 4 x pi x1 x 1 = 12.57 steradians, one steradian on a sphere with a one meter radius gives one square meter surface area.

Luminous intensity

As a standard, one candela is formally defined as the magnitude of particular electromagnetic radiation at 540 THz (556 nm wavelength), in a specified direction, that has a power level of 1/683 watt per steradian. Therefore, luminous intensity is always described associated with an angle and is irrelevant to measuring distance. For ease of illustration, Let's flatten the three dimension span into two dimension. you will easily understand that screen B would catch exactly the same amount of light rays as screen A, provided that screen A can be removed and not being a shield to screen B. This is because both screen A and B have same solid angle subtended at light source which form same angular span.

Luminous flux

Lumen and candela   integrating sphere

2) Luminous flux and Lumen: Luminous flux is measure of perceived overall light amount emitted by a light source, it is accumulating of all visible wavelengths of light emitted at one time by a light source. Unit of luminous flux is the Lumen with "lm" as symbol. One lumen is defined as light amount as a light source emitting one cd of luminous intensity over a solid angle of one steradian. That is: 1 lm = 1 cd x sr.
To better understand lumen and candela, let's Imagine a transparent 1 meter radius sphere with a candle inside in center of the sphere. the total surface area of the sphere will be: 4 x pi x1 x1 = 12.57 square meter. Assuming the candle is so perfect that its light intensity of every unit three directions space (steradian) is 1 cd (1/683 watt per steradian), thus the amount of light passing through 1 square meter surface of the sphere will be 1 lumen, that also means this particular candle’s luminous flux is 12.57 lumens.
In order to measure luminous flux of a light source, an integrating sphere should be used. It will tell how many lumens a light source can emit as overall light amount at a time, provided the light source has constant luminous flux output.

Lingting concept

3) Luminance: Luminance is a measure of luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted or is reflected from a particular area within a given solid angle. The luminance indicates how much luminous power will be detected by human eyes looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eyes. In video and photography industry, luminance is used to characterize the brightness of a display or an object. Unit for luminance is defined as candela per square meter (cd/m2)

Lux

4) Illuminance and Lux: Illuminance is the total luminous flux amount incident on a unit area surface. Illuminance is invisible! It is amount of light flux passing through space and not seen until those flux fall on a surface and being reflected. In other words we can only see "luminance". It is easy to understand that our eyes are able to sense strength (intensity) of light, but unable to count amount of light. Unit of illuminance is called Lux (lx) and defined as lumens per square meter (Lm/m2).

Illuminance and Lux are used in interior lighting design to describe what level of illumination is required to achieve by selecting and placing right wattage of light sources in right position and layout. For example, 300 lx is usually required in office and 500 lx is usually required in shop etc. Level of illuminance can be measured by an instrument such as Lux Meter.

Traditional light sources are usually with 360 degree beam angle, they rely on fixture's reflection to form certain beam angle,

6. Flicker of artificial light source
 
Flickering

Flicker is cycled light intensity change. It appears in display or some type of artificial light source such as fluorescent lamp. Flicker frequency of fluorescent light source is twice frequency of AC power supply. Cycled light intensity is direct according to cycle of supply current strength which has two peaks in opposite direction within one voltage cycle. In case of 50Hz AC power supply, flickering frequency of fluorescent light is 100Hz.

Eye strain

Human visual nerve centre is capable to distinguish light intensity change frequency below 50Hz (50 cycles a second), therefore, most of displays refresh rate are set 60Hz above in order for us to see a steady still picture, in another word, our brain can not feel flicker of fluorescent light. However our eyes themselves (rather than nerve centre) are able to sense light intensity change of 50-120Hz, that is why we will feel eye strain after watching computer screen or TV for certain time. 100Hz flickering of fluorescent light allow human to work longer than watching monitor without severe eye strain, but still some eye fatigue or even headache when much long time, it varies from person to person.
Using totally flicking- free light source such as LED light is ultimate solution of eliminating eye strain.

7. Beam and field angle of a luminaire
 
Beam and field angle

Beam angle refers to the angle between the two planes of light where the intensity is at least 50% of the maximum intensity at center beam. Field angle refers to the angle between the two planes of light where the intensity is 10% (or less) of the maximum intensity at center beam. Beam angle of a luminaire gives an idea of how the useful light beam spread away form the light beam center, while field angle tells how far the light beam can spead away from beam center.

Traditional light sources are usually with 360 degree beam angle, they rely on fixture's reflection to form certain beam angle. Depending on material and finishing of fixture, luminous flux loss from each reflection could be 5%-15%.